feat: Add mirror and mirror-token inputs for custom Python distribution sources (#1302)

* feat: Add `mirror` and `mirror-token` inputs for custom Python distribution sources

Users who need custom CPython builds (internal mirrors, GHES-hosted forks,
special build configurations, compliance builds, air-gapped runners) could not
previously point setup-python at anything other than actions/python-versions.

Adds two new inputs:
- `mirror`: base URL hosting versions-manifest.json and the Python
  distributions it references. Defaults to the existing
  https://raw.githubusercontent.com/actions/python-versions/main.
- `mirror-token`: optional token used to authenticate requests to the mirror.

If `mirror` is a raw.githubusercontent.com/{owner}/{repo}/{branch} URL, the
manifest is fetched via the GitHub REST API (authenticated rate limit applies);
otherwise the action falls back to a direct GET of {mirror}/versions-manifest.json.

Token interaction
-----------------

`token` is never forwarded to arbitrary hosts. Auth resolution is per-URL:

  1. if mirror-token is set, use mirror-token
  2. else if token is set AND the target host is github.com,
     *.github.com, or *.githubusercontent.com, use token
  3. else send no auth

Cases:

  Default (no inputs set)
    mirror = default raw.githubusercontent.com URL, mirror-token empty,
    token = github.token.
    → manifest API call and tarball downloads use `token`.
    Identical to prior behavior.

  Custom raw.githubusercontent.com mirror (e.g. personal fork)
    mirror-token empty, token = github.token.
    → manifest API call and tarball downloads use `token`
      (target hosts are GitHub-owned).

  Custom non-GitHub mirror, no mirror-token
    mirror-token empty, token = github.token.
    → manifest fetched via direct URL (no auth attached),
      tarball downloads use no auth.
    `token` is NOT forwarded to the custom host — this is the
    leak-prevention case.

  Custom non-GitHub mirror with mirror-token
    mirror-token set, token may be set.
    → manifest fetch and tarball downloads use `mirror-token`.

  Custom GitHub mirror with both tokens set
    mirror-token wins. Used for both the manifest API call and
    tarball downloads.

* fix: address mirror review feedback

- scope mirror-token to the mirror host and send it verbatim
- route non-repo mirrors straight to the URL fetch instead of throwing
- authenticate the manifest fetch
- warn on slash branches, and on mirror with PyPy/GraalPy
- memoize mirror validation
- exercise the direct-URL path in the E2E job

Addresses https://github.com/actions/setup-python/pull/1302#issuecomment-5202618946

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: correct mirror warnings, auth scoping, and integration coverage

- only warn about PyPy/GraalPy mirror when a custom mirror is set; the
  action.yml default made the warning fire on every run
- accept the refs/heads/{branch} raw URL form so it routes via the REST
  API instead of tripping the slash-branch warning
- scope mirror-token to the full mirror origin (scheme+host+port) so it
  can't leak to a same-host http download_url
- make an invalid mirror fatal on the auth path, matching getManifestUrl
- fix warning/docs that wrongly claimed the raw fallback is anonymous
- force a manifest fetch in the mirror integration job (check-latest) so
  it actually contacts the mirror instead of using the preinstalled cache

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/test-python.yml

@@ -61,6 +61,32 @@ jobs:
       - name: Run simple code
         run: python -c 'import math; print(math.factorial(5))'
 
+  setup-versions-via-mirror-input:
+    name: 'Setup via explicit mirror input: ${{ matrix.os }}'
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      # The refs/heads/ form serves the same manifest as the default mirror and
+      # exercises the refs/heads/{branch} -> {branch} parsing on the API path.
+      # check-latest forces a manifest fetch even though 3.12 is preinstalled,
+      # so the job actually contacts the mirror instead of short-circuiting on
+      # the tool cache.
+      - name: setup-python with explicit mirror
+        uses: ./
+        with:
+          python-version: 3.12
+          check-latest: true
+          mirror: https://raw.githubusercontent.com/actions/python-versions/refs/heads/main
+
+      - name: Run simple code
+        run: python -c 'import sys; print(sys.version)'
+
   setup-versions-from-file:
     name: Setup ${{ matrix.python }} ${{ matrix.os }} version file
     runs-on: ${{ matrix.os }}

__tests__/install-python-mirror.test.ts

@@ -0,0 +1,600 @@
+import {jest, describe, it, expect, beforeEach} from '@jest/globals';
+
+// Inputs are read lazily by install-python.ts, so each test can set them
+// before invoking the function under test.
+const inputs: Record<string, string> = {};
+
+// Mock @actions/http-client
+jest.unstable_mockModule('@actions/http-client', () => ({
+  HttpClient: jest.fn().mockImplementation(() => ({
+    getJson: jest.fn()
+  })),
+  HttpClientError: class HttpClientError extends Error {},
+  HttpCodes: {
+    OK: 200,
+    NotFound: 404,
+    InternalServerError: 500
+  }
+}));
+
+// Mock @actions/cache (needed transitively by utils.ts)
+jest.unstable_mockModule('@actions/cache', () => ({
+  saveCache: jest.fn(),
+  restoreCache: jest.fn(),
+  isFeatureAvailable: jest.fn()
+}));
+
+// Mock @actions/tool-cache
+jest.unstable_mockModule('@actions/tool-cache', () => ({
+  getManifestFromRepo: jest.fn(),
+  downloadTool: jest.fn(),
+  extractTar: jest.fn(),
+  extractZip: jest.fn(),
+  HTTPError: class HTTPError extends Error {}
+}));
+
+// Mock @actions/core (needed by install-python.ts)
+jest.unstable_mockModule('@actions/core', () => ({
+  info: jest.fn(),
+  warning: jest.fn(),
+  debug: jest.fn(),
+  error: jest.fn(),
+  notice: jest.fn(),
+  setFailed: jest.fn(),
+  setOutput: jest.fn(),
+  getInput: jest.fn(),
+  getBooleanInput: jest.fn(),
+  getMultilineInput: jest.fn(),
+  addPath: jest.fn(),
+  exportVariable: jest.fn(),
+  saveState: jest.fn(),
+  getState: jest.fn(),
+  setSecret: jest.fn(),
+  isDebug: jest.fn(() => false),
+  startGroup: jest.fn(),
+  endGroup: jest.fn(),
+  group: jest.fn((_name: string, fn: () => Promise<unknown>) => fn()),
+  toPlatformPath: jest.fn((p: string) => p),
+  toWin32Path: jest.fn((p: string) => p),
+  toPosixPath: jest.fn((p: string) => p)
+}));
+
+// Mock @actions/exec (needed by install-python.ts)
+jest.unstable_mockModule('@actions/exec', () => ({
+  exec: jest.fn(),
+  getExecOutput: jest.fn()
+}));
+
+// Import real utils BEFORE mock registration to get real function references
+const realUtils = await import('../src/utils.js');
+
+// Pin the platform so the download/extract assertions below behave the same
+// on every runner OS.
+jest.unstable_mockModule('../src/utils.js', () => ({
+  ...realUtils,
+  IS_WINDOWS: false,
+  IS_LINUX: false
+}));
+
+// Dynamic imports after mocking
+const core = await import('@actions/core');
+const httpm = await import('@actions/http-client');
+const tc = await import('@actions/tool-cache');
+const {
+  getManifestUrl,
+  getManifest,
+  getManifestFromRepo,
+  getManifestFromURL,
+  resolveRepoCoords,
+  isMirrorCustomized,
+  installCpythonFromRelease
+} = await import('../src/install-python.js');
+
+const DEFAULT_MIRROR =
+  'https://raw.githubusercontent.com/actions/python-versions/main';
+
+const mockManifest = [
+  {
+    version: '1.0.0',
+    stable: true,
+    files: [
+      {
+        filename: 'tool-v1.0.0-linux-x64.tar.gz',
+        platform: 'linux',
+        arch: 'x64',
+        download_url: 'https://example.com/tool-v1.0.0-linux-x64.tar.gz'
+      }
+    ]
+  }
+];
+
+function setInputs(values: Record<string, string>) {
+  Object.assign(inputs, values);
+}
+
+beforeEach(() => {
+  jest.resetAllMocks();
+  for (const key of Object.keys(inputs)) {
+    delete inputs[key];
+  }
+  (core.getInput as jest.Mock<any>).mockImplementation(
+    (name: string) => inputs[name] ?? ''
+  );
+});
+
+describe('getManifestUrl', () => {
+  it('defaults to the actions/python-versions manifest', () => {
+    expect(getManifestUrl()).toBe(`${DEFAULT_MIRROR}/versions-manifest.json`);
+  });
+
+  it('appends versions-manifest.json to a custom mirror', () => {
+    setInputs({mirror: 'https://mirror.example/py'});
+    expect(getManifestUrl()).toBe(
+      'https://mirror.example/py/versions-manifest.json'
+    );
+  });
+
+  it('strips trailing slashes from the mirror', () => {
+    setInputs({mirror: 'https://mirror.example/py///'});
+    expect(getManifestUrl()).toBe(
+      'https://mirror.example/py/versions-manifest.json'
+    );
+  });
+
+  it('throws on a mirror that is not a valid URL', () => {
+    setInputs({mirror: 'not a url'});
+    expect(() => getManifestUrl()).toThrow(/Invalid 'mirror' URL/);
+  });
+
+  it('keeps throwing the same error when called repeatedly', () => {
+    setInputs({mirror: 'not a url'});
+    expect(() => getManifestUrl()).toThrow(/Invalid 'mirror' URL/);
+    // Memoized, so the second call must not silently succeed or change shape —
+    // find-python.ts calls this while building the "version not found" message.
+    expect(() => getManifestUrl()).toThrow(/Invalid 'mirror' URL/);
+  });
+
+  it('treats an invalid mirror as fatal on the auth path too', async () => {
+    // getManifestUrl() throws on a bad mirror; the auth resolution must agree
+    // rather than swallow the error and quietly skip the mirror-token branch.
+    setInputs({'mirror-token': 'MTOK', mirror: 'not a url'});
+    (tc.downloadTool as jest.Mock<any>).mockResolvedValue('/tmp/py.tgz');
+    (tc.extractTar as jest.Mock<any>).mockResolvedValue('/tmp/extracted');
+
+    const release = {
+      version: '3.12.0',
+      stable: true,
+      files: [
+        {
+          filename: 'python-3.12.0-linux-x64.tar.gz',
+          platform: 'linux',
+          arch: 'x64',
+          download_url: 'https://cdn.example/py.tar.gz'
+        }
+      ]
+    } as any;
+
+    await expect(installCpythonFromRelease(release)).rejects.toThrow(
+      /Invalid 'mirror' URL/
+    );
+  });
+});
+
+describe('isMirrorCustomized', () => {
+  it('is false when the mirror input is empty', () => {
+    expect(isMirrorCustomized()).toBe(false);
+  });
+
+  it('is false when the mirror input equals the default', () => {
+    // action.yml gives `mirror` this exact default, so getInput() returns it on
+    // every run where the user did not set one. The PyPy/GraalPy warning must
+    // not fire in that case.
+    setInputs({mirror: DEFAULT_MIRROR});
+    expect(isMirrorCustomized()).toBe(false);
+  });
+
+  it('is false when the mirror input is the default with trailing slashes', () => {
+    setInputs({mirror: `${DEFAULT_MIRROR}///`});
+    expect(isMirrorCustomized()).toBe(false);
+  });
+
+  it('is true when the mirror input is a custom URL', () => {
+    setInputs({mirror: 'https://mirror.example/py'});
+    expect(isMirrorCustomized()).toBe(true);
+  });
+});
+
+describe('resolveRepoCoords', () => {
+  it('warns and returns null for a raw.githubusercontent.com mirror with a slash in the branch', () => {
+    setInputs({
+      mirror: 'https://raw.githubusercontent.com/foo/bar/feature/riscv'
+    });
+
+    expect(resolveRepoCoords()).toBeNull();
+    expect(core.warning).toHaveBeenCalledWith(
+      expect.stringMatching(/Branch names containing '\/' are not supported/)
+    );
+  });
+
+  it('parses the refs/heads/{branch} form to the bare branch without warning', () => {
+    setInputs({
+      mirror:
+        'https://raw.githubusercontent.com/actions/python-versions/refs/heads/main'
+    });
+
+    expect(resolveRepoCoords()).toEqual({
+      owner: 'actions',
+      repo: 'python-versions',
+      branch: 'main'
+    });
+    // refs/heads/main is a valid single branch, so it must route through the
+    // API path and never hit the slash-branch warning.
+    expect(core.warning).not.toHaveBeenCalled();
+  });
+
+  it('does not warn for a non-GitHub mirror', () => {
+    setInputs({mirror: 'https://mirror.example/py'});
+
+    expect(resolveRepoCoords()).toBeNull();
+    expect(core.warning).not.toHaveBeenCalled();
+  });
+});
+
+describe('getManifestFromRepo mirror resolution', () => {
+  it('resolves the default mirror to actions/python-versions@main with token', async () => {
+    setInputs({token: 'TKN'});
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'actions',
+      'python-versions',
+      'token TKN',
+      'main'
+    );
+  });
+
+  it('extracts owner/repo/branch from a custom raw.githubusercontent.com mirror', async () => {
+    setInputs({
+      token: 'TKN',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/dev'
+    });
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'foo',
+      'bar',
+      'token TKN',
+      'dev'
+    );
+  });
+
+  it('strips a trailing slash before extracting the branch', async () => {
+    setInputs({
+      token: 'TKN',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/main/'
+    });
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'foo',
+      'bar',
+      'token TKN',
+      'main'
+    );
+  });
+
+  it('resolves the refs/heads/{branch} form to the bare branch for the API', async () => {
+    setInputs({
+      token: 'TKN',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/refs/heads/main'
+    });
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    // The GitHub tree API takes a bare branch, so the refs/heads/ prefix must
+    // be stripped rather than passed through as part of the branch name.
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'foo',
+      'bar',
+      'token TKN',
+      'main'
+    );
+  });
+
+  it('returns null for a non-GitHub mirror so the caller uses the raw URL', () => {
+    setInputs({mirror: 'https://mirror.example/py'});
+    expect(resolveRepoCoords()).toBeNull();
+    expect(tc.getManifestFromRepo).not.toHaveBeenCalled();
+  });
+
+  it('prefers mirror-token over token for the GitHub API call', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'MTOK',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/main'
+    });
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    // The API requires the `token ` prefix, and naming a repo mirror is explicit intent to
+    // read that repo, so mirror-token is prefixed here even though downloads send it verbatim.
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'foo',
+      'bar',
+      'token MTOK',
+      'main'
+    );
+  });
+
+  it('sends no auth when neither token nor mirror-token is set', async () => {
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+
+    await getManifestFromRepo();
+
+    expect(tc.getManifestFromRepo).toHaveBeenCalledWith(
+      'actions',
+      'python-versions',
+      undefined,
+      'main'
+    );
+  });
+});
+
+describe('getManifestFromURL mirror resolution', () => {
+  it('fetches {mirror}/versions-manifest.json without auth when no mirror-token is set', async () => {
+    setInputs({token: 'TKN', mirror: 'https://mirror.example/py'});
+    const getJson = jest.fn(async () => ({result: mockManifest}));
+    (httpm.HttpClient as jest.Mock<any>).mockImplementation(() => ({getJson}));
+
+    await getManifestFromURL();
+
+    // `token` must not reach a non-GitHub mirror.
+    expect(getJson).toHaveBeenCalledWith(
+      'https://mirror.example/py/versions-manifest.json',
+      undefined
+    );
+  });
+
+  it('sends mirror-token verbatim on the manifest fetch', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'Bearer MTOK',
+      mirror: 'https://mirror.example/py'
+    });
+    const getJson = jest.fn(async () => ({result: mockManifest}));
+    (httpm.HttpClient as jest.Mock<any>).mockImplementation(() => ({getJson}));
+
+    await getManifestFromURL();
+
+    expect(getJson).toHaveBeenCalledWith(
+      'https://mirror.example/py/versions-manifest.json',
+      {authorization: 'Bearer MTOK'}
+    );
+  });
+
+  it('sends token as a prefixed header for a GitHub-hosted raw manifest', async () => {
+    // A slash-branch URL genuinely falls to the direct-URL path (it does not
+    // match the {owner}/{repo}/{branch} shape). raw.githubusercontent.com is a
+    // GitHub host, so the token is still attached here — contradicting any
+    // claim that the fallback fetch is anonymous.
+    setInputs({
+      token: 'TKN',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/feature/riscv'
+    });
+    const getJson = jest.fn(async () => ({result: mockManifest}));
+    (httpm.HttpClient as jest.Mock<any>).mockImplementation(() => ({getJson}));
+
+    await getManifestFromURL();
+
+    expect(getJson).toHaveBeenCalledWith(
+      'https://raw.githubusercontent.com/foo/bar/feature/riscv/versions-manifest.json',
+      {authorization: 'token TKN'}
+    );
+  });
+});
+
+describe('getManifest source routing', () => {
+  it('skips the GitHub API entirely for a non-GitHub mirror', async () => {
+    setInputs({mirror: 'https://mirror.example/py'});
+    const getJson = jest.fn(async () => ({result: mockManifest}));
+    (httpm.HttpClient as jest.Mock<any>).mockImplementation(() => ({getJson}));
+
+    await expect(getManifest()).resolves.toEqual(mockManifest);
+
+    // Routing straight to the URL fetch avoids 3 retries with backoff on a
+    // call that could never succeed.
+    expect(tc.getManifestFromRepo).not.toHaveBeenCalled();
+    expect(getJson).toHaveBeenCalledTimes(1);
+  });
+
+  it('uses the GitHub API for a repo mirror without touching the raw URL', async () => {
+    setInputs({token: 'TKN'});
+    (tc.getManifestFromRepo as jest.Mock<any>).mockResolvedValue(mockManifest);
+    const getJson = jest.fn(async () => ({result: mockManifest}));
+    (httpm.HttpClient as jest.Mock<any>).mockImplementation(() => ({getJson}));
+
+    await expect(getManifest()).resolves.toEqual(mockManifest);
+
+    expect(tc.getManifestFromRepo).toHaveBeenCalledTimes(1);
+    expect(getJson).not.toHaveBeenCalled();
+  });
+});
+
+describe('installCpythonFromRelease auth gating', () => {
+  const makeRelease = (downloadUrl: string) =>
+    ({
+      version: '3.12.0',
+      stable: true,
+      release_url: '',
+      files: [
+        {
+          filename: 'python-3.12.0-linux-x64.tar.gz',
+          platform: 'linux',
+          platform_version: '',
+          arch: 'x64',
+          download_url: downloadUrl
+        }
+      ]
+    }) as any;
+
+  // Returns the auth argument tc.downloadTool was called with.
+  async function downloadAuthFor(downloadUrl: string) {
+    (tc.downloadTool as jest.Mock<any>).mockResolvedValue('/tmp/py.tgz');
+    (tc.extractTar as jest.Mock<any>).mockResolvedValue('/tmp/extracted');
+
+    await installCpythonFromRelease(makeRelease(downloadUrl));
+
+    const call = (tc.downloadTool as jest.Mock<any>).mock.calls[0];
+    expect(call[0]).toBe(downloadUrl);
+    return call[2];
+  }
+
+  it('forwards token to github.com download URLs', async () => {
+    setInputs({token: 'TKN'});
+    await expect(
+      downloadAuthFor(
+        'https://github.com/actions/python-versions/releases/download/3.12.0-x/python-3.12.0-linux-x64.tar.gz'
+      )
+    ).resolves.toBe('token TKN');
+  });
+
+  it('forwards token to api.github.com download URLs', async () => {
+    setInputs({token: 'TKN'});
+    await expect(
+      downloadAuthFor('https://api.github.com/repos/x/y/tarball/main')
+    ).resolves.toBe('token TKN');
+  });
+
+  it('forwards token to *.githubusercontent.com download URLs', async () => {
+    setInputs({token: 'TKN'});
+    await expect(
+      downloadAuthFor('https://objects.githubusercontent.com/x/python.tar.gz')
+    ).resolves.toBe('token TKN');
+  });
+
+  it('does NOT forward token to a non-GitHub download URL', async () => {
+    setInputs({token: 'TKN', mirror: 'https://cdn.example'});
+    await expect(
+      downloadAuthFor('https://cdn.example/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('does NOT forward token to a lookalike host', async () => {
+    setInputs({token: 'TKN', mirror: 'https://evil-github.com'});
+    await expect(
+      downloadAuthFor('https://evil-github.com/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('forwards mirror-token verbatim to the mirror host', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'Bearer MTOK',
+      mirror: 'https://cdn.example'
+    });
+    await expect(
+      downloadAuthFor('https://cdn.example/py.tar.gz')
+    ).resolves.toBe('Bearer MTOK');
+  });
+
+  it('does not prefix or rewrite a mirror-token', async () => {
+    setInputs({
+      'mirror-token': 'Basic dXNlcjpwYXNz',
+      mirror: 'https://cdn.example'
+    });
+    await expect(
+      downloadAuthFor('https://cdn.example/py.tar.gz')
+    ).resolves.toBe('Basic dXNlcjpwYXNz');
+  });
+
+  it('withholds mirror-token from a same-host download URL on a different scheme', async () => {
+    setInputs({
+      'mirror-token': 'Bearer MTOK',
+      mirror: 'https://cdn.example'
+    });
+    // The mirror is https, but the manifest points a download_url at http on
+    // the same host. Matching on origin (scheme + host + port) rather than host
+    // alone keeps the token from going out in cleartext.
+    await expect(
+      downloadAuthFor('http://cdn.example/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('withholds mirror-token from a same-host download URL on a different port', async () => {
+    setInputs({
+      'mirror-token': 'Bearer MTOK',
+      mirror: 'https://cdn.example'
+    });
+    // Different port is a different origin, so the nominated credential must
+    // not follow.
+    await expect(
+      downloadAuthFor('https://cdn.example:8443/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('sends mirror-token to a same-origin download URL on an explicit port', async () => {
+    setInputs({
+      'mirror-token': 'Bearer MTOK',
+      mirror: 'https://cdn.example:8443'
+    });
+    await expect(
+      downloadAuthFor('https://cdn.example:8443/py.tar.gz')
+    ).resolves.toBe('Bearer MTOK');
+  });
+
+  it('withholds mirror-token from an incidental GitHub host and uses token there', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'MTOK',
+      mirror: 'https://cdn.example'
+    });
+    // A manifest hosted on the private mirror may still point release assets at
+    // GitHub; the private credential must not follow them there.
+    await expect(
+      downloadAuthFor('https://objects.githubusercontent.com/x/python.tar.gz')
+    ).resolves.toBe('token TKN');
+  });
+
+  it('withholds mirror-token from a GitHub host when no token is set', async () => {
+    setInputs({'mirror-token': 'MTOK', mirror: 'https://cdn.example'});
+    await expect(
+      downloadAuthFor('https://objects.githubusercontent.com/x/python.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('withholds mirror-token from a third host that is neither the mirror nor GitHub', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'MTOK',
+      mirror: 'https://cdn.example'
+    });
+    await expect(
+      downloadAuthFor('https://other.example/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+
+  it('uses mirror-token for a GitHub mirror host when it is the nominated host', async () => {
+    setInputs({
+      token: 'TKN',
+      'mirror-token': 'token MTOK',
+      mirror: 'https://raw.githubusercontent.com/foo/bar/main'
+    });
+    await expect(
+      downloadAuthFor('https://raw.githubusercontent.com/foo/bar/py.tar.gz')
+    ).resolves.toBe('token MTOK');
+  });
+
+  it('sends no auth when no tokens are configured', async () => {
+    await expect(
+      downloadAuthFor('https://github.com/o/r/releases/download/v/py.tar.gz')
+    ).resolves.toBeUndefined();
+  });
+});

action.yml

@@ -16,8 +16,14 @@ inputs:
     description: "Set this option if you want the action to check for the latest available version that satisfies the version spec."
     default: false
   token:
-    description: "The token used to authenticate when fetching Python distributions from https://github.com/actions/python-versions. When running this action on github.com, the default value is sufficient. When running on GHES, you can pass a personal access token for github.com if you are experiencing rate limiting."
+    description: "The token used to authenticate when fetching Python distributions from https://github.com/actions/python-versions. When running this action on github.com, the default value is sufficient. When running on GHES, you can pass a personal access token for github.com if you are experiencing rate limiting. This token is only sent to GitHub-owned hosts, never to a custom 'mirror'."
     default: ${{ github.server_url == 'https://github.com' && github.token || '' }}
+  mirror:
+    description: "Base URL for downloading Python distributions (only applies to CPython; PyPy and GraalPy are unaffected). Defaults to https://raw.githubusercontent.com/actions/python-versions/main. See docs/advanced-usage.md for details."
+    default: "https://raw.githubusercontent.com/actions/python-versions/main"
+  mirror-token:
+    description: "Token used to authenticate requests to the host named in 'mirror'. Sent verbatim as the Authorization header, so include a scheme if your mirror needs one (e.g. 'Bearer <token>')."
+    required: false
   cache-dependency-path:
     description: "Used to specify the path to dependency files. Supports wildcards or a list of file names for caching multiple dependencies."
   update-environment:

dist/setup/index.js

@@ -98843,12 +98843,114 @@ function _unique(values) {
 
 
 
-const TOKEN = getInput('token');
-const AUTH = !TOKEN ? undefined : `token ${TOKEN}`;
-const MANIFEST_REPO_OWNER = 'actions';
-const MANIFEST_REPO_NAME = 'python-versions';
-const MANIFEST_REPO_BRANCH = 'main';
-const MANIFEST_URL = `https://raw.githubusercontent.com/${MANIFEST_REPO_OWNER}/${MANIFEST_REPO_NAME}/${MANIFEST_REPO_BRANCH}/versions-manifest.json`;
+const DEFAULT_REPO_OWNER = 'actions';
+const DEFAULT_REPO_NAME = 'python-versions';
+const DEFAULT_REPO_BRANCH = 'main';
+const DEFAULT_MIRROR = `https://raw.githubusercontent.com/${DEFAULT_REPO_OWNER}/${DEFAULT_REPO_NAME}/${DEFAULT_REPO_BRANCH}`;
+// Matches https://raw.githubusercontent.com/{owner}/{repo}/{branch} and the
+// equivalent https://raw.githubusercontent.com/{owner}/{repo}/refs/heads/{branch}
+// form, capturing the bare branch in both. The GitHub tree API wants the bare
+// branch, so the optional refs/heads/ prefix is consumed, not captured.
+const REPO_COORDS_RE = /^https:\/\/raw\.githubusercontent\.com\/([^/]+)\/([^/]+)\/(?:refs\/heads\/)?([^/]+)\/?$/;
+function getToken() {
+    return getInput('token');
+}
+function getMirrorToken() {
+    return getInput('mirror-token');
+}
+// Memoized per raw input value so the mirror is validated once per run rather
+// than on every call. `getManifestUrl()` is also used to build the "version not
+// found" message in find-python.ts, where re-validating would replace the real
+// cause with an invalid-mirror error.
+const mirrorCache = new Map();
+function getMirror() {
+    const input = getInput('mirror') || DEFAULT_MIRROR;
+    let resolved = mirrorCache.get(input);
+    if (!resolved) {
+        const url = input.trim().replace(/\/+$/, '');
+        try {
+            new URL(url);
+            resolved = { url };
+        }
+        catch {
+            resolved = { error: new Error(`Invalid 'mirror' URL: "${url}"`) };
+        }
+        mirrorCache.set(input, resolved);
+    }
+    if ('error' in resolved)
+        throw resolved.error;
+    return resolved.url;
+}
+function getManifestUrl() {
+    return `${getMirror()}/versions-manifest.json`;
+}
+// Whether the user set `mirror` to something other than the built-in default.
+// action.yml gives `mirror` a default, so core.getInput('mirror') is never
+// empty; callers that want "did the user opt into a custom mirror" must compare
+// against DEFAULT_MIRROR rather than test for a falsy input. Normalizes the
+// same way getMirror() does but never throws, so a warning path can call it
+// even when the mirror is malformed.
+function isMirrorCustomized() {
+    const input = getInput('mirror');
+    if (!input)
+        return false;
+    return input.trim().replace(/\/+$/, '') !== DEFAULT_MIRROR;
+}
+// Origin (scheme + host + port) of the mirror, so `mirror-token` is matched
+// against the exact origin the user nominated. Comparing origin rather than
+// host alone means a manifest served over https that points a download_url at
+// http://same-host/... does NOT get the token — the scheme must match too.
+// Deliberately not wrapped in try/catch: an invalid mirror throws here just as
+// it does in getManifestUrl(), so both agree a bad mirror is fatal.
+function getMirrorOrigin() {
+    return new URL(getMirror()).origin;
+}
+function isGitHubHost(host) {
+    return (host === 'github.com' ||
+        host.endsWith('.github.com') ||
+        host.endsWith('.githubusercontent.com'));
+}
+// Warned at most once per distinct mirror; resolveRepoCoords() is called from
+// several paths within a single run.
+const warnedMirrors = new Set();
+function resolveRepoCoords() {
+    const mirror = getMirror();
+    const m = REPO_COORDS_RE.exec(mirror);
+    if (m)
+        return { owner: m[1], repo: m[2], branch: m[3] };
+    // A raw.githubusercontent.com URL that doesn't parse is a branch name
+    // containing a slash (e.g. .../{owner}/{repo}/feature/riscv), which is
+    // indistinguishable from a deeper path. getMirror() succeeded above, so the
+    // URL is well-formed and this parse cannot throw.
+    const isRawGitHub = new URL(mirror).host === 'raw.githubusercontent.com';
+    if (!warnedMirrors.has(mirror) && isRawGitHub) {
+        warnedMirrors.add(mirror);
+        warning(`Could not parse owner/repo/branch out of mirror "${mirror}", so the manifest will be fetched directly from the raw URL instead of through the GitHub REST API. ` +
+            `The request is still authenticated with your token, because raw.githubusercontent.com is a GitHub host. ` +
+            `Branch names containing '/' are not supported for the REST API path; use a branch without a slash if you want the manifest fetched through the API.`);
+    }
+    return null;
+}
+// Mirror origin with `mirror-token` set gets the token verbatim, so internal
+// mirrors can choose their own scheme (Bearer, Basic, ...). GitHub hosts get
+// `token ${token}`. Anything else is anonymous — neither credential is sent to
+// a host the user didn't nominate.
+function authForUrl(url) {
+    let parsed;
+    try {
+        parsed = new URL(url);
+    }
+    catch {
+        return undefined;
+    }
+    const mirrorToken = getMirrorToken();
+    if (mirrorToken && parsed.origin === getMirrorOrigin())
+        return mirrorToken;
+    const token = getToken();
+    if (token && isGitHubHost(parsed.host))
+        return `token ${token}`;
+    return undefined;
+}
 function getLinuxOsRelease() {
     try {
         const content = external_fs_namespaceObject.readFileSync('/etc/os-release', 'utf8');
@@ -98974,17 +99076,25 @@ async function fetchValidManifest(source, fetcher) {
     throw new Error(`Failed to fetch a valid manifest from ${source} after ${attempts} attempt(s): ${lastError?.message}`);
 }
 async function getManifest() {
-    try {
-        return await fetchValidManifest('the GitHub API', install_python_getManifestFromRepo);
+    // Only GitHub repo mirrors can be fetched via the API. Checking up front
+    // avoids burning MANIFEST_FETCH_MAX_ATTEMPTS with backoff on a throw that
+    // could never succeed.
+    if (resolveRepoCoords()) {
+        try {
+            return await fetchValidManifest('the GitHub API', install_python_getManifestFromRepo);
+        }
+        catch (err) {
+            core_debug('Fetching the manifest via the API failed.');
+            if (err instanceof Error) {
+                core_debug(err.message);
+            }
+            else {
+                core_debug('An unexpected error occurred while fetching the manifest.');
+            }
+        }
     }
-    catch (err) {
-        core_debug('Fetching the manifest via the API failed.');
-        if (err instanceof Error) {
-            core_debug(err.message);
-        }
-        else {
-            core_debug('An unexpected error occurred while fetching the manifest.');
-        }
+    else {
+        core_debug(`Mirror "${getMirror()}" is not a GitHub repo URL; fetching the manifest by URL.`);
     }
     try {
         return await fetchValidManifest('the raw URL', getManifestFromURL);
@@ -98996,15 +99106,32 @@ async function getManifest() {
     }
 }
 function install_python_getManifestFromRepo() {
-    core_debug(`Getting manifest from ${MANIFEST_REPO_OWNER}/${MANIFEST_REPO_NAME}@${MANIFEST_REPO_BRANCH}`);
-    return getManifestFromRepo(MANIFEST_REPO_OWNER, MANIFEST_REPO_NAME, AUTH, MANIFEST_REPO_BRANCH);
+    const coords = resolveRepoCoords();
+    if (!coords) {
+        throw new Error(`Mirror "${getMirror()}" is not a GitHub repo URL; falling back to raw URL fetch.`);
+    }
+    core_debug(`Getting manifest from ${coords.owner}/${coords.repo}@${coords.branch}`);
+    // This only runs for GitHub repo mirrors, where `mirror-token` is the user's
+    // explicit intent for that repo. The target is always api.github.com, which
+    // requires the `token ` prefix, so the host rule in authForUrl() doesn't
+    // apply here.
+    const token = getToken();
+    const mirrorToken = getMirrorToken();
+    const auth = mirrorToken
+        ? `token ${mirrorToken}`
+        : token
+            ? `token ${token}`
+            : undefined;
+    return getManifestFromRepo(coords.owner, coords.repo, auth, coords.branch);
 }
 async function getManifestFromURL() {
     core_debug('Falling back to fetching the manifest using raw URL.');
+    const manifestUrl = getManifestUrl();
     const http = new lib_HttpClient('tool-cache');
-    const response = await http.getJson(MANIFEST_URL);
+    const auth = authForUrl(manifestUrl);
+    const response = await http.getJson(manifestUrl, auth ? { authorization: auth } : undefined);
     if (!response.result) {
-        throw new Error(`Unable to get manifest from ${MANIFEST_URL}`);
+        throw new Error(`Unable to get manifest from ${manifestUrl}`);
     }
     return response.result;
 }
@@ -99047,7 +99174,7 @@ async function installCpythonFromRelease(release) {
     let pythonPath = '';
     try {
         const fileName = getDownloadFileName(downloadUrl);
-        pythonPath = await downloadTool(downloadUrl, fileName, AUTH);
+        pythonPath = await downloadTool(downloadUrl, fileName, authForUrl(downloadUrl));
         info('Extract downloaded archive');
         let pythonExtractedFolder;
         if (utils_IS_WINDOWS) {
@@ -99165,7 +99292,7 @@ async function useCpythonVersion(version, architecture, updateEnvironment, check
         if (freethreaded) {
             msg.push(`Free threaded versions are only available for Python 3.13.0 and later.`);
         }
-        msg.push(`The list of all available versions can be found here: ${MANIFEST_URL}`);
+        msg.push(`The list of all available versions can be found here: ${getManifestUrl()}`);
         throw new Error(msg.join(external_os_.EOL));
     }
     const _binDir = binDir(installDir);
@@ -99567,8 +99694,8 @@ function findPyPyInstallDirForWindows(pythonVersion) {
 
 
 
-const install_graalpy_TOKEN = getInput('token');
-const install_graalpy_AUTH = !install_graalpy_TOKEN ? undefined : `token ${install_graalpy_TOKEN}`;
+const TOKEN = getInput('token');
+const AUTH = !TOKEN ? undefined : `token ${TOKEN}`;
 async function installGraalPy(graalpyVersion, architecture, allowPreReleases, releases) {
     let downloadDir;
     releases = releases ?? (await getAvailableGraalPyVersions());
@@ -99591,7 +99718,7 @@ async function installGraalPy(graalpyVersion, architecture, allowPreReleases, re
     const downloadUrl = `${foundAsset.browser_download_url}`;
     info(`Downloading GraalPy from "${downloadUrl}" ...`);
     try {
-        const graalpyPath = await downloadTool(downloadUrl, undefined, install_graalpy_AUTH);
+        const graalpyPath = await downloadTool(downloadUrl, undefined, AUTH);
         info('Extracting downloaded archive...');
         if (utils_IS_WINDOWS) {
             downloadDir = await extractZip(graalpyPath);
@@ -99632,8 +99759,8 @@ async function installGraalPy(graalpyVersion, architecture, allowPreReleases, re
 async function getAvailableGraalPyVersions() {
     const http = new lib_HttpClient('tool-cache');
     const headers = {};
-    if (install_graalpy_AUTH) {
-        headers.authorization = install_graalpy_AUTH;
+    if (AUTH) {
+        headers.authorization = AUTH;
     }
     /*
     Get releases first.
@@ -103608,12 +103735,25 @@ function getCacheDistributor(packageManager, pythonVersion, cacheDependencyPath)
 
 
 
+
 function isPyPyVersion(versionSpec) {
     return versionSpec.startsWith('pypy');
 }
 function isGraalPyVersion(versionSpec) {
     return versionSpec.startsWith('graalpy');
 }
+// `mirror` only redirects CPython distributions. PyPy and GraalPy resolve from
+// downloads.python.org and the GitHub releases API respectively, so warn rather
+// than let the input look like it applied. Only warns when the user actually
+// set a custom mirror: action.yml gives `mirror` a default, so a plain
+// getInput() check would fire on every pypy-*/graalpy-* run.
+function warnIfMirrorUnsupported(versionSpec) {
+    if (!isMirrorCustomized()) {
+        return;
+    }
+    const implementation = isPyPyVersion(versionSpec) ? 'PyPy' : 'GraalPy';
+    warning(`The 'mirror' input only applies to CPython distributions and is ignored for ${implementation} ('${versionSpec}'), which is downloaded from its own upstream source.`);
+}
 async function cacheDependencies(cache, pythonVersion) {
     const cacheDependencyPath = getInput('cache-dependency-path') || undefined;
     const cacheDistributor = getCacheDistributor(cache, pythonVersion, cacheDependencyPath);
@@ -103675,11 +103815,13 @@ async function run() {
             startGroup('Installed versions');
             for (const version of versions) {
                 if (isPyPyVersion(version)) {
+                    warnIfMirrorUnsupported(version);
                     const installed = await findPyPyVersion(version, arch, updateEnvironment, checkLatest, allowPreReleases);
                     pythonVersion = `${installed.resolvedPyPyVersion}-${installed.resolvedPythonVersion}`;
                     info(`Successfully set up PyPy ${installed.resolvedPyPyVersion} with Python (${installed.resolvedPythonVersion})`);
                 }
                 else if (isGraalPyVersion(version)) {
+                    warnIfMirrorUnsupported(version);
                     const installed = await findGraalPyVersion(version, arch, updateEnvironment, checkLatest, allowPreReleases);
                     pythonVersion = `${installed}`;
                     info(`Successfully set up GraalPy ${installed}`);

docs/advanced-usage.md

@@ -524,6 +524,46 @@ Such a requirement on side-effect could be because you don't want your composite
 
 >**Note:** Python versions used in this action are generated in the [python-versions](https://github.com/actions/python-versions) repository. For macOS and Ubuntu images, python versions are built from the source code. For Windows, the python-versions repository uses installation executable. For more information please refer to the [python-versions](https://github.com/actions/python-versions) repository.
 
+#### Using a custom mirror
+
+The `mirror` input lets you point `setup-python` at a different location for CPython distributions — a personal fork of `actions/python-versions`, an internal mirror, or any server that hosts a `versions-manifest.json` at its root plus the tarballs referenced by that manifest. Default: `https://raw.githubusercontent.com/actions/python-versions/main`.
+
+The manifest is resolved as follows:
+
+- If `mirror` matches `https://raw.githubusercontent.com/{owner}/{repo}/{branch}`, the manifest is fetched via the GitHub REST API (giving you the 5000/hr authenticated rate limit when a token is present).
+- Otherwise, the action fetches `{mirror}/versions-manifest.json` via a direct HTTP GET.
+
+Authentication is decided by the host of each request, so neither credential reaches a server you did not nominate:
+
+- Requests to the host named in `mirror` use `mirror-token`, sent **verbatim** as the `Authorization` header. Include a scheme if your mirror expects one — `Bearer <token>`, `Basic <base64>`, or `token <token>` for a GitHub host. This covers both the manifest fetch and the tarball downloads.
+- Requests to `github.com`, `*.github.com`, or `*.githubusercontent.com` use `token`, sent as `token <token>`. A manifest that points its `download_url` at a GitHub host therefore keeps working without `mirror-token` being leaked to it.
+- Any other host is requested anonymously.
+- One exception: when `mirror` is a GitHub repo URL, the manifest is fetched from `api.github.com`, and `mirror-token` is preferred there (with the `token ` prefix the API requires) because naming a repo mirror is an explicit instruction to read that repo.
+
+Point at a personal fork of `actions/python-versions` (uses the default `token`, fetched via the GitHub API):
+
+```yaml
+- uses: actions/setup-python@v6
+  with:
+    python-version: '3.12'
+    mirror: https://raw.githubusercontent.com/my-org/python-versions/main
+```
+
+Point at an internal mirror with its own credential:
+
+```yaml
+- uses: actions/setup-python@v6
+  with:
+    python-version: '3.12'
+    mirror: https://python-mirror.internal.example
+    mirror-token: ${{ secrets.PYTHON_MIRROR_TOKEN }}
+```
+
+Caveats:
+
+- `mirror` and `mirror-token` apply to **CPython only**. PyPy resolves from `downloads.python.org` and GraalPy from the GitHub releases API; both ignore these inputs, and the action warns if you set `mirror` alongside a `pypy-*` or `graalpy-*` version.
+- Branch names containing `/` cannot be used with a `raw.githubusercontent.com` mirror, because `.../{owner}/{repo}/feature/riscv` is indistinguishable from a repo path. Such a mirror still works and is still authenticated with your `token` (raw.githubusercontent.com is a GitHub host), but the manifest is fetched directly from the raw URL rather than through the GitHub REST API; the action warns when this happens. Use a branch without a slash to get the REST API path. The `refs/heads/{branch}` form (for example `.../actions/python-versions/refs/heads/main`) is recognized and routes through the REST API.
+
 ### PyPy
 
  `setup-python` is able to configure **PyPy** from two sources:

src/find-python.ts

@@ -137,7 +137,7 @@ export async function useCpythonVersion(
       );
     }
     msg.push(
-      `The list of all available versions can be found here: ${installer.MANIFEST_URL}`
+      `The list of all available versions can be found here: ${installer.getManifestUrl()}`
     );
     throw new Error(msg.join(os.EOL));
   }

src/install-python.ts

@@ -9,12 +9,135 @@ import * as semver from 'semver';
 import {IS_WINDOWS, IS_LINUX, getDownloadFileName} from './utils.js';
 import {IToolRelease} from '@actions/tool-cache';
 
-const TOKEN = core.getInput('token');
-const AUTH = !TOKEN ? undefined : `token ${TOKEN}`;
-const MANIFEST_REPO_OWNER = 'actions';
-const MANIFEST_REPO_NAME = 'python-versions';
-const MANIFEST_REPO_BRANCH = 'main';
-export const MANIFEST_URL = `https://raw.githubusercontent.com/${MANIFEST_REPO_OWNER}/${MANIFEST_REPO_NAME}/${MANIFEST_REPO_BRANCH}/versions-manifest.json`;
+const DEFAULT_REPO_OWNER = 'actions';
+const DEFAULT_REPO_NAME = 'python-versions';
+const DEFAULT_REPO_BRANCH = 'main';
+const DEFAULT_MIRROR = `https://raw.githubusercontent.com/${DEFAULT_REPO_OWNER}/${DEFAULT_REPO_NAME}/${DEFAULT_REPO_BRANCH}`;
+
+// Matches https://raw.githubusercontent.com/{owner}/{repo}/{branch} and the
+// equivalent https://raw.githubusercontent.com/{owner}/{repo}/refs/heads/{branch}
+// form, capturing the bare branch in both. The GitHub tree API wants the bare
+// branch, so the optional refs/heads/ prefix is consumed, not captured.
+const REPO_COORDS_RE =
+  /^https:\/\/raw\.githubusercontent\.com\/([^/]+)\/([^/]+)\/(?:refs\/heads\/)?([^/]+)\/?$/;
+
+function getToken(): string {
+  return core.getInput('token');
+}
+
+function getMirrorToken(): string {
+  return core.getInput('mirror-token');
+}
+
+// Memoized per raw input value so the mirror is validated once per run rather
+// than on every call. `getManifestUrl()` is also used to build the "version not
+// found" message in find-python.ts, where re-validating would replace the real
+// cause with an invalid-mirror error.
+const mirrorCache = new Map<string, {url: string} | {error: Error}>();
+
+function getMirror(): string {
+  const input = core.getInput('mirror') || DEFAULT_MIRROR;
+  let resolved = mirrorCache.get(input);
+
+  if (!resolved) {
+    const url = input.trim().replace(/\/+$/, '');
+    try {
+      new URL(url);
+      resolved = {url};
+    } catch {
+      resolved = {error: new Error(`Invalid 'mirror' URL: "${url}"`)};
+    }
+    mirrorCache.set(input, resolved);
+  }
+
+  if ('error' in resolved) throw resolved.error;
+  return resolved.url;
+}
+
+export function getManifestUrl(): string {
+  return `${getMirror()}/versions-manifest.json`;
+}
+
+// Whether the user set `mirror` to something other than the built-in default.
+// action.yml gives `mirror` a default, so core.getInput('mirror') is never
+// empty; callers that want "did the user opt into a custom mirror" must compare
+// against DEFAULT_MIRROR rather than test for a falsy input. Normalizes the
+// same way getMirror() does but never throws, so a warning path can call it
+// even when the mirror is malformed.
+export function isMirrorCustomized(): boolean {
+  const input = core.getInput('mirror');
+  if (!input) return false;
+  return input.trim().replace(/\/+$/, '') !== DEFAULT_MIRROR;
+}
+
+// Origin (scheme + host + port) of the mirror, so `mirror-token` is matched
+// against the exact origin the user nominated. Comparing origin rather than
+// host alone means a manifest served over https that points a download_url at
+// http://same-host/... does NOT get the token — the scheme must match too.
+// Deliberately not wrapped in try/catch: an invalid mirror throws here just as
+// it does in getManifestUrl(), so both agree a bad mirror is fatal.
+function getMirrorOrigin(): string {
+  return new URL(getMirror()).origin;
+}
+
+function isGitHubHost(host: string): boolean {
+  return (
+    host === 'github.com' ||
+    host.endsWith('.github.com') ||
+    host.endsWith('.githubusercontent.com')
+  );
+}
+
+// Warned at most once per distinct mirror; resolveRepoCoords() is called from
+// several paths within a single run.
+const warnedMirrors = new Set<string>();
+
+export function resolveRepoCoords(): {
+  owner: string;
+  repo: string;
+  branch: string;
+} | null {
+  const mirror = getMirror();
+  const m = REPO_COORDS_RE.exec(mirror);
+  if (m) return {owner: m[1], repo: m[2], branch: m[3]};
+
+  // A raw.githubusercontent.com URL that doesn't parse is a branch name
+  // containing a slash (e.g. .../{owner}/{repo}/feature/riscv), which is
+  // indistinguishable from a deeper path. getMirror() succeeded above, so the
+  // URL is well-formed and this parse cannot throw.
+  const isRawGitHub = new URL(mirror).host === 'raw.githubusercontent.com';
+  if (!warnedMirrors.has(mirror) && isRawGitHub) {
+    warnedMirrors.add(mirror);
+    core.warning(
+      `Could not parse owner/repo/branch out of mirror "${mirror}", so the manifest will be fetched directly from the raw URL instead of through the GitHub REST API. ` +
+        `The request is still authenticated with your token, because raw.githubusercontent.com is a GitHub host. ` +
+        `Branch names containing '/' are not supported for the REST API path; use a branch without a slash if you want the manifest fetched through the API.`
+    );
+  }
+
+  return null;
+}
+
+// Mirror origin with `mirror-token` set gets the token verbatim, so internal
+// mirrors can choose their own scheme (Bearer, Basic, ...). GitHub hosts get
+// `token ${token}`. Anything else is anonymous — neither credential is sent to
+// a host the user didn't nominate.
+function authForUrl(url: string): string | undefined {
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return undefined;
+  }
+
+  const mirrorToken = getMirrorToken();
+  if (mirrorToken && parsed.origin === getMirrorOrigin()) return mirrorToken;
+
+  const token = getToken();
+  if (token && isGitHubHost(parsed.host)) return `token ${token}`;
+
+  return undefined;
+}
 
 interface LinuxOsRelease {
   id: string;
@@ -205,15 +328,24 @@ async function fetchValidManifest(
 }
 
 export async function getManifest(): Promise<tc.IToolRelease[]> {
-  try {
-    return await fetchValidManifest('the GitHub API', getManifestFromRepo);
-  } catch (err) {
-    core.debug('Fetching the manifest via the API failed.');
-    if (err instanceof Error) {
-      core.debug(err.message);
-    } else {
-      core.debug('An unexpected error occurred while fetching the manifest.');
+  // Only GitHub repo mirrors can be fetched via the API. Checking up front
+  // avoids burning MANIFEST_FETCH_MAX_ATTEMPTS with backoff on a throw that
+  // could never succeed.
+  if (resolveRepoCoords()) {
+    try {
+      return await fetchValidManifest('the GitHub API', getManifestFromRepo);
+    } catch (err) {
+      core.debug('Fetching the manifest via the API failed.');
+      if (err instanceof Error) {
+        core.debug(err.message);
+      } else {
+        core.debug('An unexpected error occurred while fetching the manifest.');
+      }
     }
+  } else {
+    core.debug(
+      `Mirror "${getMirror()}" is not a GitHub repo URL; fetching the manifest by URL.`
+    );
   }
 
   try {
@@ -229,24 +361,41 @@ export async function getManifest(): Promise<tc.IToolRelease[]> {
 }
 
 export function getManifestFromRepo(): Promise<tc.IToolRelease[]> {
+  const coords = resolveRepoCoords();
+  if (!coords) {
+    throw new Error(
+      `Mirror "${getMirror()}" is not a GitHub repo URL; falling back to raw URL fetch.`
+    );
+  }
   core.debug(
-    `Getting manifest from ${MANIFEST_REPO_OWNER}/${MANIFEST_REPO_NAME}@${MANIFEST_REPO_BRANCH}`
-  );
-  return tc.getManifestFromRepo(
-    MANIFEST_REPO_OWNER,
-    MANIFEST_REPO_NAME,
-    AUTH,
-    MANIFEST_REPO_BRANCH
+    `Getting manifest from ${coords.owner}/${coords.repo}@${coords.branch}`
   );
+  // This only runs for GitHub repo mirrors, where `mirror-token` is the user's
+  // explicit intent for that repo. The target is always api.github.com, which
+  // requires the `token ` prefix, so the host rule in authForUrl() doesn't
+  // apply here.
+  const token = getToken();
+  const mirrorToken = getMirrorToken();
+  const auth = mirrorToken
+    ? `token ${mirrorToken}`
+    : token
+      ? `token ${token}`
+      : undefined;
+  return tc.getManifestFromRepo(coords.owner, coords.repo, auth, coords.branch);
 }
 
 export async function getManifestFromURL(): Promise<tc.IToolRelease[]> {
   core.debug('Falling back to fetching the manifest using raw URL.');
 
+  const manifestUrl = getManifestUrl();
   const http: httpm.HttpClient = new httpm.HttpClient('tool-cache');
-  const response = await http.getJson<tc.IToolRelease[]>(MANIFEST_URL);
+  const auth = authForUrl(manifestUrl);
+  const response = await http.getJson<tc.IToolRelease[]>(
+    manifestUrl,
+    auth ? {authorization: auth} : undefined
+  );
   if (!response.result) {
-    throw new Error(`Unable to get manifest from ${MANIFEST_URL}`);
+    throw new Error(`Unable to get manifest from ${manifestUrl}`);
   }
   return response.result;
 }
@@ -291,7 +440,11 @@ export async function installCpythonFromRelease(release: tc.IToolRelease) {
   let pythonPath = '';
   try {
     const fileName = getDownloadFileName(downloadUrl);
-    pythonPath = await tc.downloadTool(downloadUrl, fileName, AUTH);
+    pythonPath = await tc.downloadTool(
+      downloadUrl,
+      fileName,
+      authForUrl(downloadUrl)
+    );
     core.info('Extract downloaded archive');
     let pythonExtractedFolder;
     if (IS_WINDOWS) {

src/setup-python.ts

@@ -2,6 +2,7 @@ import * as core from '@actions/core';
 import * as finder from './find-python.js';
 import * as finderPyPy from './find-pypy.js';
 import * as finderGraalPy from './find-graalpy.js';
+import {isMirrorCustomized} from './install-python.js';
 import * as path from 'path';
 import * as os from 'os';
 import {fileURLToPath} from 'url';
@@ -23,6 +24,21 @@ function isGraalPyVersion(versionSpec: string) {
   return versionSpec.startsWith('graalpy');
 }
 
+// `mirror` only redirects CPython distributions. PyPy and GraalPy resolve from
+// downloads.python.org and the GitHub releases API respectively, so warn rather
+// than let the input look like it applied. Only warns when the user actually
+// set a custom mirror: action.yml gives `mirror` a default, so a plain
+// getInput() check would fire on every pypy-*/graalpy-* run.
+function warnIfMirrorUnsupported(versionSpec: string) {
+  if (!isMirrorCustomized()) {
+    return;
+  }
+  const implementation = isPyPyVersion(versionSpec) ? 'PyPy' : 'GraalPy';
+  core.warning(
+    `The 'mirror' input only applies to CPython distributions and is ignored for ${implementation} ('${versionSpec}'), which is downloaded from its own upstream source.`
+  );
+}
+
 async function cacheDependencies(cache: string, pythonVersion: string) {
   const cacheDependencyPath =
     core.getInput('cache-dependency-path') || undefined;
@@ -102,6 +118,7 @@ async function run() {
       core.startGroup('Installed versions');
       for (const version of versions) {
         if (isPyPyVersion(version)) {
+          warnIfMirrorUnsupported(version);
           const installed = await finderPyPy.findPyPyVersion(
             version,
             arch,
@@ -114,6 +131,7 @@ async function run() {
             `Successfully set up PyPy ${installed.resolvedPyPyVersion} with Python (${installed.resolvedPythonVersion})`
           );
         } else if (isGraalPyVersion(version)) {
+          warnIfMirrorUnsupported(version);
           const installed = await finderGraalPy.findGraalPyVersion(
             version,
             arch,