Firestore Session Service does not handle timestamp correctly

[Dumeng]

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Describe the Bug:
A clear and concise description of what the bug is.

Steps to Reproduce:

1. Use google.adk.integrations.firestore.firestore_session_service.FirestoreSessionService as sessions service.
2. Call session_service.list_sessions

Expected Behavior:
The returned session's last_update_time should match the Firestore record

Observed Behavior:
The returned session's last_update_time is always 0.

Environment Details:

• ADK Library Version (pip show google-adk): 1.20
• Desktop OS:** [e.g., macOS, Linux, Windows]: Linux
• Python Version (python -V): 3.14

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini

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The issue is the code does not handle the attribute.

firestore_session_service.py

Regression:
Did this work in a previous version of ADK? If so, which one?

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Additional Context:
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Minimal Reproduction Code:
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How often has this issue occurred?:

• Always (100%)

[surajksharma07]

@Dumeng list_sessions() hardcodes last_update_time=0.0 in the Session constructor and never reads data.get("updateTime") from the Firestore document, while get_session() already handles this correctly with an isinstance(update_time, datetime) check (lines 337-346 of firestore_session_service.py).

A workaround worth trying: copy that same timestamp conversion block from get_session() into the for doc in docs: loop in list_sessions(), replacing the hardcoded 0.0 with the extracted value before constructing the Session.

Could you apply that locally and verify last_update_time now reflects the actual Firestore updateTime after calling list_sessions()?