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

Annotations already declare readOnly, idempotent, non-destructive, openWorld, so the safety profile is covered. The description adds genuinely useful context beyond that: it is "Free", every failed check includes a fix, and verification mirrors what a facilitator checks (recipient, amount, time window, EIP-712 domain, nonce reuse, balance). It does not detail rate limits or output shape, but the output schema covers returns.

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 essential facts ("Free.", the decode action, the optional verify). It is one long sentence but every clause carries information; the only slight cost is density, where the parameter mappings could have been split for readability.

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, return values need not be explained, and the description covers both modes plus error behavior (warnings, fixes). The one implicit gap is that it never states that at least one of url/requirement/payment must be supplied, though the schema hints at this via "Instead of url".

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. The description adds meaning on top by mapping parameters to the real protocol artifacts (PAYMENT-REQUIRED header / 402 body for requirement, PAYMENT-SIGNATURE / X-PAYMENT for payment) and by framing url as the called-once-for-402 path, which clarifies the mutually exclusive inputs better than the schema alone.

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 specific verbs and resource: decode an x402 payment request and optionally verify a signed payment. The two input paths (request decoding vs. payment verification) are named explicitly, and none of the siblings (bridge_*, payment_readiness, stocks_*) overlap this debugging role.

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?

"Decode any x402 payment request... optionally verify a signed payment" gives clear context for both operating modes. It does not name exclusions or point to an alternative tool (e.g. payment_readiness) for cases where the caller wants a broader readiness verdict, so it stops short of full when/when-not guidance.

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