diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1abb7a2..ecc2cbd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,8 +14,8 @@
 
 ### Documentation
 
-- simplify the setup story around three cases: default rootless setup, single-project fallback, and explicit `project` retries
-- clarify that issue #63 fixed the architecture and workspace-aware workflow, but issue #2 is not fully solved when the client does not provide enough project context
+- simplify the setup story around a roots-first contract: roots-capable multi-project sessions, single-project fallback, and explicit `project` retries
+- clarify that issue #63 fixed the architecture and workspace-aware workflow, but issue #2 is still only partially solved when the client does not provide roots or active-project context
 - remove the repo-local `init` / marker-file story from the public setup guidance
 
 ## [1.9.0](https://github.com/PatrickSys/codebase-context/compare/v1.8.2...v1.9.0) (2026-03-19)
diff --git a/docs/capabilities.md b/docs/capabilities.md
index 60796fb..547472a 100644
--- a/docs/capabilities.md
+++ b/docs/capabilities.md
@@ -21,6 +21,7 @@ Per-project config overrides supported today:
 - `projects[].analyzerHints.analyzer`: prefers a registered analyzer by name for that project and falls back safely when the name is missing or invalid
 - `projects[].analyzerHints.extensions`: adds project-local source extensions for indexing and auto-refresh watching without changing defaults for other projects
 
+
 Copy-pasteable client config templates are shipped in the package:
 
 - `templates/mcp/stdio/.mcp.json` — stdio setup for `.mcp.json`-style clients
@@ -104,6 +105,7 @@ Behavior matrix:
 Rules:
 
 - If the client provides workspace context, that becomes the trusted workspace boundary for the session. In practice this usually comes from MCP roots.
+- Treat seamless multi-project routing as evidence-backed only for roots-capable hosts. Without roots, explicit fallback is still required.
 - If the server still cannot tell which project to use, a bootstrap path or explicit absolute `project` path remains the fallback.
 - `project` is the canonical explicit selector when routing is ambiguous.
 - `project` may point at a project path, file path, `file://` URI, or relative subproject path.
diff --git a/docs/client-setup.md b/docs/client-setup.md
index 4acdb02..23304e6 100644
--- a/docs/client-setup.md
+++ b/docs/client-setup.md
@@ -18,6 +18,18 @@ npx -y codebase-context --http --port 4000
 
 Copy-pasteable templates: [`templates/mcp/stdio/.mcp.json`](../templates/mcp/stdio/.mcp.json) and [`templates/mcp/http/.mcp.json`](../templates/mcp/http/.mcp.json).
 
+## Project routing contract
+
+Automatic multi-project routing is evidence-backed only when the MCP host announces workspace roots. Treat that as the primary path.
+
+If the host does not send roots, or still cannot tell which project is active, use one of the explicit fallbacks instead:
+
+- start the server with a single bootstrap path
+- set `CODEBASE_ROOT`
+- retry tool calls with `project`
+
+If multiple projects are available and no active project can be inferred safely, the server returns `selection_required` instead of guessing.
+
 ## Claude Code
 
 ```bash
@@ -197,9 +209,9 @@ Check these three flows:
 
 1. **Single project** — call `search_codebase` or `metadata`. Routing is automatic.
 
-2. **Multiple projects, one server entry** — open two repos or a monorepo. Call `codebase://context`. Expected: workspace overview, then automatic routing once a project is active.
+2. **Multiple projects on a roots-capable host** — open two repos or a monorepo. Call `codebase://context`. Expected: workspace overview, then automatic routing once a project is active.
 
-3. **Ambiguous selection** — start without a bootstrap path, call `search_codebase`. Expected: `selection_required`. Retry with `project` set to `apps/dashboard` or `/repos/customer-portal`.
+3. **Ambiguous or no-roots selection** — start without a bootstrap path, call `search_codebase`. Expected: `selection_required`. Retry with `project` set to `apps/dashboard` or `/repos/customer-portal`.
 
 For monorepos, test all three selector forms:
 
diff --git a/scripts/run-vitest.mjs b/scripts/run-vitest.mjs
index 32e164e..c529a5c 100644
--- a/scripts/run-vitest.mjs
+++ b/scripts/run-vitest.mjs
@@ -1,3 +1,5 @@
+// Spawns vitest via process.execPath to avoid bin-resolution failures when
+// Node is invoked directly (e.g. `node scripts/run-vitest.mjs`) without pnpm.
 import { spawn } from 'node:child_process';
 import { fileURLToPath } from 'node:url';
 
diff --git a/src/index.ts b/src/index.ts
index a520ed2..5f1005c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1667,10 +1667,8 @@ async function applyServerConfig(
       configRoots.set(rootKey, { rootPath: proj.root });
       registerKnownRoot(proj.root);
       const runtimeOverrides = buildProjectRuntimeOverrides(proj);
-      if (Object.keys(runtimeOverrides).length > 0) {
-        const project = getOrCreateProject(proj.root);
-        project.runtimeOverrides = runtimeOverrides;
-      }
+      const project = getOrCreateProject(proj.root);
+      project.runtimeOverrides = runtimeOverrides;
     } catch {
       console.error(`[config] Skipping inaccessible project root: ${proj.root}`);
     }
diff --git a/src/utils/language-detection.ts b/src/utils/language-detection.ts
index 7999d84..2cc8bac 100644
--- a/src/utils/language-detection.ts
+++ b/src/utils/language-detection.ts
@@ -176,6 +176,9 @@ function buildCodeExtensions(extraExtensions?: Iterable<string>): Set<string> {
   return merged;
 }
 
+// Cached default set — built once at module load, reused by callers that pass no extra extensions.
+const defaultCodeExtensions: ReadonlySet<string> = buildCodeExtensions();
+
 /**
  * Detect language from file path
  */
@@ -193,7 +196,11 @@ export function isCodeFile(
 ): boolean {
   const ext = path.extname(filePath).toLowerCase();
   const supportedExtensions =
-    extensions instanceof Set ? extensions : buildCodeExtensions(extensions);
+    extensions instanceof Set
+      ? extensions
+      : extensions
+        ? buildCodeExtensions(extensions)
+        : defaultCodeExtensions;
   return supportedExtensions.has(ext);
 }
 
diff --git a/tests/mcp-client-templates.test.ts b/tests/mcp-client-templates.test.ts
index 1d40405..03cf820 100644
--- a/tests/mcp-client-templates.test.ts
+++ b/tests/mcp-client-templates.test.ts
@@ -133,4 +133,27 @@ describe('docs/capabilities.md transport documentation', () => {
     expect(caps).toContain('Codex');
     expect(caps).toContain('Windsurf');
   });
+
+  it('states the roots-first routing fallback explicitly', () => {
+    expect(caps).toContain('roots-capable hosts');
+    expect(caps).toContain('explicit fallback is still required');
+  });
+});
+
+describe('docs/client-setup.md multi-project guidance', () => {
+  const clientSetup = readText('docs/client-setup.md');
+
+  it('documents the project routing contract', () => {
+    expect(clientSetup).toContain(
+      'Automatic multi-project routing is evidence-backed only when the MCP host announces workspace roots.'
+    );
+    expect(clientSetup).toContain(
+      'the server returns `selection_required` instead of guessing'
+    );
+  });
+
+  it('keeps the three verification flows aligned with the roots-first contract', () => {
+    expect(clientSetup).toContain('Multiple projects on a roots-capable host');
+    expect(clientSetup).toContain('Ambiguous or no-roots selection');
+  });
 });
diff --git a/tests/multi-project-routing.test.ts b/tests/multi-project-routing.test.ts
index e091be9..3ee1d68 100644
--- a/tests/multi-project-routing.test.ts
+++ b/tests/multi-project-routing.test.ts
@@ -362,6 +362,45 @@ describe('multi-project routing', () => {
     }
   });
 
+  it('triggers a background rebuild for a corrupted explicit project without falling back to cwd', async () => {
+    delete process.env.CODEBASE_ROOT;
+    delete process.argv[2];
+
+    await fs.rm(path.join(secondaryRoot, CODEBASE_CONTEXT_DIRNAME, INDEX_META_FILENAME), {
+      force: true
+    });
+
+    const { server, refreshKnownRootsFromClient } = await import('../src/index.js');
+    const typedServer = server as unknown as TestServer & {
+      listRoots: () => Promise<{ roots: Array<{ uri: string; name?: string }> }>;
+    };
+    const originalListRoots = typedServer.listRoots.bind(typedServer);
+    const handler = typedServer._requestHandlers.get('tools/call');
+    if (!handler) throw new Error('tools/call handler not registered');
+
+    typedServer.listRoots = vi.fn().mockRejectedValue(new Error('roots unsupported'));
+
+    try {
+      await refreshKnownRootsFromClient();
+      const response = await callTool(handler, 21, 'search_codebase', {
+        query: 'feature',
+        project: secondaryRoot
+      });
+      const payload = parsePayload(response) as {
+        status: string;
+        message: string;
+        index?: { action?: string; reason?: string };
+      };
+
+      expect(payload.status).toBe('indexing');
+      expect(payload.message).toContain('retry shortly');
+      expect(payload.index?.action).toBe('rebuild-started');
+      expect(String(payload.index?.reason || '')).toContain('Index meta');
+    } finally {
+      typedServer.listRoots = originalListRoots;
+    }
+  });
+
   it('returns selection_required instead of silently falling back to cwd when startup is rootless and unresolved', async () => {
     delete process.env.CODEBASE_ROOT;
     delete process.argv[2];