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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint, but description adds crucial behavioral detail: full card numbers are never exposed to agents. This is a security-relevant guarantee beyond annotations and clarifies the returned data shape.

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 purpose and data limitation, then a relevant cross-reference. Zero waste, perfect length for a simple list operation.

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 parameterless read-only list with no output schema, the description covers all necessary details: what is returned, what is not exposed, and linkage to purchase_confirm_intent. Nothing an agent needs to call it correctly is missing.

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?

Tool has zero parameters, so there is nothing to explain. Baseline of 4 is appropriate; description correctly omits parameter details since none exist.

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 a specific verb (List), resource (principal's saved payment methods), and scope (last4 + brand only). Clearly distinguishes from payment_methods_create and other tools by detailing what data is returned and what is never exposed.

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?

Implies usage for viewing available payment methods and references purchase_confirm_intent's behavior when no payment_method_id is passed, giving context on when to use this tool. Lacks explicit 'when not to use' but is clear enough given no direct alternative for listing.

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