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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely useful behavior beyond that: a five-level depth cap, explicit termination states (foreign parent, listed company, PSC statement), insolvency flags, 'trouble above', and the privacy rule that individuals are counted but never named.

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?

Purpose is front-loaded in the first clause, followed by progressively finer detail about the walk, the per-company fields, and the stopping conditions. It is dense and slightly run-on, but essentially every clause carries information an agent or upstream model would want.

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 must describe return content, and it does: per-company name, number, status, insolvency flag, holdings, plus the register's endpoint and any adverse signals. Depth limit and privacy handling are also stated. Only minor gaps remain, such as what happens for non-UK or unresolvable inputs.

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?

There is a single parameter with 100% schema description coverage ('A company number or name'), so the schema already carries the semantics. The description never mentions the parameter or clarifies accepted formats (e.g. number vs name resolution behavior), so it adds nothing here — 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?

The description states a specific resource and scope: the people-with-significant-control chain followed company-by-company up to five levels, with the termination conditions spelled out. This is clear and unambiguous. It does not, however, name or contrast itself with the nearby siblings (registers, company_search, public_record), so an agent must infer the boundary itself.

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: the description makes it obvious this is for tracing control upward through corporate controllers, and the five-level cap plus termination conditions hint at scope limits. There is no explicit when-to-use guidance and no named alternative for raw register data (e.g. registers) or basic lookups (company_search).

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