diff --git a/test/helpers/touchfiles.ts b/test/helpers/touchfiles.ts
index c42a03c7..c03974f7 100644
--- a/test/helpers/touchfiles.ts
+++ b/test/helpers/touchfiles.ts
@@ -141,6 +141,10 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   'benchmark-workflow':            ['benchmark/**', 'browse/src/**'],
   'setup-deploy-workflow':         ['setup-deploy/**', 'scripts/gen-skill-docs.ts'],
 
+  // Sidebar agent
+  'sidebar-navigate':              ['browse/src/server.ts', 'browse/src/sidebar-agent.ts', 'browse/src/sidebar-utils.ts', 'extension/**'],
+  'sidebar-url-accuracy':          ['browse/src/server.ts', 'browse/src/sidebar-agent.ts', 'browse/src/sidebar-utils.ts', 'extension/background.js'],
+
   // Autoplan
   'autoplan-core':  ['autoplan/**', 'plan-ceo-review/**', 'plan-eng-review/**', 'plan-design-review/**'],
 
@@ -262,6 +266,10 @@ export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
   'benchmark-workflow': 'gate',
   'setup-deploy-workflow': 'gate',
 
+  // Sidebar agent
+  'sidebar-navigate': 'periodic',
+  'sidebar-url-accuracy': 'periodic',
+
   // Autoplan — periodic (not yet implemented)
   'autoplan-core': 'periodic',
 
diff --git a/test/skill-e2e-sidebar.test.ts b/test/skill-e2e-sidebar.test.ts
new file mode 100644
index 00000000..e986bd85
--- /dev/null
+++ b/test/skill-e2e-sidebar.test.ts
@@ -0,0 +1,215 @@
+/**
+ * Layer 4: E2E tests for the sidebar agent with real Claude.
+ * Starts browse server + fixture server + sidebar-agent, POSTs to /sidebar-command
+ * (simulating what the Chrome extension does), and verifies Claude actually processes
+ * the request and responds through the chat buffer.
+ *
+ * These tests cost ~$0.80 total and run as periodic tier.
+ */
+
+import { describe, test, expect, beforeAll, afterAll } from 'bun:test';
+import { spawn, type Subprocess } from 'bun';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import {
+  ROOT, evalsEnabled,
+  describeIfSelected, testConcurrentIfSelected,
+  logCost, recordE2E,
+  createEvalCollector, finalizeEvalCollector,
+} from './helpers/e2e-helpers';
+import { startTestServer } from '../browse/test/test-server';
+
+const evalCollector = createEvalCollector('e2e-sidebar');
+
+// --- Sidebar Agent E2E ---
+
+describeIfSelected('Sidebar agent E2E', ['sidebar-navigate', 'sidebar-url-accuracy'], () => {
+  let serverProc: Subprocess | null = null;
+  let agentProc: Subprocess | null = null;
+  let fixtureServer: { server: ReturnType<typeof Bun.serve>; url: string } | null = null;
+  let serverPort: number = 0;
+  let authToken: string = '';
+  let tmpDir: string = '';
+  let stateFile: string = '';
+  let queueFile: string = '';
+
+  async function api(pathname: string, opts: RequestInit = {}): Promise<Response> {
+    const headers: Record<string, string> = {
+      'Content-Type': 'application/json',
+      ...(opts.headers as Record<string, string> || {}),
+    };
+    if (!headers['Authorization'] && authToken) {
+      headers['Authorization'] = `Bearer ${authToken}`;
+    }
+    return fetch(`http://127.0.0.1:${serverPort}${pathname}`, { ...opts, headers });
+  }
+
+  async function resetState() {
+    await api('/sidebar-session/new', { method: 'POST' });
+    fs.writeFileSync(queueFile, '');
+  }
+
+  async function pollChatUntil(
+    predicate: (entries: any[]) => boolean,
+    timeoutMs = 60000,
+  ): Promise<any[]> {
+    const deadline = Date.now() + timeoutMs;
+    while (Date.now() < deadline) {
+      const resp = await api('/sidebar-chat?after=0');
+      const data = await resp.json();
+      if (predicate(data.entries)) return data.entries;
+      await new Promise(r => setTimeout(r, 2000));
+    }
+    const resp = await api('/sidebar-chat?after=0');
+    return (await resp.json()).entries;
+  }
+
+  beforeAll(async () => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'sidebar-e2e-'));
+    stateFile = path.join(tmpDir, 'browse.json');
+    queueFile = path.join(tmpDir, 'sidebar-queue.jsonl');
+    fs.mkdirSync(path.dirname(queueFile), { recursive: true });
+
+    // Start fixture server for test pages
+    fixtureServer = startTestServer(0);
+
+    // Start browse server (no browser — sidebar agent uses `browse` commands
+    // which will fail without a browser, but we're testing the message flow)
+    const serverScript = path.resolve(ROOT, 'browse', 'src', 'server.ts');
+    serverProc = spawn(['bun', 'run', serverScript], {
+      env: {
+        ...process.env,
+        BROWSE_STATE_FILE: stateFile,
+        BROWSE_HEADLESS_SKIP: '1',
+        BROWSE_PORT: '0',
+        SIDEBAR_QUEUE_PATH: queueFile,
+        BROWSE_IDLE_TIMEOUT: '300',
+      },
+      stdio: ['ignore', 'pipe', 'pipe'],
+    });
+
+    // Wait for server
+    const deadline = Date.now() + 15000;
+    while (Date.now() < deadline) {
+      if (fs.existsSync(stateFile)) {
+        try {
+          const state = JSON.parse(fs.readFileSync(stateFile, 'utf-8'));
+          if (state.port && state.token) {
+            serverPort = state.port;
+            authToken = state.token;
+            break;
+          }
+        } catch {}
+      }
+      await new Promise(r => setTimeout(r, 100));
+    }
+    if (!serverPort) throw new Error('Server did not start in time');
+
+    // Start sidebar-agent with real claude
+    const agentScript = path.resolve(ROOT, 'browse', 'src', 'sidebar-agent.ts');
+    const browseBin = path.resolve(ROOT, 'browse', 'dist', 'browse');
+    agentProc = spawn(['bun', 'run', agentScript], {
+      env: {
+        ...process.env,
+        BROWSE_SERVER_PORT: String(serverPort),
+        BROWSE_STATE_FILE: stateFile,
+        SIDEBAR_QUEUE_PATH: queueFile,
+        SIDEBAR_AGENT_TIMEOUT: '120000',
+        BROWSE_BIN: fs.existsSync(browseBin) ? browseBin : 'browse',
+      },
+      stdio: ['ignore', 'pipe', 'pipe'],
+    });
+
+    await new Promise(r => setTimeout(r, 1500));
+  }, 25000);
+
+  afterAll(async () => {
+    if (agentProc) { try { agentProc.kill(); } catch {} }
+    if (serverProc) { try { serverProc.kill(); } catch {} }
+    if (fixtureServer) { try { fixtureServer.server.stop(); } catch {} }
+    finalizeEvalCollector(evalCollector);
+    try { fs.rmSync(tmpDir, { recursive: true, force: true }); } catch {}
+  });
+
+  testConcurrentIfSelected('sidebar-navigate', async () => {
+    await resetState();
+    const startTime = Date.now();
+
+    // Ask Claude to describe the page at the fixture URL
+    const fixtureUrl = `${fixtureServer!.url}/basic.html`;
+    const resp = await api('/sidebar-command', {
+      method: 'POST',
+      body: JSON.stringify({
+        message: `What is the title of the page at ${fixtureUrl}? Just tell me the title text, nothing else.`,
+        activeTabUrl: fixtureUrl,
+      }),
+    });
+    expect(resp.status).toBe(200);
+
+    // Wait for Claude to finish (agent_done)
+    const entries = await pollChatUntil(
+      (entries) => entries.some((e: any) => e.type === 'agent_done'),
+      90000,
+    );
+
+    const duration = Date.now() - startTime;
+    const doneEntry = entries.find((e: any) => e.type === 'agent_done');
+    expect(doneEntry).toBeDefined();
+
+    // Claude should have responded with something about the page
+    const agentEntries = entries.filter((e: any) => e.role === 'agent' && (e.type === 'text' || e.type === 'result'));
+    expect(agentEntries.length).toBeGreaterThan(0);
+
+    // Check that Claude mentioned the page title or content
+    const allText = agentEntries.map((e: any) => e.text || '').join(' ').toLowerCase();
+    const mentionsPage = allText.includes('test page') || allText.includes('basic') || allText.includes('hello');
+
+    recordE2E(evalCollector, 'sidebar-navigate', 'Sidebar agent E2E', {
+      exitReason: doneEntry ? 'success' : 'timeout',
+      durationMs: duration,
+      toolCalls: entries.filter((e: any) => e.type === 'tool_use').length,
+      cost: 0, // we can't easily measure cost from chat entries
+    } as any);
+
+    expect(mentionsPage).toBe(true);
+  }, 120_000);
+
+  testConcurrentIfSelected('sidebar-url-accuracy', async () => {
+    await resetState();
+
+    // POST with an activeTabUrl that differs from any Playwright URL
+    const fakeExtensionUrl = `${fixtureServer!.url}/forms.html`;
+    const resp = await api('/sidebar-command', {
+      method: 'POST',
+      body: JSON.stringify({
+        message: 'What URL am I on?',
+        activeTabUrl: fakeExtensionUrl,
+      }),
+    });
+    expect(resp.status).toBe(200);
+
+    // Verify the queue entry has the extension URL, not the Playwright URL
+    await new Promise(r => setTimeout(r, 200));
+    const queueContent = fs.readFileSync(queueFile, 'utf-8').trim();
+    const lines = queueContent.split('\n').filter(Boolean);
+    expect(lines.length).toBeGreaterThan(0);
+    const lastEntry = JSON.parse(lines[lines.length - 1]);
+
+    // The prompt should contain the extension URL
+    expect(lastEntry.pageUrl).toBe(fakeExtensionUrl);
+    expect(lastEntry.prompt).toContain(fakeExtensionUrl);
+    // Should NOT contain 'about:blank' (the no-browser fallback)
+    expect(lastEntry.pageUrl).not.toBe('about:blank');
+
+    recordE2E(evalCollector, 'sidebar-url-accuracy', 'Sidebar agent E2E', {
+      exitReason: 'success',
+      durationMs: 0,
+      toolCalls: 0,
+      cost: 0,
+    } as any);
+
+    // Kill the agent so it doesn't keep running
+    await api('/sidebar-agent/kill', { method: 'POST' });
+  }, 30_000);
+});