Session initialization failed in Gemini Enterprise after Agent Engine runtime update (was working previously) #4538
Description
Description
My ADK agent deployed to Vertex AI Agent Engine works correctly in the Vertex AI Playground but fails with Session initialization failed when invoked through Gemini Enterprise (Agentspace). No code or configuration changes were made — it broke on its own around Feb 17-18, 2026, indicating an Agent Engine runtime update caused the regression.
Tested with google-adk==1.23.0 and google-adk==1.24.0 — same error on both versions.
Environment
- google-adk version: 1.24.0 (also tested 1.23.0)
- google-cloud-aiplatform version: 1.137.0
- Python version: 3.12
- Deployment: Vertex AI Agent Engine via
gcloudCLI - Project: snappy-guard-410305
- Location: us-central1
- Agent Engine ID: 157119112097562624
Error / Stack Trace
File "/code/app/api/factory/python_file_api_builder.py", line 473, in _async_stream_json_generator
async for chunk in output:
File "/home/appuser/.cache/pypoetry/virtualenvs/reasoning-engine-assembly-service-MATOk_fk-py3.12/lib/python3.12/site-packages/vertexai/agent_engines/templates/adk.py", line 1212, in streaming_agent_run_with_events
raise RuntimeError("Session initialization failed.")
RuntimeError: Session initialization failed.
Steps to Reproduce
Deploy any ADK agent to Vertex AI Agent Engine with google-adk==1.24.0
Verify it works in the Vertex AI Playground
Register it with Gemini Enterprise (Agentspace)
Invoke the agent through Gemini Enterprise
Observe "Session initialization failed" error
Additional Context
This was working until approximately Feb 17, 2026
Vertex AI Playground continues to work (likely uses the in-memory session path without session_id)