huskies/.living_spec/stories/archive/15_new_session_cancellation.md

Story 14: New Session Cancellation

User Story

As a User I want the backend to stop processing when I start a new session So that tools don't silently execute in the background and streaming doesn't leak into my new session

The Problem

Current Behavior (THE BUG):

1. User sends message → Backend starts streaming → About to execute a tool (e.g., write_file)
2. User clicks "New Session" and confirms
3. Frontend clears messages and UI state
4. Backend keeps running → Tool executes → File gets written → Streaming continues
5. Streaming tokens appear in the new session
6. User has no idea these side effects occurred in the background

Why This Is Critical:

• Tool calls have real side effects (file writes, shell commands, searches)
• These happen silently after user thinks they've started fresh
• Streaming from old session leaks into new session
• Can cause confusion, data corruption, or unexpected system state
• User expects "New Session" to mean a clean slate

Acceptance Criteria

• Clicking "New Session" and confirming cancels any in-flight backend request
• Tool calls that haven't started yet are NOT executed
• Streaming from old request does NOT appear in new session
• Backend stops processing immediately when cancellation is triggered
• New session starts with completely clean state
• No silent side effects in background after new session starts

Out of Scope

• Stop button during generation (that's Story 13)
• Improving the confirmation dialog (already done in Story 20)
• Rolling back already-executed tools (partial work stays)

Implementation Approach

Backend

• Uses same cancel_chat command as Story 13
• Same cancellation mechanism (tokio::select!, watch channel)

Frontend

• Call invoke("cancel_chat") BEFORE clearing UI state in clearSession()
• Wait for cancellation to complete before clearing messages
• Ensure old streaming events don't arrive after clear

Testing Strategy

1. Test Tool Call Prevention:

   • Send message that will use tools (e.g., "search all TypeScript files")
   • Click "New Session" while it's thinking
   • Confirm in dialog
   • Verify tool does NOT execute (check logs/filesystem)
   • Verify new session is clean

2. Test Streaming Leak Prevention:

   • Send message requesting long response
   • While streaming, click "New Session" and confirm
   • Verify old streaming stops immediately
   • Verify NO tokens from old request appear in new session
   • Type new message and verify only new response appears

3. Test File Write Prevention:

   • Ask to write a file: "Create test.txt with current timestamp"
   • Click "New Session" before tool executes
   • Check filesystem: test.txt should NOT exist
   • Verify no background file creation happens

Success Criteria

Before (BROKEN):

User: "Search files and write results.txt"
Backend: Starts streaming...
User: *clicks New Session, confirms*
Frontend: Clears UI ✓
Backend: Still running... executes search... writes file... ✗
Result: File written silently in background ✗
Old streaming tokens appear in new session ✗

After (FIXED):

User: "Search files and write results.txt"
Backend: Starts streaming...
User: *clicks New Session, confirms*
Frontend: Calls cancel_chat, waits, then clears UI ✓
Backend: Receives cancellation, stops immediately ✓
Backend: Tools NOT executed ✓
Result: Clean new session, no background activity ✓

Related Stories

• Story 13: Stop Button (shares same backend cancellation mechanism)
• Story 20: New Session confirmation dialog (UX for triggering this)
• Story 18: Streaming Responses (must not leak between sessions)