Core Web Vitals assessment

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

Annotations already indicate a safe read operation (readOnlyHint, idempotentHint, non-destructive), but the description goes far beyond by disclosing exact output format (p75, ratings, densities summing to ~1.0), thresholds, passes_core_web_vitals definition, failure mode ({no_data: true}), and data quirks (redirects, key normalization, sparse tablet/single-page data). This is rich behavioral context beyond annotations.

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?

The description is dense but every sentence earns its place: a one-line purpose, then metric/threshold details, then edge cases and input constraints. It is front-loaded and free of filler, achieving high information density without bloat.

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?

There is no output schema, but the description fully defines what the agent will receive: which metrics, thresholds, ratings, densities, pass criteria, and the no_data response. It also covers input selection and data caveats, making it self-sufficient for correct invocation despite the absence of an output schema.

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?

The input schema already provides 100% coverage with detailed field descriptions for url, origin, and form_factor, including mutual exclusivity and canonical key guidance. The description adds minimal parameter-specific value beyond reinforcing these points; extra details like timing units and CLS unitless concern the output, not the parameters.

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 opens with a specific one-call Core Web Vitals assessment for an origin or URL over the latest 28-day window, clearly identifying the resource and scope. It further distinguishes itself from siblings by detailing the snapshot metrics (p75, ratings, densities) and pass/fail logic, which differs from trend or comparison tools.

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 provides clear operational context: exactly one of origin or url, when to fall back from url to origin, and optional form_factor usage. It does not explicitly name alternative sibling tools or state when not to use this tool, but the 28-day snapshot framing implies its use case, and the origin/url guidance is strong.

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