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

With annotations declaring readOnlyHint/openWorldHint, the description still adds substantive behavior the annotations cannot: the x402 payment requirement, the isError + PaymentRequired response on unpaid calls with a fresh invoiceId, the retry path via _meta["x402/payment"] or payment_signature, and the explicit 'measurement, not advice or a quality rating' framing. No contradiction with readOnlyHint (it is a read) or idempotentHint=false (fresh invoiceId per call).

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?

Every sentence carries load: payment mechanism, usage trigger, return shape, and payment-failure/retry behavior. It is dense but not padded. Minor deduction because it opens with pricing rather than the tool's purpose, so the core value proposition is not strictly front-loaded.

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 9-parameter, no-output-schema tool with a non-trivial x402 payment flow, the description covers the return fields (pick, fallback vendor, reasons, excluded counts, sources/times) and the full payment/retry lifecycle. An agent has everything needed to call it correctly and recover from the unpaid error.

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 all nine parameters are already documented in the schema. The description only references the payment_signature argument and the output `pick` field, adding no new syntax or semantics beyond the schema. Baseline 3 is appropriate.

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 states a specific verb+resource: it returns the lowest listed price (OpenRouter listing) among models meeting hard needs, and names the output field `pick`. This is clearly distinguishable from siblings like list_models, get_model_report, or get_price_index, which do not do constraint-filtered cheapest selection.

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?

It gives an explicit trigger: 'Use when an agent needs the lowest listed price among models meeting hard needs.' The framing as 'a pre-flight price reading' reinforces when it applies. It does not name an alternative sibling or state when-not-to-use, so it stops short of a 5.

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