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

Beyond the annotations (readOnlyHint=false, openWorldHint=true, destructiveHint=false), it discloses that no account or card is needed, that the first call returns PAYMENT_REQUIRED with x402 requirements, that credits are added on the second call, and that the uc_live_ API key is shown only once but stored for the session. That is substantive behavioral context a mutation tool needs.

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?

Front-loaded with the outcome and protocol, then the two steps in order, then the pack source and the spend constraint. Every clause carries actionable information; nothing is padding.

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?

Even with no output schema, the description covers the returns (PAYMENT_REQUIRED payload, uc_live_ key shown once) and the full call flow, which is everything an agent needs to invoke a paid, two-phase tool correctly.

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 schema already documents all three parameters, including that pack accepts packageId or starter/plus/pro aliases and that acceptLawfulUse must be true. The description restates the same semantics and mainly adds workflow ordering rather than new per-parameter meaning, so baseline 3 is correct.

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 (buy a prepaid credit pack) plus the payment rail (USDC on Base via x402), which cleanly separates it from the stripe/nowpayments credit siblings. An agent can identify the tool without opening the 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?

Spells out the exact two-step call sequence (first without paymentHeader, then with the signed header), points to ucode_agent_billing as the source of pack ids, and adds the governance rule 'Only spend what the user approved.' When-to-use and prerequisites are both explicit.

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