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

Annotations already declare readOnlyHint=true, so the description's job is to characterize the value, and it does: it enumerates the three possible return states ("metric", "imperial", or null) and explains that null means follow each client's phone region. That null semantics is genuine behavioral context an agent could not infer from the schema, which is empty.

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?

One sentence, no filler, and the key information (the possible values and the null fallback) is packed in. Minor deduction because it opens with a noun phrase rather than a verb, so the action ('get') is only recoverable from the tool name.

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?

With no output schema and no parameters, the description carries the full burden of describing the result, and it does so completely for all three return states including the null case. The only gap is the absence of any note about the paired setter or how the default is changed.

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?

The tool takes zero parameters, so there is nothing to disambiguate and the baseline of 4 applies. The description does not waste space restating inputs and instead spends its words on the return value.

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 names the specific setting being read (the unit system new clients start in) and scopes it to 'this coach', which distinguishes it from the per-client get_client_weight_unit. It is clear, though it omits an explicit verb and never names the sibling getter it differs from.

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?

Usage is only implied: 'new clients start in' hints that this is the fallback default applied to new clients rather than a per-client value. There is no explicit when-to-use, no mention of when to prefer get_client_weight_unit, and no prerequisite/permission notes.

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