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

Annotations already declare readOnlyHint and openWorldHint, yet the description adds real value: ~1s latency, works for any public site including competitors, and the origin fallback when a URL has too little traffic (with the caveat that it says so). It doesn't discuss rate limits or whether the fallback is silent vs flagged in the response beyond a passing note.

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-loads the data source and metrics before routing guidance, and every sentence contributes something actionable (workflow, fallback behavior, tool alternative). No filler or restatement of the title.

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 carries the burden of describing returns and does so: percentile metrics, good/needs-improvement/poor ratings, and the pass/fail assessment verdict. Combined with the fallback note and URL limits, an agent has everything needed to call it and interpret results.

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%, so both parameters are already documented, including the PHONE default and enum values. The description reinforces the 10-URL comparison use case and full-URL vs bare-domain semantics, but never mentions formFactor, so it adds only marginal meaning beyond the schema. 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?

States a specific verb and resource: real-user Core Web Vitals from the Chrome UX Report, with named metrics (LCP, INP, CLS, FCP, TTFB) and a 28-day window. It explicitly separates itself from pagespeed_audit ('for the reasons a page is slow') and from its history sibling by framing itself as current-data only.

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 trigger scenario ('which of my busiest pages are slow?') and a full upstream workflow: pull top landing pages from run_report with landingPage or Search Console, then pass full URLs here. It also names the alternative tool and the condition that selects it.

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