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

Annotations already indicate readOnlyHint=false and destructiveHint=false, but the description adds important behavioral context: it requires a two-step process, uses _meta for the payment, and settles and starts the eSIM after payment. It also notes a critical limitation with wallet tools. No contradiction with annotations.

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 compact and well-structured, with three sentences covering purpose, usage, and a critical caveat. Each sentence adds necessary information without redundancy. Front-loaded with the core purpose.

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?

Given the tool's complexity (two-step payment process, meta usage), the description fully covers the workflow and constraints. It also clarifies the environment where it works, which is critical for the MCP client. The output schema exists, so return values don't need explanation. The description is complete for an agent to invoke correctly.

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 only parameter, order_id, is already described in the schema as 'order_id from create_checkout', which is clear. The description doesn't add much beyond that, but since schema coverage is 100%, additional description isn't necessary. The description does reinforce that order_id comes from create_checkout, which is helpful.

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 tool's purpose: 'Pay for an order in USDC over x402' and distinguishes it from siblings by noting it's for MCP clients that sign x402 payments themselves. It also differentiates from create_checkout and other order-related tools.

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?

Provides explicit usage guidelines: call once for payment requirements, then repeat with signed payment in _meta. Also tells when NOT to use it (if wallet is a separate tool, use x402_url instead), and mentions a key limitation: cannot take payment on behalf of a wallet tool.

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