** Please make sure you read the contribution guide and file the issues in the right place. **
Contribution guide.

Describe the bug
ADK crashes with the error 'dict' object has no attribute 'start_timestamp' when event compaction is enabled in v1.18.0.

To Reproduce
Please share a minimal code and data to reproduce your problem.
Steps to reproduce the behavior:

1. Install google-adk==1.18.0
2. Enable event compaction in the ADK configuration, for example: events_compaction_config=EventsCompactionConfig( compaction_interval=1, overlap_size=0, )
3. Start a session and begin chatting.
4. After follow-ups exceed the compaction interval, the crash occurs.

Expected behavior
ADK should not crash; chatting should continue normally with event compaction enabled.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. macOS, Linux, Windows]
• Python version(python -V):
• ADK version(pip show google-adk):

Model Information:

• Are you using LiteLLM: Yes/No
• Which model is being used(e.g. gemini-2.5-pro)

Additional context

• As a temporary workaround, I’ve removed the compaction config.
• A PR has been sent to fix this issue: fix(sessions): use model_validate to deserialize nested EventCompaction #3490