files_upload_v2 returns internal_error when using markdown in blocks

AsyncWebClient.files_upload_v2 から開始し、files.completeUploadExternal に送信されるリクエストを追跡して、markdown ブロックを成功するブロックと比較します。Issue にあるリクエストを再現し、SDK の payload と Slack API のどちらに原因があるかを判断します。原因が切り分けられ、issue に検証済みの修正または文書化された upstream の制限があることを完了条件とします。

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!