Summary

When a model generates incomplete or malformed JSON for a built-in tool call (e.g., missing required parameters like content in replace_note_content), OWUI's response_handler in utils/middleware.py fails to parse the arguments and terminates the SSE stream. The browser shows "Network connection lost" — leaving the user with no actionable information and no way to understand what went wrong.

Environment

• Open WebUI v0.8.5
• LLM: anthropic/claude-sonnet-4.6 via OpenRouter (with extended thinking / reasoning_effort: high)
• Built-in Notes tools (search_notes, view_note, replace_note_content)

Steps to Reproduce

1. Enable built-in Notes tools for a model with extended thinking (e.g., Claude Sonnet 4.6)
2. Ask the model to edit a note using replace_note_content
3. If the model generates an incomplete tool call (e.g., generates note_id and title but the stream is cut before the large content field is emitted)
4. → OWUI logs ERROR and terminates the SSE stream
5. → Browser shows "Network connection lost"

Log Evidence

ERROR | open_webui.utils.middleware:response_handler:4059 - Error parsing tool call arguments: {"note_id": "9af73036-6a51-4840-98d6-b348d4e26595", "title": "AI-Richtlinie"
WARNING | open_webui.utils.middleware:response_handler:4532 - Task was cancelled!

The JSON is truncated — json.loads() and ast.literal_eval() both fail. The handler currently silently drops the connection.

Code Path (v0.8.5)

In open_webui/utils/middleware.py, response_handler around line 4059:

try:
    tool_function_params = ast.literal_eval(tool_args)
except Exception as e:
    log.debug(e)
    try:
        tool_function_params = json.loads(tool_args)
    except Exception as e:
        log.error(f"Error parsing tool call arguments: {tool_args}")
        # ← After this, tool_function_params = {} and the stream terminates
        # ← User sees "Network connection lost" with zero context

Expected Behavior

When tool argument parsing fails, OWUI should:

1. Stream a clear error message to the user instead of terminating the connection:

   ⚠️ Tool call failed (replace_note_content): arguments could not be parsed. Please try again.

2. Continue the conversation turn — do not cancel the SSE stream
3. (Bonus) Validate tool call args against the tool's JSON Schema required fields before executing — reject calls with missing required params with a clear message instead of a Python exception downstream

Proposed Fix (sketch)

except Exception as e:
    log.error(f"Error parsing tool call arguments: {tool_args}")
    # Gracefully inform the user instead of crashing the stream
    await event_emitter({
        "type": "message",
        "data": {
            "content": f"\n\n⚠️ Tool call failed: could not parse arguments for `{tool_function_name}`. Please try again."
        }
    })
    continue  # skip this tool call, keep the handler alive

Impact

• Users see a cryptic "Network connection lost" instead of a clear retry prompt
• The note is not saved; the user has to manually retry without knowing why it failed
• Extended thinking models (which occasionally misformat tool call JSON) are disproportionately affected

Additional Context

This was observed with replace_note_content (built-in Notes tool) where the content parameter — which contains the full note markdown — is the last and largest JSON field. On first attempt the model omitted it; on retry (with longer thinking time) it succeeded. Better error surfacing would make this a minor UX annoyance rather than a confusing hard failure.