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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behaviour, so the safety profile is covered. The description adds real value by disclosing the composition of the verdict (tax, MOT, and gap reporting). It says nothing about caching, data freshness, or failure modes for an unknown VRM.

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?

Two sentences, no filler: the first defines the tool and its outputs, the second handles routing. Everything is front-loaded and each 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 single-parameter read-only check with full schema coverage, the description covers the essentials and even summarises the return content in the absence of an output schema. Minor gaps remain around the exact response shape and how 'what is missing' is expressed.

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% and the single nested 'vrm' parameter is fully documented with an example, so the schema does the heavy lifting. The description's 'Given a UK vehicle registration' merely restates the schema. 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?

States a specific verb and resource ('decide whether it is legal to drive now') and enumerates the sub-checks performed (tax status, MOT validity, what is missing). It also explicitly contrasts itself with vehicle_lookup, so an agent can route between the two without opening schemas.

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 concrete trigger phrasings ('is this car road legal / can I drive it / can I tax it') and names an alternative (vehicle_lookup) with a reason not to use it (avoid reasoning over fields yourself). It does not, however, disambiguate against the near-identically named sibling road_legal_check, which is the more likely routing confusion.

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