diff --git a/packages/core/src/flag/tool-choice.ts b/packages/core/src/flag/tool-choice.ts
new file mode 100644
index 0000000..01e31c0
--- /dev/null
+++ b/packages/core/src/flag/tool-choice.ts
@@ -0,0 +1,20 @@
+import { Flag } from "./flag"
+
+/**
+ * Resolve OpenAI-compatible tool_choice for one agent step.
+ *
+ * - Final max-steps turn always forces `none` so the model must answer in text.
+ * - Structured-output turns always force `required` (schema tool).
+ * - Otherwise honor OPENCODE_TOOL_CHOICE / VERDICT_FORCE_TOOL_CHOICE when set.
+ * - Default remains undefined (provider auto).
+ */
+export function resolveAgentToolChoice(input: {
+  readonly isLastStep: boolean
+  readonly structuredRequired?: boolean
+}): "auto" | "required" | "none" | undefined {
+  if (input.isLastStep) return "none"
+  if (input.structuredRequired) return "required"
+  const flag = Flag.OPENCODE_TOOL_CHOICE
+  if (flag === "required" || flag === "auto" || flag === "none") return flag
+  return undefined
+}
diff --git a/packages/core/src/session/runner/llm.ts b/packages/core/src/session/runner/llm.ts
index 2bbdd60..9560d2c 100644
--- a/packages/core/src/session/runner/llm.ts
+++ b/packages/core/src/session/runner/llm.ts
@@ -11,7 +11,7 @@ import {
 import { Cause, DateTime, Effect, FiberSet, Layer, Option, Semaphore, Stream } from "effect"
 import { AgentV2 } from "../../agent"
 import { Config } from "../../config"
-import { Flag } from "../../flag/flag"
+import { resolveAgentToolChoice } from "../../flag/tool-choice"
 import { Database } from "../../database/database"
 import { EventV2 } from "../../event"
 import { Location } from "../../location"
@@ -208,15 +208,8 @@ const layer = Layer.effect(
         tools: toolMaterialization?.definitions ?? [],
         // Default: auto (undefined). OPENCODE_TOOL_CHOICE=required forces tool
         // calls on non-final steps so weak local models cannot print JSON prose.
-        toolChoice: isLastStep
-          ? "none"
-          : Flag.OPENCODE_TOOL_CHOICE === "required"
-            ? "required"
-            : Flag.OPENCODE_TOOL_CHOICE === "none"
-              ? "none"
-              : Flag.OPENCODE_TOOL_CHOICE === "auto"
-                ? "auto"
-                : undefined,
+        // Final max-steps exit still forces none via resolveAgentToolChoice.
+        toolChoice: resolveAgentToolChoice({ isLastStep }),
       })
       if (yield* compaction.compactIfNeeded({ sessionID: session.id, entries, model, request }))
         return yield* Effect.die(continueAfterCompaction(currentStep))
diff --git a/packages/core/src/tool/registry.ts b/packages/core/src/tool/registry.ts
index 1c2dfe7..00dc815 100644
--- a/packages/core/src/tool/registry.ts
+++ b/packages/core/src/tool/registry.ts
@@ -116,7 +116,12 @@ const registryLayer = Layer.effect(
           settle: (input) => {
             const registration = registrations.get(input.call.name)
             if (registration) return settleWith(input, registration.identity)
-            return Effect.succeed({ result: { type: "error", value: `Unknown tool: ${input.call.name}` } })
+            const available = Array.from(registrations.keys()).slice(0, 48)
+            const value =
+              available.length > 0
+                ? `Unknown tool: ${input.call.name}. Available tools (${available.length}): ${available.join(", ")}`
+                : `Unknown tool: ${input.call.name}`
+            return Effect.succeed({ result: { type: "error", value } })
           },
         }
       }),
diff --git a/packages/core/test/session-runner-tool-registry.test.ts b/packages/core/test/session-runner-tool-registry.test.ts
index 82cd015..d73dde5 100644
--- a/packages/core/test/session-runner-tool-registry.test.ts
+++ b/packages/core/test/session-runner-tool-registry.test.ts
@@ -178,7 +178,10 @@ describe("ToolRegistry", () => {
           ...identity,
           call: { type: "tool-call", id: "missing", name: "missing", input: {} },
         }),
-      ).toEqual({ type: "error", value: "Unknown tool: missing" })
+      ).toMatchObject({
+        type: "error",
+        value: expect.stringMatching(/^Unknown tool: missing\. Available tools \(\d+\): /),
+      })
 
       yield* service.register({
         defect: Tool.make({
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index 7fdf4df..487b826 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -2552,7 +2552,10 @@ describe("SessionRunnerLLM", () => {
             {
               type: "tool",
               id: "call-missing",
-              state: { status: "error", error: { message: "Unknown tool: missing" } },
+              state: {
+                status: "error",
+                error: { message: expect.stringMatching(/^Unknown tool: missing\./) },
+              },
             },
           ],
         },
@@ -2809,6 +2812,57 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
+  it.effect("forces tool_choice required on non-final steps when OPENCODE_TOOL_CHOICE=required", () =>
+    Effect.gen(function* () {
+      const prev = process.env["OPENCODE_TOOL_CHOICE"]
+      const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      process.env["OPENCODE_TOOL_CHOICE"] = "required"
+      delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      try {
+        yield* setup
+        const agents = yield* AgentV2.Service
+        yield* agents.transform((editor) =>
+          editor.update(AgentV2.ID.make("build"), (agent) => {
+            agent.steps = 2
+          }),
+        )
+        const session = yield* SessionV2.Service
+        yield* session.prompt({ sessionID, prompt: Prompt.make({ text: "Require tools" }), resume: false })
+
+        requests.length = 0
+        executions.length = 0
+        responses = [
+          [
+            LLMEvent.stepStart({ index: 0 }),
+            LLMEvent.toolCall({ id: "call-required", name: "echo", input: { text: "done" } }),
+            LLMEvent.stepFinish({ index: 0, reason: "tool-calls" }),
+            LLMEvent.finish({ reason: "tool-calls" }),
+          ],
+          [
+            LLMEvent.stepStart({ index: 0 }),
+            LLMEvent.textStart({ id: "text-final" }),
+            LLMEvent.textDelta({ id: "text-final", text: "Done" }),
+            LLMEvent.textEnd({ id: "text-final" }),
+            LLMEvent.stepFinish({ index: 0, reason: "stop" }),
+            LLMEvent.finish({ reason: "stop" }),
+          ],
+        ]
+
+        yield* session.resume(sessionID)
+
+        expect(requests).toHaveLength(2)
+        expect(requests[0]?.toolChoice).toMatchObject({ type: "required" })
+        expect(requests[1]?.toolChoice).toMatchObject({ type: "none" })
+        expect(executions).toEqual(["done"])
+      } finally {
+        if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+        else process.env["OPENCODE_TOOL_CHOICE"] = prev
+        if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+        else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+      }
+    }),
+  )
+
   it.effect("forces a text response on an agent's configured final step", () =>
     Effect.gen(function* () {
       yield* setup
diff --git a/packages/core/test/tool-choice-resolve.test.ts b/packages/core/test/tool-choice-resolve.test.ts
new file mode 100644
index 0000000..edf6246
--- /dev/null
+++ b/packages/core/test/tool-choice-resolve.test.ts
@@ -0,0 +1,80 @@
+import { describe, expect, test } from "bun:test"
+import { resolveAgentToolChoice } from "../src/flag/tool-choice"
+
+describe("resolveAgentToolChoice", () => {
+  test("last step always forces none (max-steps early stop)", async () => {
+    const prev = process.env["OPENCODE_TOOL_CHOICE"]
+    const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+    try {
+      process.env["OPENCODE_TOOL_CHOICE"] = "required"
+      delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      expect(resolveAgentToolChoice({ isLastStep: true })).toBe("none")
+      expect(resolveAgentToolChoice({ isLastStep: true, structuredRequired: true })).toBe("none")
+    } finally {
+      if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+      else process.env["OPENCODE_TOOL_CHOICE"] = prev
+      if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+    }
+  })
+
+  test("structured output requires tool choice on non-final steps", async () => {
+    const prev = process.env["OPENCODE_TOOL_CHOICE"]
+    const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+    try {
+      delete process.env["OPENCODE_TOOL_CHOICE"]
+      delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      expect(resolveAgentToolChoice({ isLastStep: false, structuredRequired: true })).toBe("required")
+    } finally {
+      if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+      else process.env["OPENCODE_TOOL_CHOICE"] = prev
+      if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+    }
+  })
+
+  test("OPENCODE_TOOL_CHOICE=required forces tool calls on non-final steps", async () => {
+    const prev = process.env["OPENCODE_TOOL_CHOICE"]
+    const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+    try {
+      delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      process.env["OPENCODE_TOOL_CHOICE"] = "required"
+      expect(resolveAgentToolChoice({ isLastStep: false })).toBe("required")
+    } finally {
+      if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+      else process.env["OPENCODE_TOOL_CHOICE"] = prev
+      if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+    }
+  })
+
+  test("VERDICT_FORCE_TOOL_CHOICE=1 maps to required on non-final steps", async () => {
+    const prev = process.env["OPENCODE_TOOL_CHOICE"]
+    const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+    try {
+      delete process.env["OPENCODE_TOOL_CHOICE"]
+      process.env["VERDICT_FORCE_TOOL_CHOICE"] = "1"
+      expect(resolveAgentToolChoice({ isLastStep: false })).toBe("required")
+    } finally {
+      if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+      else process.env["OPENCODE_TOOL_CHOICE"] = prev
+      if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+    }
+  })
+
+  test("default remains undefined (provider auto)", async () => {
+    const prev = process.env["OPENCODE_TOOL_CHOICE"]
+    const prevForce = process.env["VERDICT_FORCE_TOOL_CHOICE"]
+    try {
+      delete process.env["OPENCODE_TOOL_CHOICE"]
+      delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      expect(resolveAgentToolChoice({ isLastStep: false })).toBeUndefined()
+    } finally {
+      if (prev === undefined) delete process.env["OPENCODE_TOOL_CHOICE"]
+      else process.env["OPENCODE_TOOL_CHOICE"] = prev
+      if (prevForce === undefined) delete process.env["VERDICT_FORCE_TOOL_CHOICE"]
+      else process.env["VERDICT_FORCE_TOOL_CHOICE"] = prevForce
+    }
+  })
+})
diff --git a/packages/opencode/src/session/llm.ts b/packages/opencode/src/session/llm.ts
index a99f8ac..aaccbfb 100644
--- a/packages/opencode/src/session/llm.ts
+++ b/packages/opencode/src/session/llm.ts
@@ -301,11 +301,20 @@ const live: Layer.Layer<
                 toolName: lower,
               }
             }
+            // Surface a short available-tool list so weak local models can
+            // self-correct instead of looping on invented names until timeout.
+            const available = Object.keys(prepared.tools)
+              .filter((name) => name !== "invalid")
+              .slice(0, 48)
+            const hint =
+              available.length > 0
+                ? ` Available tools (${available.length}): ${available.join(", ")}`
+                : ""
             return {
               ...failed.toolCall,
               input: JSON.stringify({
                 tool: failed.toolCall.toolName,
-                error: failed.error.message,
+                error: `${failed.error.message}.${hint}`,
               }),
               toolName: "invalid",
             }
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 6734a1f..03de3ba 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -17,6 +17,7 @@ import { SystemPrompt } from "./system"
 import { Instruction } from "./instruction"
 import { Plugin } from "../plugin"
 import { MAX_STEPS_PROMPT } from "@opencode-ai/core/session/runner/max-steps"
+import { resolveAgentToolChoice } from "@opencode-ai/core/flag/tool-choice"
 import { ToolRegistry } from "@/tool/registry"
 import { MCP } from "../mcp"
 import { LSP } from "@/lsp/lsp"
@@ -1281,7 +1282,13 @@ const layer = Layer.effect(
               ],
               tools,
               model,
-              toolChoice: format.type === "json_schema" ? "required" : undefined,
+              // Live `verdict run` path (caseforge investigate). m19 only wired
+              // OPENCODE_TOOL_CHOICE into the V2 runner; this SessionPrompt loop
+              // is what the binary actually uses for agent turns.
+              toolChoice: resolveAgentToolChoice({
+                isLastStep,
+                structuredRequired: format.type === "json_schema",
+              }),
             })
 
             if (structured !== undefined) {
diff --git a/packages/opencode/src/tool/invalid.ts b/packages/opencode/src/tool/invalid.ts
index b8d145d..2748d5f 100644
--- a/packages/opencode/src/tool/invalid.ts
+++ b/packages/opencode/src/tool/invalid.ts
@@ -14,7 +14,9 @@ export const InvalidTool = Tool.define(
     execute: (params: { tool: string; error: string }) =>
       Effect.succeed({
         title: "Invalid Tool",
-        output: `The arguments provided to the tool are invalid: ${params.error}`,
+        // Used for both unknown tool names (via experimental_repairToolCall) and
+        // schema failures — tell the model to retry with an exact advertised name.
+        output: `Tool call rejected for "${params.tool}": ${params.error}. Use an exact available tool name with valid arguments; do not invent tool names.`,
         metadata: {},
       }),
   }),