# Story 13: Stop Button

## User Story
**As a** User
**I want** a Stop button to cancel the model's response while it's generating
**So that** I can immediately stop long-running or unwanted responses without waiting for completion

## The Problem

**Current Behavior:**
- User sends message → Model starts generating
- User realizes they don't want the response (wrong question, too long, etc.)
- **No way to stop it** - must wait for completion
- Tool calls will execute even if user wants to cancel

**Why This Matters:**
- Long responses waste time
- Tool calls have side effects (file writes, searches, shell commands)
- User has no control once generation starts
- Standard UX pattern in ChatGPT, Claude, etc.

## Acceptance Criteria

- [ ] Stop button (⬛) appears in place of Send button (↑) while model is generating
- [ ] Clicking Stop immediately cancels the backend request
- [ ] Tool calls that haven't started yet are NOT executed after cancellation
- [ ] Streaming stops immediately
- [ ] Partial response generated before stopping remains visible in chat
- [ ] Stop button becomes Send button again after cancellation
- [ ] User can immediately send a new message after stopping
- [ ] Input field remains enabled during generation

## Out of Scope
- Escape key shortcut (can add later)
- Confirmation dialog (immediate action is better UX)
- Undo/redo functionality
- New Session flow (that's Story 14)

## Implementation Approach

### Backend
- Add `cancel_chat` command callable from frontend
- Use `tokio::select!` to race chat execution vs cancellation signal
- Check cancellation before executing each tool
- Return early when cancelled (not an error - expected behavior)

### Frontend
- Replace Send button with Stop button when `loading` is true
- On Stop click: call `invoke("cancel_chat")` and set `loading = false`
- Keep input enabled during generation
- Visual: Make Stop button clearly distinct (⬛ or "Stop" text)

## Testing Strategy

1. **Test Stop During Streaming:**
   - Send message requesting long response
   - Click Stop while streaming
   - Verify streaming stops immediately
   - Verify partial response remains visible
   - Verify can send new message

2. **Test Stop Before Tool Execution:**
   - Send message that will use tools
   - Click Stop while "thinking" (before tool executes)
   - Verify tool does NOT execute (check logs/filesystem)

3. **Test Stop During Tool Execution:**
   - Send message with multiple tool calls
   - Click Stop after first tool executes
   - Verify remaining tools do NOT execute

## Success Criteria

**Before:**
- User sends message → No way to stop → Must wait for completion → Frustrating UX

**After:**
- User sends message → Stop button appears → User clicks Stop → Generation cancels immediately → Partial response stays → Can send new message

## Related Stories
- Story 14: New Session Cancellation (same backend mechanism, different trigger)
- Story 18: Streaming Responses (Stop must work with streaming)