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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely useful context beyond that: no account is required, nothing is spent, and it explains the 'unlocked' field's meaning relative to the user's linked account, which no structured field would convey.

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?

Front-loaded with purpose and usage trigger in two tight sentences, then a scannable return list. Slightly padded by the 'Returns:' enumeration, since an output schema already exists, but nothing is misleading or wasted-heavy.

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?

Covers purpose, trigger, cost/auth posture, and even clarifies the ambiguous 'unlocked'/'full_report_covers' return fields. Since an output schema is present, the remaining return-field listing is mostly redundant, but nothing an agent needs to invoke it correctly is missing.

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?

Only one parameter, fully documented at 100% schema coverage including spacing/case tolerance and an example. The description adds only the loose gloss 'by its registration plate,' so per the baseline rule a 3 is appropriate when the schema does the heavy lifting.

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+resource ('free check of a UK car by its registration plate') and enumerates the exact data domains covered (make/model/engine/fuel, tax, MOT status, latest MOT test). The mention of 'full_report_covers' and 'unlocked' implicitly separates it from the paid sibling full_report, so an agent can distinguish it 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?

Explicit trigger: 'Use when the user gives a plate or asks about a car,' plus the constraint 'No account needed and nothing is spent,' which tells the agent this is the no-cost entry point. It stops short of naming an alternative (e.g. 'use full_report for a deeper history'), so it is clear context without explicit exclusions.

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