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

Annotations already declare readOnly, idempotent, closed-world and non-destructive, so the safety profile is covered. The description adds genuinely non-redundant context: the tool works offline from the plate string alone, performs no vehicle lookup, and requires no account. It does not state behavior on an unrecognized or non-conforming plate, which is the remaining gap.

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 short sentences, zero filler, with the output scope front-loaded and the exclusion (free_check) immediately after. The opening clause is a sentence fragment that takes a moment to parse, but nothing is wasted.

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?

An output schema exists, so return values need not be explained, and the description nonetheless previews the response fields. It is complete for a single-parameter, read-only decode; the only omission is what happens for plates that don't match the stated format.

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?

There is a single parameter and schema description coverage is 100% — the schema already specifies 'UK registration plate, any spacing or case' with an example. The description adds no syntax, normalization, or validation detail beyond that, so the schema carries the parameter semantics.

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 ('decode a UK plate') and enumerates exactly what it returns: format, age identifier, and the issue window as first/last month. It also pre-empts confusion with the sibling that looks up the vehicle itself, so an agent can separate decode_plate from free_check without reading either schema.

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?

Names an alternative and the condition that selects it ('it does not look the car up; use free_check for that') and adds prerequisites ('Offline', 'No account needed'). It does not address the other report-oriented siblings, but the one boundary an agent is likely to confuse is drawn explicitly.

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