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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=true, so the safety profile is covered. The description usefully identifies the backing source (Companies House, consistent with the open-world hint) but omits any error behavior (e.g. company not found) or mention of rate limits/registration requirements for the external API.

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?

A single front-loaded sentence that leads with the verb and resource and then lists outputs; nothing is padded. The trailing phrase 'quick fit check' is undefined jargon that slightly undercuts precision.

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 carries the return-value burden and does enumerate several fields (name, status, registered office, next accounts period). However, 'quick fit check' is never explained and no failure mode or lookup limits are described, leaving gaps for a one-parameter external lookup 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 description coverage is 0% for the single companyNumber parameter, so the description must carry it. Saying 'by company number' does clarify that the identifier is a company number rather than a name, but gives no format guidance (e.g. 8-digit numeric string) or validation expectations.

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?

Names a specific verb (look up), resource (UK company), source (Companies House), and key selector (company number), then enumerates the returned fields. It is clearly distinct from the CT600/self-assessment siblings, though it never names them explicitly.

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 only usage signal is the implicit 'by company number' selector plus an undefined 'quick fit check'. There is no statement of when to reach for this tool versus the sibling tax tools, and no prerequisites or exclusions.

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