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

Annotations already declare readOnly, idempotent, non-destructive, openWorld. The description adds the key behavioral constraint beyond them: return scope is limited to name and current approved-function status, 'nothing more', and that the IRN must already be known. It does not mention error behavior for unknown IRNs or auth/rate limits.

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?

Roughly three sentences, front-loaded with the purpose and followed by the exclusion and an example. Minor redundancy between 'name and current approved-function status only' and 'nothing more', but nothing is wasted overall.

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?

No output schema exists, and the description compensates by naming the returned fields and their currency ('current approved-function status'). With one documented required parameter and full annotation coverage, the only gap is unstated behavior on invalid or unknown IRNs.

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%, so the baseline is 3, but the description adds meaning the schema does not: the IRN must be a value the caller already possesses and represents exactly one individual. The worked example reinforces the expected format.

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 and resource (look up an individual's public Financial Services Register record by IRN) and delimits the returned scope to name and current approved-function status. Distinguishable from sibling firm-oriented tools (fca_firm_details, fca_firm_search, fca_firm_representatives) at a glance.

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?

Explicitly states the negative case: 'This tool does not enumerate or list individuals; it looks up exactly one IRN you already have,' which routes the agent away from using it as a search. It does not name a specific alternative tool for that case, so it stops short of full when/when-not/alternative guidance.

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