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

readOnlyHint=true already tells the agent this is a safe read with no side effects, so the description carries a lighter burden. It adds the concrete REST path (GET /1/user/-/cardioscore/date/...), which is mild context, but it says nothing about behavior when no cardio score exists for a date, whether data is device-dependent, or what the returned payload contains.

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: the first front-loads the purpose and scope, the second gives 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?

With no output schema, the description ideally would sketch the return shape (intraday values, ranges, units), which it only partially does via 'VO2 max estimate.' For a simple two-parameter read-only tool with full schema coverage it is serviceable but leaves the agent guessing about response content and empty-result behavior.

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 their yyyy-MM-dd/'today' patterns. The description only restates that the input is 'a date or range,' adding no format or semantic detail beyond the schema, which is the expected baseline when 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 names a specific resource (cardio fitness score / VO2 max estimate) and its scope (a date or range), so an agent knows exactly what data comes back. It does not, however, distinguish itself from any sibling health metric tool (hrv, heart_rate, spo2), relying on the resource name alone to differentiate.

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?

There is no when-to-use guidance, no mention of prerequisites (e.g., needing a Fitbit account or device that computes VO2 max), and no routing to alternatives. The phrase 'for a date or range' describes input shape rather than a usage condition, leaving an agent to infer when this tool is appropriate.

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