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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds real value beyond that: the data provenance ('our own unpaid probes'), the specific signals measured, and a concrete quota ('Free: 10 calls/day per client') an agent must budget for. It does not say what happens when the quota is exhausted or when a probe cannot reach the endpoint.

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?

Roughly three sentences, front-loaded with the output (score range and letter grade) and reasons, then the use case, then the quota. Every clause carries distinct information — no filler and nothing buried.

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 structure, and annotations cover safety; the remaining needs — provenance, cost, intended decision — are all addressed. Minor gap: no guidance on rate-limit exhaustion or on probing an endpoint that fails to respond, which matters for a pre-payment check.

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% for the single 'url' parameter, so the schema already documents it. The description's contribution is only the implied constraint that the URL must be a single x402 endpoint, with no format, scheme, or base-path guidance, so 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-like outcome (reputation score 0-100, A-F) scoped to exactly one x402 endpoint, plus what the score is composed of (uptime, terms validity, latency, price stability, sample size). That scope ('one endpoint') plus the 'seller reputation' framing separates it from x402_find_endpoint (discovery) and x402_status_leaderboard (aggregate ranking) without opening any 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?

Gives an explicit trigger: 'Use before paying an unfamiliar seller,' which tells the agent exactly the decision this informs. It stops short of when-not-to-use or naming a sibling alternative (e.g. the leaderboard for aggregate comparison), so it is clear context rather than full routing guidance.

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