files_upload_v2 returns internal_error when using markdown in blocks

`files_upload_v2` returns `internal_error` when using `markdown` in `blocks` parameter

Reproducible in:

slack-bolt==1.25.0
slack-sdk==3.36.0
Python 3.13.5
ProductName: macOS
ProductVersion: 15.5
BuildVersion: 24F74
Darwin Kernel Version 24.5.0

The Slack SDK version

slack-bolt==1.25.0
slack-sdk==3.36.0

Python runtime version

Python 3.13.5

OS info

ProductName:		macOS
ProductVersion:		15.5
BuildVersion:		24F74
Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:27 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6041

Steps to reproduce:

1. Code Example

from slack_sdk.web.async_client import AsyncWebClient
import json

# Initialize client
client = AsyncWebClient(token="xoxb-your-token")

# Prepare parameters
channel_id = "D099T7G7VHD"  # DM channel
blocks = [{"type": "markdown", "text": "_**Haim Raitsev** posted a new message_\\n> test"}]
files = [{"id": "F09J0L8TDRQ", "title": "screenshot.png"}]

# Attempt to upload with blocks
response = await client.files_upload_v2(
    channel=channel_id,
    file_uploads=[
        {
            "content": b"<file_bytes_here>",
            "filename": "screenshot.png",
            "title": "screenshot.png"
        }
    ],
    blocks=json.dumps(blocks)
)

2. Response

{
    "ok": False,
    "error": "internal_error"
}

Expected result:

The file should be uploaded successfully with the rich text blocks displayed in the message, as the underlying `files.completeUploadExternal`:

Actual result:

The API returns {"ok": false, "error": "internal_error"} without any additional error details.

Additional Context:

This issue is reproduced only when we have a new markdown block (https://docs.slack.dev/reference/block-kit/blocks/markdown-block/) when we use other blocks the message posted correctly.

`files.completeUploadExternal` API Call

HTTP Request

POST https://slack.com/api/files.completeUploadExternal HTTP/1.1
Host: slack.com
Authorization: Bearer xoxb-your-token-here
Content-Type: application/x-www-form-urlencoded

Request Details

Method: POST

URL: https://slack.com/api/files.completeUploadExternal

Headers:

Authorization: Bearer xoxb-your-token-here
Content-Type: application/x-www-form-urlencoded

Body (URL-encoded):

files=[{"id":"F09J0L8TDRQ","title":"screenshot.png"}]
channel_id=D099T7G7VHD
blocks=[{"type":"markdown","text":"_**Haim Raitsev** posted a new message_\\n> test"}]

Body Parameters Breakdown

Parameter	Value	Description
`files`	`[{"id":"F09J0L8TDRQ","title":"screenshot.png"}]`	JSON array of file objects (URL-encoded)
`channel_id`	`D099T7G7VHD`	DM channel ID where file will be shared
`blocks`	`[{"type":"markdown","text":"_Haim Raitsev posted a new message_\\n> test"}]`	JSON array of blocks (URL-encoded)

Example using cURL

curl -X POST https://slack.com/api/files.completeUploadExternal \
  -H "Authorization: Bearer xoxb-your-token-here" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data-urlencode 'files=[{"id":"F09J0L8TDRQ","title":"screenshot.png"}]' \
  --data-urlencode 'channel_id=D099T7G7VHD' \
  --data-urlencode 'blocks=[{"type":"markdown","text":"_**Haim Raitsev** posted a new message_\\n> test"}]'

Error Response (Current Issue)

{
    "ok": false,
    "error": "internal_error"
}

Thank you for maintaining this excellent SDK!

any update?

@haimrait

Hey @haimrait! 👋 Thanks for reporting this and a kind bump 👾 ✨

I can confirm that the files.completeUploadExternal method errors when using the markdown block which seems off to me.

We're investigating this on the backend side of things now, but I might recommend using different blocks in the meantime.

Also, we need to support this argument from the SDK itself I believe - following slackapi/node-slack-sdk#2261. This might be unrelated, but let's track it here for now 🎁

Hi, any updates? Thanks!

👋 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.

Just reporting that this is still an issue

It would be amazing if this issue could be fixed. It still exists, and it's affecting my team.

We're investigating this on the backend side of things now, but I might recommend using different blocks in the meantime.

Which blocks?

docs: note Slack markdown block failure on files.completeUploadExtern…

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files_upload_v2 returns internal_error when using markdown in blocks #1756

`files_upload_v2` returns `internal_error` when using `markdown` in `blocks` parameter

Reproducible in:

The Slack SDK version

Python runtime version

OS info

Steps to reproduce:

1. Code Example

2. Response

Expected result:

The file should be uploaded successfully with the rich text blocks displayed in the message, as the underlying `files.completeUploadExternal`:

Actual result:

Additional Context:

`files.completeUploadExternal` API Call

HTTP Request

Request Details

Body Parameters Breakdown

Example using cURL

Error Response (Current Issue)

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files_upload_v2 returns internal_error when using markdown in blocks #1756

Description

files_upload_v2 returns internal_error when using markdown in blocks parameter

Reproducible in:

The Slack SDK version

Python runtime version

OS info

Steps to reproduce:

1. Code Example

2. Response

Expected result:

The file should be uploaded successfully with the rich text blocks displayed in the message, as the underlying files.completeUploadExternal:

Actual result:

Additional Context:

files.completeUploadExternal API Call

HTTP Request

Request Details

Body Parameters Breakdown

Example using cURL

Error Response (Current Issue)

Activity

haimrait commented on Oct 19, 2025

zimeg commented on Oct 21, 2025

github-actions commented on Nov 24, 2025

razlo commented on Nov 30, 2025

github-actions commented on Jan 5, 2026

kperkins-ss commented on Jan 12, 2026

daniel-daydream commented on Jul 14, 2026

luxalpa commented on Sep 7, 2026

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`files_upload_v2` returns `internal_error` when using `markdown` in `blocks` parameter

The file should be uploaded successfully with the rich text blocks displayed in the message, as the underlying `files.completeUploadExternal`:

`files.completeUploadExternal` API Call