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Browser[X]MCP

Browser[X]MCP

by rnd-pro
OverviewSchemaSourceRelated ServersDiscussionsAdmin
JavaScript
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evaluate_in_page

Execute JavaScript code directly in the browser page to manipulate elements, extract data, and validate application behavior.

Instructions

Execute JavaScript code in the browser page context

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": {
      "code": "<code>"
    },
    "name": "evaluate_in_page"
  }
}

Input Schema

NameRequiredDescriptionDefault
codeYesJavaScript code to execute in the page

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv1.0.0-beta.1

TDQS

C2.9/5.0
Behavior2/5

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It fails to mention whether the code's return value is surfaced, how errors are handled, whether execution is asynchronous, or if there are side effects like page state changes. The only extra context is 'in the browser page context', which is minimal.

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?

The description is a single sentence that is very concise and front-loaded. However, it is perhaps too brief, lacking context that would warrant a higher score. Still, for conciseness, it earns a high score as there is no wasted wording.

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?

The tool is simple with one parameter, but it executes arbitrary JavaScript, which is powerful and potentially side-effectful. With no output schema and no annotations, the description should explain at least the return value and error behavior, but it does not. This leaves the agent uncertain about what happens after execution.

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% for the single 'code' parameter, which is described as 'JavaScript code to execute in the page'. The tool description adds no additional meaning beyond the schema, but the baseline for high coverage is 3, and there is nothing missing in parameter documentation.

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 uses the specific verb 'Execute' with the resource 'JavaScript code in the browser page context', clearly identifying the tool's function. It distinguishes from sibling tools like navigate_browser and input_text, though it doesn't explicitly call out that distinction. The name and description align well.

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?

The description provides no guidance on when to use this tool versus alternatives. It only states what it does, without mentioning exclusions, prerequisites, or scenarios where other tools like execute_atomic_action or click_element_by_xpath might be more appropriate. This is a clear gap.

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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