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

Beyond the annotations (which already cover read-only/idempotent/non-destructive), the description discloses the dual field+lab data model, the 28-day measurement window, Google's own rating bands, the pass/fail ranking-signal interpretation, and the auth/rate-limit profile (Bearer Pro key, 500 checks/minute). That is substantive context an agent cannot get from structured fields.

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, front-loaded with the capability and data sources before the auth/rate-limit aside. The closing clause about /docs and daily regression alerts is slightly promotional, but it still conveys useful operational context.

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 describing the return shape: good/needs-improvement/poor ratings for LCP, INP, and CLS, plus the overall ranking-signal verdict. Combined with the auth and rate-limit notes, an agent has everything needed to call and interpret the tool.

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?

Only one parameter, and schema description coverage is 100% ("url": "Page to check"), so the schema already carries the load. The description adds no format guidance (scheme, trailing slash, redirect handling), so 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 (Check) and resource (a page's real Core Web Vitals) and enumerates exactly which data sources are used: CrUX field data with a 28-day window plus a live Lighthouse lab run. There are no siblings, but the scope is unambiguous without opening the schema.

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?

"Call this before telling someone a page performs well" gives a concrete decision trigger, and the mention of a Pro API key plus /docs for multi-URL monitoring points to adjacent workflows. It stops short of explicit exclusions or when-not-to-use conditions, but the context is clear.

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