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

Annotations only cover the safety profile (not read-only, not destructive, open-world). The description adds real behavioral context: the credit is applied to the next invoice in the plan's currency, the reason is visible to the merchant only, and the call returns the credit given. It omits reversibility/refund behaviour, which is a minor gap for a non-destructive mutation.

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, which is good, but the 'taken off their next invoice' effect is stated twice in near-identical wording, and the return value is repeated from the opening sentence, so the text is somewhat redundant for its length.

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 mutation tool with no output schema, the description covers the essential facts an agent needs: effect timing, currency scope, audience of the reason field, and the returned value. Nothing critical is missing, though error/permission conditions and reversal behaviour would round it out.

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%, so all four parameters are already documented, including currency, merchant-only reason, and the business acting context. The description restates currency and reason semantics but adds no syntax or format detail beyond the schema, making the baseline 3 appropriate.

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?

Specific verb+resource: 'Put credit on a subscriber's account' and 'Adds credit to one subscription' clearly states the action and the scope (one subscription). It is distinguishable from sibling list/archive tools, though it never explicitly names the neighbouring tools it is not (e.g. discounts_create or subscriptions_credits).

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 implies the context of use by explaining the effect ('taken off their next invoices') and the merchant-only reason, but gives no explicit when-to-use/when-not guidance or contrast with alternatives such as issuing a discount instead of a credit.

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