fulfill_payment

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

With no annotations, the description carries the burden and does well: it declares the tool does not create an invoice or send money (non-destructive, idempotent-looking recovery), and requires artifacts from a prior call. It doesn't state auth requirements or what happens if the inputs are stale/invalid, so a small gap remains.

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 sentences, tightly front-loaded with the 'Recovery only' scope before the requirements and the negative guidance. Every sentence earns its place.

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?

No annotations and no output schema, so the description must stand alone; it covers purpose, prerequisites, and exclusions well. It stops short of describing what the returned VPN credentials look like or how errors/failed recovery are surfaced, and it ignores the optional country field.

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 0%, so the description must compensate, and it does partially: it names the four required parameters and tells the agent where they come from (prior buy_vpn/sign_and_pay). It adds no format or validation detail for invoice_no, payment_signature, settlement_tx, and never mentions the optional 'country' parameter.

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 (exchange) and resource (already-broadcast payment → VPN credentials) plus an explicit scope qualifier ('Recovery only'). It also distinguishes itself from siblings by naming buy_vpn/sign_and_pay as the source of the required inputs.

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?

Explicitly states when to use it ('when fulfill failed after payment') and when not to ('never call buy_vpn again', 'does not create a new invoice or send money'). The alternative is named and the triggering condition is given, leaving nothing to inference.

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