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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavioral facts beyond that: results are capped at 12, times are UK local with a UTC offset, and each entry carries a pre-filled booking link — all details an agent would otherwise have to discover from the response.

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 dense sentences with no filler; the core value (free times for a session), the horizon, the cap, and the timezone/link payload are all front-loaded before any secondary detail.

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 an output schema present, the description need not explain return structure, and annotations handle the safety profile; what remains is covered by the timezone note and result cap. The only soft spot is that eligibility (which service id, and whether login is needed to book) is left entirely to schema inference.

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 three parameters (id, days, lang) are already documented in the schema, which sets the baseline at 3. The description reinforces the days range but adds nothing about the id's provenance or the lang enum beyond what the schema states.

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 and output: free start times for one Sissoo session, with an explicit horizon (1-14 days) and result cap (up to 12). This is clearly distinguishable from the sibling lookup tools (get_service, get_practitioner, search_services), which return entities rather than bookable slots.

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 intent — surface bookable slots — is implied by 'Free start times ... each with a link to the booking page', so an agent can infer this is the availability-check tool. However, there is no explicit statement of when to call it versus get_service, nor any prerequisite such as needing a service id from search results first (that hint lives only in the schema).

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