Problem

When saving team state to JSON, users encountered a serialization error due to datetime objects in messages:

# This was failing with "Object of type datetime is not JSON serializable"
team_state = await team.save_state()
with open(state_file, "w") as f:
    json.dump(team_state, f)  # TypeError: Object of type datetime is not JSON serializable

The issue occurred because BaseChatMessage and BaseAgentEvent classes both have a created_at field of type datetime, which is not JSON serializable by default.

Solution

Changed the BaseMessage.dump() method to use Pydantic's JSON-aware serialization mode:

# Before
return self.model_dump()

# After  
return self.model_dump(mode="json")

This leverages Pydantic's built-in capability to automatically convert datetime objects to ISO 8601 format strings during serialization.

Benefits

• ✅ Minimal change: Only 1 line changed + documentation update
• ✅ Surgical fix: Targets the exact source of the problem
• ✅ Backward compatible: All existing functionality works unchanged
• ✅ Standards compliant: Uses ISO 8601 datetime format
• ✅ Round-trip safe: Load/dump operations preserve data correctly

Testing

Verified the fix with comprehensive tests covering:

• Original user scenario now works without errors
• All message types serialize/deserialize correctly
• Team state saving/loading works end-to-end
• Backward compatibility maintained for existing APIs
• Round-trip load/dump operations preserve all data

Example of working code after the fix:

# Create team and run task
team = Swarm([project_agent], termination_condition=termination)
result = await team.run(task="What is a Montague?")

# Save state to JSON - now works without errors!
team_state = await team.save_state()
with open(state_file, "w") as f:
    json.dump(team_state, f)  # ✅ Success!

# Load state back
with open(state_file, "r") as f:
    loaded_state = json.load(f)

Fixes #6793.

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