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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds genuinely new behavioral context: it discloses the empty-result case ("Empty when the user pinned nothing") and clarifies that only public-profile pins are returned, which prevents an agent from misreading an empty array as a failure.

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 sentences, purpose front-loaded, with the edge case and the sibling pointer following in priority order. The "username: GitHub username" clause duplicates the schema and is the one line that does not earn its place.

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, read-only lookup with an output schema already defining the return shape and annotations covering safety, the description covers purpose, edge case, and the alternative tool. Nothing critical is missing, though a note on the ordering/limits of returned pins would round it out.

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 parameter, so the schema already defines username. The description's "username: GitHub username" is a restatement rather than added meaning, so the 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 ("Returns the repositories a user pinned on their public profile") and adds the distinguishing semantic frame "showcase signal". It also names the sibling github_user_repos, so an agent can differentiate pinned repos from a full public repo listing without opening either 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 points to github_user_repos as the alternative for listing a user's public repositories, giving a clear routing signal. It stops short of stating the full when-not condition (e.g., "use this only when you need the curated pinned subset"), but the contrast is clear enough to select correctly.

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