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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the tool's safe inspection behavior is largely established. The description adds important behavioral context that is not in annotations, explicitly warning 'without spending' and indicating it probes for 'price, network, and payTo'. It doesn't state possible error responses or network dependencies, but annotations plus this disclosure adequately cover core behavioral expectations.

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 economically written sentences convey purpose, target behavior, key outputs, and the relationship to a sibling tool. Every phrase earns its place, and the main action is front-loaded so the agent immediately understands what the tool does.

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 read-only probe with a full input schema, rich output schema, and explicit annotations, this description is complete. It explains what will (price, network, payTo), what will not happen (no spending), and when to use it (before pay_and_fetch). An agent has everything needed to call this correctly without opening the schema.

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 description coverage is 100%, so the parameters' meanings are fully documented in the schema itself. The description adds no parameter-specific detail beyond what the schema already provides, which matches the baseline of 3 for a highly documented schema.

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 uses the specific verb 'probe' with the resource 'URL' and explicitly defines HTTP 402 PAYMENT-REQUIRED terms, which includes what data is inspected (price, network, payTo). It clearly stands apart from its sibling tools by narrowing the action to a pre-payment inspection step, making the tool's intent unambiguous.

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 explicitly states when to use the tool: 'Use before x402.pay_and_fetch' and what it accomplishes ('inspect... without spending'). It does not explicitly mention cases where the tool should not be used or call out alternative siblings, which keeps this from a 5, but the single-sentence context is clear and actionable.

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