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

With no annotations, the description carries the full burden. It discloses that data is current for 2026-27, sourced from gov.uk, and includes source URLs and verification dates — a concrete freshness and traceability guarantee. It stops short of describing output shape or error behavior, but for a read-only lookup the key behavioral traits are covered.

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 description is compact: a scoping sentence, a category enumeration, and a usage directive. The category list is long but necessary for disambiguation. There is slight redundancy with the schema's category list, but overall it is front-loaded and every sentence earns its place.

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 tool with one optional parameter and no output schema, the description tells an agent what the tool returns, which values are supported, and when to use it. It lacks a sample output or explicit statement that it is read-only, but these are minor gaps for such a simple, well-scoped 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?

The schema already documents the single parameter completely, including the 'omit to list all' behavior, so description coverage is 100%. The tool description repeats the category list but adds no semantic detail beyond the schema, meeting the baseline 3 without exceeding it.

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 names the resource ('UK (2026-27) rates and thresholds') and the action (lookup/return), and explicitly enumerates ten categories. It is a specific verb-resource combination, not a tautology, and the 'verified against gov.uk' detail solidifies the scope.

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 explicitly says 'Use whenever a user asks about...' and lists all triggering topics, with a rationale ('models reliably misremember these numbers'). This gives an agent clear conditions for invocation, and no exclusions are needed since there are no sibling tools.

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