diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 29ff668..785d66b 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -37,15 +37,15 @@ jobs:
         uses: ./
         with:
           use-installer: true
-          version: "1.104.0"
-      - run: sam --version | grep -F 1.104.0
+          version: "1.159.1"
+      - run: sam --version | grep -F 1.159.1
 
       - name: Test official installer (pinned version; should use cache)
         uses: ./
         with:
           use-installer: true
-          version: "1.104.0"
-      - run: sam --version | grep -F 1.104.0
+          version: "1.159.1"
+      - run: sam --version | grep -F 1.159.1
 
       - name: Test official installer (latest version)
         uses: ./
@@ -56,6 +56,53 @@ jobs:
           sam --version | grep -F "$version"
         shell: bash
 
+      - name: Test official installer (nightly version)
+        uses: ./
+        with:
+          use-installer: true
+          version: nightly
+      - run: sam --version
+
+  native-installer-windows:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - windows-latest
+          - windows-2022
+    name: Native installer (MSI) / ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v6
+
+      # Pin to an older version to exercise the downgrade path: GitHub-hosted
+      # Windows runners ship a recent SAM CLI preinstalled, and Windows
+      # Installer rejects silent downgrades unless the existing product is
+      # uninstalled first. This step verifies that uninstall-before-install
+      # works regardless of which version the runner image happens to ship.
+      - name: Test MSI installer (pinned version)
+        uses: ./
+        with:
+          use-installer: true
+          version: "1.139.0"
+      - run: sam --version | findstr /C:"1.139.0"
+        shell: cmd
+
+      - name: Test MSI installer (latest version)
+        uses: ./
+        with:
+          use-installer: true
+      - run: sam --version
+        shell: cmd
+
+      - name: Test MSI installer (nightly version)
+        uses: ./
+        with:
+          use-installer: true
+          version: nightly
+      - run: sam --version
+        shell: cmd
+
   integ:
     strategy:
       fail-fast: false
@@ -75,8 +122,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     env:
       # Set SAM versions based on Python version
-      SAM_VERSION: ${{ contains(fromJson('["3.12", "3.13"]'), matrix.python) && '1.128.0' || '1.18.2' }}
-      INSTALLER_VERSION: ${{ contains( matrix.os, '-arm') && '1.130.0' || '1.71.0' }}
+      SAM_VERSION: ${{ contains(fromJson('["3.12", "3.13"]'), matrix.python) && '1.159.1' || '1.18.2' }}
+      INSTALLER_VERSION: "1.159.1"
     steps:
       - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
diff --git a/README.md b/README.md
index 2461a2c..575fe7e 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,17 @@ Action to set up [AWS SAM CLI](https://docs.aws.amazon.com/serverless-applicatio
 
 This action enables you to run AWS SAM CLI commands in order to build, package, and deploy [serverless applications](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/what-is-sam.html) as part of your workflow.
 
+## Do you need this action?
+
+The AWS SAM CLI is **preinstalled on every GitHub-hosted runner image** (Ubuntu, Windows, and macOS) — see the [`runner-images`](https://github.com/actions/runner-images) repository (e.g. [Ubuntu 24.04](https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md), [Ubuntu 22.04](https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md), [Windows 2025](https://github.com/actions/runner-images/blob/main/images/windows/Windows2025-Readme.md), [macOS 15](https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md)) for the exact version shipped with each image. If your workflow only needs the version that comes with the runner, you can call `sam` directly without using this action.
+
+Use this action when you need:
+
+- A **specific version** of the SAM CLI (pinned via the `version` input).
+- The **`nightly` release** of the SAM CLI to validate upcoming changes before they ship.
+- A consistent SAM CLI version across runner image upgrades.
+- The native installer on a runner where SAM CLI is not preinstalled (e.g. self-hosted runners).
+
 ## Example
 
 Assuming you have a [`samconfig.toml`](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-cli-config.html) at the root of your repository:
@@ -38,11 +49,29 @@ jobs:
 
 See [AWS IAM best practices](https://docs.aws.amazon.com/IAM/latest/UserGuide/best-practices.html) for handling AWS credentials.
 
+### Installing the nightly release
+
+To validate your project against unreleased changes to the AWS SAM CLI:
+
+```yaml
+- uses: aws-actions/setup-sam@v3
+  with:
+    use-installer: true
+    version: nightly
+- run: sam --version
+```
+
 ## Inputs
 
 ### `version`
 
-The AWS SAM CLI version to install. Installs the latest version by default.
+The AWS SAM CLI version to install. Installs the latest stable version by default.
+
+Accepts:
+
+- An exact version (`x.y.z`, e.g. `1.139.0`) — pinned install.
+- A version pattern (`1.*`, `1.139.*`) — only when `use-installer` is `false` (resolved by `pip`).
+- `nightly` — installs the latest [nightly release](https://github.com/aws/aws-sam-cli/releases/tag/sam-cli-nightly) of the SAM CLI. Requires `use-installer: true`. Nightly releases are not cached because the `sam-cli-nightly` tag is updated in place each day.
 
 ### `use-installer`
 
@@ -50,8 +79,12 @@ The AWS SAM CLI version to install. Installs the latest version by default.
 >
 > This is the recommended approach on supported platforms. It does not require Python to be installed, and is faster than the default installation method.
 >
-> Currently supports Linux x86-64 and aarch64 (ARM) runners. For ARM architecture, only versions 1.104.0 and above are supported.
-> Set to `true` to set up AWS SAM CLI using a native installer. Defaults to `false`.
+> Currently supports:
+>
+> - Linux x86-64 and aarch64 (ARM) — uses the official archive installer. For ARM, only versions 1.104.0 and above are supported.
+> - Windows x86-64 — uses the official MSI installer (`AWS_SAM_CLI_64_PY3.msi`).
+
+Set to `true` to set up AWS SAM CLI using a native installer. Defaults to `false`. Required when `version` is set to `nightly`.
 
 ### `python`
 
diff --git a/action.yml b/action.yml
index 5004417..b4d4b30 100644
--- a/action.yml
+++ b/action.yml
@@ -5,7 +5,7 @@ branding:
   color: "orange"
 inputs:
   version:
-    description: "The AWS SAM CLI version to install"
+    description: 'The AWS SAM CLI version to install. Use "nightly" (with use-installer: true) to install the latest nightly release.'
     required: false
   python:
     description: "The Python interpreter to use for AWS SAM CLI"
diff --git a/dist/index.js b/dist/index.js
index 269aec8..d9c3de8 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -130,6 +130,16 @@ function isSemver(s) {
   return /^\d+\.\d+\.\d+$/.test(s);
 }
 
+const NIGHTLY = "nightly";
+const NIGHTLY_TAG = "sam-cli-nightly";
+
+/**
+ * Returns whether a version string requests the nightly release.
+ */
+function isNightly(version) {
+  return version === NIGHTLY;
+}
+
 /**
  * Get latest SAM CLI version from https://api.github.com/repos/aws/aws-sam-cli/releases/latest
  *
@@ -198,10 +208,12 @@ async function downloadAndCache(version, arch, installDir, cacheKey) {
  * Downloads SAM CLI without caching.
  *
  * @param {string} arch - The architecture (x86_64 or arm64).
+ * @param {string} releaseTag - Optional release tag (e.g. "sam-cli-nightly"). Defaults to latest.
  * @returns {Promise<string>} The directory SAM CLI is installed in.
  */
-async function downloadWithoutCache(arch) {
-  const url = `https://github.com/aws/aws-sam-cli/releases/latest/download/aws-sam-cli-linux-${arch}.zip`;
+async function downloadWithoutCache(arch, releaseTag) {
+  const tagSegment = releaseTag ? `download/${releaseTag}` : "latest/download";
+  const url = `https://github.com/aws/aws-sam-cli/releases/${tagSegment}/aws-sam-cli-linux-${arch}.zip`;
   const tempDir = mkdirTemp();
 
   try {
@@ -215,32 +227,265 @@ async function downloadWithoutCache(arch) {
 }
 
 /**
- * Installs SAM CLI using the native installers.
+ * Builds the URL to the Windows MSI asset for a given release tag or version.
  *
- * See https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/install-sam-cli.html
+ * @param {string} tagOrVersion - Either a release tag (e.g. "sam-cli-nightly")
+ *   or an empty string to use the "latest" alias.
+ * @param {boolean} isTag - True if the first arg is a release tag, false if version (x.y.z).
+ */
+function windowsMsiUrl(tagOrVersion, isTag) {
+  const asset = "AWS_SAM_CLI_64_PY3.msi";
+  if (!tagOrVersion) {
+    return `https://github.com/aws/aws-sam-cli/releases/latest/download/${asset}`;
+  }
+  const tag = isTag ? tagOrVersion : `v${tagOrVersion}`;
+  return `https://github.com/aws/aws-sam-cli/releases/download/${tag}/${asset}`;
+}
+
+/**
+ * Runs the MSI silently via msiexec. Throws on failure.
  *
- * @param {string} inputVersion - The SAM CLI version to install.
- * @param {string} token - Authentication Token to use for GITHUB Apis.
- * @returns {Promise<string>} The directory SAM CLI is installed in.
+ * @param {string} msiPath - Path to the downloaded MSI.
+ */
+async function runMsiExec(msiPath) {
+  // 3010 = ERROR_SUCCESS_REBOOT_REQUIRED; treat as success
+  const logPath = path.join(mkdirTemp(), "msi-install.log");
+  const exitCode = await exec.exec(
+    "msiexec",
+    ["/i", msiPath, "/qn", "/norestart", "/l*v", logPath],
+    { ignoreReturnCode: true },
+  );
+  if (exitCode !== 0 && exitCode !== 3010) {
+    if (fs.existsSync(logPath)) {
+      const tail = fs.readFileSync(logPath, "utf8").split("\n").slice(-30);
+      core.warning(
+        `msiexec failed (${exitCode}); last log lines:\n${tail.join("\n")}`,
+      );
+    }
+    throw new Error(`msiexec failed with exit code ${exitCode}`);
+  }
+}
+
+/**
+ * Returns the MSI install root for the given SAM CLI flavor.
+ *
+ * Stable installs to `C:\Program Files\Amazon\AWSSAMCLI`; nightly installs
+ * to `C:\Program Files\Amazon\AWSSAMCLI_NIGHTLY`.
+ *
+ * @param {boolean} nightly - Whether to return the nightly path.
+ */
+function windowsSamInstallRoot(nightly) {
+  const programFiles = process.env["ProgramFiles"] || "C:\\Program Files";
+  return path.join(
+    programFiles,
+    "Amazon",
+    nightly ? "AWSSAMCLI_NIGHTLY" : "AWSSAMCLI",
+  );
+}
+
+/**
+ * Locates the SAM CLI bin directory created by the MSI install.
+ *
+ * @param {boolean} nightly - Whether the nightly MSI was installed.
  */
-// TODO: Support more platforms
-async function installUsingNativeInstaller(inputVersion, token) {
-  // Validate platform
-  if (os.platform() !== "linux") {
-    core.setFailed("Only Linux is supported with use-installer: true");
+function findWindowsSamBinDir(nightly) {
+  const dir = path.join(windowsSamInstallRoot(nightly), "bin");
+  if (!fs.existsSync(dir)) {
+    throw new Error(`Expected SAM CLI install directory not found: ${dir}`);
+  }
+  return dir;
+}
+
+/**
+ * Uninstalls any existing AWS SAM CLI MSI whose install root matches the
+ * flavor we're about to install (stable or nightly).
+ *
+ * Windows Installer rejects silent downgrades, so if a newer SAM CLI is
+ * already installed (e.g. GitHub-hosted Windows runners ship a recent stable,
+ * or a previous step in the same workflow installed one), `msiexec /i` for
+ * an older or equal pinned version fails with exit 1603. Removing the
+ * existing product first makes the install order- and version-independent.
+ *
+ * Best-effort: logs and continues on any uninstall failure — the subsequent
+ * `msiexec /i` will surface the real error if it can't proceed.
+ *
+ * @param {boolean} nightly - Whether to remove the nightly product.
+ */
+async function uninstallExistingWindowsSamCli(nightly) {
+  // Match by DisplayName rather than InstallLocation: the SAM CLI MSI does
+  // not populate ARPINSTALLLOCATION, so the registry's InstallLocation field
+  // is empty for stable AND nightly. The DisplayName is reliable — stable is
+  // "AWS SAM Command Line Interface", nightly is the same with " Nightly".
+  // We still gate on the install root existing to skip the powershell
+  // invocation entirely on a clean machine.
+  const installRoot = windowsSamInstallRoot(nightly);
+  if (!fs.existsSync(installRoot)) {
+    return;
+  }
+
+  core.info(
+    `Found existing SAM CLI at ${installRoot}; uninstalling before reinstall to avoid downgrade rejection.`,
+  );
+
+  // Done via a script file rather than `-Command` to sidestep the fragile
+  // multi-level quoting required when passing a PowerShell script through
+  // exec args.
+  const flavor = nightly ? "nightly" : "stable";
+  const script = `$ErrorActionPreference = 'Stop'
+$flavor = $args[0]
+$paths = @(
+  'HKLM:\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\*',
+  'HKLM:\\SOFTWARE\\WOW6432Node\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\*'
+)
+$entries = Get-ItemProperty $paths -ErrorAction SilentlyContinue | Where-Object {
+  $_.DisplayName -and $_.DisplayName -like 'AWS SAM Command Line Interface*'
+}
+# Stable and nightly co-exist as separate products; keep them isolated.
+if ($flavor -eq 'nightly') {
+  $entries = $entries | Where-Object { $_.DisplayName -match '(?i)nightly' }
+} else {
+  $entries = $entries | Where-Object { $_.DisplayName -notmatch '(?i)nightly' }
+}
+$found = $false
+foreach ($entry in $entries) {
+  $found = $true
+  $code = $entry.PSChildName
+  if ($code -notmatch '^\\{[0-9A-Fa-f-]+\\}$') { continue }
+  Write-Host "Uninstalling $($entry.DisplayName) ($code)"
+  $p = Start-Process -FilePath msiexec.exe -ArgumentList '/x', $code, '/qn', '/norestart' -Wait -PassThru
+  if ($p.ExitCode -ne 0 -and $p.ExitCode -ne 3010) {
+    throw "msiexec /x $code failed with exit code $($p.ExitCode)"
+  }
+}
+if (-not $found) {
+  Write-Host "No matching $flavor SAM CLI uninstall registry entry found."
+}
+`;
+  const scriptPath = path.join(mkdirTemp(), "uninstall-sam.ps1");
+  fs.writeFileSync(scriptPath, script);
+
+  const exitCode = await exec.exec(
+    "powershell",
+    [
+      "-NoProfile",
+      "-NonInteractive",
+      "-ExecutionPolicy",
+      "Bypass",
+      "-File",
+      scriptPath,
+      flavor,
+    ],
+    { ignoreReturnCode: true },
+  );
+  if (exitCode !== 0) {
+    core.warning(
+      `Pre-install uninstall step exited with code ${exitCode}; continuing.`,
+    );
+  }
+}
+
+/**
+ * Installs SAM CLI on Windows by downloading and running the MSI.
+ *
+ * @param {string} inputVersion - The SAM CLI version to install or "nightly".
+ * @returns {Promise<string>} The directory containing sam.cmd / sam.exe.
+ */
+async function installWindowsNativeInstaller(inputVersion) {
+  // Validate version format (only x.y.z, "nightly", or empty are accepted)
+  if (inputVersion && !isNightly(inputVersion) && !isSemver(inputVersion)) {
+    core.setFailed('Version must be in the format x.y.z or "nightly"');
     return "";
   }
 
+  const nightly = isNightly(inputVersion);
+  const url = nightly
+    ? windowsMsiUrl(NIGHTLY_TAG, true)
+    : windowsMsiUrl(inputVersion, false);
+
+  if (nightly) {
+    core.info("Installing SAM CLI nightly release on Windows.");
+  } else {
+    core.info(
+      `Installing SAM CLI ${inputVersion || "latest"} on Windows via MSI.`,
+    );
+  }
+
+  try {
+    // Remove any preinstalled SAM CLI of the same flavor first so msiexec
+    // doesn't reject a downgrade or equal-version install.
+    await uninstallExistingWindowsSamCli(nightly);
+
+    // Windows Installer dispatches by file extension, so the destination
+    // path must end in `.msi` — tc.downloadTool's default UUID filename
+    // makes msiexec fail with exit code 1603.
+    const msiDest = path.join(mkdirTemp(), "AWS_SAM_CLI_64_PY3.msi");
+    const msiPath = await tc.downloadTool(url, msiDest);
+    await runMsiExec(msiPath);
+  } catch (error) {
+    core.setFailed(`Failed to install SAM CLI MSI: ${error.message}`);
+    return "";
+  }
+
+  let binDir;
+  try {
+    binDir = findWindowsSamBinDir(nightly);
+  } catch (error) {
+    core.setFailed(error.message);
+    return "";
+  }
+
+  // Nightly MSI ships sam-nightly.{cmd,exe}; copy to sam.{cmd,exe} so users
+  // can invoke `sam` regardless of which release they install.
+  if (nightly) {
+    for (const ext of ["cmd", "exe"]) {
+      const src = path.join(binDir, `sam-nightly.${ext}`);
+      const dst = path.join(binDir, `sam.${ext}`);
+      if (fs.existsSync(src) && !fs.existsSync(dst)) {
+        fs.copyFileSync(src, dst);
+      }
+    }
+  }
+
+  return binDir;
+}
+
+/**
+ * Installs SAM CLI on Linux using the official native installer archive.
+ *
+ * @param {string} inputVersion - The SAM CLI version to install or "nightly".
+ * @param {string} token - Authentication Token to use for GITHUB Apis.
+ * @returns {Promise<string>} The directory SAM CLI is installed in.
+ */
+async function installLinuxNativeInstaller(inputVersion, token) {
   if (os.arch() !== "x64" && os.arch() !== "arm64") {
     core.setFailed(
-      "Only x86-64 and aarch64 architectures are supported with use-installer: true",
+      "Only x86-64 and aarch64 architectures are supported with use-installer: true on Linux",
     );
     return "";
   }
 
+  const arch = os.arch() === "arm64" ? "arm64" : "x86_64";
+
+  // Nightly: download without caching since the "sam-cli-nightly" tag's
+  // contents change daily, so a cache hit would serve stale binaries.
+  if (isNightly(inputVersion)) {
+    core.info("Installing SAM CLI nightly release without caching.");
+    const binDir = await downloadWithoutCache(arch, NIGHTLY_TAG);
+    if (binDir) {
+      // The nightly archive ships `sam-nightly` instead of `sam`. Symlink so
+      // `sam` works on PATH without users having to change their commands.
+      const target = path.join(binDir, "sam-nightly");
+      const link = path.join(binDir, "sam");
+      if (fs.existsSync(target) && !fs.existsSync(link)) {
+        fs.symlinkSync(target, link);
+      }
+    }
+    return binDir;
+  }
+
   // Validate version format
   if (inputVersion && !isSemver(inputVersion)) {
-    core.setFailed("Version must be in the format x.y.z");
+    core.setFailed('Version must be in the format x.y.z or "nightly"');
     return "";
   }
 
@@ -258,7 +503,6 @@ async function installUsingNativeInstaller(inputVersion, token) {
   }
 
   // Validate ARM64 version requirement
-  const arch = os.arch() === "arm64" ? "arm64" : "x86_64";
   if (version && os.arch() === "arm64" && semverLt(version, "1.104.0")) {
     core.setFailed(
       "ARM64 installer is only available for versions 1.104.0 and above",
@@ -293,14 +537,44 @@ async function installUsingNativeInstaller(inputVersion, token) {
   return await downloadWithoutCache(arch);
 }
 
+/**
+ * Installs SAM CLI using the native installers.
+ *
+ * See https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/install-sam-cli.html
+ *
+ * @param {string} inputVersion - The SAM CLI version to install.
+ * @param {string} token - Authentication Token to use for GITHUB Apis.
+ * @returns {Promise<string>} The directory SAM CLI is installed in.
+ */
+// TODO: Support more platforms (macOS .pkg)
+async function installUsingNativeInstaller(inputVersion, token) {
+  if (os.platform() === "linux") {
+    return await installLinuxNativeInstaller(inputVersion, token);
+  }
+  if (os.platform() === "win32") {
+    return await installWindowsNativeInstaller(inputVersion);
+  }
+  core.setFailed(
+    "use-installer: true is only supported on Linux and Windows runners",
+  );
+  return "";
+}
+
 async function setup() {
-  const version = getInput("version", /^[\d.*]*$/, "");
+  const version = getInput("version", /^([\d.*]*|nightly)$/, "");
   // python3 isn't standard on Windows
   const defaultPython = isWindows() ? "python" : "python3";
   const python = getInput("python", /^.+$/, defaultPython);
   const useInstaller = core.getBooleanInput("use-installer");
   const token = getInput("token", /^.*$/, "");
 
+  if (isNightly(version) && !useInstaller) {
+    core.setFailed(
+      'Installing the nightly release requires "use-installer: true". The aws-sam-cli nightly release is not published to PyPI.',
+    );
+    return;
+  }
+
   const binPath = useInstaller
     ? await installUsingNativeInstaller(version, token)
     : await installSamCli(python, version);
diff --git a/lib/setup.js b/lib/setup.js
index 5012442..f9cf7bc 100644
--- a/lib/setup.js
+++ b/lib/setup.js
@@ -124,6 +124,16 @@ function isSemver(s) {
   return /^\d+\.\d+\.\d+$/.test(s);
 }
 
+const NIGHTLY = "nightly";
+const NIGHTLY_TAG = "sam-cli-nightly";
+
+/**
+ * Returns whether a version string requests the nightly release.
+ */
+function isNightly(version) {
+  return version === NIGHTLY;
+}
+
 /**
  * Get latest SAM CLI version from https://api.github.com/repos/aws/aws-sam-cli/releases/latest
  *
@@ -192,10 +202,12 @@ async function downloadAndCache(version, arch, installDir, cacheKey) {
  * Downloads SAM CLI without caching.
  *
  * @param {string} arch - The architecture (x86_64 or arm64).
+ * @param {string} releaseTag - Optional release tag (e.g. "sam-cli-nightly"). Defaults to latest.
  * @returns {Promise<string>} The directory SAM CLI is installed in.
  */
-async function downloadWithoutCache(arch) {
-  const url = `https://github.com/aws/aws-sam-cli/releases/latest/download/aws-sam-cli-linux-${arch}.zip`;
+async function downloadWithoutCache(arch, releaseTag) {
+  const tagSegment = releaseTag ? `download/${releaseTag}` : "latest/download";
+  const url = `https://github.com/aws/aws-sam-cli/releases/${tagSegment}/aws-sam-cli-linux-${arch}.zip`;
   const tempDir = mkdirTemp();
 
   try {
@@ -209,32 +221,265 @@ async function downloadWithoutCache(arch) {
 }
 
 /**
- * Installs SAM CLI using the native installers.
+ * Builds the URL to the Windows MSI asset for a given release tag or version.
  *
- * See https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/install-sam-cli.html
+ * @param {string} tagOrVersion - Either a release tag (e.g. "sam-cli-nightly")
+ *   or an empty string to use the "latest" alias.
+ * @param {boolean} isTag - True if the first arg is a release tag, false if version (x.y.z).
+ */
+function windowsMsiUrl(tagOrVersion, isTag) {
+  const asset = "AWS_SAM_CLI_64_PY3.msi";
+  if (!tagOrVersion) {
+    return `https://github.com/aws/aws-sam-cli/releases/latest/download/${asset}`;
+  }
+  const tag = isTag ? tagOrVersion : `v${tagOrVersion}`;
+  return `https://github.com/aws/aws-sam-cli/releases/download/${tag}/${asset}`;
+}
+
+/**
+ * Runs the MSI silently via msiexec. Throws on failure.
  *
- * @param {string} inputVersion - The SAM CLI version to install.
- * @param {string} token - Authentication Token to use for GITHUB Apis.
- * @returns {Promise<string>} The directory SAM CLI is installed in.
+ * @param {string} msiPath - Path to the downloaded MSI.
  */
-// TODO: Support more platforms
-async function installUsingNativeInstaller(inputVersion, token) {
-  // Validate platform
-  if (os.platform() !== "linux") {
-    core.setFailed("Only Linux is supported with use-installer: true");
+async function runMsiExec(msiPath) {
+  // 3010 = ERROR_SUCCESS_REBOOT_REQUIRED; treat as success
+  const logPath = path.join(mkdirTemp(), "msi-install.log");
+  const exitCode = await exec.exec(
+    "msiexec",
+    ["/i", msiPath, "/qn", "/norestart", "/l*v", logPath],
+    { ignoreReturnCode: true },
+  );
+  if (exitCode !== 0 && exitCode !== 3010) {
+    if (fs.existsSync(logPath)) {
+      const tail = fs.readFileSync(logPath, "utf8").split("\n").slice(-30);
+      core.warning(
+        `msiexec failed (${exitCode}); last log lines:\n${tail.join("\n")}`,
+      );
+    }
+    throw new Error(`msiexec failed with exit code ${exitCode}`);
+  }
+}
+
+/**
+ * Returns the MSI install root for the given SAM CLI flavor.
+ *
+ * Stable installs to `C:\Program Files\Amazon\AWSSAMCLI`; nightly installs
+ * to `C:\Program Files\Amazon\AWSSAMCLI_NIGHTLY`.
+ *
+ * @param {boolean} nightly - Whether to return the nightly path.
+ */
+function windowsSamInstallRoot(nightly) {
+  const programFiles = process.env["ProgramFiles"] || "C:\\Program Files";
+  return path.join(
+    programFiles,
+    "Amazon",
+    nightly ? "AWSSAMCLI_NIGHTLY" : "AWSSAMCLI",
+  );
+}
+
+/**
+ * Locates the SAM CLI bin directory created by the MSI install.
+ *
+ * @param {boolean} nightly - Whether the nightly MSI was installed.
+ */
+function findWindowsSamBinDir(nightly) {
+  const dir = path.join(windowsSamInstallRoot(nightly), "bin");
+  if (!fs.existsSync(dir)) {
+    throw new Error(`Expected SAM CLI install directory not found: ${dir}`);
+  }
+  return dir;
+}
+
+/**
+ * Uninstalls any existing AWS SAM CLI MSI whose install root matches the
+ * flavor we're about to install (stable or nightly).
+ *
+ * Windows Installer rejects silent downgrades, so if a newer SAM CLI is
+ * already installed (e.g. GitHub-hosted Windows runners ship a recent stable,
+ * or a previous step in the same workflow installed one), `msiexec /i` for
+ * an older or equal pinned version fails with exit 1603. Removing the
+ * existing product first makes the install order- and version-independent.
+ *
+ * Best-effort: logs and continues on any uninstall failure — the subsequent
+ * `msiexec /i` will surface the real error if it can't proceed.
+ *
+ * @param {boolean} nightly - Whether to remove the nightly product.
+ */
+async function uninstallExistingWindowsSamCli(nightly) {
+  // Match by DisplayName rather than InstallLocation: the SAM CLI MSI does
+  // not populate ARPINSTALLLOCATION, so the registry's InstallLocation field
+  // is empty for stable AND nightly. The DisplayName is reliable — stable is
+  // "AWS SAM Command Line Interface", nightly is the same with " Nightly".
+  // We still gate on the install root existing to skip the powershell
+  // invocation entirely on a clean machine.
+  const installRoot = windowsSamInstallRoot(nightly);
+  if (!fs.existsSync(installRoot)) {
+    return;
+  }
+
+  core.info(
+    `Found existing SAM CLI at ${installRoot}; uninstalling before reinstall to avoid downgrade rejection.`,
+  );
+
+  // Done via a script file rather than `-Command` to sidestep the fragile
+  // multi-level quoting required when passing a PowerShell script through
+  // exec args.
+  const flavor = nightly ? "nightly" : "stable";
+  const script = `$ErrorActionPreference = 'Stop'
+$flavor = $args[0]
+$paths = @(
+  'HKLM:\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\*',
+  'HKLM:\\SOFTWARE\\WOW6432Node\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\*'
+)
+$entries = Get-ItemProperty $paths -ErrorAction SilentlyContinue | Where-Object {
+  $_.DisplayName -and $_.DisplayName -like 'AWS SAM Command Line Interface*'
+}
+# Stable and nightly co-exist as separate products; keep them isolated.
+if ($flavor -eq 'nightly') {
+  $entries = $entries | Where-Object { $_.DisplayName -match '(?i)nightly' }
+} else {
+  $entries = $entries | Where-Object { $_.DisplayName -notmatch '(?i)nightly' }
+}
+$found = $false
+foreach ($entry in $entries) {
+  $found = $true
+  $code = $entry.PSChildName
+  if ($code -notmatch '^\\{[0-9A-Fa-f-]+\\}$') { continue }
+  Write-Host "Uninstalling $($entry.DisplayName) ($code)"
+  $p = Start-Process -FilePath msiexec.exe -ArgumentList '/x', $code, '/qn', '/norestart' -Wait -PassThru
+  if ($p.ExitCode -ne 0 -and $p.ExitCode -ne 3010) {
+    throw "msiexec /x $code failed with exit code $($p.ExitCode)"
+  }
+}
+if (-not $found) {
+  Write-Host "No matching $flavor SAM CLI uninstall registry entry found."
+}
+`;
+  const scriptPath = path.join(mkdirTemp(), "uninstall-sam.ps1");
+  fs.writeFileSync(scriptPath, script);
+
+  const exitCode = await exec.exec(
+    "powershell",
+    [
+      "-NoProfile",
+      "-NonInteractive",
+      "-ExecutionPolicy",
+      "Bypass",
+      "-File",
+      scriptPath,
+      flavor,
+    ],
+    { ignoreReturnCode: true },
+  );
+  if (exitCode !== 0) {
+    core.warning(
+      `Pre-install uninstall step exited with code ${exitCode}; continuing.`,
+    );
+  }
+}
+
+/**
+ * Installs SAM CLI on Windows by downloading and running the MSI.
+ *
+ * @param {string} inputVersion - The SAM CLI version to install or "nightly".
+ * @returns {Promise<string>} The directory containing sam.cmd / sam.exe.
+ */
+async function installWindowsNativeInstaller(inputVersion) {
+  // Validate version format (only x.y.z, "nightly", or empty are accepted)
+  if (inputVersion && !isNightly(inputVersion) && !isSemver(inputVersion)) {
+    core.setFailed('Version must be in the format x.y.z or "nightly"');
     return "";
   }
 
+  const nightly = isNightly(inputVersion);
+  const url = nightly
+    ? windowsMsiUrl(NIGHTLY_TAG, true)
+    : windowsMsiUrl(inputVersion, false);
+
+  if (nightly) {
+    core.info("Installing SAM CLI nightly release on Windows.");
+  } else {
+    core.info(
+      `Installing SAM CLI ${inputVersion || "latest"} on Windows via MSI.`,
+    );
+  }
+
+  try {
+    // Remove any preinstalled SAM CLI of the same flavor first so msiexec
+    // doesn't reject a downgrade or equal-version install.
+    await uninstallExistingWindowsSamCli(nightly);
+
+    // Windows Installer dispatches by file extension, so the destination
+    // path must end in `.msi` — tc.downloadTool's default UUID filename
+    // makes msiexec fail with exit code 1603.
+    const msiDest = path.join(mkdirTemp(), "AWS_SAM_CLI_64_PY3.msi");
+    const msiPath = await tc.downloadTool(url, msiDest);
+    await runMsiExec(msiPath);
+  } catch (error) {
+    core.setFailed(`Failed to install SAM CLI MSI: ${error.message}`);
+    return "";
+  }
+
+  let binDir;
+  try {
+    binDir = findWindowsSamBinDir(nightly);
+  } catch (error) {
+    core.setFailed(error.message);
+    return "";
+  }
+
+  // Nightly MSI ships sam-nightly.{cmd,exe}; copy to sam.{cmd,exe} so users
+  // can invoke `sam` regardless of which release they install.
+  if (nightly) {
+    for (const ext of ["cmd", "exe"]) {
+      const src = path.join(binDir, `sam-nightly.${ext}`);
+      const dst = path.join(binDir, `sam.${ext}`);
+      if (fs.existsSync(src) && !fs.existsSync(dst)) {
+        fs.copyFileSync(src, dst);
+      }
+    }
+  }
+
+  return binDir;
+}
+
+/**
+ * Installs SAM CLI on Linux using the official native installer archive.
+ *
+ * @param {string} inputVersion - The SAM CLI version to install or "nightly".
+ * @param {string} token - Authentication Token to use for GITHUB Apis.
+ * @returns {Promise<string>} The directory SAM CLI is installed in.
+ */
+async function installLinuxNativeInstaller(inputVersion, token) {
   if (os.arch() !== "x64" && os.arch() !== "arm64") {
     core.setFailed(
-      "Only x86-64 and aarch64 architectures are supported with use-installer: true",
+      "Only x86-64 and aarch64 architectures are supported with use-installer: true on Linux",
     );
     return "";
   }
 
+  const arch = os.arch() === "arm64" ? "arm64" : "x86_64";
+
+  // Nightly: download without caching since the "sam-cli-nightly" tag's
+  // contents change daily, so a cache hit would serve stale binaries.
+  if (isNightly(inputVersion)) {
+    core.info("Installing SAM CLI nightly release without caching.");
+    const binDir = await downloadWithoutCache(arch, NIGHTLY_TAG);
+    if (binDir) {
+      // The nightly archive ships `sam-nightly` instead of `sam`. Symlink so
+      // `sam` works on PATH without users having to change their commands.
+      const target = path.join(binDir, "sam-nightly");
+      const link = path.join(binDir, "sam");
+      if (fs.existsSync(target) && !fs.existsSync(link)) {
+        fs.symlinkSync(target, link);
+      }
+    }
+    return binDir;
+  }
+
   // Validate version format
   if (inputVersion && !isSemver(inputVersion)) {
-    core.setFailed("Version must be in the format x.y.z");
+    core.setFailed('Version must be in the format x.y.z or "nightly"');
     return "";
   }
 
@@ -252,7 +497,6 @@ async function installUsingNativeInstaller(inputVersion, token) {
   }
 
   // Validate ARM64 version requirement
-  const arch = os.arch() === "arm64" ? "arm64" : "x86_64";
   if (version && os.arch() === "arm64" && semverLt(version, "1.104.0")) {
     core.setFailed(
       "ARM64 installer is only available for versions 1.104.0 and above",
@@ -287,14 +531,44 @@ async function installUsingNativeInstaller(inputVersion, token) {
   return await downloadWithoutCache(arch);
 }
 
+/**
+ * Installs SAM CLI using the native installers.
+ *
+ * See https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/install-sam-cli.html
+ *
+ * @param {string} inputVersion - The SAM CLI version to install.
+ * @param {string} token - Authentication Token to use for GITHUB Apis.
+ * @returns {Promise<string>} The directory SAM CLI is installed in.
+ */
+// TODO: Support more platforms (macOS .pkg)
+async function installUsingNativeInstaller(inputVersion, token) {
+  if (os.platform() === "linux") {
+    return await installLinuxNativeInstaller(inputVersion, token);
+  }
+  if (os.platform() === "win32") {
+    return await installWindowsNativeInstaller(inputVersion);
+  }
+  core.setFailed(
+    "use-installer: true is only supported on Linux and Windows runners",
+  );
+  return "";
+}
+
 async function setup() {
-  const version = getInput("version", /^[\d.*]*$/, "");
+  const version = getInput("version", /^([\d.*]*|nightly)$/, "");
   // python3 isn't standard on Windows
   const defaultPython = isWindows() ? "python" : "python3";
   const python = getInput("python", /^.+$/, defaultPython);
   const useInstaller = core.getBooleanInput("use-installer");
   const token = getInput("token", /^.*$/, "");
 
+  if (isNightly(version) && !useInstaller) {
+    core.setFailed(
+      'Installing the nightly release requires "use-installer: true". The aws-sam-cli nightly release is not published to PyPI.',
+    );
+    return;
+  }
+
   const binPath = useInstaller
     ? await installUsingNativeInstaller(version, token)
     : await installSamCli(python, version);
diff --git a/test/setup.test.js b/test/setup.test.js
index fe7c0ec..b2b363d 100644
--- a/test/setup.test.js
+++ b/test/setup.test.js
@@ -377,3 +377,316 @@ test.each([["x64"], ["arm64"]])(
     }
   },
 );
+
+test.each([
+  ["x64", "x86_64"],
+  ["arm64", "arm64"],
+])(
+  "when use-installer enabled and version is nightly, downloads from sam-cli-nightly tag without caching (Linux %s)",
+  async (inputArch, expectedArch) => {
+    jest.spyOn(os, "platform").mockReturnValue("linux");
+    jest.spyOn(os, "arch").mockReturnValue(inputArch);
+
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("nightly");
+
+    tc.extractZip = jest.fn().mockResolvedValueOnce(undefined);
+    tc.downloadTool = jest
+      .fn()
+      .mockResolvedValueOnce("/path/to/downloaded/sam");
+
+    await setup();
+
+    expect(tc.downloadTool).toHaveBeenCalledWith(
+      `https://github.com/aws/aws-sam-cli/releases/download/sam-cli-nightly/aws-sam-cli-linux-${expectedArch}.zip`,
+    );
+
+    expect(cache.restoreCache).toHaveBeenCalledTimes(0);
+    expect(cache.saveCache).toHaveBeenCalledTimes(0);
+    expect(core.addPath).toHaveBeenCalled();
+    expect(core.setFailed).not.toHaveBeenCalled();
+  },
+);
+
+test("when version is nightly but use-installer is false, fails with descriptive error", async () => {
+  jest.spyOn(os, "platform").mockReturnValue("linux");
+
+  core.getBooleanInput = jest.fn().mockReturnValue(false);
+  core.getInput = jest.fn().mockReturnValueOnce("nightly");
+
+  await setup();
+
+  expect(core.setFailed).toHaveBeenCalledWith(
+    expect.stringContaining('"use-installer: true"'),
+  );
+  expect(io.which).not.toHaveBeenCalled();
+  expect(tc.downloadTool).not.toHaveBeenCalled();
+  expect(core.addPath).not.toHaveBeenCalled();
+});
+
+describe("Windows native installer", () => {
+  const fs = require("fs");
+  const path = require("path");
+
+  // path.join uses the host OS separator, so compute expected dirs the same way
+  // the production code does to keep these tests cross-platform.
+  const stableInstallRoot = path.join(
+    "C:\\Program Files",
+    "Amazon",
+    "AWSSAMCLI",
+  );
+  const nightlyInstallRoot = path.join(
+    "C:\\Program Files",
+    "Amazon",
+    "AWSSAMCLI_NIGHTLY",
+  );
+  const stableBinDir = path.join(stableInstallRoot, "bin");
+  const nightlyBinDir = path.join(nightlyInstallRoot, "bin");
+
+  let existsSyncSpy;
+  let copyFileSyncSpy;
+  let writeFileSyncSpy;
+  let originalProgramFiles;
+
+  beforeEach(() => {
+    originalProgramFiles = process.env["ProgramFiles"];
+    process.env["ProgramFiles"] = "C:\\Program Files";
+
+    jest.spyOn(os, "platform").mockReturnValue("win32");
+
+    existsSyncSpy = jest.spyOn(fs, "existsSync");
+    copyFileSyncSpy = jest
+      .spyOn(fs, "copyFileSync")
+      .mockImplementation(() => {});
+    writeFileSyncSpy = jest
+      .spyOn(fs, "writeFileSync")
+      .mockImplementation(() => {});
+
+    tc.downloadTool = jest
+      .fn()
+      .mockResolvedValue("/tmp/AWS_SAM_CLI_64_PY3.msi");
+    exec.exec = jest.fn().mockResolvedValue(0);
+  });
+
+  afterEach(() => {
+    existsSyncSpy.mockRestore();
+    copyFileSyncSpy.mockRestore();
+    writeFileSyncSpy.mockRestore();
+    if (originalProgramFiles === undefined) {
+      delete process.env["ProgramFiles"];
+    } else {
+      process.env["ProgramFiles"] = originalProgramFiles;
+    }
+  });
+
+  test("downloads MSI for a pinned version and runs msiexec", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    existsSyncSpy.mockImplementation((p) => p === stableBinDir);
+
+    await setup();
+
+    expect(tc.downloadTool).toHaveBeenCalledWith(
+      "https://github.com/aws/aws-sam-cli/releases/download/v1.139.0/AWS_SAM_CLI_64_PY3.msi",
+      expect.stringMatching(/AWS_SAM_CLI_64_PY3\.msi$/),
+    );
+    expect(exec.exec).toHaveBeenCalledWith(
+      "msiexec",
+      expect.arrayContaining([
+        "/i",
+        expect.stringMatching(/AWS_SAM_CLI_64_PY3\.msi$/),
+        "/qn",
+        "/norestart",
+      ]),
+      expect.objectContaining({ ignoreReturnCode: true }),
+    );
+    expect(core.addPath).toHaveBeenCalledWith(stableBinDir);
+    expect(copyFileSyncSpy).not.toHaveBeenCalled();
+    expect(core.setFailed).not.toHaveBeenCalled();
+  });
+
+  test("downloads MSI from latest URL when no version is provided", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("");
+
+    existsSyncSpy.mockImplementation((p) => p === stableBinDir);
+
+    await setup();
+
+    expect(tc.downloadTool).toHaveBeenCalledWith(
+      "https://github.com/aws/aws-sam-cli/releases/latest/download/AWS_SAM_CLI_64_PY3.msi",
+      expect.stringMatching(/AWS_SAM_CLI_64_PY3\.msi$/),
+    );
+    expect(core.addPath).toHaveBeenCalledWith(stableBinDir);
+  });
+
+  test("nightly: downloads from sam-cli-nightly tag and aliases sam-nightly to sam", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("nightly");
+
+    // Pretend the nightly install dir and sam-nightly.* exist; sam.* does not yet.
+    const samNightlyCmd = path.join(nightlyBinDir, "sam-nightly.cmd");
+    const samNightlyExe = path.join(nightlyBinDir, "sam-nightly.exe");
+    existsSyncSpy.mockImplementation(
+      (p) => p === nightlyBinDir || p === samNightlyCmd || p === samNightlyExe,
+    );
+
+    await setup();
+
+    expect(tc.downloadTool).toHaveBeenCalledWith(
+      "https://github.com/aws/aws-sam-cli/releases/download/sam-cli-nightly/AWS_SAM_CLI_64_PY3.msi",
+      expect.stringMatching(/AWS_SAM_CLI_64_PY3\.msi$/),
+    );
+    expect(core.addPath).toHaveBeenCalledWith(nightlyBinDir);
+    // Both sam.cmd and sam.exe should have been copied from the nightly variants
+    expect(copyFileSyncSpy).toHaveBeenCalledWith(
+      samNightlyCmd,
+      path.join(nightlyBinDir, "sam.cmd"),
+    );
+    expect(copyFileSyncSpy).toHaveBeenCalledWith(
+      samNightlyExe,
+      path.join(nightlyBinDir, "sam.exe"),
+    );
+  });
+
+  test("treats msiexec exit code 3010 (reboot required) as success", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    exec.exec = jest.fn().mockResolvedValue(3010);
+    existsSyncSpy.mockImplementation((p) => p === stableBinDir);
+
+    await setup();
+
+    expect(core.addPath).toHaveBeenCalled();
+    expect(core.setFailed).not.toHaveBeenCalled();
+  });
+
+  test("fails when msiexec returns a non-zero, non-3010 exit code", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    exec.exec = jest.fn().mockResolvedValue(1603);
+    existsSyncSpy.mockReturnValue(false);
+
+    await setup();
+
+    expect(core.setFailed).toHaveBeenCalledWith(
+      expect.stringContaining("msiexec failed with exit code 1603"),
+    );
+    expect(core.addPath).not.toHaveBeenCalled();
+  });
+
+  test("fails when expected install directory is missing after MSI completes", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    existsSyncSpy.mockReturnValue(false);
+
+    await setup();
+
+    expect(core.setFailed).toHaveBeenCalledWith(
+      expect.stringContaining("Expected SAM CLI install directory not found"),
+    );
+    expect(core.addPath).not.toHaveBeenCalled();
+  });
+
+  test("rejects invalid version string", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.2");
+
+    await setup();
+
+    expect(core.setFailed).toHaveBeenCalledWith(
+      expect.stringContaining(
+        'Version must be in the format x.y.z or "nightly"',
+      ),
+    );
+    expect(tc.downloadTool).not.toHaveBeenCalled();
+  });
+
+  test("uninstalls preinstalled stable SAM CLI before installing pinned version", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    // Pretend a stable SAM CLI is already installed (install root exists),
+    // and the bin dir exists after the new install completes.
+    existsSyncSpy.mockImplementation(
+      (p) => p === stableInstallRoot || p === stableBinDir,
+    );
+
+    await setup();
+
+    // PowerShell uninstall is invoked with the "stable" flavor argument,
+    // then msiexec /i runs for the new MSI.
+    const calls = exec.exec.mock.calls;
+    const uninstallCall = calls.find((c) => c[0] === "powershell");
+    expect(uninstallCall).toBeDefined();
+    expect(uninstallCall[1]).toEqual(expect.arrayContaining(["-File"]));
+    expect(uninstallCall[1][uninstallCall[1].length - 1]).toBe("stable");
+
+    // The script body passed via -File should match by DisplayName, not
+    // InstallLocation (the SAM MSI doesn't populate ARPINSTALLLOCATION).
+    expect(writeFileSyncSpy).toHaveBeenCalledWith(
+      expect.stringMatching(/uninstall-sam\.ps1$/),
+      expect.stringContaining(
+        "DisplayName -like 'AWS SAM Command Line Interface*'",
+      ),
+    );
+
+    const installCall = calls.find((c) => c[0] === "msiexec");
+    expect(installCall).toBeDefined();
+    expect(calls.indexOf(uninstallCall)).toBeLessThan(
+      calls.indexOf(installCall),
+    );
+    expect(core.setFailed).not.toHaveBeenCalled();
+  });
+
+  test("skips uninstall when no existing install is present", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+    // Install root does not exist before install; bin dir appears after.
+    existsSyncSpy.mockImplementation((p) => p === stableBinDir);
+
+    await setup();
+
+    expect(
+      exec.exec.mock.calls.find((c) => c[0] === "powershell"),
+    ).toBeUndefined();
+    expect(exec.exec.mock.calls.find((c) => c[0] === "msiexec")).toBeDefined();
+  });
+
+  test("uninstall targets nightly install root for nightly install", async () => {
+    core.getBooleanInput = jest.fn().mockReturnValue(true);
+    core.getInput = jest.fn().mockReturnValueOnce("nightly");
+
+    const samNightlyCmd = path.join(nightlyBinDir, "sam-nightly.cmd");
+    existsSyncSpy.mockImplementation(
+      (p) =>
+        p === nightlyInstallRoot || p === nightlyBinDir || p === samNightlyCmd,
+    );
+
+    await setup();
+
+    const uninstallCall = exec.exec.mock.calls.find(
+      (c) => c[0] === "powershell",
+    );
+    expect(uninstallCall).toBeDefined();
+    expect(uninstallCall[1][uninstallCall[1].length - 1]).toBe("nightly");
+  });
+});
+
+test("use-installer rejected on macOS", async () => {
+  jest.spyOn(os, "platform").mockReturnValue("darwin");
+  core.getBooleanInput = jest.fn().mockReturnValue(true);
+  core.getInput = jest.fn().mockReturnValueOnce("1.139.0");
+
+  await setup();
+
+  expect(core.setFailed).toHaveBeenCalledWith(
+    expect.stringContaining("only supported on Linux and Windows"),
+  );
+  expect(tc.downloadTool).not.toHaveBeenCalled();
+});