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

With no annotations, the description carries the burden and does disclose auth/rate-limit behavior ('works without an API key at low volume; set GOOGLE_PAGESPEED_API_KEY server-side for higher throughput'), which is real operational context. It stops short of describing latency, quota behavior, or failure modes for an external network call.

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 tight clauses, front-loaded with what the tool produces, then the sibling disambiguation, then the auth note. Every sentence earns its place and none repeats the schema.

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 read-only external measurement tool with no output schema, the description compensates by naming the return payload (scores, CWV, field data) and the credential requirement. Nothing an agent needs in order to call it correctly is missing.

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 'url' and the 'strategy' enum (default mobile) are already fully documented. The description adds no syntax or format detail beyond the schema, which is the expected baseline when structured data does the heavy lifting.

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 and resource ('Run Google's real PageSpeed Insights ... against a URL') and enumerates the outputs (performance score, Core Web Vitals LCP/CLS/INP/TBT, field data). It explicitly distinguishes itself from the sibling page_performance_check, so an agent can route 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?

Names the alternative (page_performance_check) and the exact condition that selects it: 'for a quick raw TTFB/size timing check from this server use page_performance_check instead.' It also states the auth prerequisite and its throughput consequence, leaving nothing to inference.

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