diff --git a/apps/next/src/app/api/threads/[threadId]/message/route.ts b/apps/next/src/app/api/threads/[threadId]/message/route.ts
index 2b0626d..d591458 100644
--- a/apps/next/src/app/api/threads/[threadId]/message/route.ts
+++ b/apps/next/src/app/api/threads/[threadId]/message/route.ts
@@ -1,3 +1,4 @@
+import type { FunctionReturnType } from 'convex/server';
 import { NextResponse } from 'next/server';
 import { proxyWorker } from '@/lib/agent-worker-proxy';
 import { convexAuthNextjsToken } from '@convex-dev/auth/nextjs/server';
@@ -35,7 +36,14 @@ export const POST = async (
         { status: 400 },
       );
     }
-    const details = await fetchQuery(api.threads.get, { threadId }, { token });
+    const details = (await fetchQuery(
+      api.threads.get,
+      { threadId },
+      { token },
+    )) as FunctionReturnType<typeof api.threads.get> | null;
+    if (details === null) {
+      return NextResponse.json({ error: 'Thread not found.' }, { status: 404 });
+    }
     const latestJob = details.latestJob;
     const canSendToWorker =
       latestJob &&
diff --git a/apps/next/src/components/agent-workspace/agent-workspace-shell.tsx b/apps/next/src/components/agent-workspace/agent-workspace-shell.tsx
index 923d300..19244f8 100644
--- a/apps/next/src/components/agent-workspace/agent-workspace-shell.tsx
+++ b/apps/next/src/components/agent-workspace/agent-workspace-shell.tsx
@@ -1,5 +1,6 @@
 'use client';
 
+import type { FunctionReturnType } from 'convex/server';
 import type { CSSProperties, PointerEvent as ReactPointerEvent } from 'react';
 import { useCallback, useEffect, useState } from 'react';
 import { useMutation, useQuery } from 'convex/react';
@@ -59,7 +60,10 @@ type PendingOverwrite = {
 };
 
 export const AgentWorkspaceShell = ({ jobId }: { jobId: Id<'agentJobs'> }) => {
-  const job = useQuery(api.agentJobs.get, { jobId });
+  const job = useQuery(api.agentJobs.get, { jobId }) as
+    | FunctionReturnType<typeof api.agentJobs.get>
+    | null
+    | undefined;
   const messages =
     useQuery(api.agentJobs.listMessages, { jobId, limit: 200 }) ?? [];
   const events =
@@ -323,6 +327,14 @@ export const AgentWorkspaceShell = ({ jobId }: { jobId: Id<'agentJobs'> }) => {
     );
   }
 
+  if (job === null) {
+    return (
+      <main className='text-muted-foreground p-6'>
+        This workspace was not found.
+      </main>
+    );
+  }
+
   const activeFile = activeFilePath ? files[activeFilePath] : undefined;
   const recoverWorkspace = async () => {
     if (!job.threadId) return;
diff --git a/apps/next/tests/component/agent-workspace-shell.test.tsx b/apps/next/tests/component/agent-workspace-shell.test.tsx
new file mode 100644
index 0000000..3e61bd9
--- /dev/null
+++ b/apps/next/tests/component/agent-workspace-shell.test.tsx
@@ -0,0 +1,31 @@
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+
+import { AgentWorkspaceShell } from '../../src/components/agent-workspace/agent-workspace-shell';
+
+const { mockUseMutation, mockUseQuery } = vi.hoisted(() => ({
+  mockUseMutation: vi.fn(() => vi.fn()),
+  mockUseQuery: vi.fn(() => null),
+}));
+
+vi.mock('convex/react', () => ({
+  useMutation: mockUseMutation,
+  useQuery: mockUseQuery,
+}));
+
+vi.mock('sonner', () => ({
+  toast: {
+    error: vi.fn(),
+    success: vi.fn(),
+  },
+}));
+
+describe('AgentWorkspaceShell', () => {
+  it('renders a missing state when the job query returns null', () => {
+    render(<AgentWorkspaceShell jobId={'job-1' as never} />);
+
+    expect(
+      screen.getByText('This workspace was not found.'),
+    ).toBeInTheDocument();
+  });
+});
diff --git a/apps/next/tests/unit/thread-message-route.test.ts b/apps/next/tests/unit/thread-message-route.test.ts
new file mode 100644
index 0000000..759a27a
--- /dev/null
+++ b/apps/next/tests/unit/thread-message-route.test.ts
@@ -0,0 +1,50 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { POST } from '../../src/app/api/threads/[threadId]/message/route';
+
+const { mockAuthToken, mockFetchMutation, mockFetchQuery, mockProxyWorker } =
+  vi.hoisted(() => ({
+    mockAuthToken: vi.fn(),
+    mockFetchMutation: vi.fn(),
+    mockFetchQuery: vi.fn(),
+    mockProxyWorker: vi.fn(),
+  }));
+
+vi.mock('@convex-dev/auth/nextjs/server', () => ({
+  convexAuthNextjsToken: mockAuthToken,
+}));
+
+vi.mock('convex/nextjs', () => ({
+  fetchMutation: mockFetchMutation,
+  fetchQuery: mockFetchQuery,
+}));
+
+vi.mock('@/lib/agent-worker-proxy', () => ({
+  proxyWorker: mockProxyWorker,
+}));
+
+describe('thread message route', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockAuthToken.mockResolvedValue('token');
+  });
+
+  it('returns 404 when the thread detail query returns null', async () => {
+    mockFetchQuery.mockResolvedValue(null);
+
+    const response = await POST(
+      new Request('http://localhost/api/threads/thread-1/message', {
+        method: 'POST',
+        body: JSON.stringify({ content: 'Hello' }),
+      }),
+      { params: Promise.resolve({ threadId: 'thread-1' }) },
+    );
+
+    expect(response.status).toBe(404);
+    await expect(response.json()).resolves.toEqual({
+      error: 'Thread not found.',
+    });
+    expect(mockFetchMutation).not.toHaveBeenCalled();
+    expect(mockProxyWorker).not.toHaveBeenCalled();
+  });
+});