Quick reference for the top 10 most common Langfuse errors and their solutions.
Check error message and code in your logs or console.
Match your error to one of the documented cases.
Follow the solution steps for your specific error.
Error Message:
Langfuse: Unauthorized - Invalid API key
Error: 401 Unauthorized # HTTP 401 Unauthorized
Cause: API key is missing, expired, or mismatched with host.
Solution:
set -euo pipefail
# 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)"
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();
});
Error Message:
FetchError: request to https://cloud.langfuse.com failed
ECONNREFUSED / ETIMEDOUT
Cause: Network connectivity or firewall issues.
Solution:
set -euo pipefail
# 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 # 30000: 30 seconds in ms
});
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,
},
});
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();
}
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();
Error Message:
TypeError: langfuse.trace is not a function
Property 'observeOpenAI' does not exist
Cause: Outdated SDK or breaking API change.
Solution:
set -euo pipefail
# 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
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" });
Error Message:
Failed to connect to localhost:3000 # 3000: 3 seconds in ms
Certificate verification failed
Cause: Self-hosted instance not running or SSL issues.
Solution:
set -euo pipefail
# Check if Langfuse is running
docker ps | grep langfuse
curl http://localhost:3000/api/public/health # 3000: 3 seconds in ms
# 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 # 3000: 3 seconds in ms
});
Error Message:
Error: 429 Too Many Requests # HTTP 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 # 30000: 30 seconds in ms
});
// See langfuse-rate-limits skill for advanced handling
set -euo pipefail
# 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 # HTTP 200 OK
langfuse-debug-bundle
For comprehensive debugging, see langfuse-debug-bundle.
Basic usage: Apply langfuse common errors to a standard project setup with default configuration options.
Advanced scenario: Customize langfuse common errors for production environments with multiple constraints and team-specific requirements.