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

Goes well beyond the readOnlyHint/openWorldHint annotations: it discloses that the trader name is null, that an HMRC outage surfaces as 503 and is never reported as 'invalid', that EU numbers get a 400, and the exact pricing/charge model (x402, failed calls not charged). This is exactly the operational context an agent needs for an external paid check.

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?

Purpose is front-loaded, followed by edge cases and pricing, with no wasted sentences. Dense but every clause carries distinct operational value.

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 an output schema present, return values needn't be explained, yet the description still clarifies the one surprising field (name is null). Error behavior (503, 400) and cost model are covered, making it complete for a paid external validation tool.

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 the single parameter is already documented, so the baseline is 3. The description adds real meaning beyond the schema example by specifying accepted formats: GB or XI prefix followed by 12 or 15 digits.

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 (Checks) and resource (UK EORI numbers) plus the authority used (HMRC's public check) and what is returned (valid or not, processing date). An agent can tell this apart from nearby validation tools like vat or lei 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?

Usage context is clear (validate UK EORI numbers against HMRC), and it explicitly states a when-not: EU EORI numbers are refused with a 400 in this version. No named alternative sibling is offered, so it stops short of full routing guidance.

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