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

With no annotations, the description carries the whole burden and mostly discharges it: cost/auth ('FREE, no key'), provenance ('Live Companies House data') and a staleness caveat ('cached up to 6 hours') are all things an agent could not infer from structured fields. It doesn't mention rate limits or error behaviour for an invalid company number, so not 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?

Two sentences, front-loaded with the highest-value constraint ('FREE, no key'), then the lookup modes and return fields. Every clause carries information; nothing is repeated from the schema verbatim.

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, so the description takes on the return-value burden and does: it enumerates the fields the caller gets in each mode. Combined with cost, freshness and no-key auth, an agent has everything needed to decide to call it and interpret the result.

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 inputs are already documented; the baseline is 3. The description does add value by framing company_number vs name as mutually alternative lookup modes with different return shapes, and gives a concrete example format, but it never explains `limit` (left entirely to the schema).

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 names a specific verb and resource ('Companies House status check for a UK company') and enumerates exactly what comes back: status, name, incorporation and cessation dates, registered office postcode, has_insolvency_history. That field list, plus the single-company lookup framing, separates it from the sibling discovery/pack tools (new_cos_discover, daily_new_companies, kyb_pack) without needing to name them.

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 gives explicit branch guidance: pass company_number when you know the entity, or pass name when you only have a name and want top matches with their numbers. That is real when-to-use-routing. It stops short of exclusions or pointing at a sibling (e.g. 'use kyb_pack for full KYB reports'), which keeps it from a 5.

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