`replaceSpecialVars` uses `dayjs()` which defaults to the server's
timezone (UTC in Docker), causing `{{current_datetime}}` and
`{{current_date}}` to always show +00:00 regardless of the user's
location. This is inconsistent with `append_current_datetime` for
assistants which already handles client timezone via `clientTimestamp`.
Add dayjs timezone plugin support and a `timezone` parameter to
`replaceSpecialVars`. The client sends its IANA timezone string
(`Intl.DateTimeFormat().resolvedOptions().timeZone`) alongside the
existing `clientTimestamp`, and the agent initialization flow passes
it through to produce timezone-aware date formatting.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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