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.