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

Annotations declare readOnlyHint/openWorldHint, and the description goes well beyond them: it guarantees the endpoint is never paid, discloses the exact cost ($0.002 USDC per call via x402 or prepaid credits), notes free-trial availability, and lists the data returned. Cost and non-mutation guarantee are exactly the operational facts an agent needs before invoking a paid probe.

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 the core action and the 'never pays' guarantee before the long output enumeration, and the pricing sentences are compact. The middle list of checked fields is dense but each item earns its place by defining the tool's output.

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 need not explain return values, and it still summarizes them; combined with the cost, safety guarantee, and usage context, an agent has everything needed to call this tool 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?

The 2-parameter schema has 100% description coverage, including a documented default and enum for method, so the schema carries the parameter burden. The prose does not add format or syntax detail beyond what the schema states, making the baseline of 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 and resource ('Live health check of any x402 endpoint') and enumerates exactly what is verified: 402 response, payment request validity, prices, pay-to wallets, x402 version, Bazaar metadata, response time, buyers. This clearly separates it from siblings like x402-find, x402-recommend, and trade-precheck.

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?

Explicitly frames the use case and timing ('before you pay it') and adds a key constraint ('Never pays the endpoint'). It does not name a specific alternative tool or a when-not-to-use condition, so it stops short of full routing guidance, but the context for invocation is clear.

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