Default MCP tab tools to the active tab

Requiring an explicit tab_id forced every navigate or close through a
preceding tabs_list call, which costs an MCP client a full round trip just to
learn the ID the browser already considers current.

navigate and tabs_close now resolve the active tab when tab_id is omitted,
matching the screenshot tool. Resolution is explicit rather than forwarding
None into the SDK, so the acting tool knows which tab it touched; tabs_close
reports it, since closing the wrong tab is not recoverable.

This stays in the MCP layer: the SDK and CLI signatures are unchanged.

README.md

@@ -174,6 +174,11 @@ The server is stateless at the MCP layer. Every tool call creates a fresh
 `browser`, `remote`, and `key` on a tool call when a specific local profile or
 authenticated browser-cli remote is required.
 
+`browser_navigate`, `browser_tabs_close`, and `browser_screenshot` take an
+optional `tab_id` and act on the active tab when it is omitted, so a caller
+does not need a preceding `browser_tabs_list` round trip. `browser_tabs_close`
+reports the tab it closed.
+
 Available tools:
 - `browser_tabs_list`, `browser_tabs_open`, `browser_tabs_close`
 - `browser_navigate`, `browser_page_info`

browser_cli/mcp/server.py

@@ -16,6 +16,7 @@ from urllib.parse import urlsplit
 from browser_cli import BrowserCLI
 from browser_cli.mcp.naming import resolve_tool_prefix, tool_name
 from browser_cli.mcp.serialization import structured
+from browser_cli.mcp.targets import resolve_tab_id
 
 ClientFactory = Callable[..., BrowserCLI]
 
@@ -93,27 +94,29 @@ def create_server(*, client_factory: ClientFactory = BrowserCLI, tool_prefix: st
 
   @mcp.tool(name=tool_name("tabs_close", prefix))
   def tabs_close(
-    tab_id: int,
+    tab_id: int | None = None,
     browser: str | None = None,
     remote: str | None = None,
     key: str | None = None,
   ) -> dict[str, int]:
-    """Close one tab by its current ID. This changes the user's real browser."""
-    closed = _client(client_factory, browser, remote, key).tabs.close(tab_id)
-    return {"closed": closed}
+    """Close a tab, defaulting to the active tab. This changes the real browser."""
+    client = _client(client_factory, browser, remote, key)
+    target = resolve_tab_id(client, tab_id)
+    return {"closed": client.tabs.close(target), "tab_id": target}
 
   @mcp.tool(name=tool_name("navigate", prefix))
   def navigate(
-    tab_id: int,
     url: str,
+    tab_id: int | None = None,
     browser: str | None = None,
     remote: str | None = None,
     key: str | None = None,
   ) -> dict[str, Any]:
-    """Navigate an existing tab to a URL and return a fresh tab snapshot."""
+    """Navigate a tab to a URL, defaulting to the active tab, and return it."""
     client = _client(client_factory, browser, remote, key)
-    client.nav.to(tab_id, url)
-    return structured(client.tabs.status(tab_id))
+    target = resolve_tab_id(client, tab_id)
+    client.nav.to(target, url)
+    return structured(client.tabs.status(target))
 
   @mcp.tool(name=tool_name("page_info", prefix))
   def page_info(

browser_cli/mcp/targets.py

@@ -0,0 +1,16 @@
+"""Tab targeting for the MCP surface.
+
+MCP callers pay a full round trip for every extra tool call, so tools that act
+on a tab accept an optional ``tab_id`` and fall back to the browser's current
+active tab. Resolution happens here rather than by forwarding ``None`` into the
+SDK, so the acting tool always knows which tab it touched and can report it.
+"""
+from __future__ import annotations
+
+from browser_cli import BrowserCLI
+
+def resolve_tab_id(client: BrowserCLI, tab_id: int | None) -> int:
+  """Return *tab_id*, or the ID of the currently active tab when it is ``None``."""
+  if tab_id is not None:
+    return tab_id
+  return client.tabs.active().id

browser_cli/remote/pool.py

@@ -26,6 +26,7 @@ from browser_cli.framing import frame
 # hand back one the server has just timed out and closed.
 _MAX_IDLE_SECONDS = max(5, REMOTE_SESSION_IDLE_TIMEOUT - 5)
 _MAX_PER_ENDPOINT = 8
+_MAX_ENDPOINTS = 64
 
 class PooledConnection:
   __slots__ = ("sock", "secret", "last_used")
@@ -56,10 +57,32 @@ def checkout(endpoint: str) -> PooledConnection | None:
       _close(conn.sock)  # too old — assume the server has dropped it
   return None
 
+def _prune_endpoints_locked(now: float) -> None:
+  """Keep the number of endpoint buckets bounded for long-running SDK users."""
+  for endpoint, bucket in list(_POOL.items()):
+    fresh = [conn for conn in bucket if now - conn.last_used <= _MAX_IDLE_SECONDS]
+    if fresh:
+      _POOL[endpoint] = fresh
+    else:
+      for conn in bucket:
+        _close(conn.sock)
+      _POOL.pop(endpoint, None)
+
+  while len(_POOL) >= _MAX_ENDPOINTS:
+    oldest_endpoint, bucket = min(
+      _POOL.items(),
+      key=lambda item: min(conn.last_used for conn in item[1]) if item[1] else 0.0,
+    )
+    for conn in bucket:
+      _close(conn.sock)
+    _POOL.pop(oldest_endpoint, None)
+
 def checkin(endpoint: str, conn: PooledConnection) -> None:
   """Return a still-healthy connection to the pool for reuse."""
   conn.last_used = time.monotonic()
   with _LOCK:
+    if endpoint not in _POOL and len(_POOL) >= _MAX_ENDPOINTS:
+      _prune_endpoints_locked(conn.last_used)
     bucket = _POOL.setdefault(endpoint, [])
     if len(bucket) >= _MAX_PER_ENDPOINT:
       _close(conn.sock)

browser_cli/serve/security.py

@@ -70,19 +70,48 @@ class RateLimiter:
   ``rate`` is the sustained refill in tokens/second; ``burst`` is the bucket
   capacity (defaults to ``rate``). ``rate <= 0`` disables limiting entirely.
   Thread-safe so it can be shared across all connections of one serve process.
+
+  The bucket table is capped. Without that bound, a long-running public server
+  could retain one entry per ever-seen identity/IP forever; GC cannot reclaim
+  those entries because the limiter still references them.
   """
 
-  def __init__(self, rate: float, burst: float | None = None) -> None:
+  def __init__(self, rate: float, burst: float | None = None, max_buckets: int = 4096) -> None:
     self.rate = float(rate)
     self.capacity = float(burst) if burst is not None else max(float(rate), 1.0)
+    self.max_buckets = max(1, int(max_buckets))
     self._buckets: dict[str, tuple[float, float]] = {}
     self._lock = threading.Lock()
 
+  def _prune_locked(self, now: float) -> None:
+    """Drop idle/full buckets, then oldest buckets, until the table is bounded."""
+    if len(self._buckets) < self.max_buckets or self.rate <= 0:
+      return
+
+    # Once a bucket has fully refilled, keeping it around carries no useful
+    # throttling state. Use at least 60s so normal active identities are not
+    # churned out aggressively on high-rate configs.
+    idle_seconds = max(60.0, (self.capacity / self.rate) * 2)
+    full_epsilon = 1e-9
+    for bucket_key, (tokens, last) in list(self._buckets.items()):
+      refilled = min(self.capacity, tokens + (now - last) * self.rate)
+      if refilled >= self.capacity - full_epsilon and now - last >= idle_seconds:
+        self._buckets.pop(bucket_key, None)
+
+    # If an attacker keeps creating fresh identities faster than they go idle,
+    # still keep memory bounded. Evict the oldest identity state; that may reset
+    # throttling for that identity, but bounded memory is more important here.
+    while len(self._buckets) >= self.max_buckets:
+      oldest_key = min(self._buckets, key=lambda k: self._buckets[k][1])
+      self._buckets.pop(oldest_key, None)
+
   def allow(self, key: str) -> bool:
     if self.rate <= 0:
       return True
     now = time.monotonic()
     with self._lock:
+      if key not in self._buckets and len(self._buckets) >= self.max_buckets:
+        self._prune_locked(now)
       tokens, last = self._buckets.get(key, (self.capacity, now))
       tokens = min(self.capacity, tokens + (now - last) * self.rate)
       if tokens < 1.0:

extension/src/classes/JobManager.ts

@@ -15,6 +15,11 @@ import type { Job, Serializable, ErrorLike, DispatchArgs } from '../types';
 // jobs only need to survive long enough for the CLI to poll their result.
 export const MAX_FINISHED_JOBS = 20;
 
+// Cap simultaneously running background jobs. A hung job has a watchdog, but a
+// command flood could still pin many timers/results for up to JOB_TIMEOUT_MS.
+// Rejecting above this bound keeps service-worker memory predictable.
+export const MAX_RUNNING_JOBS = 32;
+
 // Watchdog: if a runner never resolves/rejects (e.g. executeScript against a
 // dead tab), finalize the job as an error so its persist interval stops instead
 // of writing to api.storage.local every second forever.
@@ -77,6 +82,11 @@ export class JobManager {
   }
 
   async start(command: string, args: DispatchArgs, runner: JobRunner) {
+    const runningCount = [...this.jobs.values()].filter(job => job.status === "running").length;
+    if (runningCount >= MAX_RUNNING_JOBS) {
+      throw new Error(`too many background jobs running (${runningCount}); wait for jobs to finish or cancel one`);
+    }
+
     const jobId = `${Date.now().toString(36)}-${Math.random().toString(36).slice(2, 10)}`;
     const job: Job = {
       id: jobId,

extension/src/commands/perf.ts

@@ -1,7 +1,7 @@
 import { getLargeOperationThrottle, getPerformanceProfile, hasAudibleTabs, setPerformanceProfile } from '../core';
 import { CommandGroup } from '../classes/CommandGroup';
 import type { CommandEntry } from '../classes/CommandGroup';
-import type { PerfSetProfileArgs, JobIdArgs } from '../types';
+import type { Job, PerfSetProfileArgs, JobIdArgs } from '../types';
 
 // PerfCommands also owns the jobs.* status/cancel queries: they read the same
 // JobManager (ctx.jobs) that perf.status reports, and there is no dedicated
@@ -15,6 +15,19 @@ export class PerfCommands extends CommandGroup {
     "jobs.cancel":      (a: JobIdArgs) => this.ctx.jobs.cancel(a),
   };
 
+  private jobSummary(job: Job) {
+    return {
+      id: job.id,
+      command: job.command,
+      status: job.status,
+      phase: job.phase,
+      current: job.current,
+      total: job.total,
+      percent: job.percent,
+      cancelRequested: job.cancelRequested,
+    };
+  }
+
   private async perfStatus() {
     const profile = await getPerformanceProfile();
     const audible = await hasAudibleTabs();
@@ -23,16 +36,7 @@ export class PerfCommands extends CommandGroup {
       performanceProfile: profile,
       audible,
       throttle,
-      jobs: this.ctx.jobs.list().map(job => ({
-        id: job.id,
-        command: job.command,
-        status: job.status,
-        phase: job.phase,
-        current: job.current,
-        total: job.total,
-        percent: job.percent,
-        cancelRequested: job.cancelRequested,
-      })),
+      jobs: this.ctx.jobs.list().map(job => this.jobSummary(job)),
     };
   }
 }

extension/src/commands/windows.ts

@@ -14,9 +14,31 @@ export class WindowsCommands extends CommandGroup {
     "windows.open":   (a: WindowsOpenArgs) => this.windowsOpen(a),
   };
 
+  private async activeWindowIds(): Promise<Set<number>> {
+    const windows = await api.windows.getAll({});
+    return new Set(windows.map(w => w.id).filter(id => typeof id === "number"));
+  }
+
+  private async pruneAliases(activeIds?: Set<number>): Promise<Record<string, string>> {
+    const aliases = await getAliases();
+    const liveIds = activeIds || await this.activeWindowIds();
+    const pruned: Record<string, string> = {};
+    let changed = false;
+    for (const [id, alias] of Object.entries(aliases)) {
+      if (liveIds.has(Number(id))) {
+        pruned[id] = alias;
+      } else {
+        changed = true;
+      }
+    }
+    if (changed) await api.storage.local.set({ windowAliases: pruned });
+    return pruned;
+  }
+
   private async windowsList() {
     const windows = await api.windows.getAll({ populate: true });
-    const aliases = await getAliases();
+    const activeIds = new Set(windows.map(w => w.id).filter(id => typeof id === "number"));
+    const aliases = await this.pruneAliases(activeIds);
     return windows.map(w => ({
       id: w.id,
       alias: aliases[w.id] || null,
@@ -27,7 +49,7 @@ export class WindowsCommands extends CommandGroup {
   }
 
   private async windowsRename({ windowId, name }: WindowsRenameArgs) {
-    const aliases = await getAliases();
+    const aliases = await this.pruneAliases();
     aliases[windowId] = name;
     await api.storage.local.set({ windowAliases: aliases });
     return { windowId, name };
@@ -35,6 +57,11 @@ export class WindowsCommands extends CommandGroup {
 
   private async windowsClose({ windowId }: WindowsCloseArgs) {
     await api.windows.remove(windowId);
+    const aliases = await this.pruneAliases();
+    if (windowId in aliases) {
+      delete aliases[windowId];
+      await api.storage.local.set({ windowAliases: aliases });
+    }
     return { windowId };
   }
 

extension/test/jobs.test.ts

@@ -1,7 +1,7 @@
 // @ts-nocheck
 import { test, mock } from 'node:test';
 import assert from 'node:assert/strict';
-import { JobManager, JOB_TIMEOUT_MS, MAX_FINISHED_JOBS, pruneFinishedJobs } from '../src/classes/JobManager';
+import { JobManager, JOB_TIMEOUT_MS, MAX_FINISHED_JOBS, MAX_RUNNING_JOBS, pruneFinishedJobs } from '../src/classes/JobManager';
 import { makeChromeMock } from './chrome-mock';
 
 // Drain pending microtasks (finalize() chains several awaits). setImmediate is
@@ -129,6 +129,21 @@ test('JobManager: a runner that settles after the watchdog cannot resurrect the
   mock.timers.reset();
 });
 
+test('JobManager: rejects new background jobs above the running-job cap', async () => {
+  mock.timers.enable({ apis: ['setInterval', 'setTimeout'] });
+  globalThis.chrome = makeChromeMock();
+  const mgr = new JobManager();
+  for (let i = 0; i < MAX_RUNNING_JOBS; i++) {
+    await mgr.start(`running${i}`, {}, () => new Promise(() => {}));
+  }
+  await assert.rejects(
+    () => mgr.start('overflow', {}, async () => 'nope'),
+    /too many background jobs running/,
+  );
+  assert.equal(mgr.list().filter(job => job.status === 'running').length, MAX_RUNNING_JOBS);
+  mock.timers.reset();
+});
+
 test('JobManager: persisted set keeps running jobs even past the finished cap', async () => {
   mock.timers.enable({ apis: ['setInterval', 'setTimeout'] });
   globalThis.chrome = makeChromeMock();

extension/test/windows.test.ts

@@ -0,0 +1,69 @@
+// @ts-nocheck
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { WindowsCommands } from '../src/commands/windows';
+import { makeChromeMock } from './chrome-mock';
+
+function makeWindowsChromeMock(windows) {
+  const chrome = makeChromeMock();
+  chrome.windows = {
+    getAll: async () => windows,
+    remove: async () => {},
+    create: async () => ({ id: 99 }),
+  };
+  return chrome;
+}
+
+test('windows.list prunes aliases for closed windows', async () => {
+  globalThis.chrome = makeWindowsChromeMock([
+    { id: 1, focused: true, state: 'normal', tabs: [{ id: 10 }] },
+    { id: 2, focused: false, state: 'minimized', tabs: [] },
+  ]);
+  globalThis.chrome.storage.local._store.windowAliases = {
+    1: 'main',
+    2: 'side',
+    999: 'closed',
+  };
+
+  const commands = new WindowsCommands({ jobs: {} });
+  const result = await commands.commands['windows.list']({});
+
+  assert.deepEqual(result.map(w => [w.id, w.alias]), [[1, 'main'], [2, 'side']]);
+  assert.deepEqual(globalThis.chrome.storage.local._store.windowAliases, { 1: 'main', 2: 'side' });
+});
+
+test('windows.rename prunes stale aliases before saving the new name', async () => {
+  globalThis.chrome = makeWindowsChromeMock([
+    { id: 1, focused: true, state: 'normal', tabs: [] },
+    { id: 2, focused: false, state: 'normal', tabs: [] },
+  ]);
+  globalThis.chrome.storage.local._store.windowAliases = {
+    1: 'main',
+    999: 'closed',
+  };
+
+  const commands = new WindowsCommands({ jobs: {} });
+  await commands.commands['windows.rename']({ windowId: 2, name: 'work' });
+
+  assert.deepEqual(globalThis.chrome.storage.local._store.windowAliases, { 1: 'main', 2: 'work' });
+});
+
+test('windows.close removes the closed window alias immediately', async () => {
+  let removed = null;
+  globalThis.chrome = makeWindowsChromeMock([
+    { id: 1, focused: true, state: 'normal', tabs: [] },
+    { id: 2, focused: false, state: 'normal', tabs: [] },
+  ]);
+  globalThis.chrome.windows.remove = async id => { removed = id; };
+  globalThis.chrome.storage.local._store.windowAliases = {
+    1: 'main',
+    2: 'side',
+    999: 'closed',
+  };
+
+  const commands = new WindowsCommands({ jobs: {} });
+  await commands.commands['windows.close']({ windowId: 2 });
+
+  assert.equal(removed, 2);
+  assert.deepEqual(globalThis.chrome.storage.local._store.windowAliases, { 1: 'main' });
+});

tests/test_mcp.py

@@ -26,6 +26,7 @@ class FakeClient:
       list=self.tabs_list,
       open=self.tabs_open,
       close=self.tabs_close,
+      active=self.tabs_active,
       status=self.tabs_status,
       screenshot=self.tabs_screenshot,
     )
@@ -46,6 +47,10 @@ class FakeClient:
     self.calls.append(("close", tab_id))
     return 1
 
+  def tabs_active(self):
+    self.calls.append(("active",))
+    return SimpleNamespace(id=7)
+
   def tabs_status(self, tab_id):
     self.calls.append(("status", tab_id))
     return {"id": tab_id, "title": "Navigated", "url": "https://example.com/next"}
@@ -178,6 +183,18 @@ async def test_mutating_tools_use_sdk_and_return_fresh_state(client):
     ("navigate", 8, "https://example.com/next"), ("status", 8)
   ]
 
+@pytest.mark.anyio
+async def test_tab_tools_default_to_the_active_tab(client):
+  navigated = await client.call_tool("browser_navigate", {"url": "https://example.com/next"})
+  closed = await client.call_tool("browser_tabs_close", {})
+
+  assert navigated.structured_content["id"] == 7
+  assert closed.structured_content == {"closed": 1, "tab_id": 7}
+  assert FakeClient.instances[0].calls == [
+    ("active",), ("navigate", 7, "https://example.com/next"), ("status", 7)
+  ]
+  assert FakeClient.instances[1].calls == [("active",), ("close", 7)]
+
 @pytest.mark.anyio
 async def test_screenshot_returns_image_content(client):
   result = await client.call_tool("browser_screenshot", {"tab_id": 7, "format": "png"})

tests/test_remote_pool.py

@@ -43,6 +43,39 @@ def test_checkin_caps_pool_size():
     b.close()
   pool.close_all()
 
+def test_checkin_caps_endpoint_buckets():
+  pool.close_all()
+  peers = []
+  try:
+    for i in range(pool._MAX_ENDPOINTS + 5):
+      a, b = _socketpair()
+      peers.append(b)
+      pool.checkin(f"host-{i}:443", pool.PooledConnection(a, b"secret"))
+    assert len(pool._POOL) <= pool._MAX_ENDPOINTS
+  finally:
+    for peer in peers:
+      peer.close()
+    pool.close_all()
+
+def test_checkin_prunes_stale_endpoint_buckets():
+  pool.close_all()
+  old_a, old_b = _socketpair()
+  old = pool.PooledConnection(old_a, b"secret")
+  pool.checkin("old:443", old)
+  old.last_used -= pool._MAX_IDLE_SECONDS + 1
+  peers = [old_b]
+  try:
+    for i in range(pool._MAX_ENDPOINTS):
+      a, b = _socketpair()
+      peers.append(b)
+      pool.checkin(f"new-{i}:443", pool.PooledConnection(a, b"secret"))
+    assert "old:443" not in pool._POOL
+    assert len(pool._POOL) <= pool._MAX_ENDPOINTS
+  finally:
+    for peer in peers:
+      peer.close()
+    pool.close_all()
+
 def test_session_inner_message_strips_auth_fields():
   msg = {
     "id": "1", "command": "tabs.list", "args": {}, "user_agent": "browser-cli/1",

tests/test_serve_security.py

@@ -181,6 +181,23 @@ def test_rate_limiter_is_per_key():
   assert limiter.allow("a") is False
   assert limiter.allow("b") is False
 
+def test_rate_limiter_caps_identity_buckets():
+  limiter = RateLimiter(rate=0.0001, burst=1, max_buckets=3)
+  for i in range(10):
+    assert limiter.allow(f"key-{i}") is True
+  assert len(limiter._buckets) <= 3
+
+def test_rate_limiter_prunes_refilled_idle_buckets(monkeypatch):
+  current = 1000.0
+  monkeypatch.setattr("browser_cli.serve.security.time.monotonic", lambda: current)
+  limiter = RateLimiter(rate=1, burst=2, max_buckets=2)
+  assert limiter.allow("old") is True
+  current += 120.0
+  assert limiter.allow("a") is True
+  assert limiter.allow("b") is True
+  assert "old" not in limiter._buckets
+  assert len(limiter._buckets) <= 2
+
 # ── ServeSecurity ────────────────────────────────────────────────────────────────
 
 def test_effective_policy_prefers_per_key_override():