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

Annotations already establish readOnly/openWorld=false/destructive=false, so the safety profile is covered. The description adds genuinely useful behavior: that results are computed by Mobela by merging Garmin, Apple Watch, Health Connect, Strava and in-app GPS, and that the scope is all-time. It stops short of noting latency or edge cases (e.g., missing data sources), which keeps it from a 5.

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, purpose and scope front-loaded before the sibling reference. The distance enumeration and source list are dense but informative rather than filler; still, the sentence could compress slightly without losing value.

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 carries the burden of describing returns and does so (time, seconds, session id, date). Combined with the aggregation-source and scope notes, an agent has everything needed to call and interpret this parameterless read tool.

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?

The tool takes zero parameters, so the baseline is 4; there is nothing the description could add on parameter semantics. It correctly does not invent filtering options, keeping expectations aligned with the empty schema.

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+resource ('the user's all-time running bests') with the exact distances covered, the sources aggregated, and the returned fields. It also distinguishes itself from sibling get_session_details, so an agent can select it without opening the schema.

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?

Explicitly says 'Use for PR and race-time questions' and routes the per-session case to get_session_details with the condition ('a single run's own best efforts and any PRs it set'). Both the when-to-use and the alternative are named.

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