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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds genuinely useful behavioral context beyond that: the call is free, it performs a live probe against the seller's endpoint, and the full report is gated behind a paid call. It omits latency/timeout and rate-limit behavior, so not a 5.

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?

Two sentences, front-loaded with the free-check benefit and verdict output before the paid upsell. The parenthetical enumerating the paid report's contents ('every payment option, cheapest payable, Paddock/Settled numbers') is somewhat dense but earns its place by explaining the upgrade decision.

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 correctly carries the return-value burden by naming the verdicts and score bucket. Combined with annotations covering read-only and open-world behavior, an agent has what it needs to decide and call; only failure modes and timing behavior are unaddressed.

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% with only two parameters (url, method) fully described in the schema, so the baseline is 3. The description adds no syntax or format detail for url/method and does not explain the enum choice beyond what the schema already says.

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?

The description names a specific verb+resource ('probes the seller's live 402 challenge and settlement reputation') and a concrete output shape ('PAY / CAUTION / AVOID with a score bucket and first reasons'). The x402-endpoint resource is clearly distinct from the sibling tools (swap_guard_preview, token_risk_preview, paid_endpoints), so an agent can route without opening a 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?

It states the usage context explicitly ('before your agent pays any x402 endpoint') and names the upgrade path to the paid /x402-preflight report at $0.01 USDC on Solana or Base, with the condition that selects it. It does not, however, give any when-not guidance (e.g. when token_risk_preview or swap_guard_preview would be the better preflight).

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