Add credit

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

The description goes well beyond the annotations by explaining that credit is applied only after payment settles, that a human must complete payment, and that automated payment or URL-opening is forbidden. It also discloses the required billing:write scope, giving the agent a full behavioral picture.

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 front-loaded: primary action first, then critical safety and workflow instructions, then the auth requirement. Every sentence adds value, and there is no redundant restatement of schema details.

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?

Despite having no output schema, the description tells the agent what to expect (checkout_url), what not to do, and when credit actually lands. Combined with the annotated openWorldHint and idempotentHint behavior, the tool is fully specified 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?

Schema description coverage is 100%, so the schema fully documents amount_usd, return_url, and idempotency_key. The description adds only the confirmation guidance related to amount_usd, which is useful but not a substantial expansion of the parameter semantics.

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 opens with 'Start a top-up,' a specific verb and resource, and immediately explains the key output (a checkout_url for human payment). It is clearly distinct from siblings like get_balance, which reads rather than creates.

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 gives clear usage context: confirm the amount with the user first, and never open the URL or attempt payment yourself. It doesn't name explicit alternative tools, but no sibling appears to offer the same top-up functionality, so the guidance is sufficient.

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