---
title: How do I check whether I am on Langfuse v4?
seoTitle: "Check Whether Your Langfuse Instance or Project Runs v4"
description: How to tell whether a Langfuse Cloud organization or a self-hosted instance runs Langfuse v4 and its observations-first data model.
tags: [platform, self-hosting, v4]
---

# How do I check whether I am on Langfuse v4?

## Langfuse Cloud

- Organizations created on or after April 14, 2026 run [Langfuse v4](/docs/v4) and do not show a toggle.
- Organizations created before that date show a **Langfuse v4 preview** toggle in the bottom-left corner of the UI until the v4 cutover. When the toggle is on, you see the v4 experience.
- The **Migration Assistant** sidebar panel and the [Migration status page](https://cloud.langfuse.com/v4-migration) list the remaining actions for your projects.

## Self-hosted Langfuse

Two things decide whether your instance is on v4:

1. **Server version.** `GET /api/public/health` returns the running version in the `version` field, for example `"version": "4.75.0"`. The version is also shown in the UI sidebar. Versions starting with `4.` are Langfuse v4.
2. **Write mode.** A v4 server can still run in a migration mode. `LANGFUSE_MIGRATION_V4_WRITE_MODE` controls it:

| `LANGFUSE_MIGRATION_V4_WRITE_MODE` | Meaning                                                                                                                       |
| ---------------------------------- | ----------------------------------------------------------------------------------------------------------------------------- |
| `events_only` (default)            | Fully on v4. All users see the v4 experience.                                                                                 |
| `dual`                             | Migration in progress. Users can switch the v4 preview on and off, unless `LANGFUSE_MIGRATION_V4_ALLOW_PREVIEW_OPT_IN=false`. |
| `legacy`                           | v3 behavior on a v4 server.                                                                                                   |

For the migration steps, see [Migrate Langfuse v3 to v4](/self-hosting/upgrade/upgrade-guides/upgrade-v3-to-v4). To prepare a project, see [Make your project ready for the upgrade to Langfuse v4](/faq/all/upgrade-to-langfuse-v4).

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## Agent Instructions

This page is part of the [Langfuse](https://langfuse.com) documentation, published as plain Markdown for AI agents. Every page is available as Markdown by appending `.md` to its URL, or by sending an `Accept: text/markdown` header. This page: `https://langfuse.com/faq/all/check-if-on-langfuse-v4.md`.

### Querying these docs

If the answer is not on this page, query the documentation instead of guessing:

- **Semantic search** across all Langfuse docs, returning an answer with the relevant pages and excerpts. Ask a specific, self-contained question:

  ```bash
  curl -sG "https://langfuse.com/api/search-docs" --data-urlencode "query=How do I trace a LangGraph agent?"
  ```

- **Index of every page**: <https://langfuse.com/llms.txt>, with per-section indexes [llms-docs.txt](https://langfuse.com/llms-docs.txt), [llms-integrations.txt](https://langfuse.com/llms-integrations.txt), and [llms-self-hosting.txt](https://langfuse.com/llms-self-hosting.txt).

### Before writing Langfuse code

- **Install the [Langfuse Agent Skill](https://langfuse.com/docs/api-and-data-platform/features/agent-skill).** It encodes Langfuse's own best practices for instrumentation, prompt management, and evaluation, and materially improves results.
- **Read [What does a good trace look like?](https://langfuse.com/docs/observability/best-practices.md)** before instrumenting an application.
- **Verify endpoints, parameters, and response fields** against the [API reference](https://api.reference.langfuse.com) instead of inferring them from code examples.
- **Use the [Langfuse CLI](https://langfuse.com/docs/api-and-data-platform/features/cli)** (`npx @langfuse/cli api <resource> <action>`) to read or write traces, prompts, datasets, and scores from the terminal.

Found an error in these docs? Please open an issue at <https://github.com/langfuse/langfuse-docs/issues>.