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

Beyond the annotations, the description discloses critical behavioral traits: funds are not moved until the user's wallet pays the link, the system never pulls funds, credit is an entitlement rather than a stored balance, and it is never refunded in cash. It also explains network-dependent discounting and lists concrete refusal codes, which is substantial and useful disclosure for an agent.

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?

The description is front-loaded with the core purpose and every sentence carries meaningful behavior, return-value, or error-handling detail. It is long and dense, with some tangential specifics like the npm package version and docs reference, but these still support the payment invocation and do not feel like filler.

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?

There is no output schema, so the description carries the burden of explaining return fields. It fully covers credit_link, pay_with, credit_usdc, expires_at, and the credit's spending rules, and it documents the relevant refusal codes. Given the tool's financial complexity and sparse annotations, nothing needed to call and interpret the result is missing.

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 baseline is 3, but the description adds value beyond the schema by tying amount_usdc to the quoted credit_link and by revealing the six-decimal precision limit through the invalid_input/amount_out_of_range refusal. It does not need to restate idempotency_key semantics because the schema already documents them thoroughly.

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 opening sentence is explicit: 'Buy prepaid credit for hosting and the promoted slot (needs your API key).' It names a specific action and resource, and the surrounding detail about the x402 link and CLI command makes the tool's function unmistakable. It also distinguishes itself from sibling tools like myFees, withdraw, and promote by focusing on the prepaid-credit purchase flow.

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 gives clear context for when to use the tool: to prepay storage, delivery, and promoted-slot fees, and it even points to myFees() for reading the ledger afterward. It also states a prerequisite ('needs your API key') and the wallet restriction ('only the wallet your profile holds right now can pay it'). It does not explicitly enumerate alternatives or say 'do not use when...', so it stops just short of full routing guidance.

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