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

The readOnlyHint=true annotation already covers the safety profile, so the description's added value is the disclosure that the lookup reports whether and how the invoice can be paid via x402 (USDC on Base) — non-obvious behavioral context. It does not mention auth requirements or rate limiting, but the annotation carries the safety burden.

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?

One sentence, front-loaded with the verb and resource, ending with the most distinctive detail (x402 payability). No filler or repetition.

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, yet the description compensates by enumerating the returned fields (seller, amount, status, payment capability), and the read-only annotation covers the safety dimension. Nothing needed to call it correctly 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 coverage is 100%, so the single parameter (public token UUID or Billify link) is already fully documented. The description's phrase 'from its public link' restates rather than extends that, so baseline 3 is appropriate.

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?

Specific verb ('look up') plus resource ('Billify invoice') and scope ('from its public link'), with the returned fields enumerated (seller, amount, status, x402 payability). It is clearly a read/lookup operation distinct from the mutating pay_invoice, but the description never names or contrasts the siblings, so differentiation is inferred rather than stated.

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?

Usage is only implied: the read-only lookup of an invoice naturally precedes pay_invoice, and the x402 payability detail hints at that flow, but there is no explicit when-to-use, when-not-to-use, or named alternative. An agent must infer the ordering itself.

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