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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=false, destructiveHint=false), so the description correctly focuses on behavior the annotations cannot express: a verdict-tier output, the needs_info fallback when hasMultipleJobs is absent, and the caveat that refund figures are estimates rather than amounts owed. It does not describe the full shape of the verdict or its other fields.

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?

Three dense sentences, front-loaded with the core purpose before the edge-case rule and the estimate caveat. Every sentence carries information, though the middle sentence is packed and could be split for scanability.

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 11 parameters, no output schema, and no annotations covering inputs, the description leaves significant ground uncovered: it never explains the purpose of the many circumstance flags or how taxYear/scotland/monthsOnCode affect the result, and it only gestures at the return structure via 'verdict tier'. Adequate for the core case, incomplete for a tool of this parameter complexity.

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 description coverage is 55% across 11 parameters, so the schema documents roughly half. The description adds real value by giving concrete tax-code formats (1257L, BR, K475, 1257L W1/M1) and by explaining the role of hasMultipleJobs, but it is silent on the many boolean circumstance flags (hasCompanyCar, hasPrivateMedical, hasMarriageAllowance, receivesStatePension, scotland, monthsOnCode) that materially change the verdict.

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 (check) and resource (UK PAYE tax code), plus the three things it does: validate, explain, and estimate over/under-payment. Sibling tools are all invoicing/transaction/mileage oriented, so this tool is unambiguously distinguishable.

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?

Gives an explicit conditional rule for a hard case: flat-rate codes (BR, D0, D1) are only judged when hasMultipleJobs is known, otherwise the verdict is needs_info. That tells the agent when the tool can and cannot give a real answer. It does not name alternative tools or state broader when-not conditions, so it falls short of a 5.

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