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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, open-world). The description adds genuinely useful behavioral context beyond that: an empty list is a legitimate answer, not a failure, which prevents an agent from misinterpreting the result. It stops short of covering return shape or rate/pagination behavior.

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?

Front-loaded with the core action, then the important empty-result caveat, then a concrete example. Three tight sentences with no waste; every sentence earns its place.

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?

There is no output schema, so the description carries some burden for return semantics. It handles the empty case clearly but does not describe the shape of a non-empty AR/principal record. Given simple two-parameter input and full annotations, this is nearly complete with a minor return-format gap.

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 100%, so both parameters (frn and _apiKey) are already fully documented in the schema. The description adds no syntax or format meaning beyond what the schema provides, so the baseline 3 applies.

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 ('List') and a specific resource ('a firm's appointed representatives (principal/AR relationships)') scoped by FRN. This is clearly distinguishable from siblings like fca_firm_details, fca_firm_search, and fca_individual_lookup, which cover different resources.

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 implies when to use it (to retrieve AR relationships for a firm) and usefully frames the empty result as a valid answer rather than an error. However, it never names or contrasts with alternative tools, nor states any preconditions for choosing this over siblings, leaving usage guidance only implied.

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