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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the description doesn't need to re-explain safety. It adds genuinely useful non-obvious context instead: each call is billed against a Stratum API balance, and a test key is available for non-billable use. It does not explain behaviour on an invalid or not-found FRN, which keeps it short of a 5.

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?

One sentence, front-loaded with what the tool returns, then the billing note. Every clause carries information an agent would otherwise have to discover: the resource, the key, the returned fields, and the cost model.

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 steps in and names the fields returned, which is exactly what an agent needs to decide whether to call it. For a single-parameter read tool with full annotations that is nearly complete; only not-found/error behaviour and any response shape detail are missing.

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% and the single parameter already documents the 'Firm Reference Number, 6 or 7 digits' format. The description only restates the identifier as 'FRN' and adds no further syntax, derived-value or edge-case guidance, so the schema is doing all the work — 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 concrete resource (a single firm on the FCA register), the lookup key (its FRN), and the payload it returns (authorisation status, addresses, Warning List presence). The 'one firm ... by its FRN' phrasing implicitly separates it from the sibling fca_search_firms, though that contrast is never made explicit.

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?

Usage is only implied: an agent can infer this is for when you already hold an FRN rather than a name. There is no explicit 'use this instead of fca_search_firms when...' routing, and no statement of preconditions such as needing a valid FRN or what to do if only a firm name is known.

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