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

Annotations already cover the safe-read profile (readOnly, idempotent, non-destructive, openWorld). The description adds genuine behavioral context annotations do not: it discloses the validation method (mod-97 + country length), that a UK sort code is extracted as a side result, and a cost model (free within daily allowance, then $0.0005 USDC per call via x402). The pricing/allowance disclosure is materially useful for an agent deciding whether to call.

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?

Two sentences, zero waste; the functional purpose is front-loaded and the pricing caveat follows. Appropriately sized for a single-parameter validator, though the cost detail could arguably be deferred.

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?

An output schema exists, so return values need not be spelled out, and annotations carry the safety profile. For a low-complexity one-parameter validator, the description covers method, a notable side output, and cost. The missing piece is usage/alternative guidance, keeping it at 4.

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?

There is one parameter with 100% schema description coverage, so the baseline is 3. The description implies which aspects of the IBAN are checked (checksum and country length), marginally clarifying the input expectation, but adds no format, normalization, or example details 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?

The description names a specific verb and resource: mod-97 checksum validation, country length check, and UK sort code extraction on an IBAN. This is concrete and distinguishable from siblings like validators__email or validators__imei by the IBAN resource. It stops short of explicitly contrasting with alternatives, so a 4 rather than 5.

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?

There is no when-to-use or when-not-to-use guidance, no mention of prerequisites, and no routing to or away from sibling validators. The only context is the billing note, which says nothing about when this tool is the right choice.

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