ComfyUI Won't Start After Updating

If ComfyUI worked before an update but now crashes on startup, the update likely introduced new dependencies, changed APIs, or exposed incompatibilities with your custom nodes.

This is one of the most common ComfyUI problems. The update itself is not broken — but your existing environment may need adjustments to match the new code.

.\python_embeded\python.exe -s -m pip install -r ComfyUI\requirements.txt
.\python_embeded\python.exe -s ComfyUI\main.py --windows-standalone-build

What the errors look like

Post-update failures can show many different error messages:

ModuleNotFoundError: No module named 'somepackage'

ImportError: cannot import name 'SomeClass' from 'somemodule'

TypeError: __init__() got an unexpected keyword argument '...'

AttributeError: module 'comfy' has no attribute '...'

The common thread is that these errors did not exist before the update.

Why it happens

• New dependencies: The updated ComfyUI code requires packages that were not in the previous requirements.txt
• Version bumps: A dependency now needs a newer version than what you have installed
• Breaking API changes: ComfyUI internal APIs changed and custom nodes have not caught up
• Custom node conflicts: Your custom nodes depend on the old ComfyUI API
• Partial update: Git pull succeeded for ComfyUI but custom nodes were not updated
• Frontend changes: The comfyui-frontend-package version changed

Step-by-step fix

Step 1: Read the actual error

Do not skip this. Launch ComfyUI from the command line so you can see the full error:

.\python_embeded\python.exe -s ComfyUI\main.py --windows-standalone-build

Copy the last traceback. It will point to either a missing module, a version conflict, or a custom node failure.

Step 2: Update core requirements

.\python_embeded\python.exe -s -m pip install -r ComfyUI\requirements.txt

This installs any new dependencies added by the update. It is safe to run and should be your first step after every git pull.

Step 3: Check if a custom node is the cause

If the error traceback points to a file inside custom_nodes/, that custom node is not compatible with the updated ComfyUI.

Options:

1. Update the custom node: cd ComfyUI\custom_nodes\<node-name> && git pull
2. Temporarily disable it: Rename the folder to add .disabled suffix
3. Check the node's GitHub issues: Others may have reported the same incompatibility

To test whether custom nodes are the problem, try starting with all custom nodes disabled:

.\python_embeded\python.exe -s ComfyUI\main.py --windows-standalone-build --disable-all-custom-nodes

If ComfyUI starts successfully with this flag, one or more custom nodes are the problem.

Step 4: If torch or a core package was affected

Check for dependency conflicts:

.\python_embeded\python.exe -s -m pip check

If torch, torchvision, or torchaudio show conflicts, fix those first before addressing custom node issues. See Torch/CUDA Version Mismatch for details.

Step 5: If rolling back is faster

If you need ComfyUI working immediately and the update is not critical:

cd ComfyUI
git log --oneline -10
git checkout <previous-commit-hash>

This reverts to a known-working state. You can try the update again later when custom nodes have been updated.

When not to run pip install blindly

• Do not run pip install --upgrade on everything — this can break your torch CUDA build
• Do not install requirements from every custom node simultaneously — they may conflict
• Do not delete python_embeded and start over unless you have exhausted targeted fixes
• If the update changelog mentions breaking changes, read them before debugging

How Wonderful Launcher can help

Wonderful Launcher helps manage ComfyUI updates safely. It checks for dependency changes before applying updates, verifies custom node compatibility, and can help you revert to a working state if an update causes problems.

Download Wonderful Launcher — it's free and makes updates safer for your existing workflows and models.

Related errors

• ComfyUI Startup Failed
• ComfyUI Won't Update
• ComfyUI Dependency Conflicts
• ComfyUI Custom Nodes Broke Environment
• ComfyUI Plugin Import Failed

Source References

• ComfyUI GitHub
• ComfyUI Documentation
• ComfyUI Manager GitHub