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

With no annotations, the description carries the full burden and does so well: it discloses the validation pipeline, the diagnostic behavior on signature failure (naming the domain the signature was actually made for, with a concrete Base Sepolia vs Base example), and the cost model ($0.003 per call, first 3 priced calls per caller free). Pricing and rate-limit-like information are exactly the behavioral traits an agent needs and cannot get from structured fields.

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 with the action and its purpose, followed by the failure behavior and then pricing. Two dense sentences with little waste, though the domain-mismatch example is somewhat long relative to the rest.

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 six-parameter, nested-object tool with no annotations and no output schema, the description covers purpose, validation behavior, failure diagnostics, and cost. It stops short of describing the success return value or whether the call has any side effects, but the essential calling context is present.

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%, so the schema already documents all six parameters including the x_payment/authorization alternatives. The description reinforces the seller-side check fields (payTo, amount, asset, network) and the alternative input modes but adds no syntax, format, or constraint detail beyond what the schema provides. Baseline 3 is appropriate.

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 ('Check a signed x402 USDC payment') plus the exact moment it applies ('before submitting it'). The description also enumerates the concrete validation work performed (EIP-712 signer recovery, USDC's own checks, seller's payTo/amount/asset/network), so an agent knows precisely what this tool does and how it differs from the pipeline_* siblings.

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?

'before submitting it' establishes a clear usage context, and the parameter note 'Instead of x_payment: from, to, ...' tells the agent which input mode to pick. It stops short of naming explicit alternatives or when-not-to-use conditions, but the invocation context is unambiguous.

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