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

Annotations already declare read-only/idempotent/non-destructive, and the description adds material context beyond them: the $0.01 x402 pricing on Base, the prepaid x-credit-token header path, the 1-free-call/day allowance, and 'No API key required.' These auth/cost details are exactly the kind of behavioral disclosure an agent needs before calling.

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-loads the core operation and an example before the commercial details, with every sentence carrying information. The pricing/credit sentences are relevant but slightly expand the block; still well-structured and appropriately sized.

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?

With an output schema present, the description needn't explain return values, and it covers purpose, format example, inverse-sibling relation, and payment/auth requirements. Nothing an agent needs to invoke it correctly is missing.

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 coverage is 100%, so the baseline is 3, but the description reinforces the single required input and, more usefully, demonstrates the expected signature format (transfer(address,uint256)) which clarifies syntax the schema label 'Function signature' leaves implicit.

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 ('Computes the 4-byte selector') and resource ('function signature') with a concrete input→output example (transfer(address,uint256) → 0xa9059cbb). It also names its relationship to the decoder sibling ('the inverse of the decoder'), so an agent can distinguish it from decode_selector without opening either schema.

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?

Provides clear context ('useful for agents building or matching calldata') and implicitly routes the inverse case to decode_selector. It lacks an explicit when-not or a direct 'use X instead' instruction, so it stops short of full 5-level guidance.

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