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

Beyond the readOnly/idempotent annotations, the description discloses important behaviors: it refuses full PANs, works offline, returns results only when the prefix is known, requires no wallet or API key, and has a specific pricing/authorization model. This gives the agent materially useful behavioral context not available from annotations alone.

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 dense but front-loaded with the core purpose, followed by constraints, output examples, and then pricing/auth details. Every sentence adds useful information, though the pricing section is somewhat verbose for a tool description.

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 one-parameter tool with an output schema, the description covers input constraints, output type, offline behavior, and authentication requirements. It does not specify exact behavior for unknown prefixes or error responses, but these are minor gaps given the existing output schema and annotations.

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 schema already covers the bin parameter, but the description adds meaningful semantics: exactly 6–8 digits accepted, full PANs refused, and the expected output is a card network. This goes beyond the schema's 'BIN (6–8 digits)' by clarifying validation behavior and result meaning.

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?

Description names a specific verb+resource: an offline Issuer Identification Number check on the first 6–8 digits of a card. It clearly distinguishes itself from a live BIN registry lookup and states the exact output (card networks like Visa/Mastercard/Amex). This is not a vague restatement of the tool name.

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 gives clear context: it is offline, accepts only 6–8 digit BINs, refuses full PANs, and is not a live BIN registry lookup. It does not explicitly name sibling alternatives or state exact when-to-use conditions, but the exclusion of live lookups and required input are clearly communicated.

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