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

With no annotations, the description carries the full burden and delivers: cost of 1 CRD per call, that credits are consumed even on rejected calls, the 429 RATE_LIMITED exception, and that error results carry a request_id to quote to support. This is exactly the operational context an agent cannot get from the schema.

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?

Three sentences, front-loaded with the return shape and nullity first, then provenance, then cost/error behavior. No filler; each clause carries distinct operational information.

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 read tool with no output schema and no annotations, the description supplies the return fields, the empty case, the credit cost model, the rate-limit exception, and error payload conventions. Auth via api_key/x-api-key is already covered by the schema, so 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?

Schema coverage is 100% and each of the four parameters is documented in the schema itself, so the baseline of 3 applies. The description only obliquely touches parameters — "for a user" maps to user_id and the CRD cost implies x402_payment — but adds no syntax or format detail beyond the schema.

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 and resource ("Get the card saved for a user") and enumerates the fields returned (brand, last4, expiry), including the null case. It is clearly separable from siblings like charge_saved_card and delete_saved_payment_method without opening any schema.

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?

Provides the precondition needed to understand when this tool has data at all — the card must have been saved via create_checkout_session with save_payment_method: true — which implicitly routes the agent to that sibling when no card exists. There is no explicit when-not guidance, but the retrieval-only context makes it low-risk.

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