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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds substantial value beyond them: no credit cost, the two data sources (lab + CrUX field), and a detailed account of what resources=true changes—raw resource lists, unused CSS/JS bytes, oversized images, network requests, LCP element—with the consequence of omitting it.

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?

Front-loaded with purpose and the free/no-credit hook, and every sentence about resources genuinely earns its place given the 0% schema coverage. The middle paragraph is dense but not padded, so it is appropriately sized rather than excessively verbose.

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?

An output schema exists, so return values need no explanation; safety is covered by annotations; and all non-trivial parameters are explained. Nothing an agent needs to call this correctly is missing.

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 0%, so the description carries full burden and does so: strategy accepts 'mobile' (default) or 'desktop', resources toggles raw resource output, and limit caps rows per audit with default 20. Only url is left implicit, which is self-evident from the tool's purpose.

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?

States a specific verb+resource ("Real Core Web Vitals & performance for a page") and names the underlying source and metrics (Google PageSpeed Insights, LCP, CLS, INP/TBT, 0–100 score, opportunities). It is clearly distinguishable from adjacent siblings like seo_technical_audit or wordpress_performance.

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?

Gives a concrete when-to-use context ("quality gate when building or fixing a website") and notes it is free with no credit cost. It does not explicitly name an alternative tool or state when NOT to use it, so it stops short of the top band.

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