fix: Datetime format mismatch causing a bug in chainlit_data_layer.py

[jakubgajski]

Problem

The ChainlitDataLayer methods were generating ISO-formatted datetime strings without the Z suffix (e.g., 2026-03-02T11:54:54.670000), while the widely-used utc_now() utility function in backend/chainlit/utils.py consistently produces timestamps with the Z suffix (e.g., 2026-03-02T11:54:54.670000Z).
This inconsistency caused ValueError exceptions when parsing datetime strings:

# lines 373-375
        created_at = step_dict.get("createdAt")
        if created_at:
            timestamp = datetime.strptime(created_at, ISO_FORMAT)

ValueError: time data '2026-03-02T10:29:07.146000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'

While the original log was:

{"event": "Task exception was never retrieved\nfuture: <Task finished name='Task-359' coro=<ChainlitDataLayer.update_step() done, defined at /<project-name-redacted>/.venv/lib/python3.13/site-packages/chainlit/data/utils.py:10> exception=ValueError(\"time data '2026-03-02T11:54:54.670000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'\")>", "exc_info": ["<class 'ValueError'>", "ValueError(\"time data '2026-03-02T11:54:54.670000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'\")", "<traceback object at 0x10ef46080>"], "level": "error", "logger": "asyncio", "timestamp": "2026-03-02T12:10:30.840355Z"}

Solution

Added Z suffix to all ISO-formatted datetime strings in ChainlitDataLayer to ensure consistency with the utc_now() format used throughout the codebase.

Modified methods:
get_user() - line 98
create_user() - line 124
list_threads() - line 500
get_thread() - line 564
_convert_step_row_to_dict() - lines 684, 687, 689 (for createdAt, start, and end fields)

Changes

Fix (cc120b7): Modified 6 locations in chainlit_data_layer.py to append "Z" to .isoformat() calls

Tests (b7ebd72): Added comprehensive test coverage (206 lines) verifying:

• Z suffix presence across all affected methods
• Proper handling of None timestamps in _convert_step_row_to_dict()
• Alignment with utc_now() datetime format

Impact

Prevents ValueError exceptions during datetime parsing
Ensures consistent UTC timezone representation across the data layer
Maintains compatibility with existing parsing logic that expects the Z suffix

---

Summary by cubic

Standardize datetime strings in ChainlitDataLayer by appending the "Z" suffix to ISO timestamps to match utc_now(). This prevents parsing errors and ensures all timestamps are UTC.

• Bug Fixes
  • Append "Z" to ISO timestamps in get_user, create_user, list_threads, get_thread, and _convert_step_row_to_dict (createdAt, start, end).
  • Add tests confirming the "Z" suffix across methods and correct handling of None timestamps.

^{Written for commit 3e25918. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[dokterbob]

Thanks for the contrib. However, this seems a dup of #2798. If I am wrong, please, reopen!

[dokterbob]

The other (approved) PR is just pending merge conflicts. Feel free to solve them and shoot a new PR yourself. Or poke the original author. :)