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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds the one non-obvious behavior: the return payload is raw FIT bytes rather than JSON, which matters because there is no output schema. It says nothing about error behavior, whether workouts must be scheduled/owned, or size limits.

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?

One sentence, zero filler, and the most decision-relevant fact (FIT-file bytes) is placed at the end of a short clause where it is still immediately visible.

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 one-parameter download with no output schema, the description supplies the crucial return-format information and annotations cover safety. The remaining gap is routing: it never says how this differs from get_workout_by_id or preview_workout.

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% for the single required workoutId (typed integer|string, 'Numeric id'), so the schema does the work. The description adds no clarification on the identifier's source or format, which is the baseline expectation when schema coverage is high.

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 (downloads) and resource (workout) plus the payload form (FIT-file bytes), which separates it from read-shaped siblings like get_workout_by_id and preview_workout. It does not name those siblings explicitly, so the differentiation is inferred from the output format rather than stated.

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?

There is no guidance on when to use this versus get_workout_by_id, preview_workout, or download_activity. The agent is left to infer that this is the export path for a workout object.

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