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

Annotations declare readOnlyHint=true, and the description adds meaningful behavioral context by specifying the exact metrics, distributions, form factors, and configurable day range. It does not mention potential data availability caveats, but for a read-only data fetch, the provided transparency is solid.

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 sentences with no filler: the first states the core purpose, the second lists return metrics and configurable options. Information is front-loaded and every clause earns its place.

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 compensates by listing the return metrics and distributions. It covers all parameters and the read-only nature via annotations. It lacks caveats about data availability for new monitors, but is otherwise complete for a simple fetch tool.

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 the baseline is 3. The description reiterates form factor options and day range but adds no new meaning beyond what the schema already documents. It does not compensate with extra parameter-level detail.

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 uses a specific verb ('Get') and resource ('real-user CrUX field data for a monitor'), and explicitly lists the metrics returned (p75 LCP, FCP, CLS, INP, TTFB). It differentiates from siblings like get-page-speed by emphasizing 'real-user' field data, making the tool's purpose unmistakable.

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?

The description clearly establishes the context: this tool retrieves field data from real Chrome users, which implies it is for field metrics rather than lab-based alternatives. However, it does not explicitly name sibling tools or state when not to use it, so it falls short of a 5.

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