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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds the underlying REST endpoint (GET /1/user/-/body/log/fat/date/{date}.json), reinforcing the read semantics, but discloses nothing about return format, empty-result behavior, or units beyond what annotations and schema supply.

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 short sentences, front-loaded with the resource and scope. The REST path sentence is mildly redundant with the name/title but is compact and earns some value as an endpoint reference.

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?

For a simple single-parameter read tool with safety covered by annotations and format covered by the schema, the description is largely sufficient. It omits any hint about the returned entry structure, but with no output schema the burden there is low.

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% with a single required date parameter that already documents the yyyy-MM-dd or 'today' format via pattern. The description adds no parameter detail 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?

The description states a specific resource (body fat percentage log entries) scoped to a date, which distinguishes it from sibling log tools like fitbit_weight_log or fitbit_water_log. The verb is only implicit (retrieval via GET), but the mapping is unambiguous.

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?

There is no explicit when-to-use guidance or routing to alternatives. The agent must infer that this tool is for body fat data specifically versus the many other Fitbit log siblings; nothing is said about when-not to use it or which sibling covers adjacent metrics.

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