start_checkout

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

With no annotations, the description carries the behavioral burden. It discloses that credits are only applied after the Stripe webhook fires, that a human must open the URL, and that the tool returns a specific error when purchases are disabled. This goes well beyond a minimal description.

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 reasonably short and front-loaded, but it repeats return information: it first says 'Returns a hosted Checkout URL' and later repeats 'Returns: dict with keys: url.' Merging these would make it tighter.

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 zero-parameter tool with no output schema and no annotations, the description covers the essential invocation context: what the tool starts, what is returned, what the human must do, the asynchronous credit behavior, and the disabled-purchases error case. It is sufficiently complete for an agent to use 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 tool has zero parameters, so there is no parameter semantics burden. The description reinforces this by noting the top-up is 'server-configured,' meaning the agent should not expect or supply any arguments.

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 states a specific action: 'Open Stripe Checkout for the server-configured prepaid top-up.' The verb and resource are clear, and the purpose is distinct from siblings like billing_portal, though it does not explicitly name the distinction.

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 intended use is implied by 'prepaid top-up,' and the description notes the human must open the returned URL. However, it does not explicitly explain when to use this tool versus alternatives such as billing_portal, nor does it provide when-not-to-use conditions.

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