feat: add Hermes CDP tools for React Native runtime access#14
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cryptotavares wants to merge 1 commit into
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feat: add Hermes CDP tools for React Native runtime access#14cryptotavares wants to merge 1 commit into
cryptotavares wants to merge 1 commit into
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Add hermes_cdp and hermes_targets MCP tools that speak Chrome DevTools Protocol to the React Native Hermes JS runtime via Metro's inspector proxy (iOS and Android). Includes standalone HermesSession state for Metro port, app bundle ID, and device pinning, plus target discovery, selection, and ambiguity handling.
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What is the current state of things and why does it need to change?
The MCP server can drive native UI automation (idb/adb/Appium) but has no
way to inspect the app's JavaScript layer. For a React Native app, a large
share of state and logic lives in the Hermes JS runtime, which native automation
cannot reach. Agents debugging or validating app behavior have no way to
evaluate JavaScript, read runtime state, or diagnose the JS layer.
What is the solution your changes offer and how does it work?
Adds two MCP tools that speak raw Chrome DevTools Protocol
(CDP) to the React Native Hermes runtime via Metro's inspector proxy — on
both iOS and Android (the transport is identical HTTP/WebSocket to Metro; only
the default
appIddiffers):hermes_cdphermes_targetsHow it works: the app's Hermes runtime connects out to Metro; Metro exposes
debuggable targets over
http://localhost:<metroPort>/jsonwith a per-targetwebSocketDebuggerUrl. The server discovers targets, selects the right one, andspeaks real CDP over that WebSocket. A standalone
HermesSessionholds theresolved Metro port, expected app bundle id, and the pinned device id
(precedence: per-call param > env var > default), decoupled from
DeviceBackend.Safety model — a single Metro can host multiple apps/simulators, so five
fail-closed checks run before executing CDP:
appIdmatch (no substring, notargets[0]fallback).HermesInternalidentity probe before the user's method.reactNative.logicalDeviceIdafter first success.ws:protocol, loopback host, matching port).Destructive methods (
Runtime.terminateExecution,Inspector.detached) areblocked, and the synthetic legacy page is filtered out.
Configuration:
HERMES_APP_ID/appIdparam (defaultsio.metamask.MetaMaskiOS,
io.metamaskAndroid) andHERMES_METRO_PORT/metroPortparam (default8081). Requires a DEBUG build with Metro running; Node 20 needsNODE_OPTIONS="--experimental-websocket".Includes full unit test coverage (session resolution, CDP core, tool handlers)
and README + CHANGELOG updates.