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

Annotations declare openWorld=true, idempotent=false, readOnly=false, and the description adds real context beyond them: an external payment link is returned, an active card subscriber is switched immediately, and activation occurs automatically when paid. It also notes the call is free and requires the user's personal connection address. It stops short of describing failure modes (expired link, failed payment) or any rate/abuse limits on link generation.

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?

Front-loaded with the core action, then branch-specific behavior, then prerequisites and the alternative — a sensible ordering with no filler. The 'Card:'/'Crypto:' telegraphic style is compact but slightly dense to parse in a single block.

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 five-parameter, open-world payment tool with an output schema, the description covers the decision branches, side effects, cost, and auth requirement, which is most of what an agent needs. It omits what happens on a failed or abandoned payment and how a returned link's validity is bounded, which matters for a non-idempotent purchase flow.

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 description coverage is 100% and all five parameters (including the three enums) are already documented in the schema, so the baseline is 3. The description reinforces the card vs. crypto branch and what a crypto response contains, but adds nothing about 'billing' or when 'user_accepted_terms' must be true that the schema does not already say.

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 specific verbs and resource: upgrade, renew or switch the signed-in user's plan without the website. It also distinguishes the two payment modalities and names a sibling (start_signup) for the public-preview case, so an agent can tell this apart from check_payment, confirm_signup and start_signup.

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?

Explicitly scopes usage to the signed-in user's own plan and gives a when-not with the alternative: 'on the public preview use start_signup instead.' It also names the prerequisite (the user's personal connection address), which is a routing condition, not just a hint.

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