user_get_friends

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

readOnlyHint=true already establishes the safety profile, so the description needn't argue it is a safe read. The description adds useful context about what the response contains (usernames usable elsewhere), but says nothing about result ordering, pagination behavior across the page/limit pair, or what happens for a user with no friends — the latter matters since there is no output schema.

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 with zero filler; the resource identification is front-loaded and the downstream usage hint follows immediately. Nothing is repeated from the schema or name.

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 simple, parameter-light read tool with no output schema, the description covers what the tool returns (usernames) and how to use them downstream. The remaining gap is pagination/ordering semantics, which is partially mitigated by the fully documented page and limit parameters.

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%, with page, limit, and the LASTFM_USERNAME default behavior all documented in the schema itself. The description adds no parameter-level meaning beyond that, 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?

The description names a specific resource ('The user's Last.fm friends') and implicitly the retrieval operation, which is clearly distinct from any sibling (no other tool returns a user's friend list). The verb is omitted — it reads as a noun phrase rather than 'List the user's friends' — but the intent is unambiguous.

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?

The second sentence provides real chaining guidance: 'Their usernames can be passed to the other user_* tools,' telling the agent this is a discovery/enumeration tool that feeds the other user_* queries. It gives no explicit when-not or alternative conditions, but no sibling performs this function, so the gap is minor.

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