Python: fix: prevent colliding message_id values in incremental compaction

[LEDazzio01]

Fixes

#5237 — _ensure_message_ids assigns colliding IDs when called incrementally via annotate_message_groups, breaking SummarizationStrategy and other group-ID-dependent consumers.

Root Cause

_ensure_message_ids assigns f"msg_{index}" where index is the position within the list it receives. However, annotate_message_groups calls group_messages(messages[start_index:]) on a slice, so the slice always starts at index 0. Successive incremental calls produce msg_0, msg_1, etc., colliding with IDs already assigned by earlier calls.

The issue reporter's reproduction shows:

  0: role=user      id='msg_0'
  1: role=assistant id='msg_1'
  2: role=user      id='msg_1'  ← collision!
  3: role=assistant id='msg_2'
  4: role=user      id='msg_1'  ← collision!

Fix

Move the _ensure_message_ids(messages) call from group_messages() (which operates on the slice) to annotate_message_groups() (which has access to the full list). By running it on the full list before slicing, positional indices are globally unique.

# annotate_message_groups():
+    _ensure_message_ids(messages)  # full list — globally unique indices
     spans = group_messages(messages[start_index:])

# group_messages():
-    _ensure_message_ids(messages)  # operated on slice — colliding indices
     spans: list[dict[str, Any]] = []

Test

Added test_incremental_annotation_produces_unique_message_ids which simulates a 4-turn conversation with incremental annotation (matching the CompactionProvider usage pattern) and asserts all message_id values are unique.

[Copilot]

Pull request overview

Fixes a bug in Python compaction grouping where incremental (slice-based) annotation could assign colliding message_id values, breaking downstream group-ID-dependent logic (e.g., SummarizationStrategy).

Changes:

• Moves _ensure_message_ids() invocation from group_messages() (slice scope) to annotate_message_groups() (full-list scope) to prevent ID collisions across incremental calls.
• Updates group_messages() documentation to reflect new caller responsibility for ensuring message_id is set.
• Adds a regression test that simulates incremental annotation over multiple turns and asserts message_id uniqueness.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
python/packages/core/agent_framework/_compaction.py	Adjusts where message IDs are assigned to avoid collisions during incremental grouping/annotation.
python/packages/core/tests/core/test_compaction.py	Adds a regression test ensuring incremental annotation produces unique message_id values.

[Copilot]

group_messages() no longer calls _ensure_message_ids(), which changes behavior for any callers that previously relied on group_messages() to populate missing message_id values. Since group_messages is exported from this module, consider preserving backward compatibility (e.g., keep ensuring IDs here with an offset-aware approach) or clearly documenting this as a breaking precondition change (and/or making group_messages private if it’s not intended for external use).

[Copilot]

Calling _ensure_message_ids(messages) on the full list on every incremental annotate_message_groups() invocation makes ID assignment O(n) per call and can become O(n²) over a growing history. Consider making _ensure_message_ids accept an index offset (or otherwise assign IDs based on absolute position) so you can run it only on the suffix being re-annotated (messages[start_index:]) while still guaranteeing globally unique IDs across incremental calls.

Suggested change

	# Assign message IDs over the *full* list before slicing to avoid
	# index-based collisions across incremental calls (fixes #5237).
	_ensure_message_ids(messages)
	# Preserve globally unique, index-based message IDs without re-scanning
	# the entire history on every incremental suffix re-annotation.
	if start_index == 0:
	_ensure_message_ids(messages)
	else:
	for absolute_index, message in enumerate(messages[start_index:], start=start_index):
	if _message_id(message) is None:
	_write_message_id(message, str(absolute_index))