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evaluate

Execute custom JavaScript in the browser console and return a compact, token-efficient result summary. Includes a preview guard for large outputs.

Instructions

[may return preview+token] ⚙️ CUSTOM JAVASCRIPT EXECUTION - Execute arbitrary JavaScript in the browser console and return a compact, token-efficient summary of the result. Single expressions return their value automatically; multi-statement scripts must use return. Includes a large-output preview guard with a one-time token. ⚠️ NOT for: scroll detection (inspect_dom shows 'scrollable ↕️'), element dimensions (use measure_element), DOM inspection (use inspect_dom), CSS properties (use get_computed_styles), position comparison (use compare_element_alignment). Use ONLY when specialized tools cannot accomplish the task. Automatically detects common patterns and suggests better alternatives.

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": {
      "script": "<script>"
    },
    "name": "evaluate"
  }
}

Input Schema

Name	Required	Description	Default
`script`	Yes	JavaScript code to execute

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.13.0May 11, 2026

TDQS

A4.5/5.0

Behavior4/5

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

Annotations provide readOnlyHint=false and openWorldHint=true; description adds details like return value preview guard, token efficiency, and auto-detection of alternative tools, going beyond annotations to clarify behavior.

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

Conciseness4/5

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

Description is front-loaded with core purpose and uses clear bullet-point structure, though it could be slightly more concise without losing essential details.

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

Completeness4/5

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

Given the complexity of arbitrary JS execution, description covers usage guidelines, alternatives, and behavioral traits adequately. Lacks explicit error handling details but sufficient for agent selection.

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

Parameters4/5

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

Schema covers the 'script' parameter 100%, but description adds critical usage notes (single vs multi-statement, return requirement) that enhance understanding beyond schema.

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?

Description clearly states 'Execute arbitrary JavaScript in the browser console' and lists exclusions for sibling tools (scroll detection, element dimensions, etc.), making the purpose highly specific and distinguishable.

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

Usage Guidelines5/5

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

Explicitly states when NOT to use the tool by listing sibling tools for common tasks and concludes 'Use ONLY when specialized tools cannot accomplish the task,' providing strong decision support.

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