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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds substantial behavioral context beyond that: the per-figure evidence shape (value, unit, quoted line, source URL, effectiveFrom, prior-year value), the envelope fields (verifiedDate, freshness, citation permalink), and the explicit failure mode for unknown categories.

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 and coverage list are front-loaded with no filler sentences. However, it is delivered as one very long run-on sentence, and some of the return-shape detail is dense; a slightly more structured form would read better, though little could be deleted without losing 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?

No output schema exists, so the description carries the return-value burden and does so thoroughly — evidence fields per figure, envelope metadata, citation permalink, and the unknown-category fallback. Combined with read-only annotations and a fully documented single parameter, an agent has everything needed to invoke and interpret the result.

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%, so a 3 baseline applies, but the description goes further by explaining what each category value actually covers (e.g. ved-electric = zero-emission cars taxable since 1 April 2025, historic = 40-year MOT exemption and build-date tax exemption). That is meaningful semantics beyond the bare list in the schema.

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 resource (UK vehicle tax/VED and MOT rules) with year scope (2026-27) and source authority (gov.uk), then enumerates every category the tool can return. An agent knows exactly what it is getting and can map its question onto a specific category.

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 clear context: category values are enumerated, omitting it lists all, and an unknown category returns covered:false with the nearest keys. There are no sibling tools, so no alternative routing is needed, but the description never states when this tool is *not* appropriate (e.g. non-UK vehicles or non-2026-27 years).

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