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After you add a model in Open WebUI, you must flip two settings. The managed chat.yambr.com does this by default; self-hosters running the bundled compose stack also get it done via init.sh. If you install Open WebUI yourself, or you add a new model after the initial setup, check these by hand.
SettingValueWhy
Function CallingNativeRequired for Computer Use tools to be invoked by the model. Without it, the model has no way to call bash_tool, create_file, etc. — it silently doesn’t.
Stream Chat ResponseOnEnables real-time output streaming, including tool-call deltas. Without it, long-running tool sequences feel like the UI hung.
In Admin → Settings → Models → Advanced Params: 
function_calling: native
stream_response: true
These become DEFAULT_MODEL_PARAMS — every model picks them up without per-model clicks. The bundled init.sh writes these on first Open WebUI startup: 
POST /api/v1/configs/models
{
  "default_model_params": {
    "function_calling": "native",
    "stream_response": true
  }
}

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Set per model

If you want different behavior per model, override in the model’s edit screen: Workspace → Models → [model] → Advanced Params.

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Symptom: tools don’t fire

If the model replies in plain text every time without calling tools, 99% of the time Function Calling is Default instead of Native. The Default path relies on Open WebUI’s own tool-call parsing, which doesn’t play well with MCP tools wired through the Computer Use tool module.

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Symptom: output hangs until complete

Stream Chat Response is Off. Turn it on; tool call output will appear as it’s produced.