browser-firefox-mcp

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js_eval

Executes JavaScript expressions in the Firefox browser and returns the result, enabling client-side testing and debugging.

Instructions

Execute JavaScript expression and return result.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "expression": "<expression>"
    },
    "name": "js_eval"
  }
}

Input Schema

Name	Required	Description	Default
`expression`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Aug 20, 2026

TDQS

C2.8/5.0

Behavior2/5

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

With no annotations, the description carries full responsibility for disclosing side effects. It mentions returning a result but does not warn about potential destructive side effects of executing arbitrary JS, the execution context (page vs. browser), or any security implications. This is a significant gap for a tool that can run arbitrary code.

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?

A single, front-loaded sentence with no filler words. It efficiently communicates the core action and result, earning the highest score for conciseness.

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

Completeness2/5

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

Given the tool's power (arbitrary JS execution), the description is too sparse. It omits return value format, error handling, execution context, and safety warnings. Even with only one parameter, this is insufficient for an agent to use it safely and effectively.

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

Parameters2/5

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

Schema description coverage is 0%, so the description must compensate. However, it merely repeats the parameter name ('expression') without explaining what constitutes a valid expression, how results are returned, or whether it is a full script or a single expression. It adds minimal semantic value beyond the schema.

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

Purpose4/5

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

The description clearly states the tool's action ('Execute JavaScript expression') and outcome ('return result'). It is specific enough to distinguish from the general browser automation siblings like navigate or click, though it does not differentiate from inject_payload or inject_xss_payload which also execute scripts.

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

Usage Guidelines2/5

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

There is no guidance on when to use this tool versus alternatives like inject_payload or inject_xss_payload, nor any prerequisites, context, or exclusions. The description only states what it does, not when it is appropriate.

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