[Michael Y (Michael-Duck)]

Motivation and Context

AgentChat.add_chat_messages() acquires the activity signal before it validates the roles of the incoming messages. When that validation rejects a message (for example a system message, which cannot be part of the chat history), the AgentChatException was raised before the try/finally block that releases the signal. The chat therefore stayed "active" forever: after the caller handled the validation error, every later call failed with Unable to proceed while another agent is active. and the chat could neither be used nor reset.

A recoverable validation error became permanent, which is reported in issue #14638.

Description

The role validation now runs inside the try block that already releases the activity signal, so a rejected message always leaves the chat inactive and usable again. Acquiring the signal stays outside the try, so a rejected competing operation still cannot release the signal of another running operation.

Nothing else changes: the validation still runs before history.messages.extend(messages), so a mixed batch of user and system messages is rejected without a partial append to the history.

Fixes #14638

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

The third box is left unticked only because the complete python/tests/unit suite cannot be collected in my environment: several unrelated modules fail to import under Python 3.14 (tests/unit/test_serialization.py, tests/unit/processes/dapr_runtime/, ...). The tests that cover the changed code pass in full - see below.

Tests

Added test_add_chat_messages_releases_activity_signal_when_a_message_is_rejected to python/tests/unit/agents/test_group_chat/test_agent_chat.py. It fails on the unpatched tree (assert not True for agent_chat.is_active) and passes with the fix.

• pytest python/tests/unit/agents/test_group_chat/ -> 53 passed
• ruff format --check and ruff check on both changed files -> clean

The reproduction script from the issue was also run before and after the change: before the fix the snippet ended with Exception: Unable to proceed while another agent is active.; after the fix it completes, prints False for chat.is_active and adds the corrected message.

[Copilot]

🟢 Approval recommended

The targeted fix preserves validation-before-append behavior and activity ownership, with regression coverage and no unresolved findings.

No open issues 🎉

What changed in this PR

Fixes #14638 by ensuring AgentChat releases its activity signal after rejecting a system message, allowing subsequent operations to proceed.

Changes:

• Moves role validation inside the cleanup scope while keeping activity acquisition outside it.
• Adds a regression test confirming signal release, successful recovery, and exclusion of the rejected message from history.

File	Description
python/​tests/​unit/​agents/​test_group_chat/​test_agent_chat.py	Tests activity release and recovery after message rejection.
python/​semantic_kernel/​agents/​group_chat/​agent_chat.py	Ensures validation failures release the activity signal.

_{Review effort: Balanced}