Lesstep
GitHub Actions · Checked November 3, 2026

GitHub Actions: setup-python's "The version ... with architecture 'x64' was not found"

actions/setup-python installs a ready-made Python built for the runner's exact operating system version and processor. It looks in the runner's tool cache first, then in the list of builds that GitHub publishes in actions/python-versions. It does not compile Python or try any other source, so when neither has the version you asked for, the step fails.

What it looks like

From a public run on ubuntu-26.04 on October 29, 2026, with python-version: 3.9:

Version 3.9 was not found in the local cache
##[error]The version '3.9' with architecture 'x64' was not found for Ubuntu 26.04.
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Which versions each runner has

From versions-manifest.json on November 3, 2026:

So for 3.10, 3.11 and 3.12, no single patch release exists on both Ubuntu 26.04 and Windows. A matrix that pins 3.12.10 everywhere fails on Ubuntu 26.04, and one that pins 3.12.13 fails on Windows and macOS.

To see the list for a runner yourself (platform is linux, darwin or win32; arch is x64 or arm64; macOS and Windows have no platform_version):

gh api repos/actions/python-versions/contents/versions-manifest.json \
  -H "Accept: application/vnd.github.raw+json" \
  --jq '.[] | .version as $v | .files[]
        | select(.platform == "linux" and .platform_version == "26.04" and .arch == "x64")
        | $v'

Ask for the minor version

Write "3.12", not "3.12.8" (blueprintx #945). setup-python then takes the newest build of 3.12 on each runner. The same goes for a .python-version file, which setup-python reads when python-version is not set.

- uses: actions/setup-python@v7
  with:
    python-version: "3.12"

Keep the quotes. Unquoted, YAML reads 3.10 as the number 3.1, and the error then says '3.1' (kg-microbe #1056).

Run the old version on a runner that has it

Python 3.8 and 3.9 still have builds for ubuntu-24.04, and 3.7 for ubuntu-22.04 and the Intel Mac images, such as macos-15-intel. One public project's matrix now does exactly that (EliahKagan/version #40). For one old version, an expression in runs-on is enough:

strategy:
  matrix:
    python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
runs-on: ${{ matrix.python-version == '3.9' && 'ubuntu-24.04' || 'ubuntu-latest' }}

Install it with uv instead

uv downloads its own Python builds, which are not tied to a release of Ubuntu. Its list includes 3.8 up to 3.8.20 and 3.9 up to 3.9.25 for Linux, macOS and Windows on x64, but not 3.7. On November 1, a public job on ubuntu-26.04 got 3.8 this way and downloaded cpython-3.8.20-linux-x86_64-gnu (pyrs-yaml #696).

- uses: astral-sh/setup-uv@v10.3.3
  with:
    python-version: "3.9"
    activate-environment: true
- run: uv pip install -r requirements.txt
- run: python -m pytest

The environment uv creates has no pip, so install with uv pip install rather than pip install.

Telling it apart

See failed jobs from IntelliJ IDEA

Pipeline Inbox for GitHub Actions Workflows, a plugin by Lesstep for JetBrains IDEs 2024.1 or later, lists the runs of all your repositories in an IDE tool window, with their jobs and steps, so one matrix leg that failed at setup stands out, and your current branch's runs are in the status bar. Reading is free; re-running failed jobs, cancelling, running workflows and reviewing deployments are Pro (US$1.50 a month, with a 30-day trial).

GitHub and GitHub Actions are trademarks of GitHub, Inc.; IntelliJ IDEA is a trademark of JetBrains s.r.o.; Python is a registered trademark of the Python Software Foundation. This page is not affiliated with, sponsored or endorsed by GitHub, Inc., JetBrains or the Python Software Foundation.

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