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markdown block type not supported when sending via WebhookClient / response_url #1847
Description
The markdown block type (introduced in Block Kit - Feb 2025, expanded in Mar 2026) works correctly when sent via WebClient.chat_postMessage, but returns a 500 error when sent via WebhookClient (i.e. slash command
response_url).
Reproducible example:
from slack_sdk.webhook import WebhookClient
from slack_sdk import WebClient
blocks = [
{
"type": "markdown",
"text": "## Heading\n\n| Col A | Col B |\n|---|---|\n| 1 | 2 |"
}
]
# ✅ Works — returns 200
client = WebClient(token="xoxb-...")
client.chat_postMessage(channel="#test", blocks=blocks, text="fallback")
# ❌ Fails — returns 500
webhook = WebhookClient("https://hooks.slack.com/commands/T.../...")
webhook.send(blocks=blocks)Expected behavior: WebhookClient.send() should support the markdown block type, same as chat.postMessage.
Actual behavior: Slack's response_url endpoint returns HTTP 500 with no descriptive error body.
Context:
- slack_sdk version: 3.39.0
- Python version: 3.12.12
- This affects slash command responses where response_url is the standard reply mechanism
Category (place an x in each of the [ ])
- slack_sdk.web.WebClient (sync/async) (Web API client)
- slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
- slack_sdk.models (UI component builders)
- slack_sdk.oauth (OAuth Flow Utilities)
- slack_sdk.socket_mode (Socket Mode client)
- slack_sdk.audit_logs (Audit Logs API client)
- slack_sdk.scim (SCIM API client)
- slack_sdk.rtm (RTM client)
- slack_sdk.signature (Request Signature Verifier)
Requirements
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Activity
jcgillet commentedon Mar 30, 2026
Don't know if this is actually a bug or a FR.
zimeg commentedon Mar 30, 2026
👋 Hey @jcgillet! Thank you for taking the time to try this out and share findings here 🎁 ✨
This is a known issue with the backend right now that we're also monitoring at slackapi/slack-github-action#440 🐛
Your workaround with
chat.postMessagemight be the best workaround to use at this time, so let's keep this issue open as ongoing reference! In the meantime I'll suggest sharing backend issues similar with feedback@slack.com 💌github-actions commentedon May 4, 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.
jcgillet commentedon May 4, 2026
Still an issue, blocking a couple of improvements in adding tables as MD in responses.
codedoga commentedon Jul 21, 2026
still an issue, but I don't think this is related to python SDK but the server itself