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

The annotations declare openWorldHint=true, but the description explicitly says there is no Companies House lookup, entity-existence check, or active-registration verification. For a pure syntax checker, the open-world annotation conflicts with the stated no-lookup behavior, so the description contradicts the annotation despite readOnlyHint and idempotentHint being consistent.

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?

The description is two short sentences and is front-loaded with the core behavior. The exclusion sentence earns its place by preventing the agent from assuming lookup or existence verification.

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?

Given the simple one-parameter input, full schema coverage, and the presence of an output schema, the description provides enough context for correct invocation. Its main remaining issue is the annotation mismatch rather than missing description content.

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 company_number parameter is already documented as '8 digits, or 2 letters + 6 digits.' The description adds no additional parameter-level format detail beyond the schema, so the baseline of 3 is appropriate.

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?

The description states a specific verb and resource: 'Check supported UK company-number syntax.' It also immediately distinguishes this from Companies House lookup, entity-existence checks, and active-registration verification, so an agent can tell what class of validation this is.

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 description gives clear usage boundaries by saying it is syntax-only and does not perform lookup or existence/registration checks. It does not name an alternative sibling tool for those excluded needs, but the when-not guidance is explicit.

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