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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the bar is behavioral context. The description goes well beyond that by disclosing the origin-substitution trap: when a low-traffic page has no data, Google substitutes the origin's metrics, which 'otherwise reads as page data and misattributes the whole site's performance to one URL.' That is exactly the kind of non-obvious behavior an agent needs warned about.

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?

Content is front-loaded: the first sentence delivers resource, scope, and metric list. But both sentences are long and clause-heavy, with the substitution warning packed densely; slightly more could be trimmed without losing meaning.

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 2-parameter read-only tool with annotations carrying the safety profile and no output schema, the description is complete enough — it enumerates the returned metrics, their quality bands, the verdict, and the origin-substitution caveat. An agent has everything needed to invoke and correctly interpret the result.

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 (url and the mobile/desktop enum) are already fully documented, giving the baseline of 3. The description adds no syntax or format detail beyond the schema, though its page-vs-origin framing is loosely relevant to interpreting results.

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 resource (real-user Core Web Vitals) with explicit scope (a page AND its whole origin in one call) and names the exact metrics returned — LCP, INP, CLS, TTFB — plus the percentile split and Google's pass/fail verdict. The 'Real-user...Field' framing implicitly separates it from the lab-data sibling pagespeed_audit, so an agent can tell what this returns 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?

Explicitly frames the tool as 'AUTHORITATIVE for whether a page passes Core Web Vitals,' which tells the agent when to reach for it over other metrics sources. It does not name the sibling (pagespeed_audit) or state when-not-to-use, so it stops short of full alternative routing.

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