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

readOnlyHint=true already establishes this is a safe read, and the GET endpoint mapping corroborates that. The description adds no behavioral context beyond the annotations: no note on data availability, rate limits, or that a range request returns per-day entries.

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 short sentences, front-loaded with the resource and scope, with the endpoint as supporting detail. No filler or redundancy.

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 simple two-param read tool with full schema coverage and readOnly annotations, the description covers what is needed to invoke it correctly. Since there is no output schema, a brief note on the returned summary's shape would be the only remaining gap.

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 description coverage is 100%, so both parameters (date, endDate) are already fully documented with format and required/optional status. The description's 'date or range' phrasing adds no syntax or semantic detail beyond the schema, so baseline 3 applies.

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 resource (blood oxygen saturation / SpO2) and the operation (summary for a date or range), with the exact Fitbit REST endpoint mapping. An agent can distinguish it from siblings such as fitbit_heart_rate, fitbit_hrv, and fitbit_breathing_rate purely from the resource name.

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 phrase 'for a date or range' implies how to select single-date vs range usage, but there are no explicit when-to-use/when-not conditions or named alternatives. Usage is inferable but not stated.

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