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

Annotations already cover readOnly, idempotent, non-destructive and closed-world, so the description's added value is the auth requirement ('Requires an API key') and the reconciliation-blocking state that explains why charges may fail. It stops short of stating error shapes or whether the returned prefix is truncated, but otherwise supplies meaningful behavior beyond the annotations.

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 sentences in a deliberate order: what is returned, what it requires, when to call it, and what it cannot do. No filler, and the most load-bearing information comes first.

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 no output schema, the description compensates by naming the returned fields, and it also states the auth prerequisite and the payment-blocking condition. For a zero-param read tool this covers everything an agent needs to call it and interpret the result.

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 no parameters, so the baseline is 4; there is nothing for the description to clarify and it correctly spends no words on inputs.

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 ('Returns the account behind the API key') and enumerates the returned fields: credit balance, reconciliation block state, key prefix and scopes. That field list makes it unmistakable against siblings like get_capabilities or get_pricing.

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?

Gives explicit triggering conditions ('before starting an analysis or after a payment_required error') and an explicit out-of-scope statement ('It cannot buy credits; only a human can, in the web app'), which routes the agent away from attempting a purchase through this tool.

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