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 covered. The description adds meaningful behavior beyond that: the data source is real Chrome users (CrUX), values are rated good/needs-improvement/poor, and there is an automatic fallback to site-level data when a page has too little traffic.

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 tightly packed sentences, front-loaded with what is returned, followed by the fallback caveat and the sibling pointer. No filler, and the alternative tool mention comes last where it belongs.

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?

With no output schema, the description compensates by enumerating the returned metrics, their aggregation level (75th percentile), device breakdown, and rating scale. Combined with the fallback note, an agent has enough to call it and interpret results without a return 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 description coverage is 100% and there is only one required parameter, so the schema already carries the parameter semantics. The description implies the URL is a page address but adds no format or constraint detail beyond that, so the baseline 3 applies.

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 names the exact resource (Core Web Vitals from the Chrome UX Report), lists the specific metrics returned (LCP, INP, CLS, FCP, TTFB), the percentile, and the device split. It also states what it is not (lab checks), clearly separating it from the sibling render_page.

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 explicitly routes the agent: this tool is for real-user field data, and 'Use render_page for lab checks of the first screen' names the alternative and the condition that selects it. The fallback-to-site-level behavior also tells the agent what to expect when a URL has limited data.

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