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

Annotations already declare readOnly, idempotent, non-destructive and open-world behavior, so the safety profile is covered. The description adds genuinely new context: the accepted input span (6 to 8 digits) and an explicit prohibition on submitting full PANs, which is a privacy/security constraint not present in structured fields. It omits data source, freshness, and rate-limit behavior.

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, front-loaded with the identity of the tool and its return values, followed by usage and then the critical input constraint. No filler or repetition beyond the necessary safety reiteration.

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 listing the returned fields explicitly, and it covers the input constraint, the safety rule, and the use case. For a single-parameter lookup tool, nothing an agent needs to call it correctly is 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 description coverage is 100% and the single parameter already documents 'first 6 to 8 digits, digits only, never a full card number', so the description largely restates the schema. Baseline 3 is the right ceiling here — no additional format, example, or edge-case meaning is contributed.

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?

Names a specific verb and resource ('BIN / IIN lookup') and enumerates exactly what is returned: card network, card type, card level, issuing bank and country. The scope is precise, but it does not explicitly distinguish itself from the sibling lookup_issuer, which overlaps on 'issuing bank and country', so an agent has to infer the boundary.

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?

'Use it to identify a card or where it was issued' gives a clear usage context, and 'Never pass a full card number' is a hard, actionable constraint. It stops short of naming alternatives (e.g. lookup_issuer or lookup_mcc) or stating when-not to use this tool, so it stops at 4.

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