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

Goes well beyond the annotations: it discloses that exactly one unpaid request is sent, that no signing occurs, and gives explicit billing rules (any HTTP answer including 4xx/5xx is billed; bad input, blocked hosts, connection failures and timeouts are not), plus price and payment network constraints.

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 purpose and the no-payment guarantee before billing details. It is dense with several clauses, but each (billing rules, return fields, price, terms) carries operational value rather than filler.

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 no output schema, the description enumerates the returned fields (httpStatus, isX402, x402Version, decoded accepts, discovery metadata, pass/warn/fail checks), and it covers billing and payment requirements, leaving nothing essential for a correct call unstated.

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 url, body and method, including the 405-retry behavior. The description's parenthetical restates this rather than adding new meaning, making the baseline 3 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 (decodes) and precise resource (an x402 endpoint's 402 payment terms), with the key qualifier 'without paying'. It does not explicitly name the sibling x402_check_deep or distinguish shallow vs deep probing, so it falls short of full sibling differentiation.

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 'without paying / never pays or signs' framing clearly conveys the use case: inspecting payment terms before committing. However, it never states when to prefer x402_check_deep or http instead, so alternatives are left implicit.

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