Breakpoints set via MCP API don't pause execution (using custom stdio proxy) #123

Breakpoints registered via add_breakpoint are accepted but execution never pauses when code runs through them. Debug sessions run to completion without stopping.

Important: We wrote a custom stdio-to-HTTP+SSE proxy because DebugMCP's HTTP transport is incompatible with Claude Code's stdio-only MCP client. This proxy may be causing the issue.

Environment

• DebugMCP: 2.3.0
• VS Code: 1.132.0
• Node.js: v24.16.0
• OS: Windows 11 Enterprise
• Runtime: TypeScript via tsx v4.22.4 + vitest 4.1.9
• Project: SAP CAP with @sap/cds 10.0.3
• Custom proxy: stdio → HTTP+SSE bridge

Reproduction

1. Start DebugMCP (http://localhost:3001/mcp)
2. Start custom proxy bridging stdio → HTTP+SSE
3. Call add_breakpoin + line number →returns success ✅
4. Call start_debugging with launch config → launches successfully ✅
5. Code runs through breakpoint without stopping ❌
6. Session completes "ran to completion without stopping"

What Works

• Proxy forwards all 15 MCP tools successfully
• Debug sessions launch
• Manual breakpoints in VS Code UI work fine

What Doesn't Work

• Execution never pauses at MCP-registered breakpoints
• No stack frame for variable inspection
• Tools timeout waiting for paused state

What We Tried

• TypeScript source p
• Multiple launch configurations
• Simple test scripts
• Logpoints (also didn't trigger)
• Setting breakpoints

All fail identically.

Hypothesis

Proxy may not be maintaining breakpoint state correctly between add_breakpoint and start_debugging calls. Our proxy parses SSE responses line-by-line and tracks session IDs, but breakpoint registration state might need to persist within a single HTTP session.

Simplified proxy logic:
// Parse SSE from DebugMCP, forward JSON-RPC to Claude Code
const response = await fetch('http://localhost:3001/mcp', {
method: 'POST',
headers: { 'Accept': 'text/event-stream' },
body: JSON.stringify(request)
});
// Parse SSE events, extract session IDs, forward responses