Problem

The built-in chat UI (/admin/chat) silently dropped responses when a model decided to call an MCP tool. streamResponse only handled delta.content and delta.reasoning_content — on finish_reason: tool_calls there was no content delta, so streamingContent stayed empty and nothing was pushed to the message list.

Solution

MCP tool call loop

• Accumulate delta.tool_calls chunks during streaming (OpenAI chunked format, keyed by index)
• On finish_reason: tool_calls: push a hidden assistant message with tool_calls, execute all tools in parallel via POST /v1/mcp/execute, push hidden tool result messages, then recurse into streamResponse so the model receives full tool context and streams a final answer
• Depth limit via MAX_TOOL_DEPTH = 10 constant; abort guard before recursion respects user clicking Stop
• 30s per-tool timeout (AbortSignal.timeout + AbortSignal.any), HTTP error detection on non-2xx responses
• Bookkeeping assistant/tool protocol messages hidden from display via _ui: false
• Simplified messagesForApi to a single filter+map pass that preserves tool_calls and tool_call_id fields

Engine status timeline

• Prefill progress poller (polls /admin/api/stats every 500ms): shows Prefilling N% → Generating
• Per-tool timing logged on completion/failure with check/error icons and elapsed time
• Collapsible timeline panel (activity icon toggle in message header) — entries persist on the completed message and can be expanded post-completion
• Live status line (Starting / Prefilling / Generating / Calling tool…) with animated dots, rendered below the timeline entries

Reasoning bubble

• reasoning_content deltas accumulate live in a collapsible Thinking… bubble rendered as markdown
• Collapses to Thinking (no ellipsis) once final content arrives

Bug fixes

• Response box no longer disappears on reasoning-only or empty responses — always push an assistant message on stream completion
• Timeline toggle works consistently after content arrives — removed !finalContent guard that blocked the panel
• Alpine reactivity for _perfLog: messages.splice(i, 1, { ...msg, ...patch }) instead of direct property assignment so Alpine detects the change and re-renders the toggle button

Files changed

Testing

pytest tests/test_chat_tool_call.py -v   # 18 passed

Manually verified with Qwen3.5-9B-MLX-4bit + simultaneous filesystem and Tavily MCP servers on macOS (Apple Silicon).

Checklist

• Feature branch from main
• Tests added and passing
• SPDX-License-Identifier: Apache-2.0 present in new Python file
• Code style matches project conventions
• pyproject.toml unchanged — no new dependencies