Check that an x402 endpoint's 402 challenge is well-formed before paying it

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

With no annotations, the description carries the full burden. It discloses the monetary cost (0.010000 USDC per call on Base), the automatic payment behavior when EVM_PRIVATE_KEY is set, and the fallback behavior of returning 402 payment requirements. It does not detail what constitutes 'well-formed' or error cases, leaving some gaps.

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 with zero waste, front-loading the purpose and then the payment details. Every sentence earns its place.

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?

For a simple tool with a single parameter and no output schema, the description covers the essential operational context: cost, payment flow, and fallback. It could be more explicit about what the check returns or how 'well-formed' is evaluated, but the coverage is largely adequate.

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 single url parameter is fully documented in the schema. The description adds no syntax, format, or constraint beyond what the schema already provides, matching the baseline of 3 when the schema does the heavy lifting.

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 ('Check') and resource ('x402 endpoint's 402 challenge') and clarifies intent ('before paying it'). This clearly distinguishes it from the data-retrieval siblings (earthquakes, weather, worldbank) that share the selectarank_ prefix.

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 phrase 'before paying it' gives a clear contextual trigger for when to use the tool. It does not state exclusions or name alternatives, but no sibling overlaps exist, so the guidance is sufficient though not exhaustive.

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