Time zone and UTC offset at a point

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

Annotations already declare readOnly/idempotent/non-destructive/no-open-world, so the safety profile is covered; the description goes further by disclosing the tz-database source, deterministic resolution rules for skipped and repeated hours, and the documented meaning of `standardOffset` and `isDst`. It does not mention credit cost or failure behavior for points with no zone, which are the remaining behavioral gaps.

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-loaded with the core answer (IANA zone + UTC offset, never fixed) before the edge-case rules, and the Kyiv illustration earns its space by proving historical correctness. It is dense in a single paragraph — splitting the DST-resolution rules from the output-field definitions would read slightly better — but there is no filler.

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?

Four parameters, full schema descriptions, annotations, and an output schema, so return values need not be explained; the description nonetheless defines `standardOffset` and `isDst`. Nearly complete for the complexity — the only unaddressed case is a coordinate with no associated zone (e.g., open ocean) and what the service returns then.

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 baseline is 3, but the description adds real semantics beyond the schema: `time` defaults to 12:00 and only matters on DST-transition days, plus the exact tie-breaking rule for nonexistent and ambiguous local times. The historical example (Kyiv +4 on 15 May 1990) shows the parameters identify a historical tz entry, not a present-day offset.

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 precise verb+resource: the IANA zone and UTC offset kept at a given local date/time at a point, sourced from the tz database, explicitly 'never a fixed offset'. That phrase alone separates it from fixed-offset or solar-time siblings, so an agent can distinguish it 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?

Gives clear situational context — use when you need the civil zone/offset actually observed at a coordinate and historical date, rather than a fixed offset — and explains exactly when the `time` parameter becomes material (clock-change days). It stops short of naming an alternative sibling or stating when not to use it, so it is clear but not fully routing.

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