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

Discloses behavior no annotation covers: a first call without payment returns an x402 PaymentRequired error, the payment must be an EIP-3009 signed transfer passed in _meta['x402/payment'], gas is paid by the facilitator, and the amount range is bounded. This is exactly the non-obvious sequencing an agent needs to avoid mis-calling.

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?

Three tight sentences, front-loaded with purpose and mechanism, then the call protocol, then cost/range details. Dense but every clause carries operational information; nothing is redundant 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?

With one parameter, no output schema, and rich annotations, the description covers the full calling protocol including the error path. Minor omissions remain (credit timing, payment expiry/retry semantics) but they are not blocking for correct invocation.

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 description repeats the range, but it adds a conversion semantic the schema lacks: 1 USDC adds $1 (and no gas cost), which tells the agent how to compute the amount_usd 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 (add money to balance) and resource (balance via USDC on Base) and names the exact mechanism (x402). It is immediately distinguishable from the sibling create_top_up_link because it emphasizes self-payment from a crypto wallet rather than a hosted link.

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 an explicit when-to-use condition ('for assistants that hold a crypto wallet') and lays out the two-call sequence in detail. It does not name create_top_up_link as the alternative for non-wallet assistants, so routing between top-up methods is left partly to inference.

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