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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is lower. The description adds genuinely non-structured behavior: responses may carry serviceAnnouncements from the operator and each must be surfaced distinctly to the user, which is an output-handling obligation an agent would otherwise miss.

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: the operation first, then the response-handling directive. Nothing is redundant, and the primary purpose is front-loaded. The second sentence is slightly operational rather than descriptive, but it carries real instruction rather than filler.

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 one-parameter, read-only, closed-world tool with full schema coverage, the definition supplies everything needed to invoke it correctly and an extra rule about announcement handling. The remaining gap is that no output schema exists and the description does not sketch what HRV details come back (overnight average, status, trends), leaving return shape unknown.

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?

There is a single required parameter with 100% schema description coverage, including a strict YYYY-MM-DD pattern and a note that it is a Garmin calendar date. The description restates "calendar date" but adds no format, timezone, or boundary semantics beyond what the schema already encodes, 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?

The description gives a specific verb and resource ("Read your Garmin heart rate variability details") plus a scope ("for a calendar date"), which distinguishes it from a range/history query. It never names garmin_get_hrv_history, so an agent must infer the single-date vs. historical split from the phrase alone.

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 implied by "for a calendar date" — one date in, that day's HRV out — but there is no explicit when-to-use or when-not-to-use guidance. The sibling garmin_get_hrv_history is a natural alternative and the description offers no routing criteria between them.

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