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slack_file image blocks appended with chat.appendStream render as missing images inline #1961
Description
Description
When an image block referencing a Slack-hosted file is appended to a streaming message using chat.appendStream, Slack initially renders a missing-image placeholder.
Clicking the missing image opens and displays the correct image, which indicates that:
- The file ID is valid.
- The upload completed successfully.
- Slack generated the image thumbnail.
- The original image remains accessible.
The same streamed message's DOM contains a valid Slack thumbnail URL, but Slack replaces the inline background image with an unable-to-load-error asset.
This may be a server-side file-sharing/association issue specific to blocks appended through the streaming APIs.
Reproducible in:
The Slack SDK version
3.43.0
Bot scopes:
chat:writefiles:readfiles:write
Python runtime version
3.13.7
OS info
ProductName: macOS
ProductVersion: 26.6.2
BuildVersion: 25G83
Steps to reproduce:
(Share the commands to run, source code, and project settings (e.g., pyproject.toml))
- Upload a PNG to Slack without posting a separate file message:
upload_response = await client.files_upload_v2(
content=png_bytes,
filename="sample.png",
)
file_id = upload_response["file"]["id"]- Wait until files.info reports that the image and its thumbnail are ready:
file_info = await client.files_info(file=file_id)
file = file_info["file"]
assert file["mimetype"] == "image/png"
assert file["filetype"] == "png"
assert file["thumb_360"]
assert file["thumb_360_w"]
assert file["thumb_360_h"]- Start a streaming message:
response = await client.chat_startStream(
channel=channel_id,
thread_ts=thread_ts,
)
stream_ts = response["ts"]- Append an image block using the Slack file ID:
await client.chat_appendStream(
channel=channel_id,
ts=stream_ts,
chunks=[
{
"type": "blocks",
"blocks": [
{
"type": "image",
"slack_file": {
"id": file_id,
},
"alt_text": "Sample Image",
}
],
}
],
)- Stop the stream:
await client.chat_stopStream(
channel=channel_id,
ts=stream_ts,
)Expected result:
The Slack-hosted image should render inline in the completed streaming message.
Actual result:
Slack renders a missing-image placeholder.
Clicking the placeholder opens and displays the correct image.
Inspecting the rendered HTML shows that Slack resolved the file to a generated thumbnail URL:
<a
class="c-link c-message_attachment__image"
href="https://files.slack.com/files-tmb/.../chart_480.png"
style="
background-image:
url('https://a.slack-edge.com/.../unable-to-load-error-....png');
"
>
<span role="img" aria-label="missing image"></span>
</a>
The href points to the generated Slack thumbnail, while the inline background is replaced with Slack's unable-to-load-error image.
Requirements
For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇
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Activity
WilliamBergamin commentedon Sep 9, 2026
Thanks for the detailed repro, @sealogfish 🥇
I followed your steps but couldn't reproduce the broken image, so I'm hoping you can help close the gap.
I uploaded, polled
files.infountil the thumbnails were ready, then sent the image block viachat.appendStream(blocks chunk) in both a DM and a channel withthread_ts. It rendered fine in every case, live and finalized. On the API side,files.info(...).sharesshowed the file shared into the conversation andconversations.repliesreturned an intactimageblock with a populatedimage_url, so the association looks correct in my testing.To pin down what's different about your setup:
Happy to dig in once I can see the failing request
sealogfish commentedon Sep 10, 2026
Hi @WilliamBergamin,
c745cdd438f19de75478cb2dee3d5e2532c0c3b8@1788997653) but it was happening for both
I did try specifying the channel argument in
files_upload_v2(). This results in a upload in the channel (DM) directly, before the file is appended to the stream. However, both images still render properly. The reason for not specifying channel/thread in the function call is because I would like to do formatting of the blocks first before streaming it to the user.I was also observing the network logs and it seems like the first
files.inforeturnsfile_not_found. I don't think this is my code calling the API as there is a subsequent call after that succeeds, and I don't see any logs on my end that suggest that I tried.WilliamBergamin commentedon Sep 10, 2026
Thanks for sharing more information 🙏 I tried reproducing this again but got the same behavior as before, this could be an edge case related to how the app is installed on an organization
I reported this issue internally, hopefully we can get to the bottom of this!
There isn't much we can do inside python-slack-sdk to patch this since it seems like a server side/frontend, but you can also report this through support@slack.com
github-actions commentedon Oct 12, 2026
👋 It looks like this issue has been open for 30 days with no activity. We'll mark this as stale for now, and wait 10 days for an update or for further comment before closing this issue out. If you think this issue needs to be prioritized, please comment to get the thread going again! Maintainers also review issues marked as stale on a regular basis and comment or adjust status if the issue needs to be reprioritized.