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

With no annotations, the description carries full responsibility. It discloses the paid nature, the two-step payment mechanism, amount limits, and the immediate grace-clearing effect. This is substantial behavioral context for a payment tool, though it doesn't describe failure modes or refunds.

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 two sentences with no filler. It front-loads the critical 'PAID' and payment flow, then efficiently covers rates, limits, and side effect. Every clause adds distinct value.

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 3-parameter, no-output-schema tool with no annotations, the description covers the payment flow, constraints, and behavioral side effect. It doesn't describe return values or error handling, but the two-step flow implicitly addresses the requirements-retrieval step, making it reasonably complete.

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 33% (only 'address' described). The description compensates by explaining that 'payment_b64' is omitted in the first call and used in the second, and by giving minimum/cap context for 'amount_microusd' ($1 min, $100 balance cap). However, the exact micro-to-dollar conversion is not explicit, leaving some ambiguity.

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 clearly states the tool's purpose: adding prepaid credit for checkpoint storage, with a specific verb ('top up') and resource ('storage credit'). It distinguishes itself from siblings like locker_send (which has a similar payment flow but different outcome) and locker_status, making the function unambiguous.

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 provides explicit usage guidance for the x402 payment flow ('call without payment_b64 for requirements, retry with it'), as well as constraints (minimum $1, balance cap $100) and a side effect (clears read-only grace). It doesn't explicitly name alternatives or exclusions, but the tool's unique role among siblings makes the usage context clear.

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