black 25.9.0 throwing `BLK900 Failed to load file: decode_bytes() missing required argument 'mode' (pos 2)`

[KalebCoberly]

Describe the bug

When upgrading to black==25.9.0 (from 25.1.0), I get the following error for every file when running flake8:

BLK900 Failed to load file: decode_bytes() missing 1 required positional argument: 'mode'

To Reproduce

$ pip install black==25.9.0
$ flake8 src/
<for every file in src/> BLK900 Failed to load file: decode_bytes() missing 1 required positional argument: 'mode'

$ pip install black<25.9.0
<Installs 25.1.0>
$ flake8 src/
<no errors>

flake8 and flake8-black (and flake8-*) installed in each scenario are the same, 7.3.0 and 0.3.6 respectively.

Running with a fresh install from black main (pip install -e .[d]) does not fix the issue. (All black tests passed.)

Expected behavior

No errors.

Environment

• Black's version: 25.9.0
• OS and Python version: MacOS Sequoia 15.5/15.7, Tahoe 26.0, Python 3.11-3.13

Additional context

Just started yesterday when unpinned black dependencies started installing 25.9.0 instead of 25.1.0.

[cobaltt7]

It sounds like flake8-black is using Black's public API, which is undocumented and unstable (see #779).
This is an issue that must be brought up with the flake8-black team and fixed there.

[MeGaGiGaGon]

Yep. As part of #4710 I changed the signature of decode_bytes to require the mode (black.Mode) be passed in as a second argument.

[KalebCoberly]

Ah, so the API under semvar is only the CLI?

[MeGaGiGaGon]

Yes, see #779 for some discussion on it, but currently only the behavior of the CLI is guaranteed, and the functions inside black can change at any time.