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

Annotations only declare read-only/idempotent/closed-world; the description adds genuinely new operational facts: a concrete cost ($0.001 per check) with the payment rail (x402 in USDC on Base), a guarantee that input is never echoed or stored, and that personal identifiers are masked. These are exactly the behavioral traits 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?

Three tight sentences, front-loaded with the format rule, then pricing, then return/privacy contract. Every clause carries information; nothing is restated padding.

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 supplies the return shape ({type, valid, normalized, reason}), the cost and payment mechanism, and privacy handling, which is everything an agent needs to decide to call it and interpret the result.

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 there is only one parameter, so the schema already carries the baseline. The description nonetheless adds domain semantics the schema lacks — the 9/14-digit forms and mod-11 checksum — telling the agent what a valid 'value' looks like beyond the generic example.

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+resource — validating Polish REGON — and immediately pins down the accepted format (9 or 14 digits with a mod-11 checksum). The domain qualifier 'Polish REGON' cleanly separates it from siblings like validate_pl_nip and validate_pl_pesel without ambiguity.

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?

Usage is implied by the format spec and the per-check pricing, but there is no explicit when-to-use or when-not-to-use statement, and no sibling (e.g., validate_pl_nip, validate_pl_pesel) is named as the alternative for other Polish identifiers. An agent must infer the routing.

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