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

Annotations only declare readOnlyHint=true, so the description carries most of the behavioral burden and does so well: it explains that the collection is sampled continuously, which is why datetime (not date) parameters apply, and warns that the call can return a great many points. It also cites the underlying API endpoint for reference. It does not describe pagination mechanics or how to interpret the returned samples, which keeps it out of the top band.

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?

Three sentences, all front-loaded: what it fetches, how it is scoped, and the practical caveat. Zero filler, and the endpoint citation is compact enough to help without bloating.

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, no-required-params endpoint with a fully documented schema and no output schema, the description covers purpose, timing semantics, and volume risk. The main omission is any hint about the shape or fields of a returned sample, which matters somewhat given there is no output schema, and the sandbox parameter is left entirely to the schema.

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 the baseline is 3, but the description adds genuine meaning beyond the schema by explaining the rationale for start_datetime/end_datetime over dates and by flagging latest as the escape hatch from a large result set. It pairs the two datetime params as a window rather than leaving them as isolated fields.

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 verb and resource ('Fetch individual heart-rate samples') plus the scope constraint (DATETIME range). No sibling touches heart rate, so the resource alone cleanly separates it from the webhook/document/personal-info tools in the sibling set.

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?

Gives actionable usage guidance: narrow the window or pass latest, which tells the agent how to avoid an unusably large response. It stops short of naming explicit alternatives or when-not-to-use conditions, but the context it gives is clear and operationally useful.

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