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

Beyond the readOnly/idempotent annotations, the description discloses the underlying rule ('a spacer lowers the effective offset by its thickness'), the slip-on vs bolt-on classification, and the exact engagement thresholds (>= 1x stud diameter and >= 6.5 turns). It also makes clear that the engagement check only runs when `stud`, `stud_protrusion_mm` and `wheel_pad_thickness_mm` are supplied — conditional behavior not visible in annotations.

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, examples front-loaded, no filler. The second sentence is long and packs several clauses together, but every clause carries distinct information about outputs or thresholds.

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 read-only calculator with an output schema, the description covers the physics, the two computation modes, and the qualitative outputs (track change, slip-on vs bolt-on). The only residual gap is the sign/relationship between `offset_mm`, `spacer_mm` and `target_offset_mm` when both spacer and target are supplied.

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 only 50% (`spacer_mm`, `target_offset_mm` and `stud` are undescribed in the schema), and the description compensates by explaining that `target_offset_mm` drives the recommended spacer and which three parameters trigger the engagement check. `spacer_mm` is never explicitly described, leaving one mode of the tool to inference.

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 states a concrete verb+resource ('spacer needed for a target offset', 'thread-engagement check') and its opening quotes make the real-world questions it answers unmistakable. It is clearly distinct from siblings like wheel_fitment_check, though it never names an alternative, so it stops short of a 5.

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 three quoted user phrasings ('do I need spacers', 'what spacer gets +35 wheels to +20', 'is a 10 mm slip-on spacer safe') effectively convey when to reach for this tool. There is no explicit when-not guidance or named sibling to route away to, so context is clear but exclusions are absent.

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