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

Annotations already cover the read-only/idempotent safety profile, yet the description adds substantial operational context beyond them: the $0.003 price, free 'unknown' for unsourced countries, the api_key/x402 payment flow, and the HMAC push vs. free poll distinction for register_alert with its $0.05 post-trial delivery cost. This is exactly the kind of behavior annotations cannot express.

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 two most decision-critical facts (PAID $0.003, what it validates) before the exclusions and follow-ups. Dense but largely earns its clauses; slight redundancy in re-listing return values despite an existing output schema.

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?

Covers pricing, auth/payment fallback, registration-confirmation limitation, unknown-status fallback, and downstream series-binding options. With an output schema present it needn't enumerate return fields, and nothing an agent needs to call this 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 id format (spaces/dots/dashes/VAT prefix), country enum, and api_key are already documented in the schema. The description adds marginal framing ('as written', 'one country of this brand') but no syntax or constraint detail beyond the structured fields, so baseline 3 applies.

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'), scopes it to one country, and explicitly distinguishes itself from the registration-confirming official_lookup. An agent can tell exactly what this tool returns (valid/invalid, normalised form, id type, check level) without opening the schema.

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?

Explicit when-not ('Does not confirm registration; follow official_lookup for that'), edge-case behavior ('Countries without a sourced rule answer free with status unknown'), and follow-up routing to get_changes and register_alert. Payment/auth path is also spelled out (pass api_key, else x402).

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