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

With no annotations, the description carries the burden. It discloses the auth requirement ('Proof of npub ownership is required') and its purpose, plus the free cost. However, it does not mention whether the operation is read-only, any side effects, or behavior on failure/pending statuses. The 'check' verb implies read-only, but not explicitly.

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 short paragraphs, front-loaded with the main purpose and followed by essential usage and auth context. Every sentence earns its place with no verbosity.

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?

Given the tool's simplicity and the presence of an output schema, the description covers the essential pieces: purpose, timing, cost, and auth. It doesn't discuss error states or return value interpretation, but those are less critical given the output schema and the 'check' semantics.

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 baseline is 3. The description adds marginal context: it links invoice_id to purchase_credits and explains the npub/dpop_token auth purpose. But it doesn't add syntax-level detail beyond what the schema already provides.

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 opens with 'Check the payment status of a Lightning invoice' — a specific verb and resource that clearly distinguishes it from sibling tools like check_balance or check_price. It leaves no ambiguity about what the tool does.

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?

It explicitly states when to call: 'Call after paying the invoice from purchase_credits,' and provides cost context ('Free — no credits required'). It does not explicitly mention alternatives or when not to use, but the usage context is clear enough.

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