TL;DR: Use a Python version that actually has a prebuilt binary for your runner OS and architecture; check the versions manifest before pinning an exotic combo. setup-python only downloads builds that exist upstream; a version/arch with no installer (like old Pythons on Windows arm64) can never be satisfied.

```text
Version 3.11 was not found in the local cache
```

## The fix

1. Open the versions manifest and confirm your version/arch combo has a build: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
   Expected: You see whether the combo exists.

2. Pin python-version to one that exists for your runner (e.g. 3.11 on ubuntu x64, or a newer 3.12+ for Windows arm64).
   Expected: The setup step downloads and installs cleanly.

3. If you must have that exact version, install Python yourself (apt, pyenv, or a container image) and skip setup-python.
   Expected: python --version reports what you installed.

## When this applies

- actions/setup-python fails with Version X was not found in the local cache followed by a not-found error
- self-hosted runners or unusual architectures

## When it does NOT apply

- the download itself times out (that is network flake; re-run)
- the version exists but extraction fails (that is disk space or permissions)

## Compatibility

actions/setup-python v4/v5/v6 on GitHub-hosted and self-hosted runners, all OSes.

## Variants of this error

### `Error: The version '3.9' with architecture 'arm64' was not found`
The explicit arch form of the same failure. Same fix.

### `Version 3.12 with arch x64 not found`
Older form from when 3.12 had no stable build yet. Same fix: pick a released version.

## Why it happens

The runner's tool cache only holds what previous jobs put there; on a cold cache setup-python resolves the request against the python-versions manifest, and combos without upstream binaries (new architectures, brand-new or ancient versions) resolve to nothing.

## Edge cases and pitfalls

- Ephemeral self-hosted runners always have a cold cache, so they hit the manifest on every run; pin versions that exist.
- allow-prereleases: true can satisfy requests for beta versions.
- Specifying 3.11 lets the action pick the latest 3.11.x with a build; pinning 3.11.0 exactly is more fragile.

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_LAWCAsxE-dIoHxTMrkoI5g/versions/skv_EQ_cioa8hEhktXrI8tawYA>
- Canonical HTML skill page: <https://vectle.com/skills/skl_LAWCAsxE-dIoHxTMrkoI5g>
- Current Markdown URL: <https://vectle.com/skills/skl_LAWCAsxE-dIoHxTMrkoI5g/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://github.com/actions/setup-python/issues/976"
  ]
}
```