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

Annotations only declare readOnlyHint and openWorldHint, so the description carries the rest and does so richly: case-sensitive statsType causing a 404, Garmin 400s when end-start>27, automatic 28-day chunking with concatenated rows, dropped aggregations for chunked ranges, and [] for empty ranges. This is far beyond what annotations provide.

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-loads the purpose then packs dense, high-value API quirks (chunking, error codes, empty-range behavior) into tight sentences. Slightly dense, but nearly every clause earns its place given the tool's non-obvious behavior.

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?

No output schema exists, and the description covers return values ([] for no data vs Garmin's null) and the dropped aggregations, plus error behavior. For a quirky, open-world read tool this is complete enough for correct invocation.

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 documents start/end but leaves statsType with only an enum and no description. The description compensates by specifying statsType is exactly "CALORIES" or "STEPS" and that lowercase or other values yield a 404, adding real meaning beyond the enum.

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: per-day calorie or step totals, with the statsType distinction. It also names the overlapping siblings (getDailySteps/getCaloriesDaily), but the differentiating clause is truncated ("this one returns total/act…"), so the split from siblings is incomplete.

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?

Provides clear context: ranges over 28 days are auto-chunked and it names the alternatives it overlaps with. However the stated distinction from getDailySteps/getCaloriesDaily is cut off, so the agent can't fully resolve when to pick this one over them.

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