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

Annotations already mark the call read-only, idempotent, and non-destructive, and the description adds substantial behavioral context beyond them: it relies on an external service, returns candidate (not guaranteed) signatures, costs $0.01 per call over x402 on Base, supports a prepaid header for unlimited calls, and offers one free call per day. This covers auth, rate, and cost traits the annotations do not.

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 core purpose is front-loaded in the first clause, followed by input requirements and operational details. It is dense but every clause contributes value; only minor redundancy exists with the schema's required selector field.

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 single-parameter, read-only lookup tool with an output schema, the description is complete: it names the external data source, input formats, required field, cost, auth model, rate limit, and lack of required wallet/API key. Nothing critical is missing for an agent to invoke it correctly.

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 100% schema coverage, the baseline is 3, but the description adds meaning by stating that the selector parameter accepts either a bare selector or full calldata and by giving a concrete example (0x70a08231). This goes beyond the schema's 'Function selector' label without specifying every format edge case.

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 opens with a concrete verb and resource: 'Turn a function selector into readable signatures.' It specifies the 4-byte input, an example, the external source (4byte.directory), and the output type, making its job unambiguous and naturally distinct from inverse/related siblings like encode_selector.

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?

It gives clear operational prerequisites: required selector, accepted bare selector or full calldata, no wallet/API key, and pricing. However, it never explicitly contrasts this tool with siblings such as tx_decode or encode_selector, so the when-vs-alternatives guidance is only implied rather than stated.

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