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Browser CDP Supervisor — Design

Status: Shipped (PR 14540) Last updated: 2026-04-23 Author: @teknium1

Problem

Native JS dialogs (alert/confirm/prompt/beforeunload) and iframes are the two biggest gaps in our browser tooling:

1. Dialogs block the JS thread. Any operation on the page stalls until the dialog is handled. Before this work, the agent had no way to know a dialog was open — subsequent tool calls would hang or throw opaque errors.
2. Iframes are invisible. The agent could see iframe nodes in the DOM snapshot but could not click, type, or eval inside them — especially cross-origin (OOPIF) iframes that live in separate Chromium processes.

PR #12550 proposed a stateless browser_dialog wrapper. That doesn't solve detection — it's a cleaner CDP call for when the agent already knows (via symptoms) that a dialog is open. Closed as superseded.

Backend capability matrix (verified live 2026-04-23)

Using throwaway probe scripts against a data-URL page that fires alerts in the main frame and in a same-origin srcdoc iframe, plus a cross-origin https://example.com iframe:

Backend	Dialog detect	Dialog respond	Frame tree	OOPIF Runtime.evaluate via browser_cdp(frame_id=...)
Local Chrome (--remote-debugging-port) / /browser connect	✓	✓ full workflow	✓	✓
Browserbase	✓ (via bridge)	✓ full workflow (via bridge)	✓	✓ (document.title = "Example Domain" verified on real cross-origin iframe)
Camofox	✗ no CDP (REST-only)	✗	partial via DOM snapshot	✗

How Browserbase respond works. Browserbase's CDP proxy uses Playwright internally and auto-dismisses native dialogs within ~10ms, so Page.handleJavaScriptDialog can't keep up. To work around this, the supervisor injects a bridge script via Page.addScriptToEvaluateOnNewDocument that overrides window.alert/confirm/prompt with a synchronous XHR to a magic host (hermes-dialog-bridge.invalid). Fetch.enable intercepts those XHRs before they touch the network — the dialog becomes a Fetch.requestPaused event the supervisor captures, and respond_to_dialog fulfills via Fetch.fulfillRequest with a JSON body the injected script decodes.

Net result: from the page's perspective, prompt() still returns the agent-supplied string. From the agent's perspective, it's the same browser_dialog(action=...) API either way. Tested end-to-end against real Browserbase sessions — 4/4 (alert/prompt/confirm-accept/confirm-dismiss) pass including value round-tripping back into page JS.

Camofox stays unsupported for this PR; follow-up upstream issue planned at jo-inc/camofox-browser requesting a dialog polling endpoint.

Architecture

CDPSupervisor

One asyncio.Task running in a background daemon thread per Hermes task_id. Holds a persistent WebSocket to the backend's CDP endpoint. Maintains:

• Dialog queue — List[PendingDialog] with {id, type, message, default_prompt, session_id, opened_at}
• Frame tree — Dict[frame_id, FrameInfo] with parent relationships, URL, origin, whether cross-origin child session
• Session map — Dict[session_id, SessionInfo] so interaction tools can route to the right attached session for OOPIF operations
• Recent console errors — ring buffer of the last 50 (for PR 2 diagnostics)

Subscribes on attach:

• Page.enable — javascriptDialogOpening, frameAttached, frameNavigated, frameDetached
• Runtime.enable — executionContextCreated, consoleAPICalled, exceptionThrown
• Target.setAutoAttach {autoAttach: true, flatten: true} — surfaces child OOPIF targets; supervisor enables Page+Runtime on each

Thread-safe state access via a snapshot lock; tool handlers (sync) read the frozen snapshot without awaiting.

Lifecycle

• Start: SupervisorRegistry.get_or_start(task_id, cdp_url) — called by browser_navigate, Browserbase session create, /browser connect. Idempotent.
• Stop: session teardown or /browser disconnect. Cancels the asyncio task, closes the WebSocket, discards state.
• Rebind: if the CDP URL changes (user reconnects to a new Chrome), stop the old supervisor and start fresh — never reuse state across endpoints.

Dialog policy

Configurable via config.yaml under browser.dialog_policy:

• must_respond (default) — capture, surface in browser_snapshot, wait for explicit browser_dialog(action=...) call. After a 300s safety timeout with no response, auto-dismiss and log. Prevents a buggy agent from stalling forever.
• auto_dismiss — record and dismiss immediately; agent sees it after the fact via browser_state inside browser_snapshot.
• auto_accept — record and accept (useful for beforeunload where the user wants to navigate away cleanly).

Policy is per-task; no per-dialog overrides in v1.

Agent surface (PR 1)

One new tool

browser_dialog(action, prompt_text=None, dialog_id=None)

• action="accept" / "dismiss" → responds to the specified or sole pending dialog (required)
• prompt_text=... → text to supply to a prompt() dialog
• dialog_id=... → disambiguate when multiple dialogs queued (rare)

Tool is response-only. Agent reads pending dialogs from browser_snapshot output before calling.

browser_snapshot extension

Adds three optional fields to the existing snapshot output when a supervisor is attached:

{
  "pending_dialogs": [
    {"id": "d-1", "type": "alert", "message": "Hello", "opened_at": 1650000000.0}
  ],
  "recent_dialogs": [
    {"id": "d-1", "type": "alert", "message": "...", "opened_at": 1650000000.0,
     "closed_at": 1650000000.1, "closed_by": "remote"}
  ],
  "frame_tree": {
    "top": {"frame_id": "FRAME_A", "url": "https://example.com/", "origin": "https://example.com"},
    "children": [
      {"frame_id": "FRAME_B", "url": "about:srcdoc", "is_oopif": false},
      {"frame_id": "FRAME_C", "url": "https://ads.example.net/", "is_oopif": true, "session_id": "SID_C"}
    ],
    "truncated": false
  }
}

• pending_dialogs: dialogs currently blocking the page's JS thread. The agent must call browser_dialog(action=...) to respond. Empty on Browserbase because their CDP proxy auto-dismisses within ~10ms.

• recent_dialogs: ring buffer of up to 20 recently-closed dialogs with a closed_by tag — "agent" (we responded), "auto_policy" (local auto_dismiss/auto_accept), "watchdog" (must_respond timeout hit), or "remote" (browser/backend closed it on us, e.g. Browserbase). This is how agents on Browserbase still get visibility into what happened.

• frame_tree: frame structure including cross-origin (OOPIF) children. Capped at 30 entries + OOPIF depth 2 to bound snapshot size on ad-heavy pages. truncated: true surfaces when limits were hit; agents needing the full tree can use browser_cdp with Page.getFrameTree.

No new tool schema surface for any of these — the agent reads the snapshot it already requests.

Availability gating

Both surfaces gate on _browser_cdp_check (supervisor can only run when a CDP endpoint is reachable). On Camofox / no-backend sessions, the dialog tool is hidden and snapshot omits the new fields — no schema bloat.

Cross-origin iframe interaction

Extending the dialog-detect work, browser_cdp(frame_id=...) routes CDP calls (notably Runtime.evaluate) through the supervisor's already-connected WebSocket using the OOPIF's child sessionId. Agents pick frame_ids out of browser_snapshot.frame_tree.children[] where is_oopif=true and pass them to browser_cdp. For same-origin iframes (no dedicated CDP session), the agent uses contentWindow/contentDocument from a top-level Runtime.evaluate instead — supervisor surfaces an error pointing at that fallback when frame_id belongs to a non-OOPIF.

On Browserbase, this is the ONLY reliable path for iframe interaction — stateless CDP connections (opened per browser_cdp call) hit signed-URL expiry, while the supervisor's long-lived connection keeps a valid session.

Camofox (follow-up)

Issue planned against jo-inc/camofox-browser adding:

• Playwright page.on('dialog', handler) per session
• GET /tabs/:tabId/dialogs polling endpoint
• POST /tabs/:tabId/dialogs/:id to accept/dismiss
• Frame-tree introspection endpoint

Files touched (PR 1)

New

• tools/browser_supervisor.py — CDPSupervisor, SupervisorRegistry, PendingDialog, FrameInfo
• tools/browser_dialog_tool.py — browser_dialog tool handler
• tests/tools/test_browser_supervisor.py — mock CDP WebSocket server + lifecycle/state tests
• website/docs/developer-guide/browser-supervisor.md — this file

Modified

• toolsets.py — register browser_dialog in browser, hermes-acp, hermes-api-server, core toolsets (gated on CDP reachability)
• tools/browser_tool.py
  • browser_navigate start-hook: if CDP URL resolvable, SupervisorRegistry.get_or_start(task_id, cdp_url)
  • browser_snapshot (at ~line 1536): merge supervisor state into return payload
  • /browser connect handler: restart supervisor with new endpoint
  • Session teardown hooks in _cleanup_browser_session
• hermes_cli/config.py — add browser.dialog_policy and browser.dialog_timeout_s to DEFAULT_CONFIG
• Docs: website/docs/user-guide/features/browser.md, website/docs/reference/tools-reference.md, website/docs/reference/toolsets-reference.md

Non-goals

• Detection/interaction for Camofox (upstream gap; tracked separately)
• Streaming dialog/frame events live to the user (would require gateway hooks)
• Persisting dialog history across sessions (in-memory only)
• Per-iframe dialog policies (agent can express this via dialog_id)
• Replacing browser_cdp — it stays as the escape hatch for the long tail (cookies, viewport, network throttling)

Testing

Unit tests use an asyncio mock CDP server that speaks enough of the protocol to exercise all state transitions: attach, enable, navigate, dialog fire, dialog dismiss, frame attach/detach, child target attach, session teardown. Real-backend E2E (Browserbase + local Chrome) is manual; probe scripts from the 2026-04-23 investigation kept in-repo under scripts/browser_supervisor_e2e.py so anyone can re-verify on new backend versions.