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

The description carries real behavioral weight beyond the annotations: it discloses the $0.02 price, the USDC-on-Solana/x402 payment flow, the payment-required challenge, and the exact retry location (_meta["x402/payment"]). It does not explain result format or volume, and it leaves the tension between a 'search' and readOnlyHint=false unaddressed, which prevents 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?

Purpose is front-loaded in the first clause, and the payment mechanics follow. The x402 parenthetical is dense jargon but it earns its place because the agent cannot successfully invoke the tool without it.

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 two-parameter paid search endpoint with no output schema, the definition covers purpose, cost, and the payment retry protocol, which is the decisive operational detail. It is slightly incomplete in not characterizing returned company records or describing what happens when no match or limit is reached.

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% (q accepts name keywords or a company number; limit is 1–25, default 10), so the baseline is 3. The description adds no additional meaning about matching behavior, fuzzy vs exact name matching, or how limit interacts with result paging.

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 ('Search UK companies by name in the register'), so the agent knows exactly what the call does. It doesn't differentiate from close siblings such as companies_uk_company or companies_enrich_resolve, which also touch UK company data, so a 4 rather than a 5.

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?

There is no explicit when-to-use guidance, no exclusions, and no routing to sibling tools like companies_uk_company (lookup) or companies_uk_ownership. Usage is only implied by 'by name', and that hint is undercut by the schema accepting 'a company number' for the same q parameter.

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