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

Annotations already establish readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds useful behavioral context by stating it 'Returns track names, artists, and play counts' and by reinforcing the constraint against guessed usernames. It doesn't mention rate limits or error handling, but for a simple read operation with strong annotation coverage, the additional context is sufficient.

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?

The description is a single long sentence followed by a short second sentence. It's dense but every clause adds value: the scope, period values, usage restrictions, and return contents are all packed in without fluff. It's slightly verbose but not to the point of being bloated.

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?

The tool has only three parameters with one required, and an output schema exists, so the description needn't explain return structure in depth. The description covers the main scenarios, restrictions, and period options. It's complete for an agent to decide when to use it and what to expect.

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?

The schema only describes the 'user' parameter, so coverage is low. The description compensates by explicitly listing the accepted period values (overall/7day/1month/3month/6month/12month) and clarifying that 'user' must come from the caller, not be guessed. The 'limit' parameter isn't described, but it's a standard concept and the description's partial compensation keeps this from being a major gap.

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 uses the specific verb 'Fetch' and names the exact resource: 'a SPECIFIC NAMED Last.fm user's most-scrobbled tracks'. It also distinguishes itself from siblings by explicitly stating 'only when the question names a Last.fm username' and contrasting with chart_top_artists and tag_top_artists.

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 description provides explicit when-to-use criteria ('only when the question names a Last.fm username') and clear exclusions ('NEVER for general popular songs / top music questions'), even naming the alternative tools to use instead. It also warns against using guessed or example usernames.

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