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

Beyond the annotations (readOnly, idempotent, non-destructive, openWorld), the description discloses refusal behavior for full-PAN-looking input, the offline/non-registry nature of the check, pricing at $0.002 per call over x402, the prepaid x-credit-token header for unlimited use, and the free tier. These are exactly the operational facts an agent needs and that annotations cannot convey.

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 core purpose is front-loaded in the opening clause, and the pricing/auth tail is dense but earns its place by covering access mechanics. It is slightly longer than strictly necessary given the schema and annotations already cover safety.

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 an output schema present, return values need not be spelled out, yet the description still names the returned schemes. Input constraints, refusal behavior, cost, auth, and free-tier terms are all covered, leaving no material gap for a one-parameter tool.

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% and the single parameter already carries the 'BIN (6–8 digits)' description, so the schema does the heavy lifting. The description restates 'Required input: bin' and the digit constraint, adding only the refusal nuance, which is marginal beyond the 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?

States a specific verb and resource — an offline Issuer Identification Number check that derives the card scheme from a 6–8 digit prefix — and enumerates the exact schemes returned. It is clearly distinguishable from siblings like iban_check and bic_check, which target different identifiers.

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 line 'Not a live BIN registry lookup' gives an explicit when-not boundary, steering the agent away from expecting registry-grade data. However, it never names the alternative sibling tools an agent should pick instead for related identifier checks, so it stops short of full routing guidance.

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