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

Annotations already declare readOnlyHint=true, so the read-only nature is covered. The REST endpoint syntax adds mild confirmation that this is a GET, but the description says nothing about response shape, data granularity, or availability limits beyond the annotation baseline.

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, resource and scope front-loaded, with zero filler. The REST endpoint string is optional reference material that adds minor length without harm.

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 read-only summary tool with full schema coverage and no output schema to explain, the description covers the essentials. It leaves the choice between single-date and range queries and the nature of the returned summary unaddressed, which is a modest but real 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%, with both 'date' and 'endDate' documented with formats and required/optional status in the schema. The description's URL template echoes the same date/endDate structure but adds no format or edge-case detail beyond the schema, so the 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 scope (summary for a date or range), plus the exact REST endpoint backing it. The resource is unmistakably distinct from siblings like fitbit_heart_rate, fitbit_hrv, and fitbit_breathing_rate.

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 choose between the single-date and ranged call, but there is no explicit when-to-use guidance, no mention of which sibling to prefer for other oxygen-adjacent metrics, and no prerequisites stated.

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