Open WebUI

Open WebUI is the primary tested frontend for Computer Use. Managed Yambr runs it at chat.yambr.com. Self-hosters run it from docker-compose.webui.yml. Compatibility: Tested with Open WebUI v0.8.11–0.8.12. Set OPENWEBUI_VERSION in .env to pin. Not a fork. Everything is bolted on via the official plugin API (tools + functions) + build-time patches for missing features. Stock Open WebUI v0.8.11–0.8.12 works if you install the tool and filter. Patches are applied at Docker build time — strongly recommended; 4 of them affect user-visible UX.

What’s installed

The openwebui/ directory contains:

Component	What it does	Required?
`tools/computer_use_tools.py`	MCP client tool — thin proxy to the Computer Use Server	Yes
`functions/computer_link_filter.py`	System-prompt injector + file-link rewriter + archive button	Yes
`patches/`	Build-time fixes for artifacts, error handling, file preview, large tool results	Strongly recommended
`init.sh`	Auto-installs tool + filter on first startup	Optional — manual Workspace UI works too
`Dockerfile`	Builds a patched Open WebUI image with auto-init	Optional

Auto-init (bundled compose)

On first docker compose -f docker-compose.webui.yml up, the init script:

Creates an admin user (ADMIN_EMAIL / ADMIN_PASSWORD).
Installs the Computer Use tool via POST /api/v1/tools/create.
Installs the filter via POST /api/v1/functions/create.
Configures both Valves: ORCHESTRATOR_URL=http://computer-use-server:8081 (internal, server↔server).
Marks the tool public-read (grants for both group:* and user:*) so non-admin users see it.
Marks the filter active and global (two separate toggles — active-but-not-global silently does nothing).
Merges {function_calling: "native", stream_response: true} into DEFAULT_MODEL_PARAMS.

A marker file (.computer-use-initialized) prevents re-running.

Manual setup (standalone Open WebUI)

Workspace → Tools → Create → paste openwebui/tools/computer_use_tools.py.
Set Tool ID = ai_computer_use (the filter looks for this).
Configure Valves: ORCHESTRATOR_URL = internal URL of the Computer Use Server.
Open the tool’s ⋯ → Share → set access to Public (grants read to both group:* and user:*).
Workspace → Functions → Create → paste openwebui/functions/computer_link_filter.py.
Toggle the function Active and Global (two separate switches).
In model settings, set Function Calling = Native and Stream Chat Response = On — or globally in Admin → Settings → Models → Advanced Params.

Four traps when embedding Open WebUI in your own stack

If you run Open WebUI outside the stock compose file — downstream compose, Kubernetes, Portainer — four things silently break Computer Use. In order.

1. Build from `openwebui/Dockerfile`, don’t pull upstream

Pulling ghcr.io/open-webui/open-webui:vX.Y.Z gives you a stock image without the patches:

Patch	Without it
`fix_artifacts_auto_show`	HTML/iframe renders as raw text instead of the artifacts panel
`fix_preview_url_detection`	Preview iframe never auto-inserts after file links
`fix_tool_loop_errors`	Raw exceptions instead of banners
`fix_large_tool_results`	`TOOL_RESULT_MAX_CHARS` doesn’t truncate; large outputs wreck context

Use build: mirroring docker-compose.webui.yml:11-15:

services:
  open-webui:
    build:
      context: ./openwebui
      dockerfile: Dockerfile
      args:
        OPENWEBUI_VERSION: "0.8.12"
        COMPUTER_USE_SERVER_URL: "cu.your-domain.com"
    image: open-webui-with-cu-patches:latest

Verify patches are in the running container:

docker exec open-webui bash -c \
  'grep -l "bn.set(!0),Jr.set(!0)" /app/build/_app/immutable/chunks/*.js >/dev/null \
   && echo "patches applied" || echo "MISSING — stock upstream image"'

2. `COMPUTER_USE_SERVER_URL` must be the PUBLIC domain

This is the confusing one. COMPUTER_USE_SERVER_URL is a build argument — not a network endpoint. It’s compiled into a regex inside the minified Svelte chunks by fix_preview_url_detection. The regex matches {COMPUTER_USE_SERVER_URL}/(files|preview)/... in assistant messages. The model writes whatever URL the server injected into the system prompt — the server’s PUBLIC_BASE_URL — which is your public domain. So the regex must match that.

Environment	Correct value
Production	`cu.your-domain.com` (no scheme — regex wraps it)
Local dev	`localhost:8081`

Change after build = rebuild the image (--build). The value is compiled in, not read at runtime.

3. Three URL settings, two roles

v4.0.0 simplified the old “three FILE_SERVER_URL places” footgun. Three places, two roles — public (browser-reachable) vs internal (Docker-local):

Where	Role	Prod	Local
`PUBLIC_BASE_URL` on `computer-use-server`	PUBLIC — baked into `/system-prompt` + returned via `X-Public-Base-URL` header	`https://cu.your-domain.com`	`http://localhost:8081`
Build-arg `COMPUTER_USE_SERVER_URL`	PUBLIC — compiled into Svelte regex	`cu.your-domain.com` (no scheme)	`localhost:8081`
Valves `ORCHESTRATOR_URL`	INTERNAL — server↔server fetch	`http://computer-use-server:8081`	`http://computer-use-server:8081`

Do NOT point ORCHESTRATOR_URL at your public domain. It works but every MCP request then goes browser → CDN → Traefik → container, and any hiccup in that chain kills the stream with MCP call failed: Session terminated. Stay inside the Docker network.Do NOT set the build-arg to the internal service name. The regex will look for computer-use-server:8081/files/... but the model writes the public domain.

4. Four env vars on the `open-webui` container

services:
  open-webui:
    environment:
      - CHAT_RESPONSE_MAX_TOOL_CALL_RETRIES=200
      - TOOL_RESULT_MAX_CHARS=50000
      - TOOL_RESULT_PREVIEW_CHARS=2000
      - ORCHESTRATOR_URL=http://computer-use-server:8081

Var	Default	Effect
`CHAT_RESPONSE_MAX_TOOL_CALL_RETRIES`	`30` (upstream)	30 truncates Computer Use multi-step tasks; use `200`.
`TOOL_RESULT_MAX_CHARS`	`50000`	Truncation threshold above which results are truncated/uploaded.
`TOOL_RESULT_PREVIEW_CHARS`	`2000`	Preview size after truncation/upload.
`ORCHESTRATOR_URL`	(empty)	Seeded into both Valves; consumed by `fix_large_tool_results`. Empty = oversized results silently truncate.

Open WebUI filter reference
Claude Code gateway — routing Claude Code through LiteLLM

Getting started

Demo course

Yambr Platform

Self-hosting

Features

Skills

Integrations

Reference

What’s installed

Auto-init (bundled compose)

Manual setup (standalone Open WebUI)

Four traps when embedding Open WebUI in your own stack

1. Build from `openwebui/Dockerfile`, don’t pull upstream

2. `COMPUTER_USE_SERVER_URL` must be the PUBLIC domain

3. Three URL settings, two roles

4. Four env vars on the `open-webui` container

​What’s installed

​Auto-init (bundled compose)

​Manual setup (standalone Open WebUI)

​Four traps when embedding Open WebUI in your own stack

​1. Build from openwebui/Dockerfile, don’t pull upstream

​2. COMPUTER_USE_SERVER_URL must be the PUBLIC domain

​3. Three URL settings, two roles

​4. Four env vars on the open-webui container

​Related

What’s installed

Auto-init (bundled compose)

Manual setup (standalone Open WebUI)

Four traps when embedding Open WebUI in your own stack

1. Build from `openwebui/Dockerfile`, don’t pull upstream

2. `COMPUTER_USE_SERVER_URL` must be the PUBLIC domain

3. Three URL settings, two roles

4. Four env vars on the `open-webui` container

Related