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

The annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely useful behavioral context beyond them: 'Answers from the rule set only and fetches nothing', which tells the agent the answer is static and deterministic rather than live data. It does not cover failure modes such as an unknown checkId, which keeps it from a 5.

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?

Two sentences with the trigger condition front-loaded and the return contents enumerated compactly. The only slack is the parenthetical-style clause 'usually after score_site', which is useful but slightly lengthens the opener.

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 no explanation in prose. Between description and schema the agent knows when to call it, what to pass, what it returns, and that the answer is rule-set-derived rather than fetched. Nothing material is missing.

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 coverage is 100% and the single checkId parameter already carries a concrete example ('content-without-javascript'). The description adds no further syntax, format, or lookup hints beyond what the schema provides, so the baseline 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?

States a specific verb and resource ('Explains one check') and enumerates the exact content returned: what it asks, why an agent cares, weighting, and fix guidance. It is clearly distinct from list_checks (many checks) and score_site (a full run), so an agent can route without opening the schema.

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 explicit triggering conditions ('when the user asks what a specific Agent Readiness finding means, why it matters... or how to fix it') and even names the preceding sibling call ('usually after score_site'). This is when-to-use guidance with sibling context, not just implied usage.

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