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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered; the description adds genuinely new behavior: results are returned under a `data` field, units for distance/duration/speed/heart rate are labeled, and totals reflect later Garmin corrections. It also discloses that responses may carry serviceAnnouncements from a third-party operator (Geekist) that must be surfaced to the user — operational context nothing else conveys. Only missing element is error/failure behavior (invalid or unknown activityId).

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?

Front-loaded with purpose, then return semantics, then sibling routing, then the announcement-handling obligation — a sensible priority order with no filler sentences. The units sentence is dense but each clause carries information; minor cost is that the announcement directive sits last despite being an action item.

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 does the necessary work of sketching the return shape (payload under `data`, unit labeling, correction-aware totals) and warns about embedded service announcements. For a single-required-parameter read tool with full annotation coverage, that is close to complete; failure modes and any payload size limits are the only real gaps.

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 coverage is 100% and the schema already specifies the POSITIVE_DECIMAL_STRING pattern and tells the caller to obtain the ID from garmin_list_activities or garmin_search_activities. The description only restates "by activityId" and adds no format, range or edge-case semantics beyond the schema, so the baseline of 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?

States a specific verb and resource ("Read one current Garmin Connect activity by activityId") and immediately differentiates itself from relatives by pointing to garmin_get_activity_laps and garmin_get_activity_samples. The scope word "current" plus the note that totals include later Garmin corrections tells an agent exactly which representation of the activity it gets.

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?

Names two concrete alternatives with the condition that selects each (lap timing -> laps tool; sensor records -> samples tool), which is exactly the when-to-use-this-vs-something-else guidance an agent needs. It also implies the natural entry point via the ID's provenance (garmin_list_activities / garmin_search_activities).

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