diff --git a/docs/design/worktree-manager.md b/docs/design/worktree-manager.md
index adc102e7..ce1312b1 100644
--- a/docs/design/worktree-manager.md
+++ b/docs/design/worktree-manager.md
@@ -169,7 +169,12 @@ Intents:
   `ResourceResolver` proves a resource for that path. The core ships no
   resolver.
 - `DiffIntent {repository, worktreePath, ref, head}` for a future #332 caller.
-- Related PR is always `unavailable: No pull request evidence yet` (#338).
+- Related PR (`p`, `openPullRequest`): available only with evidence from the host's cached `/github` listing
+  (`ControllerOptions.pullRequests`, never fetched by the manager). `relatedPullRequests` counts a PR only when its
+  head branch, in the PR's own repository, is exactly the worktree's branch: fork PRs with the same branch name,
+  PRs whose head repository GitHub did not report, detached worktrees and partial names are not evidence. Whether
+  the worktree's HEAD is the PR's head commit is reported separately (`atHead`). Without a listing the action says
+  `No pull request evidence yet`.
 
 Rendering: selection is a marker (`›`/`>`) plus weight, never color alone;
 `level: 'none'` emits no escape bytes; Safe glyphs are pure ASCII (and use
@@ -212,6 +217,6 @@ terminals window whole entries around the selection with an `a-b/N` indicator.
 - NMSh session focus, tmux focus, and opening a new session for a worktree.
 - #331 sidebar quick navigation (can reuse `discoverWorktrees` and the model).
 - #332 diff panel (consumes `DiffIntent`).
-- #338 related PR evidence.
+- Passing the `/github` listing to `/worktrees` as `pullRequests` and opening the PR in `/github` on `openPullRequest` (TerminalApp).
 - #329 managed-target focus context.
 - Physical terminal QA.
diff --git a/src/githubWorkspace/fixtures.ts b/src/githubWorkspace/fixtures.ts
index 8523cf9e..6d508ad7 100644
--- a/src/githubWorkspace/fixtures.ts
+++ b/src/githubWorkspace/fixtures.ts
@@ -57,6 +57,7 @@ for (let i = 0; i < 20; i++) ISSUES.push({number: 100 - i, title: `Backlog item
 function rawPr(pr: RawPr): Record<string, unknown> {
   return {__typename: 'PullRequest', number: pr.number, title: pr.title, url: `https://github.com/${OWNER}/${REPO}/pull/${pr.number}`, state: pr.state,
     isDraft: pr.isDraft ?? false, updatedAt: hoursAgo(pr.updated), headRefName: pr.head, baseRefName: pr.base ?? 'master',
+    headRepository: {nameWithOwner: pr.number === 315 ? 'contributor/notMyShell' : `${OWNER}/${REPO}`},
     additions: pr.additions, deletions: pr.deletions, changedFiles: pr.changedFiles, reviewDecision: pr.review, mergeable: pr.mergeable,
     author: pr.author === null ? null : {login: pr.author ?? OWNER}, repository,
     commits: pr.rollup === undefined ? {nodes: []} : {nodes: [{commit: {statusCheckRollup: pr.rollup === null ? null : {state: pr.rollup}}}]}};
diff --git a/src/githubWorkspace/model.ts b/src/githubWorkspace/model.ts
index 025f23a3..6a2efb84 100644
--- a/src/githubWorkspace/model.ts
+++ b/src/githubWorkspace/model.ts
@@ -74,6 +74,10 @@ export interface PrSummary {
   state: PrState;
   draft: boolean;
   headRef?: string;
+  /** `owner/name` of the repository holding the head branch; differs from the PR's repository for a fork. */
+  headRepository?: string;
+  /** The head commit GitHub reports (40 hex), when reported. */
+  headSha?: string;
   baseRef?: string;
   updatedAt?: string;
   reviewDecision: ReviewDecision;
diff --git a/src/githubWorkspace/queries.ts b/src/githubWorkspace/queries.ts
index a787e209..977a90d6 100644
--- a/src/githubWorkspace/queries.ts
+++ b/src/githubWorkspace/queries.ts
@@ -13,7 +13,8 @@ export const SEARCH_QUERY = `query($q: String!, $first: Int!, $after: String) {
     nodes {
       __typename
       ... on PullRequest {
-        number title url state isDraft updatedAt headRefName baseRefName
+        number title url state isDraft updatedAt headRefName baseRefName headRefOid
+        headRepository { nameWithOwner }
         additions deletions changedFiles reviewDecision mergeable
         author { login }
         repository { name owner { login } }
diff --git a/src/githubWorkspace/source.ts b/src/githubWorkspace/source.ts
index 9a6488f9..484c3fd9 100644
--- a/src/githubWorkspace/source.ts
+++ b/src/githubWorkspace/source.ts
@@ -84,6 +84,8 @@ function mapPrSummary(node: Json): PrSummary | undefined {
     state: mapPrState(node.state),
     draft: node.isDraft === true,
     headRef: optionalLine(node.headRefName, LIMITS.nameLength * 2),
+    headRepository: optionalLine(obj(node.headRepository).nameWithOwner, 200),
+    headSha: /^[0-9a-f]{40}$/u.test(String(node.headRefOid)) ? String(node.headRefOid) : undefined,
     baseRef: optionalLine(node.baseRefName, LIMITS.nameLength * 2),
     updatedAt: str(node.updatedAt),
     reviewDecision: 'reviewDecision' in node ? mapReviewDecision(node.reviewDecision) : 'unknown',
diff --git a/src/worktrees/controller.ts b/src/worktrees/controller.ts
index 6d02e046..51fded2e 100644
--- a/src/worktrees/controller.ts
+++ b/src/worktrees/controller.ts
@@ -1,3 +1,5 @@
+import type {PrSummary} from '../githubWorkspace/model.js';
+import {relatedPullRequests, type RelatedPr} from './relatedPr.js';
 import type {Key} from '../terminal/keys.js';
 import type {GitRunner} from './git.js';
 import {discoverWorktrees, type DiscoveryOptions} from './discovery.js';
@@ -37,7 +39,9 @@ export type KeyOutcome =
   | {readonly kind: 'requestNewWorktree'}
   | {readonly kind: 'removed'; readonly path: string}
   | {readonly kind: 'created'; readonly path: string}
-  | {readonly kind: 'back'};
+  | {readonly kind: 'back'}
+  /** Open the related pull request in /github (the host decides how). */
+  | {readonly kind: 'openPullRequest'; readonly pr: PrSummary};
 
 export type Review =
   | {readonly kind: 'remove'; readonly plan: RemovalPlan}
@@ -63,6 +67,11 @@ export interface ControllerOptions {
   readonly currentPath?: () => string | undefined;
   readonly resources?: ResourceResolver;
   readonly discovery?: DiscoveryOptions;
+  /**
+   * Pull requests the host already holds (the /github service's cached listing; never fetched here), for related-PR
+   * evidence. Without it the related-PR action stays unavailable.
+   */
+  readonly pullRequests?: () => {readonly repository: string; readonly prs: readonly PrSummary[]} | undefined;
 }
 
 export function searchText(worktree: WorktreeRecord): string {
@@ -131,6 +140,12 @@ export class WorktreeManagerController {
     return {kind: 'diff', repository, worktreePath: worktree.path, ref: worktree.ref, head: worktree.head};
   }
 
+  /** Related pull requests for the selected worktree, from the host's cached listing (see relatedPullRequests). */
+  relatedPrs(worktree = this.selected()): RelatedPr[] {
+    const listing = this.options.pullRequests?.();
+    return worktree && listing ? relatedPullRequests(worktree, listing.repository, listing.prs) : [];
+  }
+
   /** Availability from current facts only; drives footers and key dispatch alike. */
   actions(): ActionState[] {
     const worktree = this.selected();
@@ -141,7 +156,7 @@ export class WorktreeManagerController {
       this.navigationIntent() ? {id: 'navigate', available: true} : none('navigate', 'Path is not available'),
       this.current.snapshot && !stale ? {id: 'new', available: true} : none('new', 'Refresh first'),
       this.diffIntent() ? {id: 'diff', available: true} : none('diff', 'No HEAD or path'),
-      none('relatedPr', 'No pull request evidence yet'),
+      this.relatedPrs().length ? {id: 'relatedPr', available: true} : none('relatedPr', this.options.pullRequests?.() ? 'No pull request has this branch as its head' : 'No pull request evidence yet'),
       stale ? none('remove', 'List is stale; refresh first') : removal.available ? {id: 'remove', available: true} : none('remove', removal.reason),
       {id: 'refresh', available: true},
       this.current.snapshot ? {id: 'search', available: true} : none('search', 'Nothing loaded'),
@@ -188,7 +203,7 @@ export class WorktreeManagerController {
   cancelReview(): void { this.set({review: undefined}); }
 
   /**
-   * Keyboard model: ↑/↓ select, Enter navigate, n new, x preview removal,
+   * Keyboard model: ↑/↓ select, Enter navigate, n new, x preview removal, p related pull request,
    * r refresh, / search, Esc clears search, then review, then goes back.
    */
   async handleKey(key: Key): Promise<KeyOutcome> {
@@ -222,6 +237,7 @@ export class WorktreeManagerController {
         if (value === 'r') { await this.refresh(); return {kind: 'changed'}; }
         if (value === 'n') return this.available('new') ? {kind: 'requestNewWorktree'} : {kind: 'none'};
         if (value === 'x') { await this.previewRemoval(); return {kind: 'changed'}; }
+        if (value === 'p') { const related = this.relatedPrs()[0]; return related ? {kind: 'openPullRequest', pr: related.pr} : {kind: 'none'}; }
         if (value === '/' && this.available('search')) { this.set({searching: true}); return {kind: 'changed'}; }
         return {kind: 'none'};
       }
diff --git a/src/worktrees/relatedPr.ts b/src/worktrees/relatedPr.ts
new file mode 100644
index 00000000..b05c46be
--- /dev/null
+++ b/src/worktrees/relatedPr.ts
@@ -0,0 +1,26 @@
+import type {PrSummary} from '../githubWorkspace/model.js';
+import type {WorktreeRecord} from './model.js';
+
+/**
+ * Pull requests related to a worktree, from evidence only: a PR whose head branch, in the same repository as the PR,
+ * is exactly the worktree's checked-out branch. A fork's PR with a branch of the same name is not related (its head
+ * lives elsewhere), a PR whose head repository GitHub did not report is not counted (fail closed), and nothing is
+ * inferred from titles, folder names or partial branch matches. Whether the worktree is at the PR's head commit is
+ * reported separately, because a local branch can be ahead of or behind what GitHub has.
+ */
+
+export interface RelatedPr {
+  pr: PrSummary;
+  /** The worktree's HEAD is the PR's head commit; undefined when either is unknown. */
+  atHead?: boolean;
+}
+
+export function relatedPullRequests(worktree: WorktreeRecord, repository: string, prs: readonly PrSummary[]): RelatedPr[] {
+  if (!worktree.branch || worktree.detached || worktree.bare) return [];
+  const same = (a: string, b: string) => a.toLowerCase() === b.toLowerCase();
+  return prs
+    .filter(pr => same(`${pr.ref.owner}/${pr.ref.repo}`, repository) && pr.headRepository !== undefined && same(pr.headRepository, repository) && pr.headRef === worktree.branch)
+    .map(pr => ({pr, ...(pr.headSha && worktree.head ? {atHead: pr.headSha === worktree.head} : {})}))
+    // Open first, then most recently updated.
+    .sort((a, b) => Number(b.pr.state === 'open') - Number(a.pr.state === 'open') || (b.pr.updatedAt ?? '').localeCompare(a.pr.updatedAt ?? ''));
+}
diff --git a/tests/worktreeRelatedPr.test.ts b/tests/worktreeRelatedPr.test.ts
new file mode 100644
index 00000000..cfb5bcd6
--- /dev/null
+++ b/tests/worktreeRelatedPr.test.ts
@@ -0,0 +1,41 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {mapSearch} from '../src/githubWorkspace/source.js';
+import {SEARCH_QUERY} from '../src/githubWorkspace/queries.js';
+import type {PrSummary} from '../src/githubWorkspace/model.js';
+import {relatedPullRequests} from '../src/worktrees/relatedPr.js';
+import type {WorktreeRecord} from '../src/worktrees/model.js';
+
+const HEAD = 'a'.repeat(40);
+const pr = (number: number, extra: Partial<PrSummary> = {}): PrSummary => ({ref: {kind: 'pr', owner: 'raiseCatError', repo: 'notMyShell', number}, title: `PR ${number}`,
+  state: 'open', draft: false, headRef: 'feature/x', headRepository: 'raiseCatError/notMyShell', headSha: HEAD, reviewDecision: 'none', mergeable: 'unknown', checks: 'unknown', ...extra});
+const worktree = (extra: Partial<WorktreeRecord> = {}): WorktreeRecord => ({id: '/w', path: '/w', ref: 'refs/heads/feature/x', branch: 'feature/x', head: HEAD, detached: false,
+  bare: false, main: false, locked: false, prunable: false, pathState: 'present', status: {kind: 'clean'}, ...extra});
+
+test('related pull requests: same-repository head branch only; forks, unknown heads and partial names are not evidence', () => {
+  const prs = [
+    pr(1),
+    pr(2, {headRepository: 'someone/notMyShell'}),
+    pr(3, {headRepository: undefined}),
+    pr(4, {headRef: 'feature/x-2'}),
+    pr(5, {state: 'merged', updatedAt: '2026-10-11T00:00:00Z', headSha: 'b'.repeat(40)}),
+    pr(6, {ref: {kind: 'pr', owner: 'other', repo: 'repo', number: 6}, headRepository: 'other/repo'}),
+  ];
+  const related = relatedPullRequests(worktree(), 'raiseCatError/notMyShell', prs);
+  assert.deepEqual(related.map(item => [item.pr.ref.number, item.atHead]), [[1, true], [5, false]], 'open first; the merged PR is related but its head is not this HEAD');
+  assert.deepEqual(relatedPullRequests(worktree({detached: true, branch: undefined}), 'raiseCatError/notMyShell', prs), [], 'a detached HEAD has no branch to match');
+  assert.deepEqual(relatedPullRequests(worktree(), 'RAISECATERROR/NOTMYSHELL', prs).length, 2, 'owner/name compare case-insensitively, as GitHub does');
+  assert.equal(relatedPullRequests(worktree({head: undefined}), 'raiseCatError/notMyShell', [pr(1)])[0]!.atHead, undefined, 'unknown, not false');
+});
+
+test('PR summaries carry the head repository and head commit from the search query', () => {
+  assert.match(SEARCH_QUERY, /headRefOid\s+headRepository \{ nameWithOwner \}/u);
+  const page = mapSearch({search: {issueCount: 1, pageInfo: {hasNextPage: false}, nodes: [{__typename: 'PullRequest', number: 7, title: 't', state: 'OPEN', headRefName: 'b',
+    headRefOid: HEAD, headRepository: {nameWithOwner: 'fork/notMyShell\u001b[31m'}, repository: {name: 'notMyShell', owner: {login: 'raiseCatError'}}}, {__typename: 'PullRequest', number: 8, title: 't', state: 'OPEN',
+    headRefOid: 'not-a-sha', repository: {name: 'notMyShell', owner: {login: 'raiseCatError'}}}]}});
+  const [first, second] = page.items as PrSummary[];
+  assert.equal(first!.headSha, HEAD);
+  assert.doesNotMatch(first!.headRepository!, /\u001b/u, 'hostile text is sanitized');
+  assert.equal(second!.headSha, undefined, 'a malformed commit id is not kept');
+  assert.equal(second!.headRepository, undefined);
+});