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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds genuinely useful behavioral context beyond that: the data is field data 'from real visitors', distinguishing it from lab/synthetic measurements, and it discloses the percentile aggregations returned.

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?

A single dense sentence with zero filler; the resource is front-loaded and every clause (metrics, percentiles, granularity) carries information an agent needs.

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?

No output schema exists, but the description compensates by stating what is returned — the specific metrics at p50/p75/p90, overall and per page. Combined with full schema coverage and read-only annotations, this is nearly complete; only pagination or result-size behavior is unaddressed.

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 all seven parameters (site_id, from/to, period, filters, timezone, segment_id) are already documented in the schema with defaults and format syntax. The description adds no parameter-level detail, 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?

Names a specific resource (Core Web Vitals), enumerates the exact metrics (LCP, INP, CLS, FCP, TTFB), and states the aggregation (p50/p75/p90, overall and per page). No sibling tool covers this data, and the metric/percentile detail makes it unambiguously distinguishable from get_pages or get_overview.

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?

Usage is implied — an agent can infer this is the performance/RUM tool — but the description never states when to reach for it versus get_overview, get_pages, or get_breakdown, nor any exclusions. Adequate but with a clear routing gap.

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