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

Annotations cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=true, openWorldHint=false), so the description's job is to add context — and it does: it discloses that the tool only returns a link, charges nothing, the assistant cannot complete payment, and that the user's account is required. It does not describe link expiry or whether repeated calls return the same or different links, despite idempotentHint=true.

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 tight sentences, front-loaded with the trigger condition, followed by what the tool returns and what to do with it. Every clause carries actionable information (who pays, no charge, account requirement) with no repetition of the schema or title.

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?

With no output schema, the description correctly compensates by explaining the return value (a payment-page link) and the handoff to the user. The single optional parameter is covered, and no behavioral or auth detail an agent needs to call this safely 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%, which sets the baseline at 3, but the description adds genuine meaning by framing the two modes: omitting letter_id adds credit, supplying it pays that specific draft letter. That clarifies the optional parameter's role in a way the terse schema string ('Pay this draft letter directly') does not.

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+resource (create a payment link) and immediately scopes it to two concrete outcomes: adding credit or paying one draft letter by card. It also distinguishes itself from near-siblings like create_upload_link by describing the artifact returned (a link to the payment page on printandmailcompany.com).

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 the user needs to add credit, or to pay one specific draft letter by card'), and clarifies a common misuse case by noting the assistant cannot pay and calling it charges nothing. It does not name a specific alternative tool or a when-not condition, so it falls just short of the top bar.

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