Latest Core Web Vitals + LCP breakdown (CrUX)

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

Annotations already declare readOnly/openWorld/idempotent/non-destructive, so the bar is lower. The description adds value beyond those by disclosing the required API key setup, the NOT_FOUND error meaning and the scope fallback, and by clarifying that the data is a 28-day real-user aggregate. No contradiction with annotations.

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?

Although dense, every clause adds information: metrics returned, LCP breakdown purpose, nav mix, sibling routing, auth, and error handling. The main purpose is front-loaded in the first clause, and there is no 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?

There is no output schema, so the description carries the burden of explaining return content; it names the metric set, the LCP sub-part phases, the element type, and navigation mix. It also covers prerequisites and a common error path. This is sufficient for an agent to know what the tool returns and when to call 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?

The schema already covers 100% of parameters with descriptions, including enum meanings and defaults, so baseline 3 applies. The description reinforces scope choice with the 'try scope=origin' fallback but does not materially extend the schema's parameter documentation.

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 opens by naming the resource ('Chrome UX Report'), the temporal scope ('latest 28-day'), and the level ('origin or URL'), then enumerates the returned metric families. It explicitly contrasts with crux_history, so an agent can distinguish it from its only sibling. This is a clear verb+resource+scope definition.

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?

It gives an explicit alternative: 'Use crux_history for the weekly trend,' which tells the agent when to prefer the sibling. It also provides operational direction for low-traffic results ('NOT_FOUND means too little traffic, so try scope=origin') and prerequisite auth requirements, making invocation conditions clear.

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