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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=false), and the description adds genuinely new behavioral facts: it is a paid call at $0.01 USDC after 25 free calls per day, and all checks are consolidated into one verdict rather than separate calls. It stops short of describing what the verdict response contains, which matters because there is no output schema.

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?

The precondition is front-loaded in the first clause and the enumerated checks are packed into one dense sentence with no wasted lead-in. "One call, one price" borders on marketing filler but does convey the batch-verdict and cost model.

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 five-parameter read-only checker with a fully documented schema and no output schema, the description covers the trigger, the full scope of checks, the cost model and the free allowance. An agent has everything needed to decide to call it and to assemble a correct invocation.

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 every parameter including _payment, maxPrice, knownPayees and requestedUrl is already documented in the schema. The description only reinforces paymentRequired ("pass the 402 payment request you received") and the maxPrice concept via "checks the price against your limit" — baseline 3 is appropriate 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?

Specific verb-and-resource: pass a received 402 payment request, get a single verdict, with the individual checks enumerated (price limit, USDC/token legitimacy, payee address, site mismatch, domain reputation). It is unmistakable what the tool does, but it never distinguishes itself from the sibling tools address_check and link_check, whose subject matter it subsumes.

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?

"Before you sign an x402 payment to another service" gives a clear triggering condition and an implicit ordering constraint relative to signing. There are no exclusions and no routing advice telling the agent when to prefer address_check or link_check for a narrower question.

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