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

Annotations already declare readOnly=true, idempotent=true, closed-world, non-destructive, so the safety profile is covered. The description adds real behavioral context beyond that: no authority grant, no payment, no execution, no proof verification, no delivery, no persistence, and that server credentials are never supplied. This is substantive, though the 'grants no authority' claim is restated three times rather than extended.

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?

Purpose is front-loaded in the first sentence, which is good, but the boundary statements repeat the same idea ('no authority', 'grants no authority', 'existing credentials remain required') across three sentences. The repetition pads length without adding new information, though the overall size remains moderate and no sentence is entirely wasted.

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 description is thorough about the operational boundary, which is genuinely needed for a checkout-adjacent tool, and no output schema exists so return values need not be explained. What it omits is anything about the six required parameters and, for a tool whose name says 'checkout', what the computation actually returns or how the result should be used downstream. Adequate but with clear gaps.

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 0% across 6 required parameters, including a nested items array and two nullable reference fields, so the description carries the full burden. It hints at items (via 'itemize'), USD currency, and caller-supplied amounts, but gives no meaning for mode, agent_id, payment_reference, or allowance_reference — notably why those two references are required in the schema yet described as unnecessary for this operation. It does not compensate for the coverage gap.

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?

States a concrete verb and resource: 'Itemize and total caller-supplied USD estimates for supported computations.' It also distinguishes itself from adjacent concepts by explicitly denying being a provider quote, checkout session, payment request or authority grant, which separates it from siblings like wever_x402-payment-gateway or wever_ap2-mandate-gateway. It stops short of naming which sibling to use instead, so it is clear but not fully sibling-routed.

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 'Operating boundary: Prepared computation on caller-supplied data only' line implies this is the tool for pre-computation of estimates, and the note that credentials/grants are required for any separate action implies this tool itself is not the action step. However, there is no explicit 'use this when X, use sibling Y when Z' guidance, which matters given the large set of adjacent payment/authority tools.

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