Real-user field data

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

Despite readOnlyHint and openWorldHint annotations, the description adds valuable behavioral nuance: data may be absent for many sites, and absence is 'reported plainly and is not a failure.' It also explains the history flag produces a weekly p75 time series to reveal slowdowns over months. This goes beyond the annotations without contradicting them.

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?

Three sentences, each earning its place: defines what the tool does, explains the history option, and manages expectations about missing data. Front-loaded and no waste.

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 simple two-parameter tool with a clear schema and safety annotations, the description covers the core purpose, behavioral limitations, and option semantics. It is sufficiently complete for an agent to decide when and how to invoke the tool.

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 covers both parameters with descriptions (100% coverage), so baseline is 3. The description adds extra semantic meaning for the history parameter, explaining it as 'the weekly p75 time series' and its purpose for showing whether a site is getting slower over months. It doesn't add much for the urls parameter, but schema handles that.

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?

Description clearly explains that the tool provides field data from the Chrome UX Report, and contrasts it with other evidence: 'the only evidence here about real visitors, and the only thing Google ranks on.' This distinguishes it from sibling diagnostic tools and states its core purpose.

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?

It conveys when to use it: for real-user data from CrUX, and notes that missing field data is expected for low-traffic sites. It also directs the history flag to enable the weekly p75 time series. However, it does not explicitly name alternative sibling tools or state when not to use it.

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