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

With no annotations at all, the description carries the full burden and does so well: it discloses the 1 CRD charge, that the charge applies even to rejected calls, the 429 RATE_LIMITED exception, and that errors return a request_id to quote to support. It also explains field meaning like disabled_reason being set when Cred disables a failing rule.

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?

Front-loaded with the core behavior, then costs, then error handling — a sensible ordering with no filler. It is dense and packs several ideas into one long sentence, but every clause carries 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?

There is no output schema, yet the description enumerates the returned fields (threshold, enabled, max_per_day, charges_last_24h, last_status/last_error, disabled_reason) and mentions the saved card. Combined with billing and error behavior, an agent has everything needed to call and interpret this tool.

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%, so baseline is 3, but the description adds real meaning beyond the schema by defining the semantic effect of omitting credit_type_id (all rules returned) and the null case for the single-rule result. x402_payment and api_key handling are left to 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 (get) and resource (auto top-up rule / saved card) and precisely distinguishes the two result shapes via credit_type_id. An agent can immediately tell this apart from siblings like save_auto_topup or delete_auto_topup.

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 branching on credit_type_id is spelled out explicitly, which tells the agent when to expect a single rule versus all rules. It does not name alternative tools (e.g. get_saved_payment_method) or state prerequisites such as needing an existing user, so it stops short of full when/when-not guidance.

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