langfuse/langfuse · warning

Error

Error message

Error

What it means

The feedback endpoint returns 400 when forwarding feedback to Slack does not come back with status 200 — i.e. Slack's webhook rejected or errored on the request. The Slack response object is logged via logger.error before responding.

Source

Thrown at web/src/features/feedback/server/feedbackHandler.ts:19

import { runFeedbackCorsMiddleware } from "@/src/features/feedback/server/corsMiddleware";
import { sendToSlack } from "@/src/features/slack/server/slack-webhook";
import { type NextApiRequest, type NextApiResponse } from "next";
import { logger } from "@langfuse/shared/src/server";

// Legacy endpoint used by older self-hosted Langfuse versions.
export default async function feedbackApiHandler(
  req: NextApiRequest,
  res: NextApiResponse,
) {
  await runFeedbackCorsMiddleware(req, res);

  try {
    const slackResponse = await sendToSlack(req.body);
    if (slackResponse.status === 200) {
      res.status(200).json({ status: "OK" });
    } else {
      logger.error(slackResponse);
      res.status(400).json({ status: "Error" });
    }
  } catch (error) {
    logger.error(error);
    res.status(500).json({ status: "Error" });
  }
}

View on GitHub (pinned to 59d92c7cf3)

Solutions

1. Check the web container logs — the full slackResponse (including Slack's error body) is logged
2. Verify SLACK_WEBHOOK_URL is set and still valid (test with curl -X POST)
3. Re-create the Slack incoming webhook if it was revoked and update the env var
4. Retry if Slack reported a transient 429/5xx

const webhook = process.env.SLACK_WEBHOOK_URL;
if (!webhook || !webhook.startsWith('https://hooks.slack.com/')) {
  // skip feedback send or queue locally
}

When it happens

Trigger: POST /api/feedback with SLACK_WEBHOOK_URL unset/invalid (Slack returns 4xx), malformed payload for the hook, or Slack API downtime, causing sendToSlack's response status to differ from 200.

Common situations: Self-hosted deployments with a stale or revoked Slack incoming webhook URL, missing SLACK_WEBHOOK_URL env var, or Slack rate-limiting the hook.

Related errors

• protocol-not-allowed
• port-not-allowed
• [Stripe Webhook] NEXT_PUBLIC_LANGFUSE_CLOUD_REGION is not se
• [Stripe Webhook] Stripe client not found
• [Stripe Webhook] NEXT_PUBLIC_LANGFUSE_CLOUD_REGION is not se

AI-assisted analysis of langfuse/langfuse@59d92c7cf3 (2026-08-27). Data as JSON: /api/errors/eb9219bcaaa74611. Report an issue: GitHub.