core_web_vitals

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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety and external-network behavior are covered. The description adds one genuinely useful non-annotation fact — the CRUX_API_KEY credential requirement — but says nothing about low-traffic origins returning no data, rate limits, or latency, which are the real failure modes for a CrUX query.

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 dense sentences with zero filler: the resource and metric list come first, then the source and credential prerequisite. Nothing is repeated and nothing needs to be trimmed.

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 usefully enumerates the returned metrics and their p75 aggregation, which is the minimum an agent needs. It stops short of describing the response shape (e.g. per-form-factor breakdown or URL-vs-origin granularity) and the empty-data case, but it is complete enough to call the tool correctly.

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 coverage is 100%, so 'target', 'history', and 'form_factor' are already fully documented, and the description's mention of the 'latest 28-day window' merely restates the history parameter's schema text. Its one addition is the 'p75' percentile framing, which clarifies that metrics are reported as percentiles rather than averages — useful but minor.

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?

The description names a specific resource (Core Web Vitals / CrUX field data), the exact metrics returned (LCP, INP, CLS, FCP, TTFB at p75), and the upstream source (Chrome UX Report). The word 'field data' implicitly separates it from a lab-data sibling like performance_overview or a rendered-page audit, but no sibling is named, so differentiation is inferred rather than stated.

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?

It gives implied context — real-user field data behind Search Console's CWV report — which suggests when an agent should reach for it. However, it never says when to use an alternative (e.g. lab-based speed tooling for page-level diagnostics) or when this tool is inappropriate, so the agent must infer routing from the phrase 'field data'.

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