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

Annotations already declare readOnly, idempotent, non-destructive and open-world, so the safety profile is covered. The description adds genuinely non-structured context: this is a paid, per-call billed endpoint drawing on a Stratum API balance, with a test-key path. It omits anything about depth limits, latency, or what a missing/foreign company number returns.

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?

A single sentence that leads with the resource and the derived value, then appends the cost/access note. No filler, no repetition of the title, and the most decision-relevant content is front-loaded.

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?

For a one-parameter lookup with no output schema, the description supplies a good mental model of the return (PSC list, corporate ownership chain, UBO derivation) plus the billing model. It falls short only on result shape and edge-case behavior, which are minor for a read-only lookup.

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?

One required parameter at 100% schema description coverage, so the schema already explains companyNumber. The description adds no format hints, validation behavior, or jurisdiction caveat beyond the implicit 'UK company' framing. Baseline 3 applies since the schema does the work.

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?

Names a specific resource (UK company ownership and control) and describes the actual output concept: PSCs, corporate owner chains walked up, and derived ultimate beneficial owners. That is far more specific than a name restatement, though it never names the sibling it differs from (company_profile, company_risk_score).

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 it is useful (you need ownership/UBO structure rather than a general profile), but gives no explicit when-to-use/when-not or named alternatives among the ten siblings. The only concrete guidance is the payment/test-key note, which is billing guidance rather than selection guidance.

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