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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds two pieces of behavior the annotations do not carry: the call is free (no billing impact) and it requires an account-scoped key. That is genuine added context, though nothing is said about scope limits or freshness of the returned values.

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 short fragments, each load-bearing: cost, returned fields, auth requirement. The most decision-relevant constraint ('Free') is front-loaded and there is no filler.

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?

With an output schema present, return values need not be documented, so the enumeration of fields is a bonus rather than a requirement. For a zero-parameter read tool with full annotation coverage, the description plus 'Account keys only' is nearly sufficient; only the when-to-use routing to siblings is absent.

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 takes zero parameters, so the baseline is 4 and there is no parameter semantics to compensate for. The description correctly spends no words on 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 names the exact resource (auto-refill settings) and enumerates the specific fields returned: on/off state, balance threshold, monthly caps, and card-on-file status. The verb 'get' is carried by the name, and the resource is distinct from billing siblings like list_billing_transactions, though no sibling is explicitly named.

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?

'Account keys only' and 'Free' give two call prerequisites (auth scope, no billing cost), which is more than nothing. However, there is no statement of when to reach for this tool versus get_account_overview or list_billing_transactions, so usage is only implied.

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