Evaluate JavaScript expressions in a browser tab to inspect or modify page state, retrieve data, and trigger actions via the Chrome DevTools Protocol.
Evaluate JavaScript expression in a tab.
| Name | Required | Description | Default |
|---|---|---|---|
| targetId | Yes | Target ID of the tab | |
| expression | Yes | JavaScript expression to evaluate | |
| returnByValue | No | Return result by value (default: true) |
Changes observed during successful MCP inspections.
v1.1.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden of behavioral disclosure. It only says 'Evaluate JavaScript expression in a tab' without noting that arbitrary JavaScript can mutate the page, how errors are surfaced, or whether promises are resolved. This is a significant gap for an execution tool.
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 front-loaded sentence with no filler. It communicates the core action immediately and earns its place, even though it lacks additional behavioral detail.
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?
There is no output schema and no annotations, so the description must provide enough context for safe invocation. It does not explain return semantics, error handling, side effects, or the effect of returnByValue, leaving the agent under-informed about a potentially destructive operation.
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 100%, and the schema already documents targetId, expression, and returnByValue clearly. The description adds only the 'in a tab' context, so the schema does the heavy lifting and the baseline of 3 is appropriate.
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 states a specific verb and resource: 'Evaluate JavaScript expression in a tab.' It is unambiguous against siblings like dom_query or page_reload. It loses a point because it does not explicitly distinguish itself from related evaluation or inspection tools.
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 when-to-use guidance, no exclusions, and no mention of alternatives. Given the large sibling list containing DOM interaction and console tools, the agent is left to infer when page_evaluate should be chosen.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.