Last.fm MCP Server

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It does well by mentioning authentication requirements, which is crucial behavioral context. However, it doesn't disclose other important traits like rate limits, error conditions, whether this is a write operation (implied but not stated), or what happens with duplicate tags. The confirmation statement is helpful but vague about format.

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 perfectly structured and economical. The first sentence states the core purpose, the second provides crucial behavioral context (authentication), and the Args/Returns sections are cleanly formatted. Every sentence earns its place with no wasted words or redundancy.

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?

Given this is a mutation tool with no annotations but with an output schema, the description provides good coverage. It explains authentication requirements, documents all parameters meaningfully, and mentions the return value. The main gap is lack of explicit confirmation that this is a write operation and missing details about error handling or rate limits, but the output schema will handle return format details.

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?

With 0% schema description coverage, the description must compensate, and it does so effectively by explaining all three parameters: 'artist: Artist name', 'track: Track name', and 'tags: Comma-separated list of tags (max 10)'. The tags parameter gets particularly valuable semantic context with the comma-separated format and maximum limit. This adds significant meaning beyond the bare schema.

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 clearly states the action ('Add tags') and target resource ('to a track on your Last.fm profile'), making the purpose immediately understandable. It distinguishes itself from siblings like 'remove_tag' by specifying addition rather than removal. However, it doesn't explicitly differentiate from other tagging-related tools like 'get_track_top_tags' beyond the verb difference.

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 usage guidance by stating 'Requires authentication - use authenticate_user() and get_session() first.' This clearly indicates prerequisites and references specific sibling tools to use beforehand. It establishes a clear sequence of operations for proper tool invocation.

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