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

Annotations only indicate readOnlyHint false, openWorldHint true, etc., but the description discloses the critical payment behavior: an unpaid attempt yields a Payment Required payload, and a retry with payment_signature completes the call. It does not cover failure modes beyond payment but sufficiently reveals the tool's non-read-only, payment-gated 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?

Two sentences with minimal redundancy. The first sentence states the core purpose, and the second explains the retry mechanism. Every word earns its place; no filler.

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?

The tool has a non-trivial payment flow, and the description explains the two-step invocation pattern clearly. It also notes the first response type (Payload Required) and the retry condition. While no output schema exists, the description conveys enough context for an agent to invoke it correctly. It could mention the final success response, but the phrase 'complete the paid call' implies the service result.

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 describes all four parameters, so the baseline is 3. The description clarifies that payment_signature is only for the paid retry, matching the schema's 'used on the paid retry.' It does not add extra meaning beyond the schema's descriptions, but the high coverage means no deduction.

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 opens with 'Call one stable x402 service by slug,' a specific verb and resource that clearly identifies the tool's action. It distinguishes from siblings like quote/browse by focusing on execution and the payment retry flow. The title 'Call x402 Service' reinforces this 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 provides clear guidance on the two-step payment flow: first unpaid attempt returns Payment Required, then retry with payment_signature. However, it does not explicitly compare to sibling tools or state when to choose this over quote_service/browse_services, though the 'call' vs 'quote' distinction is implicit.

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