# JavaScript Debugging Implementation - Complete Analysis
## Final Root Cause Discovered ✅
After extensive debugging, the **true root cause** of JavaScript `evaluate_expression` failures has been identified:
### The Multi-Session Problem
**js-debug uses a multi-session architecture that we haven't fully implemented:**
1. **Parent Session**: Handles initial setup (initialize, setBreakpoints, configurationDone, attach/launch)
2. **Child Session**: Created by js-debug via reverse `startDebugging(__pendingTargetId)` request
3. **Child Session**: Where actual debugging occurs (variable evaluation, stepping, etc.)
### Evidence from Logs
```
"[MinimalDapClient] No active child available for routed command 'stackTrace'. Forwarding to parent session (may return empty/unsupported)."
"[MinimalDapClient] No active child available for routed command 'evaluate'. Forwarding to parent session (may return empty/unsupported)."
```
**The child session is never created!** Variable evaluation fails because we're sending requests to the parent session, which doesn't have access to the actual program state.
### Previous Fixes Applied
✅ **Context Fix**: Changed default evaluation context from 'repl' to 'variables'
- Fixed in: `src/session/session-manager-operations.ts` (line 1766)
- Fixed in: `src/server.ts` (removed hardcoded 'repl' override)
✅ **Confirmed Working**: The context is now correctly set to 'variables'
### The Real Solution Required
To fully fix JavaScript debugging, we need to implement the **js-debug multi-session model**:
1. **Listen for reverse `startDebugging`** requests with `__pendingTargetId`
2. **Create child DAP client** when reverse request received
3. **Apply strict child sequence**:
- initialize
- wait for initialized
- setExceptionBreakpoints
- setBreakpoints
- configurationDone
- attach with `__pendingTargetId`
4. **Route debuggee requests** to child session: `evaluate`, `stackTrace`, `scopes`, `variables`, `step*`, `continue`
### Files That Need Multi-Session Implementation
- `src/proxy/minimal-dap.ts` - Handle reverse startDebugging, create child sessions
- `src/proxy/dap-proxy-worker.ts` - Route requests to appropriate session (parent vs child)
### Current Status
- ✅ Variable evaluation context fixed ('variables' instead of 'repl')
- ❌ Child session creation not implemented (main blocker)
- ❌ Request routing to child session not implemented
- ❌ Reverse startDebugging handler missing
### References
See `docs/js-debug-handoff.md` for complete multi-session implementation details and the proven working sequence from `scripts/experiments/js-debug-probe-attach.mjs`.
---
**Date**: 2025-10-07
**Status**: Root cause identified, context fixes applied, multi-session architecture identified as remaining work
