Execute JavaScript in the active page and return its result to validate behavior or extract data.
Run JavaScript on the page and return the result.
| Name | Required | Description | Default |
|---|---|---|---|
| script | Yes | JavaScript code to execute |
| Name | Required | Description | Default |
|---|---|---|---|
| result | Yes |
Changes observed during successful MCP inspections.
v0.3.0Does 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 mentions returning a result but does not disclose that arbitrary JavaScript may have side effects on the page, how errors are handled, whether the script runs in the page's main world, or whether asynchronous results are supported.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is a single, direct sentence with no filler or redundant phrasing. It front-loads the core action and result without unnecessary elaboration, making it easy for an agent to parse quickly.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
For a one-parameter tool with an output schema, the description is minimally viable, but it leaves meaningful gaps. It omits guidance about side effects, execution context, error behavior, and when this tool is the appropriate choice among the many browser siblings, making it only partially complete.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
The schema already fully documents the single 'script' parameter with 100% coverage, so the baseline is 3. The description adds minimal parameter-level meaning beyond saying the script runs 'on the page,' which mostly reinforces the schema rather than explaining syntax, expected format, or execution context.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description clearly states a specific verb ('Run JavaScript') and resource ('the page'), and indicates that a result is returned. This distinguishes it from sibling browser tools like navigation, clicking, and text extraction, which perform more targeted actions.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description implies the tool is for executing JavaScript in the current page and obtaining a result, which conveys its basic use case. However, it provides no explicit guidance on when to prefer it over other browser tools, nor does it mention any exclusions or prerequisites such as requiring an active page.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.