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

The readOnlyHint annotation already establishes this is a safe read, so the description's burden is lower. It adds the underlying Fitbit REST path, which is genuinely useful for an agent reasoning about upstream behavior, but says nothing about auth requirements, rate limits, or how missing/partial days are handled.

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?

Two tightly packed clauses with the resource and scoping constraint front-loaded and no filler. The trailing REST endpoint is slightly redundant but still informational rather than wasteful.

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 usefully enumerates the returned metrics (steps, distance, floors, calories, active minutes, logged activities), which is exactly the information an agent would otherwise lack. It is nearly complete for a single-parameter read tool, missing only failure/empty-day handling.

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% and the single required 'date' parameter is fully documented (format plus 'today'), so the schema carries the burden. The description's 'for a date' adds no syntax or constraint detail beyond that, making the baseline 3 correct.

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 resource (daily activity summary) scoped to a single date and enumerates the returned metrics, so the agent knows exactly what this tool retrieves. It distinguishes itself implicitly from siblings like fitbit_activity_time_series by the 'for a date' framing, but never names an alternative 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 implied by the single-day scoping ('for a date'), which contrasts with the time-series sibling, but the description offers no explicit when-to-use/when-not guidance or named alternatives. An agent can infer intent but receives no routing help.

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