Debugger

On Node and Deno targets, JupyterLab's visual debugger works: the bug button in the notebook toolbar, breakpoints in the cell gutter, the call stack, variables and scopes, stepping, pause on exceptions, the debug console, and the rich variable viewer. The kernel translates the Jupyter debug protocol (DAP over debug_request) to V8's Debugger domain on the session it already holds (src/debugger.ts):

• Cell paths. Each cell is named as JupyterLab names it: a temp-dir prefix plus the Murmur2 hash of its code (src/murmur2.ts, matched against ipykernel's). It's evaluated with that name as its //# sourceURL, so breakpoints bind to the cell and frames point back at it.
• Lines. Breakpoint and frame lines go through the cell's source map (src/sourcemap.ts), since esbuild rewrites the TypeScript. A breakpoint on a blank or type-only line moves to the next statement.
• Nothing runs on its own. The variable explorer between cells reads the cells' declarations with V8's side-effect check, so a getter never runs. Frames, scopes and the rich viewer evaluate on the paused frame.
• Kernelspec. metadata.debugger is set for Node and Deno kernelspecs, which is what JupyterLab checks. Reinstall older kernelspecs to get it.

CDPK_DEBUG_TRACE=1 logs every debug request, reply and event to the Jupyter server's log.

Bun, workerd and celld don't run over an inspector, so they have no debugger.