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

With no annotations, the description carries the full burden, and it does disclose one real behavioral trait: unknown extensions fall back to application/octet-stream. It also mentions the paid-call model (5 free trial calls per signed wallet). It does not say what happens with malformed input, whether lookups are case-sensitive, or what error/response shape to expect.

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?

Front-loaded and short: the purpose and default behavior come first in a single sentence. The emoji-plus-pricing tail is marketing noise that does not help an agent invoke the tool, but it costs little.

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 trivial lookup this is close, but with no annotations, no output schema, and three entirely undocumented parameters, an agent cannot tell whether to send 'ext', 'extension', or 'input'. That gap is exactly what the description should have closed.

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 0% and the description adds no parameter detail beyond the loose phrase 'file extension'. The schema exposes three overlapping candidates (ext, input, extension) with none documented, so the description does not resolve which one to populate — a genuine ambiguity for a 3-parameter tool.

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 ('Maps a file extension to its MIME type') with the fallback behavior, so the operation is unambiguous. However, it does nothing to distinguish itself from near-identical siblings such as x402-mime, x402-mime-type, x402-mime-detect, and x402-file-ext-info, leaving an agent to guess which of the clones to call.

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?

There is no when-to-use guidance, no statement of when this is preferable to the many MIME/ext siblings, and no prerequisites. The only contextual note is a pricing tagline about free trial calls, which is commercial framing rather than usage guidance.

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