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debugger-evaluate

Execute JavaScript in a mobile app's runtime to read state, call functions, or test logic, returning JSON-serializable results for debugging.

Instructions

Execute arbitrary JavaScript in the app's JS runtime via CDP — Hermes on iOS / Android / Vega, V8 on Chromium. Returns the evaluation result as a JSON-serializable value, along with deviceName, appName, and logicalDeviceId for context. Use when you need to read app state, call app functions, or test logic at runtime. The result is serialized by value, so cyclic objects (many RN runtime values — fiber nodes, navigation refs, global — are cyclic) fail with a serialization error rather than returning silently. Fails if the expression throws or the runtime is not connected.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
portNoMetro server port. Optional — omit it to use this device's port, 8081 by default. Ignored for Chromium, whose CDP port is encoded in device_id.
device_idYesDevice id from list-devices — the SAME id you passed to debugger-connect (iOS simulator UDID, Android serial, Vega serial, or Chromium device id). The logicalDeviceId debugger-connect returns also resolves here, but prefer the stable list-devices id.
expressionYesJavaScript expression to evaluate in the app runtime

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv0.25.0
    • removedInput schema / properties / port / default
      Removed value: -8081
    • changedInput schema / properties / port / description
      Previous value: -"Metro server port (ignored for Chromium)"New value: +"Metro server port. Optional — omit it to use this device's port, 8081 by default. Ignored for Chromium, whose CDP port is encoded in device_id."
    • addedInput schema / properties / port / maximum
      Added value: +65535
    • addedInput schema / properties / port / minimum
      Added value: +1
    • changedInput schema / properties / port / type
      Previous value: -"number"New value: +"integer"
  2. Changed1 schema field changedv0.17.0
    • changedInput schema / properties / device_id / description
      Previous value: -"Device id from debugger-connect (iOS simulator UDID, Android logicalDeviceId, Vega serial, or Chromium device id)."New value: +"Device id from list-devices — the SAME id you passed to debugger-connect (iOS simulator UDID, Android serial, Vega serial, or Chromium device id). The logicalDeviceId debugger-connect returns also resolves here, but prefer the stable list-devices id."
  3. Changed1 schema field changedv0.16.0
    • changedInput schema / properties / device_id / description
      Previous value: -"Device id from debugger-connect (iOS simulator UDID, Android logicalDeviceId, or Chromium device id)."New value: +"Device id from debugger-connect (iOS simulator UDID, Android logicalDeviceId, Vega serial, or Chromium device id)."
  4. First observedv0.15.0

TDQS

A4.5/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full behavioral burden and does so well: it explains serialization-by-value, cyclic object failures, expression throw behavior, and runtime disconnection failures. This is substantial beyond the basic action.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Four dense sentences, all earning their place. The core action is front-loaded, followed by return value context, usage guidance, and critical failure caveats. No filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a tool with no output schema and no annotations, the description covers the essential operational contract: runtime environment, return shape, common failure modes, and when to invoke. An agent has enough to select and call it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline is 3. The description does not add much parameter-level meaning beyond the schema, though it does clarify evaluation semantics and failure behavior for the expression parameter.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific action: execute arbitrary JavaScript in the app's JS runtime via CDP, and names the relevant runtimes (Hermes/V8). This clearly differentiates it from sibling debugger tools like debugger-connect, debugger-status, and debugger-component-tree.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly says when to use it: 'Use when you need to read app state, call app functions, or test logic at runtime.' It does not name alternatives or state when not to use it, so it falls just short of a 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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