fix: prevent crash when reloading conversations with Base64-encoded files

[hztBUAA]

Summary

Fixes #2784 — Conversations with Base64-encoded file attachments crash on reload.

Root causes identified and fixed:

• Element.from_dict() called str() on bytes content, producing "b'\x89PNG...'" instead of preserving raw bytes
• ChainlitDataLayer.get_element() converted None url/objectKey/mime to the string "None" via str(None)
• Frontend resume_thread handler did not resolve element URLs from chainlitKey, unlike the element and set_sidebar_elements handlers
• Storage client failures during get_thread crashed the entire resume flow — added try/except in both ChainlitDataLayer and DynamoDB data layers
• Socket connection_successful handler had no error handling around thread resume — added try/except with error message to the frontend
• HTTP get_thread endpoint had no error handling — added try/except with 500 response

Changes

File	Change
backend/chainlit/element.py	Preserve bytes content in from_dict() instead of str() conversion
backend/chainlit/data/chainlit_data_layer.py	Fix get_element() None-to-string bug; add storage error handling in get_thread()
backend/chainlit/data/dynamodb.py	Add storage error handling in get_thread()
backend/chainlit/socket.py	Add error handling around thread resume
backend/chainlit/server.py	Add error handling for HTTP get_thread endpoint
libs/react-client/src/useChatSession.ts	Resolve element URLs from chainlitKey in resume_thread handler
backend/tests/data/test_chainlit_data_layer.py	Add tests for element None handling and get_element

Test plan

• _convert_element_row_to_dict preserves None url/objectKey values
• get_element returns None (not string "None") for missing url/objectKey/mime
• Frontend resolves element URLs from chainlitKey on thread resume
• Storage client failures don't crash thread resume
• Manual test: create conversation with Base64 file attachment, reload page

🤖 Generated with Claude Code

---

Summary by cubic

Prevents crashes when reloading conversations with Base64-encoded file attachments. Fixes bytes/None handling, resolves missing element URLs on resume, and adds error handling across the resume/get_thread path.

• Bug Fixes
  • Preserve raw bytes in Element.from_dict() (no str() on bytes).
  • Keep None for url/objectKey/mime in get_element() (no "None" strings).
  • Client resolves element URLs from chainlitKey during resume.
  • Catch storage read-URL failures in ChainlitDataLayer and DynamoDB; continue safely.
  • Wrap thread resume in socket and GET /thread in server with try/except; surface clear errors.
  • Add tests for element None handling and get_element().

^{Written for commit c2c5722. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 7 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="backend/chainlit/socket.py">

<violation number="1" location="backend/chainlit/socket.py:204">
P2: Exception handling for resume_thread emits an error but then falls through to the existing "Thread not found" branch, resulting in two error emissions for the same failure.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: Exception handling for resume_thread emits an error but then falls through to the existing "Thread not found" branch, resulting in two error emissions for the same failure.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At backend/chainlit/socket.py, line 204:

<comment>Exception handling for resume_thread emits an error but then falls through to the existing "Thread not found" branch, resulting in two error emissions for the same failure.</comment>

<file context>
@@ -194,7 +194,15 @@ async def connection_successful(sid):
+            await context.emitter.send_resume_thread_error(
+                "Failed to load conversation history."
+            )
+            thread = None
+
         if thread:
</file context>

[dokterbob]

Would love to merge this, please run uv run ruff format over your code! :)

[dokterbob]

This import might well be at the top! Same for other places.

[dokterbob]

I notice changes in two data layers but unit tests only for one, what's the reason?