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The Computer Use Filter (openwebui/functions/computer_link_filter.py) is the only integration point between Open WebUI and the Computer Use Server. Two jobs:
  • inlet() — when the ai_computer_use tool is active, fetch the server-baked system prompt over HTTP and inject it into the conversation.
  • outlet() — decorate assistant messages that contain sandbox file URLs with a markdown preview link and an archive-download link. On patched Open WebUI builds the preview link auto-promotes to an inline artifact. On stock builds it stays a clickable link — either way the user reaches the preview without a rebuild.
The filter is the single source of truth for the client-side URL shape; the server owns the prompt content and preview rendering.

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Installation

  1. Open WebUI → Admin Panel → Functions → New Function.
  2. Paste the entire contents of openwebui/functions/computer_link_filter.py.
  3. Save; toggle the function Active and Global for every model.
  4. Override any Valve if needed — defaults work end-to-end as long as the Computer Use Server is reachable at http://localhost:8081 (or your ORCHESTRATOR_URL).
Upstream docs: Open WebUI Functions.

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Valves reference

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Two URL roles

v4.0.0 removed the old “two FILE_SERVER_URL” footgun. There is now one public URL owned by the server (PUBLIC_BASE_URL), delivered to the filter via the X-Public-Base-URL response header on /system-prompt. The filter has no public-URL Valve. See Open WebUI integration for the full embedding checklist.

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How the preview gets rendered

outlet() appends a markdown link [🖥️ Open preview]({PUBLIC_BASE_URL}/preview/{chat_id}) to every assistant message that references a sandbox file.
  • Stock Open WebUI: link stays a link. User clicks it → preview opens in a new tab.
  • Patched Open WebUI (our docker-compose.webui.yml build): fix_preview_url_detection sees /preview/{chat_id} in the message text and pushes a synthetic <iframe> into htmlGroups. fix_artifacts_auto_show then auto-opens the side panel. No click.
v4.1.0 breaking change. Earlier versions embedded a raw fenced <iframe> block in messages. That was a foot-gun (the patch’s !htmlGroups.some(o=>o.html) guard skipped detection when the block was present). v4.1.0 uses only the markdown link + the patch. PREVIEW_MODE="artifact" and "both" were removed — re-seed Valves after upgrading.

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System prompt injection

INJECT_SYSTEM_PROMPT=True (default): when the ai_computer_use tool is active and chat_id is present, the filter HTTP-fetches {ORCHESTRATOR_URL}/system-prompt and injects it into the system message. 5-minute LRU cache, stale-cache fallback on transport failure. The X-Public-Base-URL response header is cached alongside the prompt so outlet() can build browser-facing links without its own Valve. Non-http(s) schemes on ORCHESTRATOR_URL are rejected to block file:// / ftp:// information-disclosure paths.

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Archive button

ARCHIVE_BUTTON="on" (default): when an assistant message contains at least one URL matching {PUBLIC_BASE_URL}/files/{chat_id}/..., outlet() appends a markdown link to /files/{chat_id}/archive. The endpoint streams a zip of every file the sandbox has written for the chat. The append is idempotent — safe to re-run outlet() on the same body.

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Troubleshooting

Preview shows “connection refused” or a blank frame
  • PUBLIC_BASE_URL must be reachable from the user’s browser. No Valve for this — flows from server env.
  • docker compose ps — is computer-use-server healthy?
  • curl the iframe’s src= directly — SPA HTML should come back as HTTP 200.
Preview link or archive button never appears
  • The assistant message must contain at least one URL matching {PUBLIC_BASE_URL}/files/{chat_id}/... (or a <details type="tool_calls"> block referencing a browser tool). No trigger, no decoration.
  • chat_id must reach outlet() via __metadata__. Restart Open WebUI after toggling Valves if the model didn’t re-init.
  • PREVIEW_MODE="button" OR ARCHIVE_BUTTON="on". Both off = outlet() returns body unchanged.
Preview opens in a new tab instead of the artifact panel You’re on stock Open WebUI — the markdown link stays a link. Apply the patches (use docker-compose.webui.yml), or live with the click. “Non-http scheme” error in logs ORCHESTRATOR_URL is set to file:// / ftp://. Fix by pointing at http:// or https://.

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See also