diff --git a/apps/next/src/app/(app)/error.tsx b/apps/next/src/app/(app)/error.tsx
new file mode 100644
index 0000000..1690291
--- /dev/null
+++ b/apps/next/src/app/(app)/error.tsx
@@ -0,0 +1,30 @@
+'use client';
+
+import { useEffect } from 'react';
+import * as Sentry from '@sentry/nextjs';
+
+import { Button } from '@spoon/ui';
+
+const AppError = ({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) => {
+  useEffect(() => {
+    Sentry.captureException(error);
+  }, [error]);
+
+  return (
+    <main className='flex min-h-[50vh] flex-col items-center justify-center gap-4 p-6 text-center'>
+      <h1 className='text-2xl font-semibold'>Something went wrong</h1>
+      <p className='text-muted-foreground max-w-md text-sm'>
+        This page hit an unexpected error. It has been reported.
+      </p>
+      <Button onClick={() => reset()}>Try again</Button>
+    </main>
+  );
+};
+
+export default AppError;
diff --git a/apps/next/src/app/(app)/not-found.tsx b/apps/next/src/app/(app)/not-found.tsx
new file mode 100644
index 0000000..0e9a171
--- /dev/null
+++ b/apps/next/src/app/(app)/not-found.tsx
@@ -0,0 +1,17 @@
+import Link from 'next/link';
+
+import { Button } from '@spoon/ui';
+
+const NotFound = () => (
+  <main className='flex min-h-[50vh] flex-col items-center justify-center gap-4 p-6 text-center'>
+    <h1 className='text-2xl font-semibold'>Not found</h1>
+    <p className='text-muted-foreground max-w-md text-sm'>
+      This item does not exist, or you do not have access to it.
+    </p>
+    <Button asChild>
+      <Link href='/dashboard'>Back to dashboard</Link>
+    </Button>
+  </main>
+);
+
+export default NotFound;
diff --git a/apps/next/src/app/(app)/spoons/[spoonId]/agent/[jobId]/page.tsx b/apps/next/src/app/(app)/spoons/[spoonId]/agent/[jobId]/page.tsx
index 93fdbfe..29aaef5 100644
--- a/apps/next/src/app/(app)/spoons/[spoonId]/agent/[jobId]/page.tsx
+++ b/apps/next/src/app/(app)/spoons/[spoonId]/agent/[jobId]/page.tsx
@@ -1,5 +1,6 @@
 'use client';
 
+import type { FunctionReturnType } from 'convex/server';
 import { useEffect } from 'react';
 import Link from 'next/link';
 import { useParams, useRouter } from 'next/navigation';
@@ -15,13 +16,33 @@ const AgentWorkspacePage = () => {
   const router = useRouter();
   const params = useParams<{ spoonId: string; jobId: string }>();
   const jobId = params.jobId as Id<'agentJobs'>;
-  const job = useQuery(api.agentJobs.get, { jobId });
+  const job = useQuery(api.agentJobs.get, { jobId }) as
+    | FunctionReturnType<typeof api.agentJobs.get>
+    | null
+    | undefined;
 
   useEffect(() => {
     if (job?.threadId) router.replace(`/threads/${job.threadId}`);
   }, [job?.threadId, router]);
 
-  if (job?.threadId) {
+  if (job === undefined) {
+    return (
+      <main className='text-muted-foreground p-6'>Loading workspace...</main>
+    );
+  }
+
+  if (job === null) {
+    return (
+      <main className='space-y-4 p-6'>
+        <p className='text-muted-foreground'>This workspace was not found.</p>
+        <Button asChild variant='outline' size='sm'>
+          <Link href={`/spoons/${params.spoonId}`}>Back to Spoon</Link>
+        </Button>
+      </main>
+    );
+  }
+
+  if (job.threadId) {
     return (
       <main className='text-muted-foreground p-6'>
         Opening thread workspace...
diff --git a/apps/next/src/app/(app)/threads/[threadId]/page.tsx b/apps/next/src/app/(app)/threads/[threadId]/page.tsx
index add0be8..b265e0a 100644
--- a/apps/next/src/app/(app)/threads/[threadId]/page.tsx
+++ b/apps/next/src/app/(app)/threads/[threadId]/page.tsx
@@ -1,5 +1,6 @@
 'use client';
 
+import type { FunctionReturnType } from 'convex/server';
 import { useState } from 'react';
 import Link from 'next/link';
 import { useParams, useRouter } from 'next/navigation';
@@ -33,7 +34,10 @@ const ThreadDetailPage = () => {
   const router = useRouter();
   const params = useParams<{ threadId: string }>();
   const threadId = params.threadId as Id<'threads'>;
-  const details = useQuery(api.threads.get, { threadId });
+  const details = useQuery(api.threads.get, { threadId }) as
+    | FunctionReturnType<typeof api.threads.get>
+    | null
+    | undefined;
   const createJob = useMutation(api.agentJobs.createForThread);
   const markResolved = useMutation(api.threads.markResolved);
   const cancel = useMutation(api.threads.cancel);
@@ -43,6 +47,14 @@ const ThreadDetailPage = () => {
     return <main className='text-muted-foreground p-6'>Loading thread...</main>;
   }
 
+  if (details === null) {
+    return (
+      <main className='text-muted-foreground p-6'>
+        This thread was not found.
+      </main>
+    );
+  }
+
   const { thread, spoon, latestJob } = details;
   if (latestJob && spoon) {
     return (
diff --git a/packages/backend/convex/agentJobs.ts b/packages/backend/convex/agentJobs.ts
index 53d0bb8..eca649b 100644
--- a/packages/backend/convex/agentJobs.ts
+++ b/packages/backend/convex/agentJobs.ts
@@ -710,7 +710,7 @@ export const get = query({
   handler: async (ctx, { jobId }) => {
     const ownerId = await getRequiredUserId(ctx);
     const job = await ctx.db.get(jobId);
-    if (job?.ownerId !== ownerId) throw new ConvexError('Agent job not found.');
+    if (job?.ownerId !== ownerId) return null;
     return job;
   },
 });
diff --git a/packages/backend/convex/threads.ts b/packages/backend/convex/threads.ts
index cd3fc29..5525fa2 100644
--- a/packages/backend/convex/threads.ts
+++ b/packages/backend/convex/threads.ts
@@ -211,7 +211,7 @@ export const get = query({
   handler: async (ctx, { threadId }) => {
     const ownerId = await getRequiredUserId(ctx);
     const thread = await ctx.db.get(threadId);
-    if (thread?.ownerId !== ownerId) throw new ConvexError('Thread not found.');
+    if (thread?.ownerId !== ownerId) return null;
     const spoon = thread.spoonId ? await ctx.db.get(thread.spoonId) : null;
     const job = thread.latestAgentJobId
       ? await ctx.db.get(thread.latestAgentJobId)
diff --git a/packages/backend/tests/unit/detail-null.test.ts b/packages/backend/tests/unit/detail-null.test.ts
new file mode 100644
index 0000000..bfb885f
--- /dev/null
+++ b/packages/backend/tests/unit/detail-null.test.ts
@@ -0,0 +1,118 @@
+import { convexTest } from 'convex-test';
+import { describe, expect, test } from 'vitest';
+
+import type { Id } from '../../convex/_generated/dataModel.js';
+import { api } from '../../convex/_generated/api.js';
+import schema from '../../convex/schema';
+
+const modules = import.meta.glob('../../convex/**/*.*s');
+
+const createUser = async (t: ReturnType<typeof convexTest>, email: string) =>
+  await t.mutation(async (ctx) => {
+    return await ctx.db.insert('users', { email, name: email });
+  });
+
+const authed = (t: ReturnType<typeof convexTest>, userId: string) =>
+  t.withIdentity({
+    subject: `${userId}|session`,
+    issuer: 'https://convex.test',
+  });
+
+const seedDetails = async (
+  t: ReturnType<typeof convexTest>,
+  ownerId: Id<'users'>,
+) =>
+  await t.mutation(async (ctx) => {
+    const now = Date.now();
+    const spoonId = await ctx.db.insert('spoons', {
+      ownerId,
+      name: 'Detail Spoon',
+      provider: 'gitea',
+      upstreamOwner: 'upstream',
+      upstreamRepo: 'detail',
+      upstreamDefaultBranch: 'main',
+      upstreamUrl: 'https://git.example.com/upstream/detail',
+      forkOwner: 'team',
+      forkRepo: 'detail-spoon',
+      forkUrl: 'https://git.example.com/team/detail-spoon',
+      visibility: 'private',
+      maintenanceMode: 'watch',
+      syncCadence: 'daily',
+      productionRefStrategy: 'default_branch',
+      status: 'active',
+      createdAt: now,
+      updatedAt: now,
+    });
+    const threadId = await ctx.db.insert('threads', {
+      ownerId,
+      spoonId,
+      title: 'Private thread',
+      source: 'user_request',
+      status: 'open',
+      priority: 'normal',
+      createdAt: now,
+      updatedAt: now,
+    });
+    const requestId = await ctx.db.insert('agentRequests', {
+      spoonId,
+      ownerId,
+      prompt: 'Private work',
+      status: 'running',
+      createdAt: now,
+      updatedAt: now,
+    });
+    const jobId = await ctx.db.insert('agentJobs', {
+      spoonId,
+      ownerId,
+      agentRequestId: requestId,
+      threadId,
+      status: 'running',
+      prompt: 'Private work',
+      runtime: 'opencode',
+      baseBranch: 'main',
+      workBranch: 'spoon/private',
+      forkOwner: 'team',
+      forkRepo: 'detail-spoon',
+      forkUrl: 'https://git.example.com/team/detail-spoon',
+      upstreamOwner: 'upstream',
+      upstreamRepo: 'detail',
+      selectedSecretIds: [],
+      model: 'openai/gpt-5.1-codex',
+      reasoningEffort: 'medium',
+      createdAt: now,
+      updatedAt: now,
+    });
+    await ctx.db.patch(threadId, { latestAgentJobId: jobId });
+    return { threadId, jobId };
+  });
+
+describe('detail queries for unavailable records', () => {
+  test('return null for records owned by another user', async () => {
+    const t = convexTest(schema, modules);
+    const ownerId = (await createUser(t, 'owner@example.com')) as Id<'users'>;
+    const otherId = await createUser(t, 'other@example.com');
+    const { threadId, jobId } = await seedDetails(t, ownerId);
+    const other = authed(t, otherId);
+
+    await expect(
+      other.query(api.threads.get, { threadId }),
+    ).resolves.toBeNull();
+    await expect(other.query(api.agentJobs.get, { jobId })).resolves.toBeNull();
+  });
+
+  test('return null for records that do not exist', async () => {
+    const t = convexTest(schema, modules);
+    const ownerId = (await createUser(t, 'owner@example.com')) as Id<'users'>;
+    const owner = authed(t, ownerId);
+    const { threadId, jobId } = await seedDetails(t, ownerId);
+    await t.mutation(async (ctx) => {
+      await ctx.db.delete(jobId);
+      await ctx.db.delete(threadId);
+    });
+
+    await expect(
+      owner.query(api.threads.get, { threadId }),
+    ).resolves.toBeNull();
+    await expect(owner.query(api.agentJobs.get, { jobId })).resolves.toBeNull();
+  });
+});