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

Goes well beyond the annotations by disclosing that nothing is charged/paid for the check, that exactly one unpaid request is made (GET, or POST on 404/405), and the exact error semantics: 422 for private/non-https, 503 for unreadable index or own network failure (not charged), and a non-answering endpoint counts as a charged result. It also adds cost (USD 0.001), a free tier (10 index calls/IP/UTC day), latency expectations (0.5-3 s, up to ~12 s), the caveat that this is automated/heuristic and not an audit, and a prompt-injection warning about untrusted page text and on-chain strings.

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 verdict and the sibling routing, and nearly every clause carries operational information (cost, error codes, latency, trust warning). It is dense and semicolon-chained toward the end, which costs some readability, but little is genuinely expendable.

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?

An output schema exists, yet the description still sketches the shape of the answer (reasons with evidence from the live 402 and the agent index: payTo history, price, listing age, domain-published payTo, farm score) and the added `policy` block. Combined with pricing, failure modes, timing, and the trust caveat, an agent has everything needed to decide and invoke 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?

Schema coverage is 100%, so the baseline is 3; the description nevertheless adds non-obvious semantics, notably that `policy` is judged on what was already read rather than triggering extra reads, and that a bad policy is a 422 and not charged. The `url` constraints are already fully documented in the schema, so no extra credit there.

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 an x402 endpoint before paying it') and immediately gives the verdict taxonomy (pay, caution or avoid). It actively distinguishes itself from two siblings by name (x402_seller_report for sellers, benchmark_x402_price for listed prices), so an agent can route 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?

Explicit when-to-use ('before paying it') plus explicit alternatives with the condition that selects them ('For a seller checking their own listing use x402_seller_report; for listed prices alone use benchmark_x402_price'). The optional `policy` clause is also framed as conditional ('adds policy.result pass or fail').

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