fix(git): harden nested repository discovery

- discard an in-flight discovery when the runtime switches, mirroring the
  isRequestCurrent pattern, so a late completion cannot repopulate the
  cleared map and suppress a fresh scan
- treat a 501 from /api/fs/git-dirs as an explicit 'unsupported' marker
  instead of a generic failure; the VS Code webview now answers the route
  with unsupportedWebRouteResponse so non-repo roots show the honest
  not-a-repository state without a futile Retry

packages/ui/src/lib/gitApiHttp.ts

@@ -116,8 +116,18 @@ export async function checkIsGitRepository(directory: string): Promise<boolean>
   }
 }
 
+export class GitDirectoriesUnsupportedError extends Error {
+  constructor() {
+    super('Nested git repository discovery is not supported by this runtime');
+    this.name = 'GitDirectoriesUnsupportedError';
+  }
+}
+
 export async function listGitDirectories(root: string): Promise<string[]> {
   const response = await runtimeFetch('/api/fs/git-dirs', { query: { path: root } });
+  if (response.status === 501) {
+    throw new GitDirectoriesUnsupportedError();
+  }
   if (!response.ok) {
     throw new Error(`Failed to list git directories: ${response.statusText}`);
   }

packages/ui/src/stores/useGitStore.test.ts

@@ -1,8 +1,22 @@
-import { beforeEach, describe, expect, test } from 'bun:test';
+import { beforeEach, describe, expect, mock, test } from 'bun:test';
 import type { GitStatus } from '@/lib/api/types';
 import { useGitStore } from './useGitStore';
 import { getRuntimeKey } from '@/lib/runtime-switch';
 
+// The real transport has no server in tests and fails as a generic error.
+// Tests that exercise other failure modes swap this implementation; the
+// default keeps every pre-existing expectation (generic failure → null).
+const listGitDirectoriesControl: { impl: (root: string) => Promise<string[]> } = {
+  impl: async () => {
+    throw new Error('network unavailable');
+  },
+};
+class TestGitDirectoriesUnsupportedError extends Error {}
+mock.module('@/lib/gitApiHttp', () => ({
+  GitDirectoriesUnsupportedError: TestGitDirectoriesUnsupportedError,
+  listGitDirectories: (root: string) => listGitDirectoriesControl.impl(root),
+}));
+
 type Deferred<T> = {
   promise: Promise<T>;
   resolve: (value: T) => void;
@@ -338,6 +352,9 @@ describe('useGitStore', () => {
 
 describe('useGitStore nested repository discovery', () => {
   beforeEach(() => {
+    listGitDirectoriesControl.impl = async () => {
+      throw new Error('network unavailable');
+    };
     useGitStore.getState().resetForRuntimeSwitch(getRuntimeKey());
   });
 
@@ -379,12 +396,47 @@ describe('useGitStore nested repository discovery', () => {
     expect(useGitStore.getState().nestedReposByRoot.size).toBe(0);
   });
 
+  test('discards an in-flight discovery result when the runtime switches', async () => {
+    const stale = useGitStore.getState().ensureNestedRepos('/root-a');
+    useGitStore.getState().resetForRuntimeSwitch('runtime-b');
+    await stale;
+
+    // The old runtime's late completion must not repopulate the cleared map.
+    expect(useGitStore.getState().nestedReposByRoot.has('/root-a')).toBe(false);
+
+    // Discovery started under the new runtime still commits normally.
+    await useGitStore.getState().ensureNestedRepos('/root-a');
+    expect(useGitStore.getState().nestedReposByRoot.get('/root-a')).toBeNull();
+  });
+
   test('marks discovery failure as a failed marker, not an empty success', async () => {
     await useGitStore.getState().ensureNestedRepos('/root-a');
 
     expect(useGitStore.getState().nestedReposByRoot.get('/root-a')).toBeNull();
   });
 
+  test('marks a 501 runtime as unsupported instead of failed', async () => {
+    listGitDirectoriesControl.impl = async () => {
+      throw new TestGitDirectoriesUnsupportedError();
+    };
+
+    await useGitStore.getState().ensureNestedRepos('/root-a');
+
+    expect(useGitStore.getState().nestedReposByRoot.get('/root-a')).toBe('unsupported');
+  });
+
+  test('unsupported does not clobber a previous successful discovery', async () => {
+    listGitDirectoriesControl.impl = async () => ['/root-a/one'];
+    await useGitStore.getState().ensureNestedRepos('/root-a');
+
+    listGitDirectoriesControl.impl = async () => {
+      throw new TestGitDirectoriesUnsupportedError();
+    };
+    await useGitStore.getState().ensureNestedRepos('/root-a', { force: true });
+
+    expect(useGitStore.getState().nestedReposByRoot.get('/root-a')).toEqual(['/root-a/one']);
+  });
+
   test('dedupes concurrent discovery runs for the same root', async () => {
     const first = useGitStore.getState().ensureNestedRepos('/root-a');
     const second = useGitStore.getState().ensureNestedRepos('/root-a');

packages/ui/src/stores/useGitStore.ts

@@ -9,7 +9,7 @@ import type {
 } from '@/lib/api/types';
 import { getDeferredSafeStorage } from '@/stores/utils/safeStorage';
 import { getRuntimeKey } from '@/lib/runtime-switch';
-import { listGitDirectories } from '@/lib/gitApiHttp';
+import { GitDirectoriesUnsupportedError, listGitDirectories } from '@/lib/gitApiHttp';
 
 const LOG_STALE_THRESHOLD = 10000;
 const REPO_CHECK_STALE_THRESHOLD = 60_000;
@@ -28,6 +28,11 @@ const DIFF_CACHE_MAX_TOTAL_SIZE_BYTES = 20 * 1024 * 1024; // 20MB
 const DIFF_CACHE_MAX_GLOBAL_ENTRIES = 200;
 type GitStatusFetchMode = 'full' | 'light';
 
+// Discovery outcome for a root that is not itself a git repository. The three
+// states are mutually exclusive: a repository list (possibly empty), a failed
+// scan (`null`), or a runtime without the discovery route (`'unsupported'`).
+export type NestedRepoDiscovery = string[] | null | 'unsupported';
+
 interface DirectoryGitState {
   isGitRepo: boolean | null;
   status: GitStatus | null;
@@ -81,8 +86,9 @@ interface GitStore {
   // Nested repository discovery: when the root directory is not itself a git
   // repository, these hold the discovered repositories and the user's pick.
   // `nestedReposByRoot` values are `null` when discovery failed — never a
-  // valid empty result — and absent when discovery has not run yet.
-  nestedReposByRoot: Map<string, string[] | null>;
+  // valid empty result — `'unsupported'` when the runtime has no discovery
+  // route, and absent when discovery has not run yet.
+  nestedReposByRoot: Map<string, NestedRepoDiscovery>;
   nestedRepoSelection: Map<string, string>;
   ensureNestedRepos: (root: string, options?: { force?: boolean }) => Promise<void>;
   selectNestedRepo: (root: string, repository: string) => void;
@@ -1203,6 +1209,7 @@ export const useGitStore = create<GitStore>()(
         if (!root) return;
         const { force = false } = options;
         const runtimeKey = getRuntimeKey();
+        const runtimeGeneration = gitRuntimeGeneration;
         const key = runtimeDirectoryKey(runtimeKey, root);
         const current = get().nestedReposByRoot.get(root);
         if (!force && (current !== undefined || inFlightNestedRepoDiscovery.has(key))) {
@@ -1217,16 +1224,30 @@ export const useGitStore = create<GitStore>()(
 
         const discovery = (async () => {
           let repositories: string[] | null = null;
+          let unsupported = false;
           try {
             repositories = await listGitDirectories(root);
           } catch (error) {
-            console.error('Failed to discover nested git repositories:', error);
+            if (error instanceof GitDirectoriesUnsupportedError) {
+              unsupported = true;
+            } else {
+              console.error('Failed to discover nested git repositories:', error);
+            }
             repositories = null;
           }
 
-          // A failed retry must not clobber an earlier successful discovery.
+          // A runtime switch invalidates the discovery: resetForRuntimeSwitch
+          // already cleared the map, and committing old-runtime data here would
+          // both leak it and suppress a fresh scan for this root.
+          if (runtimeKey !== getRuntimeKey() || runtimeGeneration !== gitRuntimeGeneration) return;
+
           const previous = get().nestedReposByRoot.get(root);
-          const nextValue = repositories ?? previous ?? null;
+          // An authoritative "unsupported" answer replaces only unknown or
+          // failed state; like a failed retry, it must not clobber an earlier
+          // successful discovery.
+          const nextValue: NestedRepoDiscovery = unsupported
+            ? (previous ?? 'unsupported')
+            : (repositories ?? previous ?? null);
           const next = new Map(get().nestedReposByRoot);
           next.set(root, nextValue);
           set({ nestedReposByRoot: next });
@@ -1368,8 +1389,9 @@ export const useEffectiveGitDirectory = (root: string | null) => {
   });
 };
 
-// `undefined` = discovery not run yet, `null` = discovery failed, otherwise
-// the discovered nested repository paths (possibly empty).
+// `undefined` = discovery not run yet, `null` = discovery failed,
+// `'unsupported'` = the runtime has no discovery route, otherwise the
+// discovered nested repository paths (possibly empty).
 export const useNestedRepos = (root: string | null) => {
   return useGitStore((state) => {
     if (!root) return undefined;

packages/vscode/webview/main.tsx

@@ -388,6 +388,10 @@ const handleLocalApiRequest = async (input: RequestInfo | URL, url: URL, init: R
     return unsupportedWebRouteResponse('Scheduled tasks');
   }
 
+  if (normalizedPathname === '/api/fs/git-dirs') {
+    return unsupportedWebRouteResponse('Nested git repository discovery');
+  }
+
   if (normalizedPathname === '/api/sessions/snapshot' && method === 'GET') {
     const activity = await sendBridgeMessage<Record<string, { type: 'idle' | 'busy' | 'cooldown' }>>('api:session-activity:get')
       .catch(() => ({}));