CrUX: compare origins

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

Annotations already declare the tool read-only, open-world, idempotent, and non-destructive. The description adds valuable behavioral details: returns one row per origin, includes pass/fail status, and reports origins without CrUX records rather than silently dropping them. This goes beyond annotation cues.

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 compact and well-structured: it opens with purpose, then explains input format, and closes with output behavior. Every sentence carries meaningful information without redundancy or fluff.

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 an output schema present, the description need not explain return structure. It adequately covers purpose, input format, and edge-case handling for missing data. A brief mention of form_factor would improve completeness, but the current level suffices for a moderately simple comparison 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?

The description thoroughly explains the 'origins' parameter with a comma-separated format and a concrete example, which is critical since schema descriptions are absent. However, 'form_factor' is not documented in the description, and the schema only provides a default. Partial compensation for a 0% schema coverage.

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 clearly states the tool compares Core Web Vitals across multiple origins, using a specific verb ('compare') and resource ('Core Web Vitals'). This distinctly separates it from siblings like crux_record (single origin) and crux_history (time series).

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 phrase 'several origins — yours against competitors' implies the intended comparison use case. It does not explicitly name alternative tools or exclusion conditions, but the contrast with sibling tool names and the context of competitor benchmarking provide strong guidance.

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