Attachments disappear when resuming thread

[maxchen-002]

Describe the bug
File attachments (message.elements/cl.Message.elements) disappear when resuming thread, even though they can be accessed correctly during live chat sessions. The attachment data are missing from messages after page refresh or thread resumption. message.elements became empty list but file is still showing on the frontend.

To Reproduce
Steps to reproduce the behavior:

1. Start a new Chainlit chat session
2. Upload a file attachment to a message
3. Send the message (message.elements should now have the data)
4. Refresh the page OR navigate to thread history and resume the thread
5. message.elements is now an empty list in the backend code

Expected behavior
File attachment data should still be accessible when resuming thread. message.elements should contain the attachment data

Screenshots
N/A

Desktop (please complete the following information):

• OS: Linux/Windows/macOS (affects all)
• Browser: Chrome, Firefox, Safari (affects all)
• Chainlit Version: 2.9.4+ (Probably older version too)

Additional context
Some what similar to #2584 and #2698. But I'm using the official data layer

[dosubot]

Hi @maxchen-002! I'm Dosu and I’m helping the chainlit team.

This is a known issue with multiple root causes. PR #2799 addresses it comprehensively but is still under review.

Root causes:

1. Missing URL generation during thread resume — During live chat, URLs are generated in useChatSession.ts, but this logic is missing when loading elements from thread history. Elements have chainlitKey but url: null, so the backend sees an empty list.

2. Element serialization bug — Element.from_dict() calls str() on bytes content, corrupting Base64-encoded attachments during deserialization.

3. None value handling — ChainlitDataLayer.get_element() converts None url/objectKey/mime to the string "None", making elements invalid.

The fix in PR #2799 adds URL generation to the resume_thread handler:

if (!element.url && element.chainlitKey) {
  element.url = client.getElementUrl(element.chainlitKey, sessionId);
}

Workaround until the PR is merged: If you're using a custom data layer, ensure you generate read URLs for elements in your get_thread() implementation, similar to how the official ChainlitDataLayer handles it.

This is tracked in #2584 and affects the official data layer, not just custom implementations.

To reply, just mention @dosu.

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