web_vitals_summary

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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds a genuinely useful behavioral precondition — the required custom dimensions and event parameters — implying results are empty or unavailable if the site isn't instrumented. It stops short of saying what happens when the prerequisite is unmet or what auth scope is needed.

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?

Three short sentences, front-loaded with what the tool returns, then prerequisites, then the alternative. No filler and no repetition of the schema.

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?

An output schema exists, so return values need not be described, and the prerequisite and alternative-tool information is present. The remainder of the gap is the undocumented date-range and property parameters, which the description leaves entirely to the schema.

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 0% and the description says nothing about `property_id`, `date_start` or `date_end` semantics (formats, defaults, timezone). The two parameters it does name, `web_vital_metric` and `web_vital_rating`, are GA4 event dimensions, not inputs to this tool, so they add no invocation guidance and risk being mistaken for arguments.

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?

States a specific resource (Core Web Vitals) and its exact data source (GA4 events sent by the user's own site), which separates it from the other GA4 report tools and from the PSI siblings. Naming psi_field_data_only as the contrasting tool makes the distinction unambiguous.

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?

Gives an explicit prerequisite (site must send web-vitals events with `web_vital_metric` and `web_vital_rating` registered as event-scoped custom dimensions) and an explicit routing rule: 'For Google's field data use psi_field_data_only instead.' An agent can decide when this tool applies without guessing.

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