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

Goes far beyond annotations, which only declare read-only/idempotent/open-world. The description discloses that the tool is a paid call ($0.003), the exact return payload (valid/invalid, normalised form, id type, check level, failure reason), that unsourced countries return 'unknown' for free, the api_key vs x402 payment paths, and the recommended follow-up binding tools. That is substantial behavioral context.

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?

Front-loads the paid price and core purpose well, but the sentence packs several distinct ideas (validation result shape, registration caveat, unknown-country rule, payment options, follow-up tool binding) into a dense block. Every idea is useful, but the flowing single paragraph could be structured into clearer chunks.

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?

For a read-only validation tool with a full schema, output schema present and annotations covering safety, the description supplies the remaining decision-critical context: result shape, failure semantics, unknown handling, pricing/payment paths and the recommended follow-up tools. Nothing essential 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 coverage is 100%, so the schema already documents id, country enum and api_key. The description adds some behavioral color (sourced-rule gating, payment options for api_key) but little syntax detail beyond the schema. Baseline 3 is 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 a specific verb and resource ('Offline format and check-digit validation of a business tax ID or VAT number') with a precise scope limited to 'one country of this brand'. The distinction from registration verification ('Does not confirm registration; follow official_lookup for that') makes the boundary explicit.

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?

Clearly explains when to use it (format/check-digit validation), what it does not do (confirm registration) and points to the alternative path (official_lookup for registration). Also specifies what happens for unsourced countries and payment flow options.

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