Servo MCP

Servo MCP is a Cloudflare Worker MCP App for running Servo WASM as an isolated, headless browser. It exposes the browser engine through Streamable HTTP at /mcp and provides a small MCP Apps UI for loading pages and viewing rendered screenshots.

The project files in this repository are MIT licensed. servo-wasm/ is a Git submodule and keeps its own license and notices; the MIT license here does not change the license of Servo or its other dependencies.

MCP tools

The server exposes focused tools. Every browser-action tool accepts a sessions array, so one call can run the same kind of action on up to 20 different tabs, with different parameters for each. servo_session_create also accepts an array of independent create options; status and close accept sessionIds. Results include a sessionId and per-session success or error so one failed tab does not hide the others.

• servo_session_create, servo_session_status, servo_session_close

• servo_navigate, servo_reload, servo_history

• servo_inspect, servo_evaluate, servo_get_capabilities

• servo_click, servo_type_text, servo_press_key, servo_scroll, servo_wait

• servo_screenshot, servo_register_font

For example, create several tabs with servo_session_create using { "sessions": [{ "url": "https://example.com" }, { "url": "https://example.org" }] }. Then inspect them together with { "sessions": [{ "sessionId": "…" }, { "sessionId": "…" }] }, or navigate them with per-tab URLs using { "sessions": [{ "sessionId": "…", "url": "https://example.net" }] }. Keep each returned sessionId mapped to its tab; there is no session-list tool. Use servo_session_close with a sessionIds array when those tabs are done.

Each session is routed to its own SQLite-backed Durable Object, which serializes operations and stores a serialized restore snapshot. The live Servo WASM runtime is only cached for 90 seconds after activity; later browser-tool calls automatically start a fresh runtime and reopen the current tab. The checkpoint preserves URL, viewport, scroll position, common form values, local/session storage, script-visible cookies, registered fonts, and the source for inline HTML pages. It does not preserve the JavaScript heap, HttpOnly cookies, browser history, arbitrary DOM mutations, or application state held only in memory. Restoring reloads the page and runs its scripts again; it never replays prior tool actions. Sessions are deleted after 30 days without use or when closed. Closing explicitly also deletes the snapshot and saved assets. The 90-second runtime cache still incurs Durable Object duration while resident, and Durable Objects do not remove the Worker CPU-time requirements of instantiating Servo or processing page actions.

servo_evaluate runs JavaScript in the page realm and awaits a returned promise within the call's time budget. It can inspect Servo's DOM, CSS, canvas and browser APIs. Results are WebDriver-style JSON clones. Servo reports supported, partial, unsupported and unverified features through servo_get_capabilities.

The endpoint currently has no authentication. A sessionId is therefore a bearer capability: anyone who obtains it can use or close that session. Do not share it or use this testing endpoint for sensitive browsing.

The server limits a pump to 15 seconds, resumable inline HTML to 1 MiB, response bodies to 8 MiB and each browser session to 50 subrequests. Only public HTTP(S) URLs are allowed. The host also sets Cloudflare's global_fetch_strictly_public flag so Worker fetches cannot connect to private network targets after DNS resolution. Do not remove either layer when deploying.

Related MCP server: Browser MCP

Local setup

The servo-wasm/ submodule is pinned to the engine revision this app targets. Initialize it, install the Rust target, and build the production artifact incrementally:

git submodule update --init --depth 1
rustup target add wasm32-unknown-unknown
npm install
npm run engine:build
npm run dev

The local health endpoint is http://127.0.0.1:8788/health; the MCP endpoint is http://127.0.0.1:8788/mcp. Connect ChatGPT Developer Mode or MCP Inspector to the HTTPS /mcp URL exposed by your development tunnel. The app uses current Cloudflare createMcpHandler with stateless Streamable HTTP and the MCP Apps UI resource convention. Browser-session continuity lives in per-session Durable Objects, separate from the stateless MCP transport. The ServoBrowserSession SQLite class is declared in wrangler.jsonc; apply its migration when deploying.

Run npm run typecheck and npm test for the app checks. npm run deploy:dry-run builds Servo and asks Wrangler to calculate the bundle without deploying. The Servo binary is close to Cloudflare's 64 MiB Worker bundle ceiling, so the combined Worker bundle must be measured before any deployment.

Test deployment

Deployed on 2026-09-27 to the authenticated Cloudflare account. The live Worker uses Durable Object-backed browser sessions and the focused multi-session MCP tools documented above.

• MCP endpoint: https://servo-mcp.defcronyke.workers.dev/mcp

• Health: https://servo-mcp.defcronyke.workers.dev/health

• Deployed Worker version: 40a51194-dd4c-4639-b546-8661c3096c3e (servo-wasm a175271c7, ABI 10)

The /health endpoint and MCP tools/list response were verified against the live Worker. The endpoint currently has no authentication and is public for testing. Before broader publication, decide the access policy; add OAuth and consent before private or user-scoped use. Measure CPU and total isolate memory on the intended Workers plan and review Cloudflare and ChatGPT submission requirements before publication. The deployed bundle was 49,946 KiB (15,263 KiB gzip) in Wrangler's dry run, below the 64 MiB Worker limit.

The MCP handler validates localhost and workers.dev Host/Origin defaults; custom domains should also be protected by Cloudflare routing and deployment policy.

License

MIT for Servo MCP code in this repository. The Servo WASM submodule and all third-party packages remain under their own license terms.