"Error: WARNING: Cache entry deserialization failed, entry ignored" turns builds red

[beamerblvd]

Description:
When using actions/setup-python@v6, sometimes the build output will include the warning "WARNING: Cache entry deserialization failed, entry ignored." Something then treats this as an error and stylizes it as "Error: WARNING: Cache entry deserialization failed, entry ignored." This results in a "red" annotation even though the build succeeded.

Action version:
6

Platform:

• Ubuntu
• macOS
• Windows

I have only seen this on macos-26 (ARM). I'm running my builds on multiple Ubuntu versions, multiple Windows versions, and macos15-intel, macos-26, and macos-26-intel, but I have only ever seen this problem on macos-26. It's possible it could occur on other platforms as well, but it happens every run with macos-26.

Runner type:

• Hosted
• Self-hosted

Tools version:
This is occurring on Python 3.10. I am also testing on 3.12 and 3.14, both on macos-26, but this occurs only on Python 3.10. It's possible it could occur on other versions as well, but it happens every run with 3.10.

Repro steps:
Add this to your workflow using macos-26:

      - name: Installing Python 3.10
        uses: actions/setup-python@v6
        with:
          python-version: "3.10"
          cache: "pip"

You can view an example of the problem here: https://github.com/OddSource/ifaddrs4u/actions/runs/26189279490

Here's the raw log output from that example: https://productionresultssa6.blob.core.windows.net/actions-results/dfb8936d-094e-4cd2-9f35-3410202f148e/workflow-job-run-9d4e464c-c0b6-5d27-8a60-7be7d2ac21f9/logs/job/job-logs.txt?rsct=text%2Fplain&se=2026-05-20T21%3A43%3A19Z&sig=ycaulFWWEQHuTLznY37a1StspVeKesa34TcByAGL3Mw%3D&ske=2026-05-21T00%3A53%3A48Z&skoid=ca7593d4-ee42-46cd-af88-8b886a2f84eb&sks=b&skt=2026-05-20T20%3A53%3A48Z&sktid=398a6654-997b-47e9-b12b-9515b896b4de&skv=2025-11-05&sp=r&spr=https&sr=b&st=2026-05-20T21%3A33%3A14Z&sv=2025-11-05

And the workflow in question is here: https://github.com/OddSource/ifaddrs4u/blob/main/.github/workflows/matrix-gh-runers.yml

Expected behavior:
For the action, when used as documented with supported options values, to not cause output that GitHub Actions treats as an error unless something actually went wrong.

Actual behavior:
The action succeeds, but results in "error" output that causes red annotations on the builds.

[chiranjib-swain]

Hy @beamerblvd ,
Thank you for raising this. We will investigate it and get back to you as soon as we have some feedback.