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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld). The description adds real behavioral context beyond them: a 20-60 second latency window, the public-URL-only restriction, and a detailed enumeration of what the report contains. It does not mention rate limits or failure modes.

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?

Front-loaded with the core action and the tool's output, with the timing and URL constraint placed at the end where they belong. It is a long run-on enumeration of report contents, but each clause conveys distinct, useful information rather than filler.

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 fully carries the burden of explaining return values — score, content timing, responsiveness, layout shift, real-user data, third-party apps, platform detection, and ranked fixes. Combined with the timing and input constraint, an agent has everything needed to call and interpret it.

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 are already documented in the schema. The description restates the competitor limit ('up to two') and comparison purpose but adds no syntax or format detail beyond what the schema provides, matching the baseline for fully-covered schemas.

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 (runs Google PageSpeed Insights mobile lab test) and resource (public store/website), plus the scope of what it reports. None of the sibling tools overlap, so the agent can identify it immediately.

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?

Gives clear usage context and constraints: 'only public web addresses are accepted' and an expected runtime of 20-60 seconds, plus the optional competitor comparison mode. It doesn't name an alternative tool or state when not to use it, but no sibling competes for this task.

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