Run custom JavaScript in the active browser page to inspect or modify the DOM, extract data, and automate dynamic interactions.
Evaluate JavaScript in current page.
| Name | Required | Description | Default |
|---|---|---|---|
| script | Yes | ||
| session_id | Yes |
| Name | Required | Description | Default |
|---|---|---|---|
No arguments | |||
Changes observed during successful MCP inspections.
v0.5.1Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description carries the full behavioral burden, and it discloses almost nothing: it does not state whether the script runs in page context, whether it is sandboxed or blocked by CSP, whether async/promises are awaited, or how errors surface. For an arbitrary-code-execution tool this is a significant transparency gap.
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?
A single short sentence with no wasted words and the core purpose front-loaded. It is efficient, though bordering on under-specification rather than genuine conciseness.
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?
An output schema exists, so return values need not be described, but with zero annotations and zero schema coverage the definition is incomplete for a tool that executes arbitrary code: execution context, sandboxing, async handling, and error behavior are all unstated.
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?
Schema description coverage is 0% for two required parameters, and the description adds no meaning beyond the names. It does not explain the expected form of 'script' (expression vs function body, return semantics) or what 'session_id' refers to.
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 gives a specific verb ('Evaluate') and resource ('JavaScript in current page'), which is clear enough for an agent to distinguish it from siblings like browser_click or browser_html. However, it does not differentiate itself from the related browser_wait_for_function or browser_query tools, so it stops short of a 5.
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?
There is no guidance on when to use this tool versus alternatives such as browser_query, browser_wait_for_function, or browser_console_messages. No prerequisites, exclusions, or context conditions are stated.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.