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

With no annotations provided, the description carries the full behavioral burden and delivers: cost behavior ('Free — no credits required'), an auth requirement ('Proof of npub ownership is required'), and the security rationale ('to prevent credit-grant front-running by an observer of the invoice ID'). This explains why the oddly strict dpop_token parameter exists, which is genuinely useful context. It doesn't disclose failure-mode behavior, but the disclosures present are substantial.

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?

Four sentences, with purpose front-loaded first, then usage timing, cost, and auth rationale. Every sentence earns its place; the only minor waste is 'Free — no credits required,' which says the same thing twice. The structure is efficient and well-ordered.

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 tool, the description covers the full call context: what it does, when to call it (after purchase_credits), cost, and the auth prerequisite. Combined with 100% schema parameter coverage and an existing output schema, nothing essential for invoking it correctly is missing. The only gap is clarity on edge outcomes (e.g., unpaid/expired invoice behavior), which is minor.

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 100%, so per the baseline rule the description needn't add parameter detail — and it doesn't. The schema itself already documents invoice_id's provenance (returned by purchase_credits), npub's format, and dpop_token's exact framing requirements. The description adds no new parameter semantics, so the 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?

The first sentence uses a specific verb and resource: 'Check the payment status of a Lightning invoice.' Among many sibling check tools (check_balance, check_proof_status, check_unlock), this one is identifiable by its unique resource — Lightning invoice payment status. It stops short of 5 because it doesn't explicitly name a sibling it is not, and 'check' is a generic verb shared with several peers.

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 gives explicit temporal guidance: 'Call after paying the invoice from purchase_credits,' which anchors the tool in a clear workflow and ties it to a named sibling. It doesn't provide exclusions or state when not to use it vs other check_* tools, so it earns a 4 rather than a 5, but the when-to-use context is unambiguous.

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