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

With no annotations, the description carries the full burden and does well: it discloses confirm-gating, idempotency ('safe to repeat'), the refusal condition for cancelled sessions, and the downstream effect on the client's used session count. It does not state ownership/authorization requirements or what the preview returns, which keeps it short of 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?

Four short clauses, front-loaded with the action and scope, then the gate, the side effect, and the id lookup. Nothing is repeated from the schema and no sentence is filler.

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?

For a two-parameter mutation with no annotations and no output schema, the description covers safety, idempotency, failure mode, and side effects. Only the shape of the preview response and permission requirements are left unspecified, which is minor given the tool's simplicity.

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 50%, with only 'confirm' documented in the schema; the description supplies the confirm-gate semantics in prose and gives the provenance for the undocumented sessionId parameter ('from list_sessions'). That adds meaning beyond the bare minLength-1 string in the schema.

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 (mark as done/completed) and resource (one of the coach's own sessions), with an explicit scope restriction to the coach's own sessions. An agent can distinguish it from cancel_session, reschedule_session and schedule_session without opening any 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?

It gives the prerequisite explicitly ('Get the session id from list_sessions') and states the operating conditions ('safe to repeat; refused for cancelled sessions'), plus a secondary use case (correcting a wrongly recorded no-show). It never names cancel_session as the alternative for the opposite intent, so the routing guidance is clear but not complete.

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