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!

Start at AsyncWebClient.files_upload_v2 and trace the request sent to files.completeUploadExternal, comparing the markdown block case with blocks that succeed. Reproduce the request from the issue and determine whether the SDK payload or the Slack API is responsible. Done means the cause is isolated and the issue has a verified fix or a documented upstream limitation.