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 two useful facts beyond them: the payload is GPX bytes (not JSON), and it is fetched via client.download from the course-service gpx endpoint. It does not say how bytes are encoded or what happens if the course id is invalid.

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 short sentences, zero waste, with what-it-does front-loaded and the endpoint/mechanism as supporting 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 no output schema, the description usefully discloses the return type ('GPX bytes'). The remaining gap is minor but real: byte payloads must be encoded somehow (e.g. base64) for an MCP text response, and that is not stated.

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% (the single courseId parameter), so the schema already carries the load. The description only echoes the placeholder {courseId} and adds no format or id-source detail, so baseline 3 is correct.

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?

The description states a specific verb+resource+output format: 'Downloads a saved course as GPX bytes.' That distinguishes it from the richer get_course (structured course data) and from the unrelated download_activity/download_workout siblings, though it never names those alternatives explicitly.

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?

No when-to-use or when-not-to-use guidance. An agent must infer that this is for retrieving a course as a GPX file rather than as structured JSON (get_course) or as an import (import_course_gpx); nothing routes it between those options.

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