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

Annotations already declare readOnlyHint, idempotentHint, and a closed-world scope, so safety is covered for free. The description adds genuine value beyond that: data is verified against gov.uk, accompanied by source URLs and verification dates, and pinned to the 2026-27 tax year. That provenance and freshness context is exactly the kind of behavioral detail annotations cannot carry.

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 verification claim is front-loaded and useful, but the category list is written out twice — once as slugs and once as prose ('Covers UK income tax, National Insurance, minimum wage...'). That second sentence largely restates the first and consumes most of the description without adding new information.

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?

This is a low-complexity, read-only, single-optional-parameter tool with full schema coverage, so little is required. The description covers data scope, provenance, and tax year, but with no output schema it never sketches the return shape (keys, per-category structure, how sources/dates are presented), leaving the agent to discover the payload blind.

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 100% and the single category parameter already enumerates its legal values. The description's category list duplicates the schema rather than adding syntax, defaults, or error behavior. With the schema fully carrying parameter meaning, baseline 3 is appropriate.

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 (current UK 2026-27 rates and thresholds) and frames it as a verified lookup against gov.uk. It covers 17 named categories, so an agent knows exactly what domain content it returns. It stops short of distinguishing itself from siblings like get_fact, resolve_fact, or take_home_pay, which is the only thing keeping it from 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 statement of when to use this tool versus alternatives. Siblings such as get_fact, resolve_fact, and take_home_pay overlap in the 'answer a UK factual question' space, and the description gives no routing guidance. The only usage hint ('omit to list all') lives in the schema, not the description.

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