Core Web Vitals

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

With no annotations, the description carries the full behavioral burden. It does this well by disclosing that the tool is slow ('takes up to a minute') and explaining the conceptual difference between lab and field data. It does not mention error cases or rate limits, but for this simple measurement tool the disclosed latency and data-source distinction are substantial.

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 tight sentences, each earning its place: the first states the core action, the second explains the key data distinction, and the third sets latency expectations. It is front-loaded with the primary purpose and contains no filler or repetition.

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 two-parameter tool with no output schema, this description gives an agent enough to select and invoke it: what it measures, the input URL requirement, the default mobile strategy from the schema, and the latency caveat. It could be more explicit about the exact metrics returned, but 'report Core Web Vitals' is a reasonable summary for this simple case.

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?

The input schema already has 100% parameter description coverage, including the 'url' and 'strategy' defaults, so the baseline is 3. The description adds no additional parameter-level meaning; it only reinforces the lab/field distinction conceptually rather than explaining how 'strategy' or 'url' behave.

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 clearly states the action ('Run PageSpeed Insights on a URL and report Core Web Vitals') and names the specific resource being measured. It is more specific than a generic audit tool because it focuses on Core Web Vitals, but it does not explicitly contrast itself with the sibling 'audit_page' tool or name alternatives.

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?

The description gives useful context: field data is what Google ranks on, so a user wanting ranking-relevant CWV should prefer that. However, it never explicitly says when to use this tool instead of siblings like 'audit_page', 'crawl_site', or 'compare_competitor', so usage guidance is only implied rather than spelled out.

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