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

Goes well beyond the readOnlyHint/openWorldHint annotations by disclosing the return shape for each mode ({ invoice } vs { valid }), the billing model (1 credit = 1 CAD cent per successful call), and the failure policy (schema-invalid input is rejected before billing). These are exactly the facts an agent needs before invoking a paid operation.

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 tight sentences, front-loaded with the core action and output format, followed by return values and then the cost caveat. No filler or redundancy.

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 fully covers return values for both modes, the cost, and the billing-failure rule. All four parameters are documented at 100% schema coverage, leaving nothing an agent needs 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%, so the baseline is 3, but the description adds real meaning by showing how prefix, yyyymm, and sequence compose into the output string (including a five-digit zero-padded sequence and an appended check digit) that the schema itself does not state.

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 (formats/validates) and resource (invoice number), and spells out the exact output template PREFIX-YYYYMM-NNNNN-C with a Luhn check digit. The dual formatting/validation mode is disambiguated in the first sentence, so an agent knows precisely what it gets.

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?

Clearly indicates the two usage modes and that they are mutually exclusive by mode ('or validates one'), which the schema reinforces with the validate field ignoring the others. There are no close sibling alternatives in the catalog (uuid, hash serve different purposes), so explicit exclusion guidance is not needed.

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