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

With no annotations provided, the description carries the transparency burden. It discloses the approximate cost ($25), instant QR delivery, the fact that it's a real eSIM, and the critical warning to call only after user confirmation, which implies irreversibility. It does not detail error conditions or refund policies, but covers the essential behavioral traits.

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 packed with relevant information in five sentences, but there is slight redundancy, such as mentioning 'real eSIM' twice. Still, every sentence serves a purpose: purpose, cost/delivery, api_key usage, request_id usage, and confirmation warning.

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?

Even without an output schema, the description tells the agent what to expect (a QR code), the cost, the prerequisite key, idempotency handling, and the important confirmation step. It lacks details on error handling or next steps with the QR, but is reasonably complete for a 2-parameter purchase tool.

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?

The input schema already documents both parameters with thorough descriptions (source of api_key and idempotency of request_id). The description merely repeats these details without adding new semantic meaning, so the score stays at the baseline for 100% schema coverage.

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 'Buy' and a well-defined resource 'UK +44 Number eSIM', clarifying exactly what the tool does. It distinguishes itself from the sibling tool 'purchase_esim' by emphasizing the UK +44 number and the real mobile line for incoming verification SMS and calls.

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 provides clear usage context: requiring the api key from create_wallet, using request_id for safe retries, and stressing that it should only be called once the user has confirmed. However, it does not explicitly mention when not to use this tool or name alternatives for other regions/types.

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