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

Annotations already declare the mutation profile (readOnly=false, destructive=true, openWorld=false). The description adds real behavioral context beyond them: the change retroactively redefines what 'today' means for all totals and streaks, and invalid input fails with example-bearing errors rather than silently. It stops short of stating reversibility or auth requirements.

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 tight sentences with zero filler; the consequence of the change is front-loaded before the input-format note.

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 a single fully documented parameter, an output schema that covers return values, and annotations covering the safety profile, the description carries what remains: impact scope and validation behavior. Only the absence of any note on auth or whether prior entries are re-bucketed keeps it from a 5.

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 coverage is 100%, so the IANA examples and format are already documented. The description still adds meaning the schema cannot: that the value determines the definition of 'today' for aggregates, and that invalid values are rejected with example output, which tells the agent how failure surfaces.

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 (changes) and a precise resource (the account timezone), and immediately names the scope of the effect (every total and streak). An agent can distinguish this from get_account and the other update_* siblings 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?

The description implies when to reach for it (when 'today' should mean something different) and gives a constraint on input format, but never states explicit when-to-use/when-not conditions or points at alternatives. Usage is inferable rather than prescribed.

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