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browser_evaluate

Execute a JavaScript function in a remote browser and retrieve the JSON-serializable result, enabling dynamic page interaction and data extraction.

Instructions

Evaluate a JavaScript function and return its (JSON-serializable) result.

function is a JS function expression, e.g. "() => document.title" or, when ref is given, "(el) => el.textContent" receiving the element.

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": {
      "function": "<function>"
    },
    "name": "browser_evaluate"
  }
}

Input Schema

Name	Required	Description	Default
`function`	Yes
`session_id`	No
`ref`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Jun 21, 2026

TDQS

A3.7/5.0

Behavior2/5

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

No annotations are provided, so the description must disclose behavioral traits. It mentions that the function's return value is JSON-serializable, but does not specify whether the function runs synchronously, has side effects, or requires a loaded page. Safety implications are absent.

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?

The description is extremely concise—two sentences with an inline example—providing essential information without wasted words. It is front-loaded with the core purpose and parameter details.

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

Completeness3/5

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

The description covers core functionality well (evaluation, return value, parameter examples), but omits explanation for `session_id` and lacks broader behavioral context (e.g., execution environment, error handling). Given the complexity of a JS execution tool, it is adequate but not fully comprehensive.

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 coverage is 0%, so the description compensates by explaining the `function` parameter as a JS function expression with examples, and the `ref` parameter for element selection. However, the `session_id` parameter is left entirely undocumented, leaving a gap.

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?

The description clearly states the tool evaluates a JavaScript function and returns a JSON-serializable result, with specific examples of function expressions. It distinguishes from sibling browser action tools (e.g., browser_click, browser_navigate) by focusing on evaluation rather than interaction.

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

Usage Guidelines3/5

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

The description provides context for when to use the `ref` parameter (receiving an element), but gives no explicit guidance on when to use this tool versus alternatives like `browser_get_text` or `browser_wait_for`. Usage context is implied but not fully elaborated.

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