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

Annotations already declare readOnly, idempotent, non-destructive, openWorld, so the safety profile is covered. The description adds genuinely useful behavior beyond annotations: the trailing-28-day measurement window, the 75th-percentile aggregation, and the fact that a missing low-traffic page is reported plainly rather than as an error.

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 sentences, each front-loaded with the essential constraint first (one page) then the data window, then routing and the failure mode. No filler or repetition 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 compensates well by stating the metric set, aggregation, and time window, and by explaining the absence case for low-traffic pages. The only gap is that the response shape itself isn't sketched, which is minor for a read-only query 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?

Schema description coverage is 100%, with url, metrics and form_factor all documented including enum values and defaults, so the schema does the heavy lifting. The description names LCP/INP/CLS but adds no syntax or format detail beyond the schema, 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?

States a specific verb+resource+scope: real-user Core Web Vitals for ONE specific page from Chrome UX Report, and names the exact metrics (75th-percentile LCP, INP, CLS) and time window. It explicitly contrasts itself with crux_query_origin, so the agent can distinguish the two page-vs-site tools without opening either 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?

Gives explicit when-to-use ('a single page — a product page, a landing page, a checkout step — is the question') and a named alternative ('use crux_query_origin for the site as a whole'). It also discloses the edge case that low-traffic pages may be absent, telling the agent not to treat that as a failure.

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