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

readOnlyHint=true is already supplied by annotations, and the REST GET endpoint reinforces the read-only nature. Beyond that, it discloses no return shape, pagination, quota, or data-availability context, which for a read tool is acceptable but not rich.

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 tight sentences, front-loaded with the resource and purpose, followed by the underlying endpoint. 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 read-only metric tool with full parameter coverage and annotations covering safety, the description supplies enough to call it correctly. It does not describe the returned HRV fields, but no output schema is required and this is a minor 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 fully documented including format and optionality. The description adds only the notion that endDate enables a range, so baseline 3 applies where 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?

The description states a specific resource (Heart rate variability / HRV) and action (summary) with clear scope (a date or range). It is distinguishable from siblings like fitbit_heart_rate or fitbit_breathing_rate by its unique metric. It stops short of explicitly contrasting with siblings, hence a 4.

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 the calling context (single-day vs range queries), but there is no explicit when-to-use or when-not-to-use guidance, and no alternatives are named. Usage is only implied.

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