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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds real behavioral context beyond that: it runs on both mobile and desktop, and field data is only present 'when Google has it' — a conditional-availability disclosure that prevents the agent misreading an empty field-data section 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?

Two tight sentences, zero filler. The core capability is front-loaded, with the sibling routing placed last as a pointer rather than padding.

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?

No output schema exists, so the description correctly compensates by listing the metrics returned and flagging the conditional field data. For a one-parameter read tool that is close to complete; only the response shape/pagination details are absent, which is minor here.

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 at 100% schema coverage, so the schema carries the burden and the baseline is already high. The description still adds meaning by clarifying that the single URL input yields results for both mobile and desktop, which the schema does not convey.

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?

Names a specific verb and resource ('Quick Core Web Vitals check for a URL') and enumerates the exact outputs (performance score, LCP, CLS, FCP, TBT, TTFB, field data). It also distinguishes itself from the nearest sibling by naming run_pagespeed_insights as the fuller alternative.

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 routes the agent: use this for a quick check, use run_pagespeed_insights for the full report. That is a clear alternative-selection rule, though it doesn't state explicit exclusions or prerequisites (auth, rate limits, quota cost).

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