Web Vitals & runtime metrics

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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds value by naming exactly which metrics are captured, including the LCP element and shifting elements for CLS, but says nothing about whether measuring forces layout/reflow, how long it takes, or whether values reflect load-time or current state.

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?

One dense sentence, front-loaded with the primary purpose (Core Web Vitals) before the secondary runtime metrics. The parenthetical lists are long but each item carries information; nothing is redundant.

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 and low complexity, the description covers what is measured but not how results are returned (units, structure, whether element/attribution data is nested). For a metrics tool with no output contract, at least a hint of the return shape would help.

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?

There is a single optional parameter with 100% schema coverage, and the schema already explains that tabId defaults to the session target tab. The description adds no parameter-level meaning at all, so the baseline of 3 applies.

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?

A specific verb ("Measures") plus enumerated resources (LCP, CLS, INP, FCP, TTFB, JS heap, DOM nodes, layouts, style recalcs, script time) makes the purpose unambiguous. It does not, however, distinguish itself from close siblings like perf_trace, memory, or coverage, which overlap on heap/timing data.

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 gives no when-to-use guidance, no exclusions, and never names an alternative such as perf_trace (detailed tracing) or memory (heap inspection). An agent must infer the choice from the name alone.

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