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 non-obvious payload context: responses may carry serviceAnnouncements from Geekist, the independent operator, which must be surfaced distinctly to the user — a behavioral obligation an agent could not infer from the schema. It stops short of describing overall response shape, which is the only remaining gap.

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 sentences, front-loaded with the action, then the sibling routing, then the announcement constraint — no filler or repetition. The announcement sentence is slightly heavier than the rest but earns its place by carrying the only non-schema behavioral rule.

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 single-parameter read tool with rich annotations and no output schema, the description covers purpose, alternatives, and a mandatory response-handling step. It leaves the content of the raw detail payload unspecified, which a consumer might want, but the JSON passthrough nature keeps that a minor omission.

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?

With a single parameter at 100% schema description coverage, the schema already carries the semantics (positive decimal string, obtainable from an activity listing). The description only restates "one activity ID" and adds no format, range, or sourcing detail beyond the schema, so 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 ("Read Garmin's activity detail response for one activity ID") and immediately contrasts it with three siblings that share the same underlying activity. An agent can distinguish this raw-detail read from the unit-labelled, laps, and samples variants without opening a 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?

Explicitly names three alternatives (garmin_get_activity, garmin_get_activity_laps, garmin_get_activity_samples) and the condition each one serves, implicitly defining when this tool is the right choice (raw detail for a single ID). This is genuine routing guidance rather than implied usage.

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