Lookup Company

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

Annotations already declare readOnly, openWorld, and idempotent, so the description does not need to restate them; instead it adds value by emphasizing 'Read-only — it does NOT save anything', directing the agent to link_company for saving, and describing the candidate vs. full-record response behaviors. No contradiction exists between the description and the annotations.

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?

Every sentence earns its place: the first states the action and modes, the second gives the business context, the third handles persistence via link_company, and the fourth handles geography. The parenthetical field list is useful because there is no output schema to document the return payload.

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?

Despite the absence of an output schema, the description compensates by enumerating the returned fields, candidate count, and the distinction between search and fetch results. It covers when to use, how to invoke by either parameter, read-only behavior, follow-up action, and the non-UK fallback, leaving no practical gap for an agent deciding whether to call this 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?

The input schema already has 100% coverage with rich descriptions of both parameters, including when to use each and what each returns. The description reinforces that behavior but does not add meaningful parameter-level information beyond the schema, so the high-coverage 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 opens with a specific verb and resource: 'Look up a UK company on Companies House' and immediately separates two modes—search by name for candidates vs. fetch by company number for the full record. It names the exact fields returned, so an agent can tell what this tool does and how it differs from the cluster of document/wiki/VAT lookup siblings.

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?

It explicitly states when to use the tool ('during onboarding to find and confirm the user's company'), how to distinguish the two invocation modes, and when not to use it ('UK only... for non-UK companies, ask the user for their details directly'). It also routes follow-up persistence to link_company, which is clear alternative guidance.

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