x402 Preflight Trust Check

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

With no annotations provided, the description carries the full burden of disclosing behavior. It explicitly states the call is free, requires no credentials, and returns a detailed verdict, including the recommendation and the fact that paid-tier fields are null. This is thorough and transparent about the tool's non-invasive, read-only nature.

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 description is well-structured, starting with a benefit statement and then enumerating the key return fields and limitations. It's slightly dense but every sentence adds value, and it avoids unnecessary fluff. It could be more concise, but the structure is effective.

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?

Given that there is an output schema (not shown here) and no annotations, the description is remarkably complete. It explains the purpose, the exact checks performed, the returned recommendation, and the free-tier limitations. This provides sufficient context for an agent to decide when and how to invoke the tool, and what to expect in return.

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 input schema already provides 100% coverage for the single 'url' parameter with a clear description. The tool description does not add new semantic information about the parameter itself, but it does contextualize its use (probes the URL). This aligns with the baseline of 3 for full schema coverage.

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 clearly states a specific action: 'Probes the URL' and returns a 'trust-preview.v1 verdict' with detailed criteria. It distinguishes itself as a preflight check before payment, which is a unique and well-defined purpose.

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 description opens with 'One free call before your agent pays,' which clearly indicates when to use the tool. It also notes that no wallet, payment, or auth is involved, implying it's safe for initial checks. However, it does not explicitly mention when not to use it or provide alternatives, since no sibling tools exist.

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