Breakpoint is not hit when debugging a cell

[kayoch1n]

Environment data

• VS Code version: 1.102.1
• Jupyter Extension version (available under the Extensions sidebar): 2025.6.0
• Python Extension version (available under the Extensions sidebar): 2025.10.1
• OS (Windows | Mac | Linux distro) and version: macOS 14.7.6 (23H626) Darwin arm64
• Python and/or Anaconda version: 3.9.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): No virtual env is used
• Jupyter server running: Local

Expected behaviour

Breakpoint will be hit

Actual behaviour

Breakpoint is not hit

Steps to reproduce:

[NOTE: Self-contained, minimal reproducing code samples are extremely helpful and will expedite addressing your issue]

1. Create a new folder and open it via vscode. Add a ipynb file with the any python code, such as

2. Add a breakpoint. Debug the cell by right clicking the "more" menu and the "Debug Cell" button

3. The breakpoint set above is never hit. But the threads in the call stack panel are running.

Logs

Visual Studio Code (1.102.1, undefined, desktop)
Jupyter Extension Version: 2025.6.0.
Python Extension Version: 2025.10.1.
Python Environment Extension Version: 1.0.0.
Pylance Extension Version: 2025.6.2.
Platform: darwin (arm64).
Home = /Users/zhijianluo
Temp Storage folder ~/Library/Application Support/Code/User/globalStorage/ms-toolsai.jupyter/version-2025.6.0
Workspace folder ~/Documents/Projects/vscode-jupyter-issue

16:31:25.747 [info] Starting debugging with mode 1
16:31:27.000 [info] Ipykernel check failed: ControllerNotSelected
16:31:27.036 [info] Starting Kernel (Python Path: /usr/bin/python3, Unknown, 3.9.6) for '~/Documents/Projects/vscode-jupyter-issue/test.ipynb' (disableUI=true)

16:31:27.057 [warn] Failed to get activated env vars for /usr/bin/python3 in 22ms
16:31:27.059 [warn] Failed to get activated env vars for /usr/bin/python3 in 16ms
16:31:27.060 [info] Process Execution: /usr/bin/python3 -c "import site;print("USER_BASE_VALUE");print(site.USER_BASE);print("USER_BASE_VALUE");"
16:31:27.084 [info] Process Execution: /usr/bin/python3 -m pip list
16:31:27.084 [info] Process Execution: /usr/bin/python3 -c "import ipykernel; print(ipykernel.version); print("5dc3a68c-e34e-4080-9c3e-2a532b2ccb4d"); print(ipykernel.file)"
16:31:27.084 [info] Process Execution: /usr/bin/python3 -m ipykernel_launcher --f=~/Library/Jupyter/runtime/kernel-v355ba5a6a71a9eb15234852802f8d6d4d985512ca.json

> cwd: ~/Documents/Projects/vscode-jupyter-issue

16:31:27.532 [info] Kernel successfully started
16:31:27.534 [info] Process Execution: /usr/bin/python3 ~/.vscode/extensions/ms-toolsai.jupyter-2025.6.0-darwin-arm64/pythonFiles/printJupyterDataDir.py
16:31:34.480 [info] Starting debugging with mode 1
16:31:40.424 [info] Ending debug session 439fc23f-e4bc-43f0-9a85-0c0f2d5d2de0
16:31:40.424 [info] Ending debug session 439fc23f-e4bc-43f0-9a85-0c0f2d5d2de0
16:37:21.487 [info] Starting debugging with mode 1

[speediedan]

same here, have tried a variety of different configurations (downgrading vscode to 1.100, downgrading jupyter to 2025.4.0)

You can get the debugger to attach within the debug cell context if you:

1. Run a cell with "Debug Cell"
2. manually pause a thread in the Call Stack pane
3. "execute" the cell with the run button
   That allows us to capture the call stack below, and then proceed with debugging the cell per normal (hitting requested breakpoints).

select (/usr/lib/python3.12/selectors.py:471)
_run_once (/usr/lib/python3.12/asyncio/base_events.py:1954)
run_forever (/usr/lib/python3.12/asyncio/base_events.py:640)
start (/home/speediedan/.venvs/it_latest/lib/python3.12/site-packages/tornado/platform/asyncio.py:211)
start (/home/speediedan/.venvs/it_latest/lib/python3.12/site-packages/ipykernel/kernelapp.py:739)
launch_instance (/home/speediedan/.venvs/it_latest/lib/python3.12/site-packages/traitlets/config/application.py:1075)
<module> (/home/speediedan/.venvs/it_latest/lib/python3.12/site-packages/ipykernel_launcher.py:18)

However, after all breakpoints have been hit, the cell will hang and attempting to pause the debugger session will fail warning a debugger is already running.

Stopping the debugging session yields the following jupyter output log:

11:51:55.819 [info] Cannot start debugging. Already debugging this notebook document.
11:52:21.349 [error] Failed to disconnect debug session Error [CodeExpectedError]: custom request failed
    at vscode-file://vscode-app/usr/share/code/resources/app/out/vs/workbench/workbench.desktop.main.js:1254:32328
11:52:21.350 [info] Ending debug session 76c98f0b-5629-4055-bafc-2780ab7576a3

As well as the following output in the cell:
ERROR:tornado.general:SEND Error: Host unreachable

@kayoch1n if you need a temporary workaround, you can start a cell with a desired breakpoint using "Debug Cell", then run the cell with normal "execute" button and it should hit the breakpoints (subsequently hanging as discussed above.

@DonJayamanne Any suggestions on other workarounds and/or the root cause? I've tried vscode versions 0.99.3, 1.100, and the latest 1.102.2 as well as jupyter extension versions 2025.3.0, 2025.4.0 2025.6.0.
Thanks!

[fleming79]

May be related to ipython/ipykernel#1412?

[kayoch1n]

May be related to ipython/ipykernel#1412?

After downgrading to 6.29.5, the button "Debug Cell" worked again. For those who have the same trouble, try downgrading to 6.29.5