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Langfuse is an open-source LLM engineering platform for tracing, evaluating, and debugging LLM applications.

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Step 1: Create a Langfuse API key

In Langfuse, go to your project’s Settings > API Keys and create a new key pair. Copy both the Secret Key and Public Key.
Langfuse API Keys

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Step 2: Enable Broadcast in OpenRouter

Go to Settings > Observability and toggle Enable Broadcast.
Enable Broadcast

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Step 3: Configure Langfuse

Click the edit icon next to Langfuse and enter:
  • Secret Key: Your Langfuse Secret Key
  • Public Key: Your Langfuse Public Key
  • Base URL (optional): Default is https://us.cloud.langfuse.com. Change for other regions or self-hosted instances
Langfuse Configuration

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Step 4: Test and save

Click Test Connection to verify the setup. The configuration only saves if the test passes.
Langfuse Configured

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Step 5: Send a test trace

Make an API request through OpenRouter and view the trace in Langfuse.
Langfuse Trace

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Custom Metadata

Langfuse supports rich trace hierarchies and metadata. Use the trace field to customize how your traces appear in Langfuse.

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Supported Metadata Keys

KeyLangfuse MappingDescription
trace_idTrace IDGroup multiple requests into a single trace
trace_nameTrace NameCustom name displayed in the Langfuse trace list
span_nameSpan NameName for intermediate spans in the hierarchy
generation_nameGeneration NameName for the LLM generation observation
parent_span_idParent Observation IDLink to an existing span in your trace hierarchy
environmentEnvironmentPopulates the first-class Environment field used by the Langfuse project filter (e.g. production, staging). Emitted as both a resource attribute and observation attribute.
releaseReleaseApplication release/version associated with the trace. Emitted as both a resource attribute and observation attribute.

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Example

{
  "model": "openai/gpt-4o",
  "messages": [{ "role": "user", "content": "Summarize this document..." }],
  "user": "user_12345",
  "session_id": "session_abc",
  "trace": {
    "trace_id": "workflow_12345",
    "trace_name": "Document Processing Pipeline",
    "span_name": "Summarization Step",
    "generation_name": "Generate Summary",
    "environment": "production",
    "release": "2.1.0",
    "pipeline_version": "2.1.0"
  }
}
This creates a hierarchical trace structure in Langfuse: 
Document Processing Pipeline (trace)
└── Summarization Step (span)
    └── Generate Summary (generation)

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Additional Context

  • The user field maps to Langfuse’s User ID for user-level analytics
  • The session_id field maps to Langfuse’s Session ID for grouping conversations
  • Any additional keys in trace are passed as trace metadata and can be used for filtering and analysis in Langfuse

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Privacy Mode

When Privacy Mode is enabled for this destination, prompt and completion content is excluded from traces. All other trace data — token usage, costs, timing, model information, and custom metadata — is still sent normally. See Privacy Mode for details.