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

Annotations already establish this as a safe read (readOnlyHint=true), so the description's added value is the volume warning: the collection is sampled continuously and 'can return a great many points,' which implies pagination pressure and motivates narrowing the window or using latest. It stops short of describing pagination mechanics or response shape, but it goes meaningfully beyond the annotations.

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, zero filler, and the core action plus temporal scope is front-loaded. The volume caution and the parameter rationale each earn their place, and the endpoint reference is a compact trailing tag.

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, fully schema-documented tool with no output schema, the description covers purpose, scope, volume behavior, and mitigation. The only unaddressed area is return format and pagination mechanics, which is largely covered by the next_token description in the schema, so nothing essential is missing.

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 each of the five parameters is already documented in the schema, establishing a baseline of 3. The description adds only the rationale for why start/end datetimes are used instead of dates, and hints that latest is the shortcut for small windows, which is marginal added value.

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 temporal scope ('over a DATETIME range'), and cites the underlying endpoint GET /v2/usercollection/heartrate. No sibling tool covers heart-rate data, so the purpose is unambiguous against the rest of the toolset.

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 guidance ('narrow the window or pass latest') and explains why the datetime parameters exist for this collection vs date-based siblings. It does not name a specific alternative tool or state when not to call it, but the usage context is clear.

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