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

Annotations already declare readOnly, idempotent, non-destructive and closed-world. The description adds genuinely useful context beyond them: the tool does not execute payment, and payment would require an x402 client plus explicit user approval — a meaningful safety boundary for a purchase-related tool.

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 short sentences with zero filler; the return value is stated first and the non-payment constraint immediately after. Every sentence carries information an agent needs.

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 names what is returned (REST purchase URL and USDC terms) and rules out the payment side-effect. It leaves minor gaps such as whether the terms expire or how the URL should be handed off, but it is sufficient for correct invocation.

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 tool takes zero parameters (empty schema, 100% coverage vacuously), so there is nothing for the description to clarify. Baseline 4 applies since no parameter guidance is needed.

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 and resource: returns the x402 REST purchase URL and exact USDC terms for the kit. The output-oriented framing ('terms', 'purchase URL') separates it implicitly from kit_quote and kit_describe, but no sibling is named, so an agent must infer the boundary.

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?

'Does not pay. Paying needs an x402 client and explicit approval from the user' clearly tells the agent this tool only retrieves terms and that actual payment happens elsewhere. It gives a strong when-not condition, though it does not point to a specific alternative tool for the payment step.

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