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

Annotations already declare it is a non-read-only, open-world, non-idempotent, non-destructive call, and the description adds genuinely useful lifecycle context: it returns an invoice URL, nothing is charged until payment, and the plan activates automatically afterward. It does not warn that repeated calls produce multiple separate invoices (relevant given idempotentHint=false), nor does it mention auth or invoice expiry.

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 what the tool does and what it returns; the tier/pricing breakdown follows as supporting detail. No filler or repetition of the schema.

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 one-parameter purchase tool with no output schema, the description covers the essential facts: output is a URL, no immediate charge, automatic activation, and per-tier pricing. Minor gaps remain around what the agent should do with the URL and whether calling twice creates duplicate invoices.

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% and the enum labels are already in the schema, so the baseline is 3. The description earns extra credit by attaching real prices and billing cadence to each tier ($15/month, $100/month, $1,000 once-per-device then $200/month), which is decision-relevant meaning the schema does not carry.

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 a specific verb+resource ("Creates a crypto payment invoice") and identifies the processor (NOWPayments), the payment rails (USDT and other coins), and the scope ("YOUR OWN account"), which separates it from client-facing siblings like client_report or find_clients. An agent can distinguish this from the display-only `plans` sibling without opening either schema.

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 "for YOUR OWN account" qualifier and the note that nothing is charged until payment give clear context for when this is the right call. It stops short of naming an alternative (e.g., check `plans` first, or `account_status` to see if already subscribed) or excluding cases where an invoice should not be created.

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