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

Goes well past the annotations: discloses that nothing is charged at call time, exactly what is returned (checkout link, card payment page, top-up id), the credit conversion rate, the required credential and scope (Last Price with price:compute), and the possibility of a refusal with alternatives. That is the full behavioral picture for a mutation-ish creation 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?

Purpose is front-loaded and every sentence carries information (amount format, no-charge guarantee, return shape, polling loop, availability, auth). It is on the long side and the critical 'Nothing is charged by this call' sits mid-paragraph rather than up front.

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?

No output schema exists, and the description compensates by naming all three return values and the follow-up polling tool. Combined with the auth scope and availability caveat, an agent has everything needed to call and then complete the flow.

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 coverage is 100% and the schema already documents the amount format and pattern, so baseline is 3; the description adds meaning beyond the schema by tying the amount to the credit rate ('one USDC per dollar') and restating the whole-cents convention. It does not add new syntax, but the conversion semantics are genuine value.

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 specific verb+resource ('Add to your workspace's prepaid balance') plus the mechanism ('by card') and the condition that selects it over the x402/crypto path. An agent can tell this apart from top_up_balance without opening either schema.

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?

Gives the when ('no crypto wallet at hand to sign an x402 payment'), the follow-up workflow ('Give the link to the person who runs you... Poll get_card_topup with the top-up id until its status is paid'), and the fallback ('a refusal lists the other ways to pay'). Exclusions and alternatives are explicit.

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