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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