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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral context by specifying that results are paginated and listing the returned fields (name, number, status, SIC codes, incorporation date, registered address). This goes beyond the annotations without contradicting them.

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 sentences long, front-loaded with the primary purpose, and every sentence earns its place. The first sentence states what it does and what it returns; the second provides alternative tool guidance and pagination instructions. There is no filler or repetition.

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 output schema exists, the description does not need to enumerate return types. It covers the core search purpose, pagination behavior, and directs to company_profile for full records. This is sufficient for an agent to select and invoke the tool correctly in the context of its sibling tools.

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?

Input schema covers 100% of parameters with descriptions. The description adds meaning beyond the schema by explicitly teaching the pagination relationship between start_index and items_per_page. It also re-emphasizes that 'query' is a company name or keyword, reinforcing the schema's semantics.

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: 'Search the Companies House register by company name or keyword.' It clearly differentiates from siblings by advising 'Use company_profile for the full record once you have the company number,' which distinguishes this tool from the related profile tool.

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 provides explicit alternative guidance: 'Use company_profile for the full record once you have the company number.' It also gives explicit pagination instructions with 'Re-call with start_index=start_index+items_per_page to fetch the next page,' telling the agent how to retrieve additional results.

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