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

No contradiction with the destructiveHint=true annotation; charging money is exactly a destructive, irreversible operation. The description goes well beyond the annotations by disclosing that only the supplied SPT is charged (never a saved card), the 402 challenge handshake, idempotency-key secrecy and reuse rules, and that success yields an access_token. It stops short of stating charge amounts, refund behavior, or what a failed retry does.

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?

Information-dense and front-loaded: the buy action comes first, then the payment handshake, then the idempotency caveat, then the return value. A few clauses (e.g., the 'secret' warning) could be tightened, but no sentence is redundant given the payment stakes.

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 destructive, open-world payment tool with no output schema and low schema coverage, this covers the critical unknowns: the challenge flow, idempotency, the charge scope, and the returned access_token. The remaining gap is tokenHours and credit-amount semantics, which an agent would have to read from 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 coverage is only 20%, so the description must carry the load. It explains idempotencyKey semantics very well (random UUID, secret, reuse for retries/status only) and clarifies paymentCredential's role in the challenge flow, but it says nothing about tokenHours (despite its default and 1–720 range) and gives no credit-quantity bounds. Roughly three of five parameters are enriched; two are left to the 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 states a concrete verb and resource — buying API lookup credits or a one-off plan — and names the payment rail (Stripe MPP card). The 'MPP' framing implicitly separates it from the x402/payment-purchase siblings, but it never names them explicitly, so the differentiation is inferential rather than stated.

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 unusually precise two-step protocol: call without paymentCredential to receive the 402 challenge, authorize with an MPP-compatible wallet, then retry the IDENTICAL purchase with paymentCredential. What is missing is any statement of when to prefer this over x402-credit-purchase or payment-purchase, or prerequisites like an existing balance.

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