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

The description adds behavioral context beyond annotations by disclosing that the tool 'charges the saved card off-session' and 'returns immediately on success.' This is important for a financial side effect. It doesn't mention failure scenarios, but the core behavior is covered.

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?

Two sentences, front-loaded with the primary action. Every word earns its place: the first sentence states the purpose, and the second provides the essential conditional workflow. No redundancy or fluff.

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?

The description covers the core workflow, including the critical prerequisite and the quick-return behavior. It doesn't explain the response format or verification steps, but for a simple purchase tool with no output schema, this is sufficient. The reference to start_checkout helps complete the context.

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 input schema provides 100% coverage, with a full enum description for tier_key including price information. The description adds no extra parameter semantics, so the baseline of 3 applies.

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 clearly states the tool's action ('Buy a credit pack') with a specific verb and resource. It distinguishes itself from the sibling start_checkout by detailing the off-session charge behavior, which sets it apart as the actual payment step.

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 provides when-not guidance: 'If no card is saved yet, run start_checkout first to capture one.' This names the alternative tool and gives a clear conditional workflow, which is exactly the kind of usage direction the dimension asks for.

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