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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.

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What’s installed

The openwebui/ directory contains:
ComponentWhat it doesRequired?
tools/computer_use_tools.pyMCP client tool — thin proxy to the Computer Use ServerYes
functions/computer_link_filter.pySystem-prompt injector + file-link rewriter + archive buttonYes
patches/Build-time fixes for artifacts, error handling, file preview, large tool resultsStrongly recommended
init.shAuto-installs tool + filter on first startupOptional — manual Workspace UI works too
DockerfileBuilds a patched Open WebUI image with auto-initOptional

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Auto-init (bundled compose)

On first docker compose -f docker-compose.webui.yml up, the init script:
  1. Creates an admin user (ADMIN_EMAIL / ADMIN_PASSWORD).
  2. Installs the Computer Use tool via POST /api/v1/tools/create.
  3. Installs the filter via POST /api/v1/functions/create.
  4. Configures both Valves: ORCHESTRATOR_URL=http://computer-use-server:8081 (internal, server↔server).
  5. Marks the tool public-read (grants for both group:* and user:*) so non-admin users see it.
  6. Marks the filter active and global (two separate toggles — active-but-not-global silently does nothing).
  7. Merges {function_calling: "native", stream_response: true} into DEFAULT_MODEL_PARAMS.
A marker file (.computer-use-initialized) prevents re-running.

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Manual setup (standalone Open WebUI)

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

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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.

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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:
PatchWithout it
fix_artifacts_auto_showHTML/iframe renders as raw text instead of the artifacts panel
fix_preview_url_detectionPreview iframe never auto-inserts after file links
fix_tool_loop_errorsRaw exceptions instead of banners
fix_large_tool_resultsTOOL_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"'

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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.
EnvironmentCorrect value
Productioncu.your-domain.com (no scheme — regex wraps it)
Local devlocalhost:8081
Change after build = rebuild the image (--build). The value is compiled in, not read at runtime.

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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):
WhereRoleProdLocal
PUBLIC_BASE_URL on computer-use-serverPUBLIC — baked into /system-prompt + returned via X-Public-Base-URL headerhttps://cu.your-domain.comhttp://localhost:8081
Build-arg COMPUTER_USE_SERVER_URLPUBLIC — compiled into Svelte regexcu.your-domain.com (no scheme)localhost:8081
Valves ORCHESTRATOR_URLINTERNAL — server↔server fetchhttp://computer-use-server:8081http://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.

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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
VarDefaultEffect
CHAT_RESPONSE_MAX_TOOL_CALL_RETRIES30 (upstream)30 truncates Computer Use multi-step tasks; use 200.
TOOL_RESULT_MAX_CHARS50000Truncation threshold above which results are truncated/uploaded.
TOOL_RESULT_PREVIEW_CHARS2000Preview size after truncation/upload.
ORCHESTRATOR_URL(empty)Seeded into both Valves; consumed by fix_large_tool_results. Empty = oversized results silently truncate.