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

Annotations indicate a write operation (readOnlyHint=false) and non-idempotent (idempotentHint=false). The description adds the two-step execution model and the balance-spending aspect, which are not captured in annotations. It also includes the user-facing instruction to disclose cost, which is a behavioral requirement beyond the structured data.

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 sentences, tightly packed with essential information: the action, the two-step flow, and the cost disclosure requirement. No filler or redundancy, and the critical steps are front-loaded.

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 write operation with no output schema, the description explains the essential flow, the required idempotency_key, and the cost disclosure. It does not cover error handling or return values, but those are less critical for invocation. The sibling context is clear, and the tool can be used correctly based on this description.

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 schema covers 75% of parameters, with confirm and idempotency_key already documented. The description adds the relationship between these two (preview then confirm with the key), which is helpful but not extensive. Quantity and package_code are adequately covered by the schema. Overall, the description complements the schema without over-explaining.

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 clearly states the action: 'Buy one or more eSIMs of a package from the account balance.' This is a specific verb (buy) + resource (eSIMs) + context (from balance), and it distinguishes the tool from siblings like topup_esim (topping up an existing eSIM) and search_packages (finding packages).

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 explicitly outlines the two-step process: first call returns a preview with idempotency_key, then call again with confirm:true and that key to execute. It also instructs to tell the user the cost before confirming. This gives clear sequencing and prerequisites, leaving no ambiguity about how to use the tool.

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