docs: add webhook implementation details for remote SDK experiments

[daffaalex22]

Summary

This PR expands the documentation for "Trigger SDK Experiment from UI" to provide a complete guide for developers setting up remote experiment triggers. It fills the gap between configuring the UI and actually implementing the webhook receiver that processes the trigger.

Core Highlight: Webhook Payload

The most important addition is the technical specification of the webhook payload. Specifically:

• Stringified JSON: It explicitly documents that the payload field is sent as a stringified JSON string (not a nested object). This is a critical detail for proper parsing on the receiver side (e.g., using JSON.parse or json.loads).
• Flexible Schema: It clarifies that the contents of the payload field are entirely user-defined and optional. Langfuse does not enforce a schema; it simply passes through the JSON configuration defined in the UI.

Detailed Changes

• Webhook Payload Section: Added a JSON example showing the raw structure, a field reference table, and a callout about the stringified payload.
• Implementation Examples: Added minimal, functional examples in both Python (Flask) and JS/TS (Express). These show how to:
  • Parse the stringified payload.
  • Respond with 200 OK promptly (required by Langfuse).
  • Execute the experiment runner asynchronously in the background.
• Workflow Overview: Added a step-by-step "End-to-End Workflow" to help developers visualize the data flow from the Langfuse UI to their server and back.
• UI Step Refinements: Polished the existing instructions for setting up the trigger in the Langfuse dashboard to ensure clarity.

Rationale

While the "Custom Experiment" trigger is a powerful feature, developers often struggle with the "how-to" of the receiving end. Providing the exact payload shape makes it much easier for users to integrate their own evaluation pipelines.

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Important

Enhances documentation with detailed webhook setup and implementation examples for remote SDK experiments in Langfuse.

• Documentation Enhancements:
  • Adds detailed instructions for setting up webhook triggers in Langfuse UI in experiments-via-sdk.mdx.
  • Specifies that the webhook payload is a stringified JSON, requiring parsing on the receiver side.
  • Clarifies that the payload schema is user-defined and optional.
• Implementation Examples:
  • Provides Python (Flask) and JS/TS (Express) examples for webhook receivers.
  • Demonstrates parsing the payload, responding with 200 OK, and running experiments asynchronously.
• Workflow Overview:
  • Describes end-to-end workflow from Langfuse UI trigger to server processing and result tracking.

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[daffaalex22]

@jannikmaierhoefer @Steffen911 @felixkrrr

I was trying to implement the Webhook for Remote SDK Experiment Run and confused with the request body Langfuse sends. This will hopefully help others like me

https://langfuse.com/docs/evaluation/experiments/experiments-via-sdk#optional-trigger-sdk-experiment-from-ui