langfuse-common-errors

name	langfuse-common-errors
description	Diagnose and fix common Langfuse errors and exceptions. Use when encountering Langfuse errors, debugging missing traces, or troubleshooting integration issues. Trigger with phrases like "langfuse error", "fix langfuse", "langfuse not working", "debug langfuse", "traces not appearing".
allowed-tools	Read, Grep, Bash(curl:*)
version	1.0.0
license	MIT
author	Jeremy Longshore <[e-mail]>

Langfuse Common Errors

Overview

Quick reference for the top 10 most common Langfuse errors and their solutions.

Prerequisites

• Langfuse SDK installed
• API credentials configured
• Access to error logs

Instructions

Step 1: Identify the Error

Check error message and code in your logs or console.

Step 2: Find Matching Error Below

Match your error to one of the documented cases.

Step 3: Apply Solution

Follow the solution steps for your specific error.

Output

• Identified error cause
• Applied fix
• Verified resolution

Error Handling

1. Authentication Failed

Error Message:

Langfuse: Unauthorized - Invalid API key
Error: 401 Unauthorized

Cause: API key is missing, expired, or mismatched with host.

Solution:

# Verify environment variables are set
echo "Public: $LANGFUSE_PUBLIC_KEY"
echo "Secret: ${LANGFUSE_SECRET_KEY:0:10}..."  # Partial for security
echo "Host: $LANGFUSE_HOST"

# Test connection
curl -X GET "$LANGFUSE_HOST/api/public/health" \
  -H "Authorization: Basic $(echo -n "$LANGFUSE_PUBLIC_KEY:$LANGFUSE_SECRET_KEY" | base64)"

---

2. Traces Not Appearing in Dashboard

Symptom: Code runs without errors but traces don't show up.

Cause: Data not flushed before process exits.

Solution:

// Always flush before exit
await langfuse.flushAsync();

// Or use shutdown for clean exit
await langfuse.shutdownAsync();

// Register shutdown handler
process.on("beforeExit", async () => {
  await langfuse.shutdownAsync();
});

---

3. Network/Connection Errors

Error Message:

FetchError: request to https://cloud.langfuse.com failed
ECONNREFUSED / ETIMEDOUT

Cause: Network connectivity or firewall issues.

Solution:

# Test connectivity
curl -v https://cloud.langfuse.com/api/public/health

# Check DNS resolution
nslookup cloud.langfuse.com

# For self-hosted, verify host URL
curl -v $LANGFUSE_HOST/api/public/health

// Increase timeout if needed
const langfuse = new Langfuse({
  requestTimeout: 30000, // 30 seconds
});

---

4. Missing Token Usage

Symptom: Generations appear but token counts are 0.

Cause: Usage data not captured from LLM response.

Solution:

// For streaming, enable usage in options
const stream = await openai.chat.completions.create({
  model: "gpt-4",
  messages,
  stream: true,
  stream_options: { include_usage: true }, // Important!
});

// Manually add usage if not available
generation.end({
  output: content,
  usage: {
    promptTokens: response.usage?.prompt_tokens || 0,
    completionTokens: response.usage?.completion_tokens || 0,
  },
});

---

5. Trace/Span Not Ending

Symptom: Traces show as "in progress" indefinitely.

Cause: Missing .end() call on spans/generations.

Solution:

// Always end in try/finally
const span = trace.span({ name: "operation" });
try {
  const result = await doWork();
  span.end({ output: result });
  return result;
} catch (error) {
  span.end({ level: "ERROR", statusMessage: String(error) });
  throw error;
}

// Or use finally
const span = trace.span({ name: "operation" });
try {
  return await doWork();
} finally {
  span.end();
}

---

6. Duplicate Traces

Symptom: Same operation creates multiple traces.

Cause: Client instantiated multiple times or missing singleton.

Solution:

// Use singleton pattern
let langfuseInstance: Langfuse | null = null;

export function getLangfuse(): Langfuse {
  if (!langfuseInstance) {
    langfuseInstance = new Langfuse();
  }
  return langfuseInstance;
}

// Don't create new instance per request
// BAD: const langfuse = new Langfuse(); // in handler
// GOOD: const langfuse = getLangfuse();

---

7. SDK Version Mismatch

Error Message:

TypeError: langfuse.trace is not a function
Property 'observeOpenAI' does not exist

Cause: Outdated SDK or breaking API change.

Solution:

# Check current version
npm list langfuse

# Update to latest
npm install langfuse@latest

# Check changelog for breaking changes
# https://github.com/langfuse/langfuse-js/releases

---

8. Environment Variable Not Loaded

Error Message:

Langfuse: Missing required configuration - publicKey

Cause: .env file not loaded or wrong variable names.

Solution:

# Verify .env file exists and has correct names
cat .env | grep LANGFUSE

# Required variable names:
# LANGFUSE_PUBLIC_KEY=pk-lf-...
# LANGFUSE_SECRET_KEY=sk-lf-...
# LANGFUSE_HOST=https://cloud.langfuse.com

// Load .env in your app
import "dotenv/config"; // At top of entry file

// Or specify env file
import { config } from "dotenv";
config({ path: ".env.local" });

---

9. Self-Hosted Connection Issues

Error Message:

Failed to connect to localhost:3000
Certificate verification failed

Cause: Self-hosted instance not running or SSL issues.

Solution:

# Check if Langfuse is running
docker ps | grep langfuse
curl http://localhost:3000/api/public/health

# For HTTPS without valid cert
export NODE_TLS_REJECT_UNAUTHORIZED=0  # Development only!

// Verify host URL doesn't have trailing slash
const langfuse = new Langfuse({
  baseUrl: "http://localhost:3000", // No trailing slash
});

---

10. Rate Limiting

Error Message:

Error: 429 Too Many Requests
Retry-After: 60

Cause: Exceeded Langfuse API rate limits.

Solution:

// Increase batch size to reduce requests
const langfuse = new Langfuse({
  flushAt: 50,        // Batch more events
  flushInterval: 30000, // Flush less often
});

// See langfuse-rate-limits skill for advanced handling

Quick Diagnostic Commands

# Check Langfuse cloud status
curl -s https://status.langfuse.com

# Verify API connectivity
curl -I https://cloud.langfuse.com/api/public/health

# Check local environment
env | grep LANGFUSE

# Test auth
curl -X GET "https://cloud.langfuse.com/api/public/traces" \
  -H "Authorization: Basic $(echo -n "$LANGFUSE_PUBLIC_KEY:$LANGFUSE_SECRET_KEY" | base64)" \
  | head -c 200

Escalation Path

1. Collect evidence with langfuse-debug-bundle
2. Check Langfuse Status
3. Search GitHub Issues
4. Join Discord Community
5. Contact support with debug bundle

Resources

• Langfuse Troubleshooting
• Langfuse GitHub Issues
• Langfuse Discord

Next Steps

For comprehensive debugging, see langfuse-debug-bundle.