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

With no annotations, the description must carry the burden of behavioral disclosure. It adds context about cost/free-trial status, which is useful and contextually relevant for a paid x402 utility. However, it doesn't say whether the operation is read-only, whether it requires authentication, or what happens if an unknown extension is passed. It is not misleading, but it keeps disclosure shallow.

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 short and front-loaded: the core behavior is stated in the first sentence, and the cost/trial note is compact. The emoji and 'FREE utility' callout add marketing noise but don't bloat it. It earns its length overall, though the sibling differentiation that's missing would have added more value.

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 4 optional parameters with circular descriptions and no usage guidance among them, this simple-looking tool is actually ambiguous for an agent to invoke correctly. The missing details — whether parameters are alternatives, which to pass, and behavior on unknown extensions — are more important than the simple output. The cost note is a plus, but it does not compensate for the invocation ambiguity.

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%, but each parameter description is just a tautology ('Ext to process', 'Name to process', etc.) — the schema itself adds almost no meaning. The description mentions 'filename extension' as the source, but doesn't explain the relationship among the four parameters (e.g., are they alternatives? precedence? required in any combination?). With 0 required params, an agent can't tell which field(s) to supply, which is a significant 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 clearly states the task: 'Detect MIME type from filename extension.' It uses a specific verb and resource, and the emoji/label signals it's a lightweight utility. However, it doesn't differentiate from closely named siblings like x402-mime and x402-mime-detect, which likely perform the same or overlapping operation, so an agent cannot be confident which one to pick based on the description alone.

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 gives no guidance on when to use this tool versus alternatives (x402-mime, x402-mime-detect) and no context on when this is the right choice. The schema has 4 optional parameters (ext, name, filename, extension) with none required, but the description doesn't clarify whether they're alternatives or which one to prefer. The 'free trial calls per registered wallet' note is informative for cost, not for usage routing.

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