Core Web Vitals field history (CrUX)

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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and destructiveHint false, so the safety profile is clear. The description adds valuable context beyond annotations: API enablement requirement, key environment variables, and the 404 error behavior with a suggested workaround. This enriches behavioral understanding.

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?

Two sentences: first covers the core functionality and metrics, second covers prerequisites and error handling. Front-loaded with the most important info, no fluff. Minor density but well-structured.

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?

Given no output schema, the description should detail the return format. It mentions metrics and time period but not the response structure (e.g., JSON shape, per-week array). Also the '~25 weeks' vs schema 'weeks' default (12) creates ambiguity. These are notable gaps for a tool with 4 parameters and no output schema.

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 coverage is only 25% (only target has a description). The description clarifies scope (origin vs URL) but does not explain the 'weeks' parameter (says 'last ~25 weeks' while schema default is 12 and max 40) or 'formFactor'. It partially compensates for low schema coverage but leaves gaps for two key parameters.

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 exactly what the tool returns: weekly p75 for LCP, INP, CLS, FCP, TTFB and the share of good/needs-improvement/poor for an origin or URL. This is specific and clearly distinguishes it from sibling tools like pagespeed (single snapshot) and geo_page_score.

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 provides practical guidance: mentions prerequisite (API enabled, key) and a fallback when 404 ('try the origin instead'). However, it does not explicitly compare to alternatives like pagespeed or other CrUX-related tools, so the agent must infer when to prefer this tool.

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