422 Unprocessable Entity in versions 2.6.0, 2.7.0, 2.8.0 when trying to save an eval case

[shivammathur33]

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Describe the Bug:
#6336 seems to still persist in versions 2.6.0, 2.7.0, 2.8.0.
Upon trying to edit an eval case on an existing eval set, and hitting save (on the top right) this error is seen:
HTTP/1.1" 422 Unprocessable Entity

Steps to Reproduce:
Similar to in #6336 but adding steps anyway:

• adk web
• Go to localhost:8000
• Create a new session
• Once agent responds, got to evals tab and create a new eval (+ new)
• Open eval set, and hit +current session (adds the session to eval set)
• Once added, session shows in the evalset, hit edit and try to edit some message in the session
• Once edit done, hit save on the top right
• Save does not happen. I see this on the logs: HTTP/1.1" 422 Unprocessable Entity

Expected Behavior:
The eval set should update the eval case and give 200 OK

Observed Behavior:
HTTP/1.1" 422 Unprocessable Entity

Environment Details:

• ADK Library Version (pip show google-adk): 2.6.0, 2.7.0, 2.8.0
• Desktop OS:** macOS
• Python Version (python -V): 3.11.6

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-pro

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Regression:
Did this work in a previous version of ADK? If so, which one?
no

[UniversePeak]

I traced the current Web UI update path: the browser sends the edited conversation, sessionInput, and creationTimestamp to the eval-case PUT endpoint, while the backend validates the request as an EvalCase. The reported 422 appears to be in that request-model boundary, but I have not yet reproduced it against the affected releases. Would you be comfortable with me preparing a focused regression test and fix for this issue?

[shivammathur33]

Yes.

[ALDRIN121]

Taking the RubricContent half of this. Checked first: no PR references #7019 and it's unassigned (the earlier #6515 fix only patched InvocationEvent.content).

Root cause I found and reproduced: RubricContent.text_property is typed Optional[str] with no default, so Pydantic v2 still treats it as required. The Web UI GET serializes with response_model_exclude_none=True, dropping text_property: None, and the save PUT then fails validation with 422 — the same required-Optional trap #6515 fixed for InvocationEvent.content but missed here.

PR up shortly with a regression test.