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

Annotations already cover readOnly/idempotent/non-destructive safety, so the description only needs to add extra behavior. It does so by disclosing the not_found response for missing accounts and the cost implication ('Always free'). No contradiction with annotations exists.

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?

The description is a single compact sentence that front-loads the endpoint, cost, and purpose, then adds the key fallback behavior. Every phrase adds information 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?

For a simple read-only balance check with rich annotations and 100% schema coverage, the description is complete. It covers the return concept, failure mode, cost, and how to handle a missing account, so an agent can invoke it correctly without further inference.

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 description coverage is 100%, so the baseline is 3; the description adds little beyond the schema. It clarifies the balance is returned 'in cents and USD', but client_key and payment_token semantics are already documented in the input 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?

The description uses a specific verb and resource: 'Check the prepaid credit balance for a client key (in cents and USD).' It differentiates itself from the sibling billing_purchase_credits by explicitly routing account creation there. The scope and output unit are unambiguous.

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 description states when the tool is appropriate, notes it is 'Always free', and gives an explicit fallback: when no account exists, 'buy credits via billing_purchase_credits to create one.' This provides clear tool-selection guidance for the main edge case.

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