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

readOnlyHint=true already covers the safety profile, so the bar is lower. The description adds a genuine constraint (max 1095 days) and the underlying REST route, but says nothing about pagination, rate limits, or result ordering.

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 compact clauses with zero filler: scope+example, hard limit, and exact route. Front-loaded with the operation and its constraint.

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, but 'time series' plus the REST route conveys the return shape well enough for a 3-param, fully-documented read tool. Only the precise response structure is left implicit.

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 resource enum, date patterns and 'today' handling are already documented. The description's REST path echoes the parameters but adds no new per-parameter meaning, so the baseline 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 (fetch) and resource (activity metric time series over a date range), with a concrete example ('daily steps for a week'). It is reasonably distinguishable from siblings like fitbit_daily_activity_summary by emphasizing a multi-day range, though it never names or contrasts a sibling explicitly.

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?

Usage is only implied: it is a range-based metric fetch, and the max-1095-days limit hints at scope. There is no explicit when-to-use vs alternatives (e.g. daily summary vs time series vs lifetime stats) and no stated exclusions.

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