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

Annotations cover the safety profile (readOnly, idempotent, non-destructive, openWorld), and the description adds substantial behavioral context beyond that: a 24-hour cache, the meaning of verification_status values including the counterintuitive case that 'pending' and 'unknown' are not compliance failures, and explicit warnings that a null company_number is not a missing value. This is rich, non-obvious disclosure.

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?

The purpose is front-loaded in the first sentence and the remaining content is dense, field-level semantics rather than filler. It is long but nearly every clause carries decision-relevant meaning; the trailing 'Cached for 24 hours' is slightly orphaned from the rest.

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 carries the full burden of explaining return values and does so thoroughly: field-by-field semantics for individual vs corporate PSCs, null-handling rules, the kind/kind_raw fallback, and registry_is_companies_house tri-state meaning. Nothing an agent needs to interpret the response is 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?

There is a single parameter at 100% schema description coverage, and the schema already documents the format, pattern and zero-padding rule. The description adds nothing specific about the company_number argument itself, so the baseline of 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 and resource ('Get the PSC (Persons with Significant Control) register for a UK company') and immediately defines scope by describing the entity types and thresholds returned. This is clearly distinguishable from siblings like get_directors or get_psc_chain without opening any schema.

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 the data is relevant (ownership/control analysis, ECCTA compliance checks) but never explicitly says when to use this versus get_psc_chain or get_directors, nor does it state any prerequisites or exclusions. Usage must be inferred from the returned-content description.

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