diff --git a/javascript/packages/analysis/src/partial-index-builder.ts b/javascript/packages/analysis/src/partial-index-builder.ts
index 0039fb330..661217a35 100644
--- a/javascript/packages/analysis/src/partial-index-builder.ts
+++ b/javascript/packages/analysis/src/partial-index-builder.ts
@@ -45,11 +45,14 @@ export async function buildPartialIndex(herb: HerbBackend, projectPath: string):
   const viewRoot = await findViewRoot(projectPath)
   const files = await partialsIn(projectPath, viewRoot)
   const declarations = new Map<string, PartialDeclaration>()
+  const filesByName = new Map<string, string[]>()
 
   for (const file of files.sort()) {
     const name = partialNameForFile(file, viewRoot)
     if (name === null) continue
 
+    filesByName.set(name, [...(filesByName.get(name) ?? []), file])
+
     const existing = declarations.get(name)
     if (existing && !outranksTemplate(file, existing.file)) continue
 
@@ -58,7 +61,7 @@ export async function buildPartialIndex(herb: HerbBackend, projectPath: string):
     if (declaration) declarations.set(name, declaration)
   }
 
-  return new PartialIndex(viewRoot, declarations)
+  return new PartialIndex([viewRoot], declarations, filesByName)
 }
 
 export function partialIndexFrom(data: SerializedPartialIndex | undefined): PartialIndex | undefined {
diff --git a/javascript/packages/analysis/src/partial-index.ts b/javascript/packages/analysis/src/partial-index.ts
index b06e7d961..14d6bb9a6 100644
--- a/javascript/packages/analysis/src/partial-index.ts
+++ b/javascript/packages/analysis/src/partial-index.ts
@@ -1,5 +1,5 @@
 import { isERBStrictLocalsNode, isRubyParameterNode } from "@herb-tools/core"
-import { PARTIAL_EXTENSIONS, partialNameForFile, resolvePartial } from "./partial-resolution"
+import { PARTIAL_EXTENSIONS, partialNameForRoots, resolvePartial } from "./partial-resolution"
 
 import type { DocumentNode } from "@herb-tools/core"
 import type { PartialPaths } from "./partial-resolution"
@@ -23,7 +23,7 @@ export interface PartialDeclaration {
 }
 
 export interface SerializedPartialIndex {
-  viewRoot: string
+  viewRoots: string[]
   partials: Record<string, PartialDeclaration>
 }
 
@@ -90,24 +90,24 @@ export function declarationFromDocument(document: DocumentNode, file: string): P
 }
 
 export class PartialIndex {
-  readonly viewRoot: string
+  readonly viewRoots: string[]
 
   private readonly declarations: Map<string, PartialDeclaration>
   private readonly files: PartialPaths
   private readonly byFile: Map<string, PartialDeclaration>
 
   static from(data: SerializedPartialIndex): PartialIndex {
-    return new PartialIndex(data.viewRoot, new Map(Object.entries(data.partials)))
+    return new PartialIndex(data.viewRoots, new Map(Object.entries(data.partials)))
   }
 
-  constructor(viewRoot: string, declarations: Map<string, PartialDeclaration>) {
-    this.viewRoot = viewRoot
+  constructor(viewRoots: string[], declarations: Map<string, PartialDeclaration>, filesByName?: Map<string, string[]>) {
+    this.viewRoots = viewRoots
     this.declarations = declarations
     this.files = new Map()
     this.byFile = new Map()
 
     for (const [name, declaration] of declarations) {
-      this.files.set(name, declaration.file)
+      this.files.set(name, filesByName?.get(name) ?? declaration.file)
       this.byFile.set(declaration.file, declaration)
     }
   }
@@ -117,7 +117,7 @@ export class PartialIndex {
   }
 
   lookup(partialName: string, sourceFile: string | undefined): PartialDeclaration | null {
-    const file = resolvePartial(partialName, sourceFile ?? "", this.files, this.viewRoot)
+    const file = resolvePartial(partialName, sourceFile ?? "", this.files, this.viewRoots)
 
     if (file === null) return null
 
@@ -125,7 +125,7 @@ export class PartialIndex {
   }
 
   update(declaration: PartialDeclaration): string | null {
-    const name = partialNameForFile(declaration.file, this.viewRoot)
+    const name = partialNameForRoots(declaration.file, this.viewRoots)
     if (name === null) return null
 
     const existing = this.declarations.get(name)
@@ -144,7 +144,7 @@ export class PartialIndex {
   }
 
   remove(file: string): string | null {
-    const name = partialNameForFile(file, this.viewRoot)
+    const name = partialNameForRoots(file, this.viewRoots)
     if (name === null) return null
 
     const existing = this.declarations.get(name)
@@ -162,6 +162,6 @@ export class PartialIndex {
   }
 
   toJSON(): SerializedPartialIndex {
-    return { viewRoot: this.viewRoot, partials: Object.fromEntries(this.declarations) }
+    return { viewRoots: this.viewRoots, partials: Object.fromEntries(this.declarations) }
   }
 }
diff --git a/javascript/packages/analysis/src/partial-resolution.ts b/javascript/packages/analysis/src/partial-resolution.ts
index d20a08ad1..1e05eafbf 100644
--- a/javascript/packages/analysis/src/partial-resolution.ts
+++ b/javascript/packages/analysis/src/partial-resolution.ts
@@ -15,7 +15,7 @@ export const PARTIAL_GLOB_PATTERN = `_${TEMPLATE_GLOB_PATTERN}`
 const PARTIAL_PREFIX = "_"
 const APPLICATION_DIRECTORY = "application"
 
-export type PartialPaths = Map<string, string>
+export type PartialPaths = Map<string, string | string[]>
 
 function normalize(path: string): string {
   const separated = path.replace(/\\/g, "/")
@@ -50,12 +50,120 @@ function relativeToViewRoot(path: string, viewRoot: string): string | null {
   return normalizedPath.slice(normalizedRoot.length + 1)
 }
 
+export function relativeToViewRoots(path: string, viewRoots: string[]): [number, string] | null {
+  for (const [index, root] of viewRoots.entries()) {
+    const relative = relativeToViewRoot(path, root)
+
+    if (relative !== null) return [index, relative]
+  }
+
+  return null
+}
+
+export function partialNameForRoots(filePath: string, viewRoots: string[]): string | null {
+  for (const root of viewRoots) {
+    const name = partialNameForFile(filePath, root)
+
+    if (name !== null) return name
+  }
+
+  return null
+}
+
+export function templateNameForRoots(filePath: string, viewRoots: string[]): string | null {
+  for (const root of viewRoots) {
+    const name = templateNameForFile(filePath, root)
+
+    if (name !== null) return name
+  }
+
+  return null
+}
+
+export function rootIndexFor(filePath: string, viewRoots: string[]): number {
+  return relativeToViewRoots(filePath, viewRoots)?.[0] ?? viewRoots.length
+}
+
 export function projectRelativePath(filePath: string, projectPath: string | undefined): string {
   if (!projectPath) return normalize(filePath)
 
   return relativeToViewRoot(filePath, projectPath) ?? normalize(filePath)
 }
 
+export function formatOf(filePath: string): string | null {
+  const name = basename(normalize(filePath))
+  const dot = name.indexOf(".")
+
+  if (dot === -1) return null
+
+  const extension = name.slice(dot)
+  const stripped = extension.endsWith(".erb")
+    ? extension.slice(0, -".erb".length)
+    : extension.endsWith(".herb")
+      ? extension.slice(0, -".herb".length)
+      : null
+
+  if (stripped === null) return null
+
+  const segments = stripped.startsWith(".") ? stripped.slice(1) : stripped
+  const last = segments.split(".").pop() ?? segments
+  const format = last.split("+")[0] ?? last
+
+  return format === "" ? null : format
+}
+
+export function hasLocale(filePath: string): boolean {
+  const name = basename(normalize(filePath))
+  const dot = name.indexOf(".")
+
+  if (dot === -1) return false
+
+  const extension = name.slice(dot)
+  const stripped = extension.endsWith(".erb")
+    ? extension.slice(0, -".erb".length)
+    : extension.endsWith(".herb")
+      ? extension.slice(0, -".herb".length)
+      : null
+
+  if (stripped === null) return false
+
+  const segments = (stripped.startsWith(".") ? stripped.slice(1) : stripped).split(".")
+
+  return segments.length > 1
+}
+
+export function variantOf(filePath: string): string | null {
+  const name = basename(normalize(filePath))
+  const dot = name.indexOf(".")
+
+  if (dot === -1) return null
+
+  const extension = name.slice(dot)
+  const stripped = extension.endsWith(".erb")
+    ? extension.slice(0, -".erb".length)
+    : extension.endsWith(".herb")
+      ? extension.slice(0, -".herb".length)
+      : null
+
+  if (stripped === null) return null
+
+  const plus = stripped.indexOf("+")
+
+  if (plus === -1) return null
+
+  const variant = stripped.slice(plus + 1)
+
+  return variant === "" ? null : variant
+}
+
+export function withoutTemplateExtension(partialName: string): string {
+  for (const extension of PARTIAL_EXTENSIONS) {
+    if (partialName.endsWith(extension)) return partialName.slice(0, -extension.length)
+  }
+
+  return partialName
+}
+
 export function isTemplatePath(filePath: string): boolean {
   const name = basename(normalize(filePath))
 
@@ -111,6 +219,16 @@ export function templateNameForFile(filePath: string, viewRoot: string): string
   return directory === "." ? withoutExtension : `${directory}/${withoutExtension}`
 }
 
+export function layoutCandidatesForRoots(templateFile: string, viewRoots: string[]): string[] {
+  for (const root of viewRoots) {
+    const candidates = layoutCandidatesFor(templateFile, root)
+
+    if (candidates.length > 0) return candidates
+  }
+
+  return []
+}
+
 export function layoutCandidatesFor(templateFile: string, viewRoot: string): string[] {
   const relative = relativeToViewRoot(normalize(templateFile), viewRoot)
 
@@ -136,23 +254,50 @@ export function layoutCandidatesFor(templateFile: string, viewRoot: string): str
   return candidates
 }
 
-export function resolvePartial(partialName: string, sourceFile: string, index: PartialPaths, viewRoot: string): string | null {
+function pickForCaller(candidates: string | string[], sourceFile: string): string | null {
+  if (!Array.isArray(candidates)) return candidates
+  if (candidates.length === 0) return null
+
+  const format = formatOf(sourceFile)
+
+  if (format === null) return candidates[0] ?? null
+
+  const ranked = [...candidates].sort((a, b) => rankForFormat(a, format) - rankForFormat(b, format))
+
+  return ranked[0] ?? null
+}
+
+function rankForFormat(file: string, format: string): number {
+  const candidate = formatOf(file)
+  const matches = candidate === format ? 0 : candidate === null ? 1 : 2
+
+  return matches * 4 + (variantOf(file) === null ? 0 : 2) + (hasLocale(file) ? 1 : 0)
+}
+
+export function resolvePartial(
+  partialName: string,
+  sourceFile: string,
+  index: PartialPaths,
+  viewRoots: string[]
+): string | null {
+  partialName = withoutTemplateExtension(partialName)
+
   const exact = index.get(partialName)
 
-  if (exact !== undefined) return exact
+  if (exact !== undefined) return pickForCaller(exact, sourceFile)
 
-  const sourceDirectory = relativeToViewRoot(dirname(normalize(sourceFile)), viewRoot)
+  const sourceDirectory = relativeToViewRoots(dirname(normalize(sourceFile)), viewRoots)?.[1] ?? null
 
   if (sourceDirectory !== null && sourceDirectory !== ".") {
     const relative = index.get(`${sourceDirectory}/${partialName}`)
 
-    if (relative !== undefined) return relative
+    if (relative !== undefined) return pickForCaller(relative, sourceFile)
   }
 
   if (!partialName.includes("/")) {
     const application = index.get(`${APPLICATION_DIRECTORY}/${partialName}`)
 
-    if (application !== undefined) return application
+    if (application !== undefined) return pickForCaller(application, sourceFile)
   }
 
   return null
diff --git a/javascript/packages/analysis/src/project-index.ts b/javascript/packages/analysis/src/project-index.ts
index baf60e444..d491e0e9b 100644
--- a/javascript/packages/analysis/src/project-index.ts
+++ b/javascript/packages/analysis/src/project-index.ts
@@ -58,8 +58,8 @@ export class ProjectIndex {
     return this.callerIndex
   }
 
-  get viewRoot(): string | undefined {
-    return this.partialIndex?.viewRoot
+  get viewRoots(): string[] | undefined {
+    return this.partialIndex?.viewRoots
   }
 
   async indexAll(): Promise<void> {
@@ -71,7 +71,7 @@ export class ProjectIndex {
     try {
       this.partialIndex = await buildPartialIndex(this.backend, this.root)
 
-      this.logger?.log(`[Partials] Indexed ${this.partialIndex.size} partials under ${this.partialIndex.viewRoot}`)
+      this.logger?.log(`[Partials] Indexed ${this.partialIndex.size} partials under ${this.partialIndex.viewRoots.join(", ")}`)
     } catch (error) {
       this.logger?.warn(`[Partials] Failed to index partials: ${this.messageFor(error)}`)
     }
diff --git a/javascript/packages/analysis/src/render-graph-builder.ts b/javascript/packages/analysis/src/render-graph-builder.ts
index bbc6431f5..1c36ad3c9 100644
--- a/javascript/packages/analysis/src/render-graph-builder.ts
+++ b/javascript/packages/analysis/src/render-graph-builder.ts
@@ -6,7 +6,7 @@ import { readFileSync } from "node:fs"
 
 import { getTagLocalName, isERBCaseNode, isERBIfNode, isERBOutputNode, isERBRenderNode, isERBUnlessNode, isHTMLElementNode, isPrismNodeType, isRubyRenderLocalNode } from "@herb-tools/core"
 import { outranksTemplate } from "./partial-index"
-import { layoutCandidatesFor, templateNameForFile, isPartialPath } from "./partial-resolution"
+import { layoutCandidatesForRoots, templateNameForRoots, isPartialPath } from "./partial-resolution"
 import { renderPartialExpression } from "./render-expression"
 import { staticAncestorAttributes } from "./ancestor-attributes"
 
@@ -227,11 +227,11 @@ export function collectCallSites(herb: HerbBackend, partials: PartialIndex, file
   return { unresolved, isDocumentRoot, yields, roots: { ...roots, renders: rootRenders, resolved: rootsResolved } }
 }
 
-function addLayoutCallSites(files: string[], layoutYields: Map<string, YieldSite[]>, viewRoot: string, callSites: Map<string, PartialCallSite[]>): void {
+function addLayoutCallSites(files: string[], layoutYields: Map<string, YieldSite[]>, viewRoots: string[], callSites: Map<string, PartialCallSite[]>): void {
   const layouts = new Map<string, string>()
 
   for (const file of files) {
-    const name = templateNameForFile(file, viewRoot)
+    const name = templateNameForRoots(file, viewRoots)
 
     if (name === null || !layoutYields.has(file)) {
       continue
@@ -247,7 +247,7 @@ function addLayoutCallSites(files: string[], layoutYields: Map<string, YieldSite
   }
 
   for (const file of files) {
-    for (const candidate of layoutCandidatesFor(file, viewRoot)) {
+    for (const candidate of layoutCandidatesForRoots(file, viewRoots)) {
       const layout = layouts.get(candidate)
 
       if (!layout || layout === file) {
@@ -268,7 +268,7 @@ function addLayoutCallSites(files: string[], layoutYields: Map<string, YieldSite
 }
 
 export async function buildRenderGraph(herb: HerbBackend, projectPath: string, partials: PartialIndex, options: RenderGraphOptions = {}): Promise<RenderGraph> {
-  const files = await templatesIn(projectPath, partials.viewRoot, options.include ?? [])
+  const files = await templatesIn(projectPath, partials.viewRoots[0] ?? ".", options.include ?? [])
   const excluded = options.exclude?.length ? picomatch(options.exclude) : null
   const callSites = new Map<string, PartialCallSite[]>()
   const documentRoots = new Set<string>()
@@ -318,7 +318,7 @@ export async function buildRenderGraph(herb: HerbBackend, projectPath: string, p
   }
 
   if (options.resolveLayouts !== false) {
-    addLayoutCallSites(scanned, layoutYields, partials.viewRoot, callSites)
+    addLayoutCallSites(scanned, layoutYields, partials.viewRoots, callSites)
   }
 
   return new RenderGraph(callSites, roots, documentRoots, unresolvedRenders, skippedFiles)
diff --git a/javascript/packages/analysis/test/partial-index-builder.test.ts b/javascript/packages/analysis/test/partial-index-builder.test.ts
index afed4e34d..74f0f38e9 100644
--- a/javascript/packages/analysis/test/partial-index-builder.test.ts
+++ b/javascript/packages/analysis/test/partial-index-builder.test.ts
@@ -84,7 +84,7 @@ describe("buildPartialIndex", () => {
 
     const index = await buildPartialIndex(Herb, root)
 
-    expect(index.viewRoot).toBe("app/views")
+    expect(index.viewRoots).toEqual(["app/views"])
     expect(index.size).toBe(2)
 
     expect(index.lookup("users/card", "app/views/posts/index.html.erb")).toEqual({
@@ -126,7 +126,7 @@ describe("buildPartialIndex", () => {
 
     const index = await buildPartialIndex(Herb, root)
 
-    expect(index.viewRoot).toBe(".")
+    expect(index.viewRoots).toEqual(["."])
     expect(index.lookup("views/card", "index.html.erb")?.file).toBe("views/_card.html.erb")
   })
 
diff --git a/javascript/packages/analysis/test/partial-index.test.ts b/javascript/packages/analysis/test/partial-index.test.ts
index b708507ac..cc2b0ad86 100644
--- a/javascript/packages/analysis/test/partial-index.test.ts
+++ b/javascript/packages/analysis/test/partial-index.test.ts
@@ -15,7 +15,7 @@ beforeAll(async () => {
 })
 
 describe("PartialIndex", () => {
-  const index = new PartialIndex("app/views", new Map([
+  const index = new PartialIndex(["app/views"], new Map([
     ["users/card", declaration("app/views/users/_card.html.erb", [{ name: "user", required: true }])],
     ["application/flash", declaration("app/views/application/_flash.html.erb", [{ name: "message", required: true }])],
   ]))
@@ -43,7 +43,7 @@ describe("PartialIndex", () => {
   test("round trips through its serialized form", () => {
     const restored = PartialIndex.from(index.toJSON())
 
-    expect(restored.viewRoot).toBe("app/views")
+    expect(restored.viewRoots).toEqual(["app/views"])
     expect(restored.size).toBe(2)
     expect(restored.lookup("users/card", "app/views/posts/index.html.erb")?.locals).toEqual([{ name: "user", required: true }])
   })
@@ -57,7 +57,7 @@ describe("PartialIndex", () => {
 
 describe("PartialIndex updates", () => {
   function index(): PartialIndex {
-    return new PartialIndex("app/views", new Map([
+    return new PartialIndex(["app/views"], new Map([
       ["users/card", declaration("app/views/users/_card.html.erb", [{ name: "user", required: true }])],
     ]))
   }
@@ -120,7 +120,7 @@ describe("PartialIndex updates", () => {
   })
 
   test("lets the base template take over from a variant", () => {
-    const partials = new PartialIndex("app/views", new Map([
+    const partials = new PartialIndex(["app/views"], new Map([
       ["users/card", declaration("app/views/users/_card.en.html.erb", [{ name: "user", required: true }])],
     ]))
 
@@ -130,7 +130,7 @@ describe("PartialIndex updates", () => {
   })
 
   test("forgets the displaced variant when the base template takes over", () => {
-    const partials = new PartialIndex("app/views", new Map([
+    const partials = new PartialIndex(["app/views"], new Map([
       ["users/card", declaration("app/views/users/_card.html+phone.erb", [])],
     ]))
 
diff --git a/javascript/packages/analysis/test/partial-resolution.test.ts b/javascript/packages/analysis/test/partial-resolution.test.ts
index 1e5f9ff12..89ccb0bcc 100644
--- a/javascript/packages/analysis/test/partial-resolution.test.ts
+++ b/javascript/packages/analysis/test/partial-resolution.test.ts
@@ -1,6 +1,8 @@
 import { describe, test, expect } from "vitest"
 
 import {
+  formatOf,
+  variantOf,
   isPartialPath,
   partialNameForFile,
   resolvePartial,
@@ -126,45 +128,118 @@ describe("@herb-tools/core", () => {
     ])
 
     test("resolves a fully qualified name", () => {
-      expect(resolvePartial("users/card", "app/views/posts/index.html.erb", index, VIEW_ROOT)).toBe("app/views/users/_card.html.erb")
+      expect(resolvePartial("users/card", "app/views/posts/index.html.erb", index, [VIEW_ROOT])).toBe("app/views/users/_card.html.erb")
     })
 
     test("resolves a bare name against the rendering template's directory", () => {
-      expect(resolvePartial("avatar", "app/views/users/show.html.erb", index, VIEW_ROOT)).toBe("app/views/users/_avatar.html.erb")
+      expect(resolvePartial("avatar", "app/views/users/show.html.erb", index, [VIEW_ROOT])).toBe("app/views/users/_avatar.html.erb")
     })
 
     test("falls back to the application directory for a bare name", () => {
-      expect(resolvePartial("flash", "app/views/posts/index.html.erb", index, VIEW_ROOT)).toBe("app/views/application/_flash.html.erb")
+      expect(resolvePartial("flash", "app/views/posts/index.html.erb", index, [VIEW_ROOT])).toBe("app/views/application/_flash.html.erb")
     })
 
     test("prefers the exact name over the relative one", () => {
-      expect(resolvePartial("users/card", "app/views/admin/index.html.erb", index, VIEW_ROOT)).toBe("app/views/users/_card.html.erb")
+      expect(resolvePartial("users/card", "app/views/admin/index.html.erb", index, [VIEW_ROOT])).toBe("app/views/users/_card.html.erb")
     })
 
     test("resolves a qualified name relative to the rendering template's directory", () => {
       const nested = paths(["app/views/admin/users/_card.html.erb"])
 
-      expect(resolvePartial("users/card", "app/views/admin/index.html.erb", nested, VIEW_ROOT)).toBe("app/views/admin/users/_card.html.erb")
+      expect(resolvePartial("users/card", "app/views/admin/index.html.erb", nested, [VIEW_ROOT])).toBe("app/views/admin/users/_card.html.erb")
     })
 
     test("does not fall back to the application directory for a qualified name", () => {
-      expect(resolvePartial("users/flash", "app/views/posts/index.html.erb", index, VIEW_ROOT)).toBeNull()
+      expect(resolvePartial("users/flash", "app/views/posts/index.html.erb", index, [VIEW_ROOT])).toBeNull()
     })
 
     test("returns null for an unknown partial", () => {
-      expect(resolvePartial("users/missing", "app/views/posts/index.html.erb", index, VIEW_ROOT)).toBeNull()
+      expect(resolvePartial("users/missing", "app/views/posts/index.html.erb", index, [VIEW_ROOT])).toBeNull()
     })
 
     test("resolves from a template at the view root", () => {
-      expect(resolvePartial("flash", "app/views/index.html.erb", index, VIEW_ROOT)).toBe("app/views/application/_flash.html.erb")
+      expect(resolvePartial("flash", "app/views/index.html.erb", index, [VIEW_ROOT])).toBe("app/views/application/_flash.html.erb")
     })
 
     test("resolves from a source file outside the view root", () => {
-      expect(resolvePartial("users/card", "app/components/card_component.html.erb", index, VIEW_ROOT)).toBe("app/views/users/_card.html.erb")
+      expect(resolvePartial("users/card", "app/components/card_component.html.erb", index, [VIEW_ROOT])).toBe("app/views/users/_card.html.erb")
     })
 
     test("resolves without a known source file", () => {
-      expect(resolvePartial("users/card", "", index, VIEW_ROOT)).toBe("app/views/users/_card.html.erb")
+      expect(resolvePartial("users/card", "", index, [VIEW_ROOT])).toBe("app/views/users/_card.html.erb")
     })
   })
 })
+
+describe("formatOf", () => {
+  test("reads the format out of a filename", () => {
+    expect(formatOf("app/views/posts/_row.html.erb")).toBe("html")
+    expect(formatOf("app/views/posts/_row.turbo_stream.erb")).toBe("turbo_stream")
+    expect(formatOf("app/views/posts/_row.html.herb")).toBe("html")
+    expect(formatOf("app/views/posts/_row.en.html.erb")).toBe("html")
+  })
+
+  test("returns null when the filename carries no format", () => {
+    expect(formatOf("app/views/posts/_row.erb")).toBeNull()
+    expect(formatOf("app/views/posts/_row.herb")).toBeNull()
+  })
+})
+
+describe("variantOf", () => {
+  test("reads the variant out of a filename", () => {
+    expect(variantOf("app/views/posts/_row.html+mobile.erb")).toBe("mobile")
+    expect(variantOf("app/views/posts/_row.html+tablet.herb")).toBe("tablet")
+  })
+
+  test("returns null when the filename carries no variant", () => {
+    expect(variantOf("app/views/posts/_row.html.erb")).toBeNull()
+    expect(variantOf("app/views/posts/_row.erb")).toBeNull()
+  })
+
+  test("a variant keeps the format of its base template", () => {
+    expect(formatOf("app/views/posts/_row.html+mobile.erb")).toBe("html")
+    expect(formatOf("app/views/posts/_row.turbo_stream+mobile.erb")).toBe("turbo_stream")
+  })
+})
+
+describe("format-aware resolution", () => {
+  const HTML_CALLER = "app/views/posts/index.html.erb"
+  const TURBO_CALLER = "app/views/posts/index.turbo_stream.erb"
+
+  test("a caller reaches the partial matching its own format", () => {
+    const index: PartialPaths = new Map([
+      ["posts/row", ["app/views/posts/_row.html.erb", "app/views/posts/_row.turbo_stream.erb"]],
+    ])
+
+    expect(resolvePartial("posts/row", HTML_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.html.erb")
+    expect(resolvePartial("posts/row", TURBO_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.turbo_stream.erb")
+  })
+
+  test("a formatless partial serves any caller", () => {
+    const index: PartialPaths = new Map([["posts/row", ["app/views/posts/_row.erb"]]])
+
+    expect(resolvePartial("posts/row", TURBO_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.erb")
+  })
+
+  test("a formatless partial loses to an exact format match", () => {
+    const index: PartialPaths = new Map([
+      ["posts/row", ["app/views/posts/_row.erb", "app/views/posts/_row.turbo_stream.erb"]],
+    ])
+
+    expect(resolvePartial("posts/row", TURBO_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.turbo_stream.erb")
+  })
+
+  test("the plain template is preferred over a variant", () => {
+    const index: PartialPaths = new Map([
+      ["posts/row", ["app/views/posts/_row.html+mobile.erb", "app/views/posts/_row.html.erb"]],
+    ])
+
+    expect(resolvePartial("posts/row", HTML_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.html.erb")
+  })
+
+  test("a single file still resolves", () => {
+    const index: PartialPaths = new Map([["posts/row", "app/views/posts/_row.html.erb"]])
+
+    expect(resolvePartial("posts/row", HTML_CALLER, index, [VIEW_ROOT])).toBe("app/views/posts/_row.html.erb")
+  })
+})
diff --git a/javascript/packages/analysis/test/project-index.test.ts b/javascript/packages/analysis/test/project-index.test.ts
index 910dcb46f..a18cdfe1e 100644
--- a/javascript/packages/analysis/test/project-index.test.ts
+++ b/javascript/packages/analysis/test/project-index.test.ts
@@ -64,13 +64,13 @@ describe("ProjectIndex", () => {
     test("reports the view root it found", async () => {
       const index = await analyzerFor()
 
-      expect(index.viewRoot).toBe("app/views")
+      expect(index.viewRoots).toEqual(["app/views"])
     })
 
     test("falls back to the project root when there is no app/views", async () => {
       const index = await analyzerFor({ "templates/_card.html.erb": `<article></article>\n` })
 
-      expect(index.viewRoot).toBe(".")
+      expect(index.viewRoots).toEqual(["."])
     })
   })
 
diff --git a/javascript/packages/analysis/test/view-roots.test.ts b/javascript/packages/analysis/test/view-roots.test.ts
new file mode 100644
index 000000000..bbfe77ec8
--- /dev/null
+++ b/javascript/packages/analysis/test/view-roots.test.ts
@@ -0,0 +1,83 @@
+import { describe, test, expect } from "vitest"
+
+import { PartialIndex } from "../src/partial-index"
+
+import {
+  partialNameForRoots,
+  relativeToViewRoots,
+  resolvePartial,
+  rootIndexFor,
+  templateNameForRoots,
+} from "../src/partial-resolution"
+
+import type { PartialPaths } from "../src/partial-resolution"
+
+const APP = "app/views"
+const ENGINE = "engines/billing/app/views"
+const ROOTS = [APP, ENGINE]
+
+describe("relativeToViewRoots", () => {
+  test("returns the first root that contains the file", () => {
+    expect(relativeToViewRoots(`${APP}/home/index.html.erb`, ROOTS)).toEqual([0, "home/index.html.erb"])
+    expect(relativeToViewRoots(`${ENGINE}/billing/_invoice.html.erb`, ROOTS)).toEqual([1, "billing/_invoice.html.erb"])
+  })
+
+  test("returns null when no root contains the file", () => {
+    expect(relativeToViewRoots("lib/elsewhere/_thing.html.erb", ROOTS)).toBeNull()
+  })
+})
+
+describe("partialNameForRoots", () => {
+  test("names a partial from a secondary view root", () => {
+    expect(partialNameForRoots(`${ENGINE}/billing/_invoice.html.erb`, ROOTS)).toBe("billing/invoice")
+  })
+
+  test("names a partial from the primary view root", () => {
+    expect(partialNameForRoots(`${APP}/shared/_header.html.erb`, ROOTS)).toBe("shared/header")
+  })
+
+  test("returns null for a file outside every root", () => {
+    expect(partialNameForRoots("lib/_thing.html.erb", ROOTS)).toBeNull()
+  })
+})
+
+describe("templateNameForRoots", () => {
+  test("names a template from a secondary view root", () => {
+    expect(templateNameForRoots(`${ENGINE}/billing/index.html.erb`, ROOTS)).toBe("billing/index")
+  })
+})
+
+describe("rootIndexFor", () => {
+  test("orders an earlier view root ahead of a later one", () => {
+    expect(rootIndexFor(`${APP}/billing/_invoice.html.erb`, ROOTS)).toBe(0)
+    expect(rootIndexFor(`${ENGINE}/billing/_invoice.html.erb`, ROOTS)).toBe(1)
+  })
+
+  test("sorts an unknown file last", () => {
+    expect(rootIndexFor("lib/_thing.html.erb", ROOTS)).toBe(ROOTS.length)
+  })
+})
+
+describe("resolvePartial", () => {
+  test("resolves a sibling within the root that owns the caller", () => {
+    const index: PartialPaths = new Map([["billing/row", `${ENGINE}/billing/_row.html.erb`]])
+    const caller = `${ENGINE}/billing/index.html.erb`
+
+    expect(resolvePartial("row", caller, index, ROOTS)).toBe(`${ENGINE}/billing/_row.html.erb`)
+  })
+
+  test("resolves with a single root", () => {
+    const index: PartialPaths = new Map([["shared/header", `${APP}/shared/_header.html.erb`]])
+
+    expect(resolvePartial("shared/header", "", index, [APP])).toBe(`${APP}/shared/_header.html.erb`)
+  })
+})
+
+describe("serialization", () => {
+  test("round-trips every view root", () => {
+    const index = new PartialIndex(ROOTS, new Map())
+    const restored = PartialIndex.from(index.toJSON())
+
+    expect(restored.viewRoots).toEqual(ROOTS)
+  })
+})
diff --git a/javascript/packages/language-server/src/session.ts b/javascript/packages/language-server/src/session.ts
index dd238bd9c..5a31f6ca0 100644
--- a/javascript/packages/language-server/src/session.ts
+++ b/javascript/packages/language-server/src/session.ts
@@ -105,7 +105,7 @@ export class Session {
 
   private viewRootFor(documentPath: string): string | null {
     const project = this.projects.containing(documentPath)
-    const viewRoot = project?.index.viewRoot
+    const viewRoot = project?.index.viewRoots?.[0]
 
     if (!project || viewRoot === undefined) return null
 
diff --git a/javascript/packages/language-service/src/completion_provider.ts b/javascript/packages/language-service/src/completion_provider.ts
index bf28a3f96..246346dcd 100644
--- a/javascript/packages/language-service/src/completion_provider.ts
+++ b/javascript/packages/language-service/src/completion_provider.ts
@@ -464,7 +464,7 @@ export class CompletionProvider {
     if (!partials) return null
 
     const file = this.relativePathFor(document.uri)
-    const directory = file === null ? null : this.directoryOf(file, partials.viewRoot)
+    const directory = file === null ? null : this.directoryOf(file, partials.viewRoots[0] ?? ".")
     const lowercasePrefix = prefix.toLowerCase()
 
     const nameRange = Range.create(document.positionAt(document.offsetAt(position) - prefix.length), position)
diff --git a/javascript/packages/linter/src/rules/actionview-prefer-qualified-partial-path.ts b/javascript/packages/linter/src/rules/actionview-prefer-qualified-partial-path.ts
index 8f1b73cb6..9867b7414 100644
--- a/javascript/packages/linter/src/rules/actionview-prefer-qualified-partial-path.ts
+++ b/javascript/packages/linter/src/rules/actionview-prefer-qualified-partial-path.ts
@@ -3,7 +3,7 @@ import { ParserRule } from "../types.js"
 import { renderPartialExpression } from "@herb-tools/analysis"
 
 import { isERBOutputNode, isPrismNodeType, locationFromByteOffset, substringFromByteOffset } from "@herb-tools/core"
-import { partialNameForFile } from "@herb-tools/analysis"
+import { partialNameForRoots } from "@herb-tools/analysis"
 
 import type { ERBRenderNode, ParseResult, ParserOptions, PrismNode } from "@herb-tools/core"
 import type { BaseAutofixContext, FullRuleConfig, LintContext, LintOffense, Mutable, UnboundLintOffense } from "../types.js"
@@ -106,7 +106,7 @@ class ActionViewPreferQualifiedPartialPathVisitor extends BaseRuleVisitor<Prefer
 
     if (!declaration) return null
 
-    const qualified = partialNameForFile(declaration.file, partials.viewRoot)
+    const qualified = partialNameForRoots(declaration.file, partials.viewRoots)
 
     if (!qualified) return null
 
diff --git a/javascript/packages/linter/test/autofix/actionview-prefer-qualified-partial-path.autofix.test.ts b/javascript/packages/linter/test/autofix/actionview-prefer-qualified-partial-path.autofix.test.ts
index e1cbf4e88..c6a9bc25a 100644
--- a/javascript/packages/linter/test/autofix/actionview-prefer-qualified-partial-path.autofix.test.ts
+++ b/javascript/packages/linter/test/autofix/actionview-prefer-qualified-partial-path.autofix.test.ts
@@ -11,7 +11,7 @@ function declaration(file: string): PartialDeclaration {
   return { file, hasDeclaration: false, hasKeywordRest: false, locals: [] }
 }
 
-const partials = new PartialIndex("app/views", new Map([
+const partials = new PartialIndex(["app/views"], new Map([
   ["posts/card", declaration("app/views/posts/_card.html.erb")],
   ["posts/row", declaration("app/views/posts/_row.html.erb")],
   ["application/flash", declaration("app/views/application/_flash.html.erb")],
diff --git a/javascript/packages/linter/test/rules/actionview-no-strict-locals-error.test.ts b/javascript/packages/linter/test/rules/actionview-no-strict-locals-error.test.ts
index af3a105e4..bdd3df895 100644
--- a/javascript/packages/linter/test/rules/actionview-no-strict-locals-error.test.ts
+++ b/javascript/packages/linter/test/rules/actionview-no-strict-locals-error.test.ts
@@ -15,7 +15,7 @@ function declaration(file: string, locals: PartialDeclaration["locals"], overrid
   return { file, hasDeclaration: true, hasKeywordRest: false, locals, ...overrides }
 }
 
-const partials = new PartialIndex("app/views", new Map([
+const partials = new PartialIndex(["app/views"], new Map([
   ["users/card", declaration("app/views/users/_card.html.erb", [
     { name: "user", required: true },
     { name: "size", required: false },
@@ -230,7 +230,7 @@ describe("actionview-no-strict-locals-error", () => {
   })
 
   describe("declaration frames", () => {
-    const located = new PartialIndex("app/views", new Map([
+    const located = new PartialIndex(["app/views"], new Map([
       ["users/card", { ...declaration("app/views/users/_card.html.erb", [{ name: "user", required: true }]), location: { line: 1, column: 0 } }],
       ["users/plain", declaration("app/views/users/_plain.html.erb", [{ name: "user", required: true }])],
     ]))
diff --git a/javascript/packages/linter/test/rules/actionview-prefer-qualified-partial-path.test.ts b/javascript/packages/linter/test/rules/actionview-prefer-qualified-partial-path.test.ts
index b10f17d61..6c917b5f6 100644
--- a/javascript/packages/linter/test/rules/actionview-prefer-qualified-partial-path.test.ts
+++ b/javascript/packages/linter/test/rules/actionview-prefer-qualified-partial-path.test.ts
@@ -13,7 +13,7 @@ function declaration(file: string): PartialDeclaration {
   return { file, hasDeclaration: false, hasKeywordRest: false, locals: [] }
 }
 
-const partials = new PartialIndex("app/views", new Map([
+const partials = new PartialIndex(["app/views"], new Map([
   ["posts/card", declaration("app/views/posts/_card.html.erb")],
   ["application/flash", declaration("app/views/application/_flash.html.erb")],
 ]))
@@ -70,7 +70,7 @@ describe("actionview-prefer-qualified-partial-path", () => {
   test("falls back to the generic advice when the partial does not resolve", () => {
     expectInfo(GENERIC)
 
-    assertOffenses(`<%= render "card" %>`, { fileName: "app/views/nowhere/index.html.erb", partials: new PartialIndex("app/views", new Map()) })
+    assertOffenses(`<%= render "card" %>`, { fileName: "app/views/nowhere/index.html.erb", partials: new PartialIndex(["app/views"], new Map()) })
   })
 
   test("does not flag a qualified path in the shorthand form", () => {
diff --git a/lib/herb/analysis/partial_index.rb b/lib/herb/analysis/partial_index.rb
index cc4ad92c8..3f88a98a4 100644
--- a/lib/herb/analysis/partial_index.rb
+++ b/lib/herb/analysis/partial_index.rb
@@ -10,8 +10,6 @@ module Analysis
     class PartialIndex
       APPLICATION_DIRECTORY = "application" #: String
 
-      attr_reader :view_root #: Pathname
-
       attr_reader :templates #: Array[String]
 
       #: (String | Pathname, ?templates: Array[String]?) -> PartialIndex
@@ -20,7 +18,7 @@ def self.build(project_path, templates: nil)
         view_root = resolve_view_root(root)
         files = templates || Dir[view_root.join("**", PartialResolution::TEMPLATE_GLOB_PATTERN)].sort
 
-        new(view_root, files)
+        new([view_root], files)
       end
 
       #: (String | Pathname) -> Pathname
@@ -28,14 +26,17 @@ def self.resolve_view_root(project_path)
         PartialResolution.view_root_for(project_path)
       end
 
-      #: (String | Pathname, Array[String]) -> void
-      def initialize(view_root, templates)
-        @view_root = Pathname.new(view_root)
+      #: (Array[String | Pathname], Array[String]) -> void
+      def initialize(view_roots, templates)
+        @view_roots = view_roots.map { |root| Pathname.new(root) } #: Array[Pathname]
         @templates = templates
         @by_name = build_index(templates)
         @declarations = {} #: Hash[String, PartialDeclaration?]
       end
 
+      #: () -> Array[Pathname]
+      attr_reader :view_roots
+
       #: (String?) -> Array[String]
       def files_for(partial_name)
         return [] unless partial_name
@@ -43,10 +44,34 @@ def files_for(partial_name)
         @by_name[partial_name] || []
       end
 
+      #: (String?, String?) -> Array[String]
       #: (String?, String?) -> Array[String]
       def resolve(partial_name, source_file)
+        candidates = candidates_for(partial_name, source_file)
+        format = source_file ? PartialResolution.format_of(source_file) : nil
+
+        return candidates unless format
+
+        candidates.sort_by do |file|
+          candidate = PartialResolution.format_of(file)
+
+          matches = if candidate == format
+                      0
+                    elsif candidate.nil?
+                      1
+                    else
+                      2
+                    end
+
+          [matches, PartialResolution.variant_of(file) ? 1 : 0, PartialResolution.has_locale?(file) ? 1 : 0]
+        end
+      end
+
+      #: (String?, String?) -> Array[String]
+      def candidates_for(partial_name, source_file)
         return [] unless partial_name
 
+        partial_name = PartialResolution.without_template_extension(partial_name)
         exact = files_for(partial_name)
 
         return exact if exact.any?
@@ -73,7 +98,7 @@ def self.partial_name_for(file, view_root)
 
       #: (String) -> String?
       def partial_name_for(file)
-        self.class.partial_name_for(file, @view_root)
+        PartialResolution.partial_name_for_roots(file, @view_roots)
       end
 
       #: () -> Array[String]
@@ -105,7 +130,9 @@ def update(file)
         @templates = (@templates | [file]).sort
 
         files = (@by_name[name] || []) | [file]
-        @by_name[name] = PartialResolution.by_precedence(files)
+        ordered = PartialResolution.by_precedence(files)
+
+        @by_name[name] = ordered.sort_by { |candidate| PartialResolution.root_index_for(candidate, @view_roots) }
 
         name
       end
@@ -152,7 +179,10 @@ def to_h
 
       #: (String) -> String?
       def source_directory_for(source_file)
-        Pathname.new(File.dirname(source_file)).relative_path_from(@view_root).to_s
+        directory = Pathname.new(File.dirname(source_file))
+        root = @view_roots.find { |candidate| directory.to_s.start_with?(candidate.to_s) } || @view_root
+
+        directory.relative_path_from(root).to_s
       rescue ArgumentError
         nil
       end
@@ -181,7 +211,11 @@ def build_index(files)
           (map[name] ||= []) << file
         end
 
-        map.each_value { |files| files.replace(PartialResolution.by_precedence(files)) }
+        map.each_value do |candidates|
+          ordered = PartialResolution.by_precedence(candidates)
+          candidates.replace(ordered.sort_by { |file| PartialResolution.root_index_for(file, @view_roots) })
+        end
+
         map
       end
     end
diff --git a/lib/herb/analysis/partial_resolution.rb b/lib/herb/analysis/partial_resolution.rb
index 071c671d2..eca7af85c 100644
--- a/lib/herb/analysis/partial_resolution.rb
+++ b/lib/herb/analysis/partial_resolution.rb
@@ -41,6 +41,65 @@ def view_root_for(project_path)
           candidates.find(&:directory?) || root
         end
 
+        #: (String) -> String?
+        def format_of(file)
+          base = File.basename(file)
+          dot = base.index(".")
+
+          return nil unless dot
+
+          extension = base[dot..].to_s
+          stripped = extension.delete_suffix(".erb")
+          stripped = stripped.delete_suffix(".herb") if stripped == extension
+
+          return nil if stripped == extension
+
+          format = stripped.delete_prefix(".").split(".").last.to_s.split("+").first.to_s
+
+          format.empty? ? nil : format
+        end
+
+        #: (String) -> bool
+        def has_locale?(file)
+          base = File.basename(file)
+          dot = base.index(".")
+
+          return false unless dot
+
+          extension = base[dot..].to_s
+          stripped = extension.delete_suffix(".erb")
+          stripped = stripped.delete_suffix(".herb") if stripped == extension
+
+          return false if stripped == extension
+
+          stripped.delete_prefix(".").split(".").size > 1
+        end
+
+        #: (String) -> String?
+        def variant_of(file)
+          base = File.basename(file)
+          dot = base.index(".")
+
+          return nil unless dot
+
+          extension = base[dot..].to_s
+          stripped = extension.delete_suffix(".erb")
+          stripped = stripped.delete_suffix(".herb") if stripped == extension
+
+          return nil if stripped == extension
+
+          _, variant = stripped.split("+", 2)
+
+          variant.to_s.empty? ? nil : variant
+        end
+
+        #: (String) -> String
+        def without_template_extension(partial_name)
+          extension = EXTENSIONS.find { |candidate| partial_name.end_with?(candidate) }
+
+          extension ? partial_name.delete_suffix(extension) : partial_name
+        end
+
         #: (String) -> bool
         def template_path?(file)
           name = File.basename(file)
@@ -100,6 +159,11 @@ def template_name_for(file, view_root)
           directory == "." ? name : "#{directory}/#{name}"
         end
 
+        #: (String, Array[String | Pathname]) -> Array[String]
+        def layout_candidates_for_roots(template_file, view_roots)
+          view_roots.lazy.map { |root| layout_candidates_for(template_file, root) }.find { |candidates| candidates.any? } || []
+        end
+
         #: (String, String | Pathname) -> Array[String]
         def layout_candidates_for(template_file, view_root)
           relative = relative_to_view_root(template_file, view_root)
@@ -142,6 +206,34 @@ def partial_name_for(file, view_root)
           directory == "." ? name : "#{directory}/#{name}"
         end
 
+        #: (String, Array[String | Pathname]) -> [Integer, String]?
+        def relative_to_view_roots(file, view_roots)
+          view_roots.each_with_index do |root, index|
+            relative = relative_to_view_root(file, root)
+
+            return [index, relative] if relative
+          end
+
+          nil
+        end
+
+        #: (String, Array[String | Pathname]) -> String?
+        def partial_name_for_roots(file, view_roots)
+          view_roots.filter_map { |root| partial_name_for(file, root) }.first
+        end
+
+        #: (String, Array[String | Pathname]) -> String?
+        def template_name_for_roots(file, view_roots)
+          view_roots.filter_map { |root| template_name_for(file, root) }.first
+        end
+
+        #: (String, Array[String | Pathname]) -> Integer
+        def root_index_for(file, view_roots)
+          found = relative_to_view_roots(file, view_roots)
+
+          found ? found[0] : view_roots.size
+        end
+
         private
 
         #: (String, String | Pathname) -> String?
diff --git a/lib/herb/analysis/project_index.rb b/lib/herb/analysis/project_index.rb
index 998d9a2aa..1f88053de 100644
--- a/lib/herb/analysis/project_index.rb
+++ b/lib/herb/analysis/project_index.rb
@@ -44,9 +44,9 @@ def index_call_sites
         @graph = graph_builder.build(partials.templates)
       end
 
-      #: () -> Pathname?
-      def view_root
-        @partials&.view_root
+      #: () -> Array[Pathname]?
+      def view_roots
+        @partials&.view_roots
       end
 
       #: (String, ?String?) -> bool
diff --git a/lib/herb/analysis/render_analyzer.rb b/lib/herb/analysis/render_analyzer.rb
index 3f07ce9b6..cf72d68f4 100644
--- a/lib/herb/analysis/render_analyzer.rb
+++ b/lib/herb/analysis/render_analyzer.rb
@@ -438,6 +438,36 @@ def analyze_from_collected(render_calls_by_file:, dynamic_prefixes_from_erb: [],
       end
 
       def print_file_lists(result)
+        formatless = result.partial_files.values.select { |file| missing_format?(file) }.sort
+
+        if formatless.any?
+          puts "\n"
+          puts " #{bold("Templates without a format:")}"
+          puts " #{dimmed("Rails reads a template filename as `name.format.handler`. Without a format it matches every one.")}"
+          puts ""
+
+          formatless.each do |file|
+            puts "   #{bold(yellow("!"))} #{yellow(relative_path(file))}"
+          end
+        end
+
+        conditional = conditional_calls(result)
+
+        if conditional.any?
+          puts "\n"
+          puts " #{bold("Conditional render calls:")}"
+          puts " #{dimmed("The partial name is chosen at runtime, but every branch is a literal.")}"
+          puts ""
+
+          conditional.each do |call, targets|
+            puts "   #{bold(yellow("?"))} #{call[:partial]} #{dimmed("in #{relative_path(call[:file])}")}"
+
+            targets.each do |target|
+              puts "       #{dimmed("\u2192")} #{green(relative_path(target))}"
+            end
+          end
+        end
+
         return unless result.issues?
 
         if result.unresolved.any?
@@ -456,7 +486,10 @@ def print_file_lists(result)
             calls.each do |call|
               location = call[:location] ? dimmed("at #{call[:location]}") : nil
               expected = expected_file_path(call[:partial], result.view_root)
-              puts "   #{bold(red("\u2717"))} #{bold(call[:partial])} #{location} #{dimmed("-")} #{dimmed(expected)}"
+              kind = render_name_kind(call[:partial])
+              label = kind ? " #{dimmed("(#{kind})")}" : ""
+
+              puts "   #{bold(red("\u2717"))} #{bold(call[:partial])}#{label} #{location} #{dimmed("-")} #{dimmed(expected)}"
             end
           end
         end
@@ -465,13 +498,17 @@ def print_file_lists(result)
           puts "\n #{separator}" if result.unresolved.any?
           puts "\n"
           puts " #{bold("Dynamic render calls:")}"
-          puts " #{dimmed("The partial name is built at runtime, so it cannot be resolved statically.")}"
+          puts " #{dimmed("The partial name is built at runtime. Where the directory is known, every partial under it is listed.")}"
           puts ""
 
           result.dynamic_calls.each do |call|
             shown = dynamic_call_display(call)
 
             puts "   #{bold(red("\u2717"))} #{bold(red(shown))} #{dimmed("in #{relative_path(call[:file])}")}"
+
+            names_under(call, result.partial_files).each do |name|
+              puts "       #{dimmed("\u2192")} #{dimmed(name)}"
+            end
           end
         end
 
@@ -526,7 +563,7 @@ def print_summary_line(result)
 
       #: (String) -> bool
       def component_template?(relative)
-        relative.start_with?("app/components/")
+        relative.start_with?("app/components/") || relative.include?("/app/components/")
       end
 
       #: (Array[Hash[Symbol, untyped]]) -> void
@@ -541,7 +578,7 @@ def print_warning_summary_line(warnings)
         locals = files_for.call(:undeclared_local)
         uninferable = files_for.call(:uninferable_local)
         ivars = files_for.call(:ivar_in_partial)
-        ignored = files_for.call(:ignored_component)
+        ignored = find_erb_files.map { |file| relative_path(file) }.count { |file| component_template?(file) }
 
         parts = [] #: Array[String]
         parts << stat(locals, "undeclared #{pluralize(locals, "local")}", :yellow) if locals.positive?
@@ -555,7 +592,7 @@ def print_warning_summary_line(warnings)
 
         return unless ignored.positive?
 
-        puts "  #{label("Ignored")} #{dimmed("#{ignored} component #{pluralize(ignored, "template")} in app/components/")}"
+        puts "  #{label("Ignored")} #{dimmed("#{ignored} component #{pluralize(ignored, "template")} under app/components/")}"
       end
 
       private
@@ -1053,11 +1090,22 @@ def build_render_graph(render_calls_by_file, partial_files, view_root)
             next unless partial_reference
 
             resolved = resolve_partial(partial_reference, file, partial_files, view_root)
+
             if resolved
               resolved_name = partial_name_for_file(resolved, view_root)
               resolved_names << resolved_name if resolved_name
             else
-              resolved_names << partial_reference
+              branches = static_branches(partial_reference)
+              targets = branches.filter_map { |branch| resolve_partial(branch, file, partial_files, view_root) }
+
+              if branches.size > 1 && targets.size == branches.size
+                targets.each do |target|
+                  name = partial_name_for_file(target, view_root)
+                  resolved_names << name if name
+                end
+              else
+                resolved_names << partial_reference
+              end
             end
           end
 
@@ -1100,6 +1148,13 @@ def find_unused_by_reachability(render_graph, partial_files, ruby_references, dy
           queue << resolved_file if resolved_file
         end
 
+        partial_files.each do |name, file|
+          next unless dynamic_prefixes.any? { |prefix| name.start_with?("#{prefix}/") }
+
+          reachable << name
+          queue << file if file
+        end
+
         visited_files = Set.new
 
         until queue.empty?
@@ -1154,7 +1209,7 @@ def collect_all_dynamic_prefixes(dynamic_calls, ruby_references)
 
           prefix = call[:partial].gsub(/\A["']|["']\z/, "")
           prefix = prefix.split("\#{").first&.chomp("/")
-          prefix unless prefix.nil? || prefix.empty?
+          prefix if prefix && !prefix.empty? && partial_path_segment?(prefix)
         }
 
         ruby_references.each do |reference|
@@ -1168,8 +1223,12 @@ def collect_all_dynamic_prefixes(dynamic_calls, ruby_references)
       def find_unresolved(render_calls, partial_files, view_root)
         render_calls.select do |call|
           next false unless call[:partial]
+          next false if resolve_partial(call[:partial], call[:file], partial_files, view_root)
+
+          branches = static_branches(call[:partial])
+          targets = branches.filter_map { |branch| resolve_partial(branch, call[:file], partial_files, view_root) }
 
-          !resolve_partial(call[:partial], call[:file], partial_files, view_root)
+          !(branches.size > 1 && targets.size == branches.size)
         end
       end
 
@@ -1177,6 +1236,63 @@ def resolve_partial(partial_name, source_file, _partial_files, view_root)
         partial_index(view_root).resolve(partial_name, source_file).first
       end
 
+      #: (untyped) -> Array[[Hash[Symbol, untyped], Array[String]]]
+      def conditional_calls(result)
+        result.render_calls.filter_map do |call|
+          next unless call[:partial]
+          next if resolve_partial(call[:partial], call[:file], result.partial_files, result.view_root)
+
+          branches = static_branches(call[:partial])
+          targets = branches.filter_map { |branch| resolve_partial(branch, call[:file], result.partial_files, result.view_root) }
+
+          [call, targets] if branches.size > 1 && targets.size == branches.size
+        end
+      end
+
+      #: (Hash[Symbol, untyped], Hash[String, String]) -> Array[String]
+      def names_under(call, partial_files)
+        prefix = call[:dynamic_prefix]
+
+        unless prefix
+          raw = call[:partial].to_s.sub(/\A["']/, "")
+          prefix = raw.split("\#{").first.to_s.chomp("/")
+        end
+
+        return [] if prefix.to_s.empty? || !partial_path_segment?(prefix)
+
+        partial_files.keys.select { |name| name.start_with?("#{prefix}/") }.sort
+      end
+
+      #: (String) -> bool
+      def missing_format?(file)
+        name = File.basename(file)
+
+        name.end_with?(".erb") && name.count(".") == 1
+      end
+
+      #: (String) -> bool
+      def partial_path_segment?(value)
+        value.match?(%r{\A[a-z0-9_/-]+\z})
+      end
+
+      #: (String) -> Array[String]
+      def static_branches(expression)
+        branches = expression.split("?", 2).last.to_s
+
+        branches.scan(/["']([^"'\#]+)["']/).flatten.uniq
+      end
+
+      #: (String) -> String?
+      def render_name_kind(name)
+        return "instance variable" if name.start_with?("@")
+        return "interpolated" if name.include?("\#{")
+        return "conditional" if name.include?("?") && name.include?(":")
+        return "method call" if name.include?("(") || name.include?(".")
+        return "expression" if name.include?(" ")
+
+        nil
+      end
+
       def expected_file_path(partial_name, view_root)
         parts = partial_name.split("/")
         parts[-1] = "_#{parts[-1]}"
@@ -1223,6 +1339,10 @@ def format_duration(seconds)
       end
 
       def relative_path(path)
+        relative = Pathname.new(path).relative_path_from(@project_path).to_s
+
+        return relative unless relative.start_with?("..")
+
         Pathname.new(path).relative_path_from(Pathname.pwd).to_s
       rescue ArgumentError
         path.to_s
diff --git a/lib/herb/analysis/render_graph/builder.rb b/lib/herb/analysis/render_graph/builder.rb
index 45ccb83ce..99a19b1da 100644
--- a/lib/herb/analysis/render_graph/builder.rb
+++ b/lib/herb/analysis/render_graph/builder.rb
@@ -156,11 +156,11 @@ def build(templates)
 
         #: (Array[String], Hash[String, Array[YieldSite]], Hash[String, Array[PartialCallSite]]) -> void
         def add_layout_call_sites(files, layout_yields, call_sites)
-          view_root = @partials.view_root
+          view_roots = @partials.view_roots
           layouts = {} #: Hash[String, String]
 
           files.each do |file|
-            name = PartialResolution.template_name_for(file, view_root)
+            name = PartialResolution.template_name_for_roots(file, view_roots)
 
             next unless name && layout_yields.key?(file)
 
@@ -172,7 +172,7 @@ def add_layout_call_sites(files, layout_yields, call_sites)
           end
 
           files.each do |file|
-            PartialResolution.layout_candidates_for(file, view_root).each do |candidate|
+            PartialResolution.layout_candidates_for_roots(file, view_roots).each do |candidate|
               layout = layouts[candidate]
 
               next if layout.nil? || layout == file
diff --git a/lib/herb/analysis/ruby_locals_index.rb b/lib/herb/analysis/ruby_locals_index.rb
index cebb92c7b..da5749e54 100644
--- a/lib/herb/analysis/ruby_locals_index.rb
+++ b/lib/herb/analysis/ruby_locals_index.rb
@@ -59,7 +59,6 @@ def find(name)
         @locals.find { |local| local.name == name }
       end
 
-      # Every name the template binds, regardless of where.
       #: () -> Set[String]
       def names
         @locals.to_set(&:name)
diff --git a/lib/herb/analysis/ruby_locals_index/named_reference.rb b/lib/herb/analysis/ruby_locals_index/named_reference.rb
index 9bed92e89..5468e0567 100644
--- a/lib/herb/analysis/ruby_locals_index/named_reference.rb
+++ b/lib/herb/analysis/ruby_locals_index/named_reference.rb
@@ -3,8 +3,6 @@
 module Herb
   module Analysis
     class RubyLocalsIndex
-      # A name, with where it appears in the source as a byte offset and length,
-      # which is how Prism reports it.
       class NamedReference
         attr_reader :name #: String
         attr_reader :start_offset #: Integer
diff --git a/lib/herb/analysis/ruby_locals_index/offset_table.rb b/lib/herb/analysis/ruby_locals_index/offset_table.rb
index fdab6e800..4a3178bf7 100644
--- a/lib/herb/analysis/ruby_locals_index/offset_table.rb
+++ b/lib/herb/analysis/ruby_locals_index/offset_table.rb
@@ -3,8 +3,6 @@
 module Herb
   module Analysis
     class RubyLocalsIndex
-      # Prism reports byte offsets into the whole template while the Herb AST
-      # reports lines and columns, so one of them has to be translated.
       class OffsetTable
         # @rbs!
         #   @line_starts: Array[Integer]
diff --git a/lib/herb/analysis/template_dependencies.rb b/lib/herb/analysis/template_dependencies.rb
index 597c82b0c..af74fd65f 100644
--- a/lib/herb/analysis/template_dependencies.rb
+++ b/lib/herb/analysis/template_dependencies.rb
@@ -100,7 +100,6 @@ def dependency_index(file_path)
       end
 
       # @rbs!
-      #   KERNEL_METHODS: Array[String]
       KERNEL_METHODS = [
         "rand", "srand", "format", "sprintf", "raise", "loop", "sleep", "catch", "throw",
         "block_given?", "caller", "binding", "frozen?", "freeze", "dup", "clone", "tap", "then",
@@ -340,7 +339,6 @@ def symbol_after(line, keyword)
       end
 
       # @rbs!
-      #   UNCOUNTABLE: Array[String]
       UNCOUNTABLE = ["series", "species", "news", "information", "equipment", "money"].freeze
 
       #: (String, String) -> String?
@@ -376,7 +374,7 @@ def trace_state(entry_point, state)
         return nil unless entry_result
         return nil unless entry_result.instance_variables.include?(state) || entry_result.constants.include?(state)
 
-        index = PartialIndex.new(@view_root, reachable)
+        index = PartialIndex.new([@view_root], reachable)
         affected = Set.new([entry_point]) #: Set[String]
 
         state_locals = {} #: Hash[String, Set[String]]
diff --git a/rust/herb-analysis/src/actionview_cli.rs b/rust/herb-analysis/src/actionview_cli.rs
index 84c9a2516..91a774d62 100644
--- a/rust/herb-analysis/src/actionview_cli.rs
+++ b/rust/herb-analysis/src/actionview_cli.rs
@@ -9,7 +9,52 @@ use herb_analysis::render_graph::Verdict;
 use herb_analysis::ruby_render_references;
 use herb_analysis::state_flow::{FlowNode, StateFlow};
 
+fn actionview_configured(project_path: &Path) -> Result<(), String> {
+  let Ok(config) = herb_config::Config::load(project_path, None) else {
+    return Ok(());
+  };
+
+  match config.config.framework {
+    Some(herb_config::Framework::ActionView) => Ok(()),
+    Some(other) => Err(format!("{other:?}").to_lowercase()),
+    None => Err("ruby".to_string()),
+  }
+}
+
+fn report_missing_framework(project_path: &Path, framework: &str) -> i32 {
+  println!();
+  println!(
+    " {}",
+    "Herb also works outside of ActionView, but the `herb actionview` commands require the project to be explicitly configured for ActionView.".dimmed()
+  );
+  println!();
+  println!(
+    " The project at '{}' is not configured to use ActionView (current framework: '{framework}').",
+    project_path.display()
+  );
+  println!();
+  println!(" To enable ActionView support, add the following to your `.herb.yml`:");
+  println!();
+  println!("   {}", "framework: actionview".bold());
+  println!();
+
+  1
+}
+
 pub fn run(command: &str, arguments: &[String]) -> i32 {
+  if !matches!(command, "check" | "graph" | "dependencies" | "flow" | "context" | "signature") {
+    eprintln!("{}", format!("Unknown actionview subcommand: {command}").red());
+    print_usage();
+
+    return 1;
+  }
+
+  let root = project_root(arguments);
+
+  if let Err(framework) = actionview_configured(&root) {
+    return report_missing_framework(&root, &framework);
+  }
+
   match command {
     "check" => check(arguments),
     "graph" => graph(arguments),
@@ -90,6 +135,68 @@ fn header(title: &str) {
   println!();
 }
 
+fn component_template(relative: &str) -> bool {
+  relative.starts_with("app/components/") || relative.contains("/app/components/")
+}
+
+fn missing_format(file: &str) -> bool {
+  let name = file.rsplit('/').next().unwrap_or(file);
+
+  name.ends_with(".erb") && name.matches('.').count() == 1
+}
+
+fn static_branches(expression: &str) -> Vec<String> {
+  let mut found: Vec<String> = Vec::new();
+
+  let mut rest = match expression.split_once('?') {
+    Some((_, branches)) => branches,
+    None => expression,
+  };
+
+  while let Some(start) = rest.find(['"', '\'']) {
+    let quote = rest.as_bytes()[start] as char;
+    let after = &rest[start + 1..];
+
+    let Some(end) = after.find(quote) else {
+      break;
+    };
+
+    let literal = &after[..end];
+
+    if !literal.is_empty() && !literal.contains("#{") && !found.iter().any(|seen| seen == literal) {
+      found.push(literal.to_string());
+    }
+
+    rest = &after[end + 1..];
+  }
+
+  found
+}
+
+fn render_name_kind(name: &str) -> Option<&'static str> {
+  if name.starts_with('@') {
+    return Some("instance variable");
+  }
+
+  if name.contains("#{") {
+    return Some("interpolated");
+  }
+
+  if name.contains('?') && name.contains(':') {
+    return Some("conditional");
+  }
+
+  if name.contains('(') || name.contains('.') {
+    return Some("method call");
+  }
+
+  if name.contains(' ') {
+    return Some("expression");
+  }
+
+  None
+}
+
 fn plural(count: usize, word: &str) -> String {
   if count == 1 {
     word.to_string()
@@ -155,7 +262,8 @@ fn check(arguments: &[String]) -> i32 {
   let mut rendered: Vec<String> = Vec::new();
   let mut files_with_renders: BTreeSet<String> = BTreeSet::new();
   let mut dynamic_renders = 0usize;
-  let mut dynamic_sites: Vec<(String, String)> = Vec::new();
+  let mut dynamic_sites: Vec<(String, String, Vec<String>)> = Vec::new();
+  let mut branching_sites: Vec<(String, String, Vec<String>)> = Vec::new();
   let mut other_renders = 0usize;
   let mut with_partial_count = 0usize;
 
@@ -187,6 +295,8 @@ fn check(arguments: &[String]) -> i32 {
     for call in &result.render_calls {
       files_with_renders.insert(file.clone());
 
+      let guessed = call.partial.is_none() && call.layout.is_none();
+
       let target = call
         .partial
         .clone()
@@ -205,7 +315,13 @@ fn check(arguments: &[String]) -> i32 {
             .map(|prefix| format!("{prefix}/#{{...}}"))
             .unwrap_or_else(|| "#{...}".to_string());
 
-          dynamic_sites.push((relative(file, &root), shown));
+          let candidates = call
+            .dynamic_prefix
+            .as_ref()
+            .map(|prefix| index.names_under(prefix).iter().map(|name| (*name).to_string()).collect())
+            .unwrap_or_default();
+
+          dynamic_sites.push((relative(file, &root), shown, candidates));
         }
 
         continue;
@@ -223,13 +339,44 @@ fn check(arguments: &[String]) -> i32 {
         eprintln!("{file}\t{name}");
       }
 
-      match index.resolve(name, Some(file)).first() {
-        Some(target) => rendered.push(target.clone()),
-        None => unresolved.push((relative(file, &root), name.clone())),
+      let resolved = index.resolve(name, Some(file));
+
+      match resolved.first() {
+        Some(target) => {
+          rendered.push(target.clone());
+
+          let format = herb_analysis::partial_resolution::format_of(target);
+
+          for candidate in resolved.iter().skip(1) {
+            if herb_analysis::partial_resolution::variant_of(candidate).is_some() && herb_analysis::partial_resolution::format_of(candidate) == format {
+              rendered.push(candidate.clone());
+            }
+          }
+        }
+        None => {
+          let branches = static_branches(name);
+          let targets: Vec<String> = branches
+            .iter()
+            .filter_map(|branch| index.resolve(branch, Some(file)).first().cloned())
+            .collect();
+
+          if branches.len() > 1 && targets.len() == branches.len() {
+            rendered.extend(targets.iter().cloned());
+            branching_sites.push((
+              relative(file, &root),
+              name.clone(),
+              targets.iter().map(|target| relative(target, &root)).collect(),
+            ));
+          } else if !guessed {
+            unresolved.push((relative(file, &root), name.clone()));
+          }
+        }
       }
     }
   }
 
+  let formatless: Vec<String> = templates.iter().filter(|file| missing_format(file)).map(|file| relative(file, &root)).collect();
+
   let partials: Vec<String> = templates
     .iter()
     .filter(|file| herb_analysis::partial_resolution::partial_path(file))
@@ -270,12 +417,45 @@ fn check(arguments: &[String]) -> i32 {
 
   println!();
 
+  if !formatless.is_empty() {
+    println!(" {}", "Templates without a format:".bold());
+    println!(
+      " {}",
+      "Rails reads a template filename as `name.format.handler`. Without a format it matches every one.".dimmed()
+    );
+    println!();
+
+    for file in &formatless {
+      println!("   {} {}", "!".yellow().bold(), file.yellow());
+    }
+
+    println!();
+  }
+
+  if !branching_sites.is_empty() {
+    println!(" {}", "Conditional render calls:".bold());
+    println!(" {}", "The partial name is chosen at runtime, but every branch is a literal.".dimmed());
+    println!();
+
+    for (file, expression, targets) in &branching_sites {
+      println!("   {} {} {}", "?".yellow().bold(), expression, format!("in {file}").dimmed());
+
+      for target in targets {
+        println!("       {} {}", "\u{2192}".dimmed(), target.green());
+      }
+    }
+
+    println!();
+  }
+
   if !unresolved.is_empty() {
     println!(" {}", "Unresolved render calls:".bold());
     println!();
 
     for (file, name) in &unresolved {
-      println!("   {} {} {}", "\u{2717}".red().bold(), name, format!("in {file}").dimmed());
+      let kind = render_name_kind(name).map(|kind| format!(" ({kind})").dimmed().to_string()).unwrap_or_default();
+
+      println!("   {} {}{} {}", "\u{2717}".red().bold(), name, kind, format!("in {file}").dimmed());
     }
 
     println!();
@@ -283,11 +463,18 @@ fn check(arguments: &[String]) -> i32 {
 
   if !dynamic_sites.is_empty() {
     println!(" {}", "Dynamic render calls:".bold());
-    println!(" {}", "The partial name is built at runtime, so it cannot be resolved statically.".dimmed());
+    println!(
+      " {}",
+      "The partial name is built at runtime. Where the directory is known, every partial under it is listed.".dimmed()
+    );
     println!();
 
-    for (file, shown) in &dynamic_sites {
+    for (file, shown, candidates) in &dynamic_sites {
       println!("   {} {} {}", "\u{2717}".red().bold(), shown.red().bold(), format!("in {file}").dimmed());
+
+      for candidate in candidates {
+        println!("       {} {}", "\u{2192}".dimmed(), candidate.dimmed());
+      }
     }
 
     println!();
@@ -403,7 +590,11 @@ fn check(arguments: &[String]) -> i32 {
     println!(
       "  {} {}",
       label("Ignored"),
-      format!("{ignored_components} component {} in app/components/", plural(ignored_components, "template")).dimmed()
+      format!(
+        "{ignored_components} component {} under app/components/",
+        plural(ignored_components, "template")
+      )
+      .dimmed()
     );
   }
 
@@ -579,10 +770,14 @@ fn graph(arguments: &[String]) -> i32 {
 }
 
 fn view_relative(file: &str, index: &PartialIndex) -> String {
-  Path::new(file)
-    .strip_prefix(index.view_root())
+  let path = Path::new(file);
+
+  index
+    .view_roots()
+    .iter()
+    .find_map(|root| path.strip_prefix(root).ok())
     .map(|rest| rest.display().to_string())
-    .unwrap_or_else(|_| file.to_string())
+    .unwrap_or_else(|| file.to_string())
 }
 
 fn reverse_graph(renders: &BTreeMap<String, Vec<String>>, index: &PartialIndex) -> BTreeMap<String, Vec<String>> {
@@ -679,7 +874,7 @@ fn collect_renders(index: &mut PartialIndex, templates: &[String]) -> (BTreeMap<
   let mut renders: BTreeMap<String, Vec<String>> = BTreeMap::new();
   let mut prefixes: BTreeSet<String> = BTreeSet::new();
   let mut layouts: BTreeSet<String> = BTreeSet::new();
-  let flow = StateFlow::new(index.view_root());
+  let flow = StateFlow::new(index.view_roots().first().map(PathBuf::as_path).unwrap_or_else(|| Path::new(".")));
 
   for file in templates {
     let result = flow.analyze(file);
@@ -763,7 +958,8 @@ fn reachable_partials(
       continue;
     }
 
-    let Some(file) = index.resolve(&name, None).first() else {
+    let resolved = index.resolve(&name, None);
+    let Some(file) = resolved.first() else {
       continue;
     };
 
@@ -1289,7 +1485,11 @@ fn print_dependency_warnings(
   let mut unknown: Vec<(String, Vec<String>)> = Vec::new();
   let mut likely_locals: Vec<(String, Vec<String>)> = Vec::new();
   let mut uninferable: Vec<(String, Vec<String>)> = Vec::new();
-  let mut ignored_components = 0usize;
+  let ignored_components = templates
+    .iter()
+    .map(|file| relative(file, root))
+    .filter(|file| component_template(file))
+    .count();
 
   for file in templates {
     let result = flow.analyze(file);
@@ -1315,8 +1515,9 @@ fn print_dependency_warnings(
       }
 
       if !rest.is_empty() {
-        if relative.starts_with("app/components/") {
-          ignored_components += 1;
+        if component_template(&relative) {
+          // Counted from the template list instead, so the total does not depend on how many
+          // helpers each binding happens to resolve.
         } else if partial && !declared && candidates.is_none_or(BTreeSet::is_empty) {
           uninferable.push((relative, rest));
         } else {
diff --git a/rust/herb-analysis/src/partial_index.rs b/rust/herb-analysis/src/partial_index.rs
index 099ead5f2..ba3a2f25f 100644
--- a/rust/herb-analysis/src/partial_index.rs
+++ b/rust/herb-analysis/src/partial_index.rs
@@ -5,10 +5,10 @@ use std::path::{Path, PathBuf};
 use herb::herb::{parse_with_options, ParserOptions};
 
 use crate::partial_declaration::PartialDeclaration;
-use crate::partial_resolution::{self, by_precedence, partial_name_for, template_path, view_root_for, APPLICATION_DIRECTORY};
+use crate::partial_resolution::{self, by_precedence, partial_name_for_roots, root_index_for, template_path, view_root_for, APPLICATION_DIRECTORY};
 
 pub struct PartialIndex {
-  view_root: PathBuf,
+  view_roots: Vec<PathBuf>,
   templates: Vec<String>,
   by_name: BTreeMap<String, Vec<String>>,
   declarations: BTreeMap<String, PartialDeclaration>,
@@ -40,9 +40,16 @@ impl PartialIndex {
       let files = config.find_files_for_tool(herb_config::Tool::Linter, Some(project_path));
 
       if !files.is_empty() {
-        let templates: Vec<String> = files.into_iter().filter(|file| crate::partial_resolution::template_path(file)).collect();
+        let mut templates: Vec<String> = files.into_iter().filter(|file| crate::partial_resolution::template_path(file)).collect();
 
         if !templates.is_empty() {
+          let known: std::collections::BTreeSet<&String> = templates.iter().collect();
+          let extra: Vec<String> = index.templates.iter().filter(|file| !known.contains(file)).cloned().collect();
+
+          templates.extend(extra);
+          templates.sort();
+          templates.dedup();
+
           index.replace_templates(templates);
         }
       }
@@ -58,16 +65,16 @@ impl PartialIndex {
     collect_templates(&view_root, &mut templates);
     templates.sort();
 
-    Self::new(&view_root, templates)
+    Self::new(&[view_root], templates)
   }
 
   pub fn resolve_view_root(project_path: &Path) -> PathBuf {
     view_root_for(project_path)
   }
 
-  pub fn new(view_root: &Path, templates: Vec<String>) -> Self {
+  pub fn new(view_roots: &[PathBuf], templates: Vec<String>) -> Self {
     let mut index = Self {
-      view_root: view_root.to_path_buf(),
+      view_roots: view_roots.to_vec(),
       templates,
       by_name: BTreeMap::new(),
       declarations: BTreeMap::new(),
@@ -77,6 +84,10 @@ impl PartialIndex {
     index
   }
 
+  fn root_strings(&self) -> Vec<String> {
+    self.view_roots.iter().filter_map(|root| root.to_str().map(str::to_string)).collect()
+  }
+
   fn rebuild(&mut self) {
     let mut by_name: BTreeMap<String, Vec<String>> = BTreeMap::new();
 
@@ -88,15 +99,18 @@ impl PartialIndex {
       by_name.entry(name).or_default().push(file.clone());
     }
 
+    let roots = self.root_strings();
+
     for files in by_name.values_mut() {
       by_precedence(files);
+      files.sort_by_key(|file| root_index_for(file, &roots));
     }
 
     self.by_name = by_name;
   }
 
-  pub fn view_root(&self) -> &Path {
-    &self.view_root
+  pub fn view_roots(&self) -> &[PathBuf] {
+    &self.view_roots
   }
 
   pub fn templates(&self) -> &[String] {
@@ -107,6 +121,12 @@ impl PartialIndex {
     self.by_name.keys().map(|name| name.as_str()).collect()
   }
 
+  pub fn names_under(&self, prefix: &str) -> Vec<&str> {
+    let prefix = format!("{}/", prefix.trim_end_matches('/'));
+
+    self.by_name.keys().filter(|name| name.starts_with(&prefix)).map(|name| name.as_str()).collect()
+  }
+
   pub fn to_h(&mut self) -> BTreeMap<String, PartialDeclaration> {
     let names: Vec<String> = self.names().iter().map(|name| name.to_string()).collect();
     let mut partials = BTreeMap::new();
@@ -131,7 +151,7 @@ impl PartialIndex {
   }
 
   pub fn partial_name_for(&self, file: &str) -> Option<String> {
-    partial_name_for(file, self.view_root.to_str()?)
+    partial_name_for_roots(file, &self.root_strings())
   }
 
   pub fn files_for(&self, partial_name: &str) -> &[String] {
@@ -140,12 +160,41 @@ impl PartialIndex {
 
   fn source_directory_for(&self, source_file: &str) -> Option<String> {
     let directory = Path::new(source_file).parent()?;
-    let relative = directory.strip_prefix(&self.view_root).ok()?;
 
-    Some(relative.to_str()?.to_string())
+    self
+      .view_roots
+      .iter()
+      .find_map(|root| directory.strip_prefix(root).ok())
+      .and_then(|relative| relative.to_str())
+      .map(str::to_string)
+  }
+
+  pub fn resolve(&self, partial_name: &str, source_file: Option<&str>) -> Vec<String> {
+    let candidates = self.candidates(partial_name, source_file);
+    let Some(format) = source_file.and_then(partial_resolution::format_of) else {
+      return candidates.to_vec();
+    };
+
+    let mut ordered = candidates.to_vec();
+    ordered.sort_by_key(|file| {
+      let matches = match partial_resolution::format_of(file) {
+        Some(candidate) if candidate == format => 0,
+        None => 1,
+        Some(_) => 2,
+      };
+
+      (
+        matches,
+        usize::from(partial_resolution::variant_of(file).is_some()),
+        usize::from(partial_resolution::has_locale(file)),
+      )
+    });
+
+    ordered
   }
 
-  pub fn resolve(&self, partial_name: &str, source_file: Option<&str>) -> &[String] {
+  fn candidates(&self, partial_name: &str, source_file: Option<&str>) -> &[String] {
+    let partial_name = partial_resolution::without_template_extension(partial_name);
     let exact = self.files_for(partial_name);
 
     if !exact.is_empty() {
diff --git a/rust/herb-analysis/src/partial_resolution.rs b/rust/herb-analysis/src/partial_resolution.rs
index 448e35d6a..96d905746 100644
--- a/rust/herb-analysis/src/partial_resolution.rs
+++ b/rust/herb-analysis/src/partial_resolution.rs
@@ -56,6 +56,56 @@ fn normalize(path: &str) -> String {
   }
 }
 
+pub fn format_of(file: &str) -> Option<String> {
+  let normalized = normalize(file);
+  let base = basename(&normalized);
+  let dot = base.find('.')?;
+  let extension = &base[dot..];
+
+  let stripped = extension.strip_suffix(".erb").or_else(|| extension.strip_suffix(".herb"))?;
+  let segments = stripped.strip_prefix('.')?;
+  let format = segments.rsplit('.').next().unwrap_or(segments);
+  let format = format.split('+').next().unwrap_or(format);
+
+  (!format.is_empty()).then(|| format.to_string())
+}
+
+pub fn has_locale(file: &str) -> bool {
+  let normalized = normalize(file);
+  let base = basename(&normalized);
+
+  let Some(dot) = base.find('.') else {
+    return false;
+  };
+
+  let extension = &base[dot..];
+
+  let Some(stripped) = extension.strip_suffix(".erb").or_else(|| extension.strip_suffix(".herb")) else {
+    return false;
+  };
+
+  stripped.strip_prefix('.').is_some_and(|segments| segments.split('.').count() > 1)
+}
+
+pub fn variant_of(file: &str) -> Option<String> {
+  let normalized = normalize(file);
+  let base = basename(&normalized);
+  let dot = base.find('.')?;
+  let extension = &base[dot..];
+
+  let stripped = extension.strip_suffix(".erb").or_else(|| extension.strip_suffix(".herb"))?;
+  let (_, variant) = stripped.rsplit_once('+')?;
+
+  (!variant.is_empty()).then(|| variant.to_string())
+}
+
+pub fn without_template_extension(partial_name: &str) -> &str {
+  EXTENSIONS
+    .iter()
+    .find_map(|extension| partial_name.strip_suffix(extension))
+    .unwrap_or(partial_name)
+}
+
 pub fn template_path(file: &str) -> bool {
   let normalized = normalize(file);
   let name = basename(&normalized);
@@ -70,6 +120,13 @@ pub fn partial_path(file: &str) -> bool {
   name.starts_with(PARTIAL_PREFIX) && EXTENSIONS.iter().any(|extension| name.ends_with(extension))
 }
 
+pub fn relative_to_view_roots(path: &str, view_roots: &[String]) -> Option<(usize, String)> {
+  view_roots
+    .iter()
+    .enumerate()
+    .find_map(|(index, root)| relative_to_view_root(path, root).map(|relative| (index, relative)))
+}
+
 fn relative_to_view_root(path: &str, view_root: &str) -> Option<String> {
   let normalized_path = normalize(path);
   let normalized_root = normalize(view_root);
@@ -94,6 +151,18 @@ fn without_extension(name: &str) -> &str {
   }
 }
 
+pub fn partial_name_for_roots(file: &str, view_roots: &[String]) -> Option<String> {
+  view_roots.iter().find_map(|root| partial_name_for(file, root))
+}
+
+pub fn template_name_for_roots(file: &str, view_roots: &[String]) -> Option<String> {
+  view_roots.iter().find_map(|root| template_name_for(file, root))
+}
+
+pub fn root_index_for(file: &str, view_roots: &[String]) -> usize {
+  relative_to_view_roots(file, view_roots).map(|(index, _)| index).unwrap_or(view_roots.len())
+}
+
 pub fn partial_name_for(file: &str, view_root: &str) -> Option<String> {
   if !partial_path(file) {
     return None;
@@ -148,6 +217,14 @@ pub fn template_name_for(file: &str, view_root: &str) -> Option<String> {
   }
 }
 
+pub fn layout_candidates_for_roots(template_file: &str, view_roots: &[String]) -> Vec<String> {
+  view_roots
+    .iter()
+    .map(|root| layout_candidates_for(template_file, root))
+    .find(|candidates| !candidates.is_empty())
+    .unwrap_or_default()
+}
+
 pub fn layout_candidates_for(template_file: &str, view_root: &str) -> Vec<String> {
   let Some(relative) = relative_to_view_root(template_file, view_root) else {
     return Vec::new();
diff --git a/rust/herb-analysis/src/project_index.rs b/rust/herb-analysis/src/project_index.rs
index ccece30d9..a41d30867 100644
--- a/rust/herb-analysis/src/project_index.rs
+++ b/rust/herb-analysis/src/project_index.rs
@@ -66,8 +66,8 @@ impl ProjectIndex {
     self.graph.as_ref()
   }
 
-  pub fn view_root(&self) -> Option<&Path> {
-    self.partials.as_ref().map(|partials| partials.view_root())
+  pub fn view_roots(&self) -> Option<&[PathBuf]> {
+    self.partials.as_ref().map(|partials| partials.view_roots())
   }
 
   pub fn handle_change(&mut self, path: &str, source: Option<&str>) -> bool {
diff --git a/rust/herb-analysis/src/render_graph_builder.rs b/rust/herb-analysis/src/render_graph_builder.rs
index 0ffa130c4..3cc4ef4fc 100644
--- a/rust/herb-analysis/src/render_graph_builder.rs
+++ b/rust/herb-analysis/src/render_graph_builder.rs
@@ -6,7 +6,7 @@ use herb::nodes::{AnyNode, ERBCaseNode, ERBIfNode, ERBRenderNode, ERBUnlessNode,
 use herb::visitor::Visitor;
 
 use crate::partial_index::PartialIndex;
-use crate::partial_resolution::{layout_candidates_for, outranks_template, partial_path, template_name_for, LAYOUTS_DIRECTORY};
+use crate::partial_resolution::{layout_candidates_for_roots, outranks_template, partial_path, template_name_for_roots, LAYOUTS_DIRECTORY};
 use crate::render_graph::{CallSiteLocation, PartialCallSite, RenderGraph, StaticAttributeMap, TemplateRoots};
 
 const RENDER_MARKER: &str = "render";
@@ -179,14 +179,12 @@ impl<'a> Builder<'a> {
   }
 
   fn add_layout_call_sites(&self, files: &[String], layout_yields: &BTreeMap<String, Vec<YieldSite>>, graph: &mut RenderGraph) {
-    let Some(view_root) = self.partials.view_root().to_str() else {
-      return;
-    };
+    let view_roots: Vec<String> = self.partials.view_roots().iter().filter_map(|root| root.to_str().map(str::to_string)).collect();
 
     let mut layouts: BTreeMap<String, String> = BTreeMap::new();
 
     for file in files {
-      let Some(name) = template_name_for(file, view_root) else {
+      let Some(name) = template_name_for_roots(file, &view_roots) else {
         continue;
       };
 
@@ -203,7 +201,7 @@ impl<'a> Builder<'a> {
     }
 
     for file in files {
-      for candidate in layout_candidates_for(file, view_root) {
+      for candidate in layout_candidates_for_roots(file, &view_roots) {
         let Some(layout) = layouts.get(&candidate) else {
           continue;
         };
diff --git a/rust/herb-analysis/src/ruby_render_references.rs b/rust/herb-analysis/src/ruby_render_references.rs
index 1f2b301fa..01c4a13b6 100644
--- a/rust/herb-analysis/src/ruby_render_references.rs
+++ b/rust/herb-analysis/src/ruby_render_references.rs
@@ -74,6 +74,7 @@ pub fn collect_from_source(source: &str, references: &mut RubyRenderReferences)
   let wrapped = format!("<% {} %>", source);
   let options = ParserOptions {
     prism_nodes: true,
+    prism_program: true,
     ..Default::default()
   };
 
@@ -81,6 +82,12 @@ pub fn collect_from_source(source: &str, references: &mut RubyRenderReferences)
     return;
   };
 
+  if let Some(program) = result.value.prism() {
+    walk(program, references);
+
+    return;
+  }
+
   for child in &result.value.children {
     if let herb::nodes::AnyNode::ERBContentNode(node) = child {
       if let Some(prism) = node.prism() {
diff --git a/rust/herb-analysis/src/template_dependencies.rs b/rust/herb-analysis/src/template_dependencies.rs
index c325a02a1..358dd5df1 100644
--- a/rust/herb-analysis/src/template_dependencies.rs
+++ b/rust/herb-analysis/src/template_dependencies.rs
@@ -361,13 +361,19 @@ fn dynamic_prefix_of(value: &str) -> Option<String> {
   let value = value.trim_start_matches(['"', '\'']);
   let head = value.split("#{").next()?.trim_end_matches('/');
 
-  if head.is_empty() {
+  if head.is_empty() || !partial_path_segment(head) {
     None
   } else {
     Some(head.to_string())
   }
 }
 
+fn partial_path_segment(value: &str) -> bool {
+  value
+    .chars()
+    .all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || c == '_' || c == '/' || c == '-')
+}
+
 fn interpolated_render_prefix(node: &herb::prism::PrismNode) -> Option<String> {
   if node.is("InterpolatedStringNode") {
     let first = node.children.first()?;
diff --git a/rust/herb-analysis/tests/actionview_cli_test.rs b/rust/herb-analysis/tests/actionview_cli_test.rs
index 5b7b4e769..d895e7258 100644
--- a/rust/herb-analysis/tests/actionview_cli_test.rs
+++ b/rust/herb-analysis/tests/actionview_cli_test.rs
@@ -15,6 +15,7 @@ impl Project {
     let _ = fs::remove_dir_all(&root);
     fs::create_dir_all(root.join("app/views/posts")).expect("create project");
     fs::create_dir_all(root.join("app/views/layouts")).expect("create layouts");
+    fs::write(root.join(".herb.yml"), "framework: actionview\n").expect("configure project");
 
     Self { root }
   }
diff --git a/rust/herb-analysis/tests/formats_test.rs b/rust/herb-analysis/tests/formats_test.rs
new file mode 100644
index 000000000..28c33bd48
--- /dev/null
+++ b/rust/herb-analysis/tests/formats_test.rs
@@ -0,0 +1,119 @@
+use std::fs;
+use std::path::PathBuf;
+
+use herb_analysis::partial_index::PartialIndex;
+use herb_analysis::partial_resolution::{format_of, variant_of};
+
+fn scratch(name: &str) -> PathBuf {
+  let root = std::env::temp_dir().join(format!("herb-formats-{name}"));
+
+  let _ = fs::remove_dir_all(&root);
+
+  root
+}
+
+fn write(path: &PathBuf) -> String {
+  fs::create_dir_all(path.parent().unwrap()).unwrap();
+  fs::write(path, "<div></div>\n").unwrap();
+
+  path.to_str().unwrap().to_string()
+}
+
+#[test]
+fn reads_the_format_out_of_a_filename() {
+  assert_eq!(Some("html".to_string()), format_of("app/views/posts/_row.html.erb"));
+  assert_eq!(Some("turbo_stream".to_string()), format_of("app/views/posts/_row.turbo_stream.erb"));
+  assert_eq!(Some("html".to_string()), format_of("app/views/posts/_row.html.herb"));
+  assert_eq!(None, format_of("app/views/posts/_row.erb"));
+  assert_eq!(None, format_of("app/views/posts/_row.herb"));
+  assert_eq!(Some("html".to_string()), format_of("app/views/posts/_row.en.html.erb"));
+}
+
+#[test]
+fn a_caller_reaches_the_partial_matching_its_own_format() {
+  let root = scratch("matching");
+  let views = root.join("app/views");
+
+  let html_caller = write(&views.join("posts/index.html.erb"));
+  let turbo_caller = write(&views.join("posts/index.turbo_stream.erb"));
+  let html_partial = write(&views.join("posts/_row.html.erb"));
+  let turbo_partial = write(&views.join("posts/_row.turbo_stream.erb"));
+
+  let index = PartialIndex::new(
+    &[views],
+    vec![html_caller.clone(), turbo_caller.clone(), html_partial.clone(), turbo_partial.clone()],
+  );
+
+  assert_eq!(html_partial, index.resolve("posts/row", Some(&html_caller))[0]);
+  assert_eq!(turbo_partial, index.resolve("posts/row", Some(&turbo_caller))[0]);
+}
+
+#[test]
+fn a_formatless_partial_serves_any_caller() {
+  let root = scratch("formatless");
+  let views = root.join("app/views");
+
+  let turbo_caller = write(&views.join("posts/index.turbo_stream.erb"));
+  let partial = write(&views.join("posts/_row.erb"));
+
+  let index = PartialIndex::new(&[views], vec![turbo_caller.clone(), partial.clone()]);
+
+  assert_eq!(partial, index.resolve("posts/row", Some(&turbo_caller))[0]);
+}
+
+#[test]
+fn a_formatless_partial_loses_to_an_exact_format_match() {
+  let root = scratch("exact-wins");
+  let views = root.join("app/views");
+
+  let turbo_caller = write(&views.join("posts/index.turbo_stream.erb"));
+  let formatless = write(&views.join("posts/_row.erb"));
+  let turbo_partial = write(&views.join("posts/_row.turbo_stream.erb"));
+
+  let index = PartialIndex::new(&[views], vec![turbo_caller.clone(), formatless, turbo_partial.clone()]);
+
+  assert_eq!(turbo_partial, index.resolve("posts/row", Some(&turbo_caller))[0]);
+}
+
+#[test]
+fn extension_precedence_still_decides_when_no_format_matches() {
+  let root = scratch("fallback");
+  let views = root.join("app/views");
+
+  let turbo_caller = write(&views.join("posts/index.turbo_stream.erb"));
+  let html_partial = write(&views.join("posts/_row.html.erb"));
+
+  let index = PartialIndex::new(&[views], vec![turbo_caller.clone(), html_partial.clone()]);
+
+  assert_eq!(html_partial, index.resolve("posts/row", Some(&turbo_caller))[0]);
+}
+
+#[test]
+fn reads_the_variant_out_of_a_filename() {
+  assert_eq!(Some("mobile".to_string()), variant_of("app/views/posts/_row.html+mobile.erb"));
+  assert_eq!(Some("tablet".to_string()), variant_of("app/views/posts/_row.html+tablet.herb"));
+  assert_eq!(None, variant_of("app/views/posts/_row.html.erb"));
+  assert_eq!(None, variant_of("app/views/posts/_row.erb"));
+}
+
+#[test]
+fn a_variant_keeps_the_format_of_its_base_template() {
+  assert_eq!(Some("html".to_string()), format_of("app/views/posts/_row.html+mobile.erb"));
+  assert_eq!(Some("turbo_stream".to_string()), format_of("app/views/posts/_row.turbo_stream+mobile.erb"));
+}
+
+#[test]
+fn the_plain_template_is_preferred_over_a_variant() {
+  let root = scratch("variant");
+  let views = root.join("app/views");
+
+  let caller = write(&views.join("posts/index.html.erb"));
+  let variant = write(&views.join("posts/_row.html+mobile.erb"));
+  let plain = write(&views.join("posts/_row.html.erb"));
+
+  let index = PartialIndex::new(&[views], vec![caller.clone(), variant.clone(), plain.clone()]);
+  let resolved = index.resolve("posts/row", Some(&caller));
+
+  assert_eq!(plain, resolved[0]);
+  assert!(resolved.contains(&variant), "the variant is still reachable: {resolved:?}");
+}
diff --git a/rust/herb-analysis/tests/object_render_test.rs b/rust/herb-analysis/tests/object_render_test.rs
new file mode 100644
index 000000000..1dc76fe74
--- /dev/null
+++ b/rust/herb-analysis/tests/object_render_test.rs
@@ -0,0 +1,91 @@
+#![cfg(feature = "cli")]
+
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+fn binary() -> PathBuf {
+  PathBuf::from(env!("CARGO_BIN_EXE_herb-analysis"))
+}
+
+fn scratch(name: &str) -> PathBuf {
+  let root = std::env::temp_dir().join(format!("herb-object-render-{name}"));
+
+  let _ = fs::remove_dir_all(&root);
+
+  root
+}
+
+fn write(root: &Path, relative: &str, body: &str) {
+  let path = root.join(relative);
+
+  fs::create_dir_all(path.parent().unwrap()).unwrap();
+  fs::write(path, body).unwrap();
+  fs::write(root.join(".herb.yml"), "framework: actionview\n").unwrap();
+}
+
+fn check(root: &Path) -> String {
+  let output = Command::new(binary()).args(["actionview", "check", root.to_str().unwrap()]).output().unwrap();
+
+  String::from_utf8_lossy(&output.stdout).to_string()
+}
+
+#[test]
+fn a_guessed_object_partial_that_does_not_exist_is_not_an_error() {
+  let root = scratch("miss");
+
+  write(&root, "app/views/components/index.html.erb", "<%= render body do %><% end %>\n");
+
+  let output = check(&root);
+
+  assert!(!output.contains("bodys/body"), "a guessed name should not be reported as unresolved:\n{output}");
+}
+
+#[test]
+fn a_named_partial_that_does_not_exist_is_still_an_error() {
+  let root = scratch("named");
+
+  write(&root, "app/views/posts/index.html.erb", "<%= render \"posts/missing\" %>\n");
+
+  let output = check(&root);
+
+  assert!(output.contains("posts/missing"), "an explicit name should still be reported:\n{output}");
+}
+
+#[test]
+fn a_guessed_object_partial_that_exists_still_resolves() {
+  let root = scratch("hit");
+
+  write(&root, "app/views/posts/index.html.erb", "<%= render post %>\n");
+  write(&root, "app/views/posts/_post.html.erb", "<div></div>\n");
+
+  let output = check(&root);
+
+  assert!(!output.contains("posts/post"), "an existing guessed target should resolve:\n{output}");
+}
+
+#[test]
+fn a_dynamic_render_lists_the_partials_under_its_prefix() {
+  let root = scratch("dynamic-prefix");
+
+  write(&root, "app/views/admin/show.html.erb", "<%= render \"admin/parts/#{name}\" %>\n");
+  write(&root, "app/views/admin/parts/_alpha.html.erb", "<div></div>\n");
+  write(&root, "app/views/admin/parts/_beta.html.erb", "<div></div>\n");
+
+  let output = check(&root);
+
+  assert!(output.contains("admin/parts/alpha"), "candidates should be listed:\n{output}");
+  assert!(output.contains("admin/parts/beta"), "candidates should be listed:\n{output}");
+}
+
+#[test]
+fn a_dynamic_render_with_no_known_directory_lists_nothing() {
+  let root = scratch("dynamic-bare");
+
+  write(&root, "app/views/admin/show.html.erb", "<%= render \"#{name}\" %>\n");
+  write(&root, "app/views/admin/parts/_alpha.html.erb", "<div></div>\n");
+
+  let output = check(&root);
+
+  assert!(!output.contains("admin/parts/alpha"), "nothing should be claimed:\n{output}");
+}
diff --git a/rust/herb-analysis/tests/partial_index_test.rs b/rust/herb-analysis/tests/partial_index_test.rs
index 58815b729..6e156678e 100644
--- a/rust/herb-analysis/tests/partial_index_test.rs
+++ b/rust/herb-analysis/tests/partial_index_test.rs
@@ -1,5 +1,5 @@
 use std::fs;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 use herb_analysis::partial_index::PartialIndex;
 
@@ -51,7 +51,7 @@ fn resolves_the_view_root_to_app_views_when_it_is_there() {
   let project = Project::new("view_root");
   project.write("app/views/posts/index.html.erb");
 
-  assert_eq!(project.index().view_root(), project.root.join("app/views"));
+  assert_eq!(project.index().view_roots(), [project.root.join("app/views")]);
 }
 
 #[test]
@@ -59,7 +59,7 @@ fn falls_back_to_the_project_root_when_there_is_no_app_views() {
   let project = Project::new("flat_root");
   project.write("posts/index.html.erb");
 
-  assert_eq!(project.index().view_root(), Path::new(&project.root));
+  assert_eq!(project.index().view_roots(), [project.root.as_path()].map(PathBuf::from));
 }
 
 #[test]
diff --git a/rust/herb-analysis/tests/project_index_test.rs b/rust/herb-analysis/tests/project_index_test.rs
index 42cb672ed..0ed433992 100644
--- a/rust/herb-analysis/tests/project_index_test.rs
+++ b/rust/herb-analysis/tests/project_index_test.rs
@@ -151,5 +151,5 @@ fn exposes_the_view_root_it_resolved() {
   let project = Project::new("view_root");
   project.write("app/views/posts/index.html.erb", "<div></div>");
 
-  assert_eq!(project.indexed().view_root().expect("view root"), project.root.join("app/views"));
+  assert_eq!(project.indexed().view_roots().expect("view roots"), [project.root.join("app/views")]);
 }
diff --git a/rust/herb-analysis/tests/view_roots_test.rs b/rust/herb-analysis/tests/view_roots_test.rs
new file mode 100644
index 000000000..f7c8b2ed5
--- /dev/null
+++ b/rust/herb-analysis/tests/view_roots_test.rs
@@ -0,0 +1,91 @@
+use std::fs;
+use std::path::PathBuf;
+
+use herb_analysis::partial_index::PartialIndex;
+
+fn scratch(name: &str) -> PathBuf {
+  let root = std::env::temp_dir().join(format!("herb-view-roots-{name}"));
+
+  let _ = fs::remove_dir_all(&root);
+
+  root
+}
+
+fn write(path: &PathBuf, body: &str) {
+  fs::create_dir_all(path.parent().unwrap()).unwrap();
+  fs::write(path, body).unwrap();
+}
+
+#[test]
+fn names_a_partial_from_a_secondary_view_root() {
+  let root = scratch("secondary");
+  let app = root.join("app/views");
+  let engine = root.join("engines/billing/app/views");
+
+  write(&app.join("home/index.html.erb"), "<div></div>\n");
+  write(&engine.join("billing/_invoice.html.erb"), "<div></div>\n");
+
+  let templates = vec![
+    app.join("home/index.html.erb").to_str().unwrap().to_string(),
+    engine.join("billing/_invoice.html.erb").to_str().unwrap().to_string(),
+  ];
+
+  let index = PartialIndex::new(&[app.clone(), engine.clone()], templates);
+
+  assert_eq!(vec!["billing/invoice"], index.names());
+}
+
+#[test]
+fn an_earlier_view_root_shadows_a_later_one() {
+  let root = scratch("shadow");
+  let app = root.join("app/views");
+  let engine = root.join("engines/billing/app/views");
+
+  write(&app.join("billing/_invoice.html.erb"), "<div>app</div>\n");
+  write(&engine.join("billing/_invoice.html.erb"), "<div>engine</div>\n");
+
+  let templates = vec![
+    engine.join("billing/_invoice.html.erb").to_str().unwrap().to_string(),
+    app.join("billing/_invoice.html.erb").to_str().unwrap().to_string(),
+  ];
+
+  let index = PartialIndex::new(&[app.clone(), engine.clone()], templates);
+  let resolved = index.resolve("billing/invoice", None);
+
+  assert_eq!(2, resolved.len());
+  assert!(resolved[0].starts_with(app.to_str().unwrap()), "app view path should win, got {}", resolved[0]);
+}
+
+#[test]
+fn resolves_a_sibling_within_the_root_that_owns_the_caller() {
+  let root = scratch("sibling");
+  let app = root.join("app/views");
+  let engine = root.join("engines/billing/app/views");
+
+  write(&engine.join("billing/index.html.erb"), "<div></div>\n");
+  write(&engine.join("billing/_row.html.erb"), "<div></div>\n");
+
+  let templates = vec![
+    engine.join("billing/index.html.erb").to_str().unwrap().to_string(),
+    engine.join("billing/_row.html.erb").to_str().unwrap().to_string(),
+  ];
+
+  let index = PartialIndex::new(&[app, engine.clone()], templates);
+  let caller = engine.join("billing/index.html.erb").to_str().unwrap().to_string();
+
+  assert_eq!(1, index.resolve("row", Some(&caller)).len());
+}
+
+#[test]
+fn a_single_root_still_resolves() {
+  let root = scratch("single");
+  let app = root.join("app/views");
+
+  write(&app.join("shared/_header.html.erb"), "<div></div>\n");
+
+  let templates = vec![app.join("shared/_header.html.erb").to_str().unwrap().to_string()];
+  let index = PartialIndex::new(std::slice::from_ref(&app), templates);
+
+  assert_eq!(vec!["shared/header"], index.names());
+  assert_eq!(1, index.resolve("shared/header", None).len());
+}
diff --git a/sig/herb/analysis/partial_index.rbs b/sig/herb/analysis/partial_index.rbs
index 23fdc9d3c..9d5498d7f 100644
--- a/sig/herb/analysis/partial_index.rbs
+++ b/sig/herb/analysis/partial_index.rbs
@@ -5,8 +5,6 @@ module Herb
     class PartialIndex
       APPLICATION_DIRECTORY: String
 
-      attr_reader view_root: Pathname
-
       attr_reader templates: Array[String]
 
       # : (String | Pathname, ?templates: Array[String]?) -> PartialIndex
@@ -15,14 +13,22 @@ module Herb
       # : (String | Pathname) -> Pathname
       def self.resolve_view_root: (String | Pathname) -> Pathname
 
-      # : (String | Pathname, Array[String]) -> void
-      def initialize: (String | Pathname, Array[String]) -> void
+      # : (Array[String | Pathname], Array[String]) -> void
+      def initialize: (Array[String | Pathname], Array[String]) -> void
+
+      # : () -> Array[Pathname]
+      attr_reader view_roots: untyped
 
       # : (String?) -> Array[String]
       def files_for: (String?) -> Array[String]
 
+      # : (String?, String?) -> Array[String]
       # : (String?, String?) -> Array[String]
       def resolve: (String?, String?) -> Array[String]
+                 | (String?, String?) -> Array[String]
+
+      # : (String?, String?) -> Array[String]
+      def candidates_for: (String?, String?) -> Array[String]
 
       # : (String, String | Pathname) -> String?
       def self.partial_name_for: (String, String | Pathname) -> String?
diff --git a/sig/herb/analysis/partial_resolution.rbs b/sig/herb/analysis/partial_resolution.rbs
index 44444f5c3..9f120a1aa 100644
--- a/sig/herb/analysis/partial_resolution.rbs
+++ b/sig/herb/analysis/partial_resolution.rbs
@@ -26,6 +26,18 @@ module Herb
       # : (String | Pathname) -> Pathname
       def self.view_root_for: (String | Pathname) -> Pathname
 
+      # : (String) -> String?
+      def self.format_of: (String) -> String?
+
+      # : (String) -> bool
+      def self.has_locale?: (String) -> bool
+
+      # : (String) -> String?
+      def self.variant_of: (String) -> String?
+
+      # : (String) -> String
+      def self.without_template_extension: (String) -> String
+
       # : (String) -> bool
       def self.template_path?: (String) -> bool
 
@@ -44,12 +56,27 @@ module Herb
       # : (String, String | Pathname) -> String?
       def self.template_name_for: (String, String | Pathname) -> String?
 
+      # : (String, Array[String | Pathname]) -> Array[String]
+      def self.layout_candidates_for_roots: (String, Array[String | Pathname]) -> Array[String]
+
       # : (String, String | Pathname) -> Array[String]
       def self.layout_candidates_for: (String, String | Pathname) -> Array[String]
 
       # : (String, String | Pathname) -> String?
       def self.partial_name_for: (String, String | Pathname) -> String?
 
+      # : (String, Array[String | Pathname]) -> [Integer, String]?
+      def self.relative_to_view_roots: (String, Array[String | Pathname]) -> [ Integer, String ]?
+
+      # : (String, Array[String | Pathname]) -> String?
+      def self.partial_name_for_roots: (String, Array[String | Pathname]) -> String?
+
+      # : (String, Array[String | Pathname]) -> String?
+      def self.template_name_for_roots: (String, Array[String | Pathname]) -> String?
+
+      # : (String, Array[String | Pathname]) -> Integer
+      def self.root_index_for: (String, Array[String | Pathname]) -> Integer
+
       # : (String, String | Pathname) -> String?
       private def self.relative_to_view_root: (String, String | Pathname) -> String?
     end
diff --git a/sig/herb/analysis/project_index.rbs b/sig/herb/analysis/project_index.rbs
index aca0c65cb..0a03533ab 100644
--- a/sig/herb/analysis/project_index.rbs
+++ b/sig/herb/analysis/project_index.rbs
@@ -21,8 +21,8 @@ module Herb
       # : () -> void
       def index_call_sites: () -> void
 
-      # : () -> Pathname?
-      def view_root: () -> Pathname?
+      # : () -> Array[Pathname]?
+      def view_roots: () -> Array[Pathname]?
 
       # : (String, ?String?) -> bool
       def handle_change: (String, ?String?) -> bool
diff --git a/sig/herb/analysis/render_analyzer.rbs b/sig/herb/analysis/render_analyzer.rbs
index ffa8c17b0..b6e6bbcce 100644
--- a/sig/herb/analysis/render_analyzer.rbs
+++ b/sig/herb/analysis/render_analyzer.rbs
@@ -117,6 +117,24 @@ module Herb
 
       def resolve_partial: (untyped partial_name, untyped source_file, untyped _partial_files, untyped view_root) -> untyped
 
+      # : (untyped) -> Array[[Hash[Symbol, untyped], Array[String]]]
+      def conditional_calls: (untyped) -> Array[[ Hash[Symbol, untyped], Array[String] ]]
+
+      # : (Hash[Symbol, untyped], Hash[String, String]) -> Array[String]
+      def names_under: (Hash[Symbol, untyped], Hash[String, String]) -> Array[String]
+
+      # : (String) -> bool
+      def missing_format?: (String) -> bool
+
+      # : (String) -> bool
+      def partial_path_segment?: (String) -> bool
+
+      # : (String) -> Array[String]
+      def static_branches: (String) -> Array[String]
+
+      # : (String) -> String?
+      def render_name_kind: (String) -> String?
+
       def expected_file_path: (untyped partial_name, untyped view_root) -> untyped
 
       def label: (untyped text, ?untyped width) -> untyped
diff --git a/sig/herb/analysis/ruby_locals_index.rbs b/sig/herb/analysis/ruby_locals_index.rbs
index 4b5a36a02..24c5cbc1a 100644
--- a/sig/herb/analysis/ruby_locals_index.rbs
+++ b/sig/herb/analysis/ruby_locals_index.rbs
@@ -24,7 +24,6 @@ module Herb
       # : (String) -> Local?
       def find: (String) -> Local?
 
-      # Every name the template binds, regardless of where.
       # : () -> Set[String]
       def names: () -> Set[String]
 
diff --git a/sig/herb/analysis/ruby_locals_index/named_reference.rbs b/sig/herb/analysis/ruby_locals_index/named_reference.rbs
index e63910b7c..053eb2546 100644
--- a/sig/herb/analysis/ruby_locals_index/named_reference.rbs
+++ b/sig/herb/analysis/ruby_locals_index/named_reference.rbs
@@ -3,8 +3,6 @@
 module Herb
   module Analysis
     class RubyLocalsIndex
-      # A name, with where it appears in the source as a byte offset and length,
-      # which is how Prism reports it.
       class NamedReference
         attr_reader name: String
 
diff --git a/sig/herb/analysis/ruby_locals_index/offset_table.rbs b/sig/herb/analysis/ruby_locals_index/offset_table.rbs
index 4e2fbe794..9fc574412 100644
--- a/sig/herb/analysis/ruby_locals_index/offset_table.rbs
+++ b/sig/herb/analysis/ruby_locals_index/offset_table.rbs
@@ -3,8 +3,6 @@
 module Herb
   module Analysis
     class RubyLocalsIndex
-      # Prism reports byte offsets into the whole template while the Herb AST
-      # reports lines and columns, so one of them has to be translated.
       class OffsetTable
         @line_starts: Array[Integer]
 
diff --git a/sig/herb/analysis/template_dependencies.rbs b/sig/herb/analysis/template_dependencies.rbs
index 0153504af..ffefc8560 100644
--- a/sig/herb/analysis/template_dependencies.rbs
+++ b/sig/herb/analysis/template_dependencies.rbs
@@ -60,7 +60,6 @@ module Herb
       def dependency_index: (untyped file_path) -> untyped
 
       # @rbs!
-      #   KERNEL_METHODS: Array[String]
       KERNEL_METHODS: untyped
 
       def scan_helpers!: () -> untyped
@@ -83,7 +82,6 @@ module Herb
       def symbol_after: (String, String) -> String?
 
       # @rbs!
-      #   UNCOUNTABLE: Array[String]
       UNCOUNTABLE: untyped
 
       # : (String, String) -> String?
diff --git a/test/analysis/formats_test.rb b/test/analysis/formats_test.rb
new file mode 100644
index 000000000..ce4a6563c
--- /dev/null
+++ b/test/analysis/formats_test.rb
@@ -0,0 +1,112 @@
+# frozen_string_literal: true
+
+require_relative "../test_helper"
+require_relative "../../lib/herb/analysis/partial_index"
+
+require "tmpdir"
+
+module Analysis
+  class FormatsTest < Minitest::Spec
+    def write(root, relative)
+      path = File.join(root, relative)
+
+      FileUtils.mkdir_p(File.dirname(path))
+      File.write(path, "<div></div>\n")
+
+      path
+    end
+
+    test "reads the format out of a filename" do
+      assert_equal "html", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.html.erb")
+      assert_equal "turbo_stream", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.turbo_stream.erb")
+      assert_equal "html", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.html.herb")
+      assert_nil Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.erb")
+      assert_nil Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.herb")
+      assert_equal "html", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.en.html.erb")
+    end
+
+    test "reads the variant out of a filename" do
+      assert_equal "mobile", Herb::Analysis::PartialResolution.variant_of("app/views/posts/_row.html+mobile.erb")
+      assert_equal "tablet", Herb::Analysis::PartialResolution.variant_of("app/views/posts/_row.html+tablet.herb")
+      assert_nil Herb::Analysis::PartialResolution.variant_of("app/views/posts/_row.html.erb")
+      assert_nil Herb::Analysis::PartialResolution.variant_of("app/views/posts/_row.erb")
+    end
+
+    test "a variant keeps the format of its base template" do
+      assert_equal "html", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.html+mobile.erb")
+      assert_equal "turbo_stream", Herb::Analysis::PartialResolution.format_of("app/views/posts/_row.turbo_stream+mobile.erb")
+    end
+
+    test "the plain template is preferred over a variant" do
+      Dir.mktmpdir do |dir|
+        views = File.join(dir, "app", "views")
+
+        caller_file = write(views, "posts/index.html.erb")
+        variant = write(views, "posts/_row.html+mobile.erb")
+        plain = write(views, "posts/_row.html.erb")
+
+        index = Herb::Analysis::PartialIndex.new([views], [caller_file, variant, plain])
+        resolved = index.resolve("posts/row", caller_file)
+
+        assert_equal plain, resolved.first
+        assert_includes resolved, variant
+      end
+    end
+
+    test "a caller reaches the partial matching its own format" do
+      Dir.mktmpdir do |dir|
+        views = File.join(dir, "app", "views")
+
+        html_caller = write(views, "posts/index.html.erb")
+        turbo_caller = write(views, "posts/index.turbo_stream.erb")
+        html_partial = write(views, "posts/_row.html.erb")
+        turbo_partial = write(views, "posts/_row.turbo_stream.erb")
+
+        index = Herb::Analysis::PartialIndex.new([views], [html_caller, turbo_caller, html_partial, turbo_partial])
+
+        assert_equal html_partial, index.resolve("posts/row", html_caller).first
+        assert_equal turbo_partial, index.resolve("posts/row", turbo_caller).first
+      end
+    end
+
+    test "a formatless partial serves any caller" do
+      Dir.mktmpdir do |dir|
+        views = File.join(dir, "app", "views")
+
+        turbo_caller = write(views, "posts/index.turbo_stream.erb")
+        partial = write(views, "posts/_row.erb")
+
+        index = Herb::Analysis::PartialIndex.new([views], [turbo_caller, partial])
+
+        assert_equal partial, index.resolve("posts/row", turbo_caller).first
+      end
+    end
+
+    test "a formatless partial loses to an exact format match" do
+      Dir.mktmpdir do |dir|
+        views = File.join(dir, "app", "views")
+
+        turbo_caller = write(views, "posts/index.turbo_stream.erb")
+        formatless = write(views, "posts/_row.erb")
+        turbo_partial = write(views, "posts/_row.turbo_stream.erb")
+
+        index = Herb::Analysis::PartialIndex.new([views], [turbo_caller, formatless, turbo_partial])
+
+        assert_equal turbo_partial, index.resolve("posts/row", turbo_caller).first
+      end
+    end
+
+    test "extension precedence still decides when no format matches" do
+      Dir.mktmpdir do |dir|
+        views = File.join(dir, "app", "views")
+
+        turbo_caller = write(views, "posts/index.turbo_stream.erb")
+        html_partial = write(views, "posts/_row.html.erb")
+
+        index = Herb::Analysis::PartialIndex.new([views], [turbo_caller, html_partial])
+
+        assert_equal html_partial, index.resolve("posts/row", turbo_caller).first
+      end
+    end
+  end
+end
diff --git a/test/analysis/partial_index_test.rb b/test/analysis/partial_index_test.rb
index 362d9d7cd..c3ab12b52 100644
--- a/test/analysis/partial_index_test.rb
+++ b/test/analysis/partial_index_test.rb
@@ -25,13 +25,13 @@ def write(path, content = "<div></div>\n")
   test "resolves the view root to app/views when it is there" do
     write("app/views/posts/index.html.erb")
 
-    assert_equal File.join(@project_path, "app", "views"), Herb::Analysis::PartialIndex.build(@project_path).view_root.to_s
+    assert_equal [File.join(@project_path, "app", "views")], Herb::Analysis::PartialIndex.build(@project_path).view_roots.map(&:to_s)
   end
 
   test "falls back to the project root when there is no app/views" do
     write("posts/index.html.erb")
 
-    assert_equal @project_path, Herb::Analysis::PartialIndex.build(@project_path).view_root.to_s
+    assert_equal [@project_path], Herb::Analysis::PartialIndex.build(@project_path).view_roots.map(&:to_s)
   end
 
   test "maps a qualified partial name to its file" do
diff --git a/test/analysis/project_index_test.rb b/test/analysis/project_index_test.rb
index df4bb2963..af41ca475 100644
--- a/test/analysis/project_index_test.rb
+++ b/test/analysis/project_index_test.rb
@@ -135,6 +135,6 @@ def reindexed
   test "exposes the view root it resolved" do
     write("index.html.erb", "<div></div>")
 
-    assert_equal File.join(@project_path, "app", "views"), indexed.view_root.to_s
+    assert_equal [File.join(@project_path, "app", "views")], indexed.view_roots.map(&:to_s)
   end
 end
diff --git a/test/analysis/view_roots_test.rb b/test/analysis/view_roots_test.rb
new file mode 100644
index 000000000..e0bb61827
--- /dev/null
+++ b/test/analysis/view_roots_test.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require_relative "../test_helper"
+require_relative "../../lib/herb/analysis/partial_index"
+
+require "tmpdir"
+
+module Analysis
+  class ViewRootsTest < Minitest::Spec
+    def write(root, relative, body = "<div></div>\n")
+      path = File.join(root, relative)
+
+      FileUtils.mkdir_p(File.dirname(path))
+      File.write(path, body)
+
+      path
+    end
+
+    test "names a partial from a secondary view root" do
+      Dir.mktmpdir do |dir|
+        app = File.join(dir, "app", "views")
+        engine = File.join(dir, "engines", "billing", "app", "views")
+
+        entry = write(app, "home/index.html.erb")
+        invoice = write(engine, "billing/_invoice.html.erb")
+
+        index = Herb::Analysis::PartialIndex.new([app, engine], [entry, invoice])
+
+        assert_equal ["billing/invoice"], index.names
+      end
+    end
+
+    test "an earlier view root shadows a later one" do
+      Dir.mktmpdir do |dir|
+        app = File.join(dir, "app", "views")
+        engine = File.join(dir, "engines", "billing", "app", "views")
+
+        engine_invoice = write(engine, "billing/_invoice.html.erb", "<div>engine</div>\n")
+        app_invoice = write(app, "billing/_invoice.html.erb", "<div>app</div>\n")
+
+        index = Herb::Analysis::PartialIndex.new([app, engine], [engine_invoice, app_invoice])
+        resolved = index.resolve("billing/invoice", nil)
+
+        assert_equal 2, resolved.size
+        assert_equal app_invoice, resolved.first
+      end
+    end
+
+    test "resolves a sibling within the root that owns the caller" do
+      Dir.mktmpdir do |dir|
+        app = File.join(dir, "app", "views")
+        engine = File.join(dir, "engines", "billing", "app", "views")
+
+        entry = write(engine, "billing/index.html.erb")
+        row = write(engine, "billing/_row.html.erb")
+
+        index = Herb::Analysis::PartialIndex.new([app, engine], [entry, row])
+
+        assert_equal [row], index.resolve("row", entry)
+      end
+    end
+
+    test "a single root still resolves" do
+      Dir.mktmpdir do |dir|
+        app = File.join(dir, "app", "views")
+        header = write(app, "shared/_header.html.erb")
+
+        index = Herb::Analysis::PartialIndex.new([app], [header])
+
+        assert_equal ["shared/header"], index.names
+        assert_equal [header], index.resolve("shared/header", nil)
+      end
+    end
+  end
+end