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

Annotations cover safety (readOnly, idempotent, closed-world), but the description adds genuinely non-obvious traits: paid via x402 in USDC on Base at $0.001 per check, input never echoed or stored, and PII masked. That's real behavioral context beyond the annotations, though the description omits rate limits or failure modes.

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?

Compact but crams four distinct concerns (format scope, payment, return shape, privacy) into one telegraphic sentence with semicolons. Front-loads the resource but the payment/privacy clauses feel appended rather than structured.

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 helpfully enumerates the return object shape ({type, valid, normalized, reason}) and covers payment and privacy concerns. It stops short of explaining the 'reason' values or failure semantics, but is largely complete for a single-param validator.

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 documents the single 'value' param with an example (PL5260250274), so the description needn't add syntax. It contributes nothing further about parameter handling, making baseline 3 correct.

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 specific verb+resource (validate EU VAT number) and a distinctive scope detail: offline per-country format checks with no VIES lookup, plus Poland-specific NIP checksum. This clearly separates it from siblings like validate_pl_nip and validate_iban.

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 'offline, no VIES' note implies when this tool is appropriate (fast local format validation) versus a live registry lookup, and the PL/NIP mention hints at overlap with validate_pl_nip. However, no explicit when-to-use/when-not or alternative routing is stated.

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