diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 0bb0109..cb9067e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -25,6 +25,7 @@ jobs:
       - name: Syntax check
         run: |
           bash -n scripts/setup.sh
+          bash -n scripts/test-setup.sh
           sh -n installer/install.sh
 
       - name: Install ShellCheck
@@ -33,6 +34,7 @@ jobs:
       - name: ShellCheck
         run: |
           shellcheck --severity=style scripts/setup.sh
+          shellcheck --severity=style scripts/test-setup.sh
           shellcheck --shell=sh --severity=style installer/install.sh
 
   lint-powershell:
@@ -53,13 +55,14 @@ jobs:
         shell: pwsh
         run: |
           Invoke-ScriptAnalyzer -Path scripts/setup.ps1 -Settings PSGallery -EnableExit
+          Invoke-ScriptAnalyzer -Path scripts/test-setup.ps1 -Settings PSGallery -EnableExit
           Invoke-ScriptAnalyzer -Path installer/install.ps1 -Settings PSGallery -EnableExit
 
       - name: AST parse check
         shell: pwsh
         run: |
           $ErrorActionPreference = "Stop"
-          foreach ($file in @("scripts/setup.ps1", "installer/install.ps1")) {
+          foreach ($file in @("scripts/setup.ps1", "scripts/test-setup.ps1", "installer/install.ps1")) {
             $tokens = $null
             $errors = $null
             [void][System.Management.Automation.Language.Parser]::ParseFile($file, [ref]$tokens, [ref]$errors)
@@ -72,6 +75,30 @@ jobs:
             Write-Output "OK: $file"
           }
 
+  test-setup:
+    name: Setup regression tests (${{ matrix.os }})
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 5
+    steps:
+      - name: Checkout
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          persist-credentials: false
+
+      - name: Test Bash setup
+        if: runner.os != 'Windows'
+        shell: bash
+        run: bash scripts/test-setup.sh
+
+      - name: Test PowerShell setup
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: pwsh -NoProfile -File scripts/test-setup.ps1
+
   installer-drift:
     name: Vendored installer drift check
     runs-on: ubuntu-latest
@@ -114,7 +141,7 @@ jobs:
       # values are enough to test the install path without secrets.
       - name: Install Cloudsmith CLI
         id: setup
-        uses: ./
+        uses: $/
         with:
           oidc-namespace: placeholder-namespace
           oidc-service-slug: placeholder-service
@@ -159,7 +186,7 @@ jobs:
 
       - name: Install Cloudsmith CLI
         id: setup
-        uses: ./
+        uses: $/
         with:
           # Latest version published to the pre-release test repository.
           # Bump to the first production release when the vendored installer
@@ -198,19 +225,19 @@ jobs:
       - name: No authentication inputs should fail
         id: no-auth
         continue-on-error: true
-        uses: ./
+        uses: $/
 
       - name: Incomplete OIDC pair should fail
         id: partial-oidc
         continue-on-error: true
-        uses: ./
+        uses: $/
         with:
           oidc-namespace: placeholder-namespace
 
       - name: verify-auth with a bad API key should fail
         id: bad-key
         continue-on-error: true
-        uses: ./
+        uses: $/
         with:
           api-key: placeholder-invalid-key
           verify-auth: "true"
@@ -272,7 +299,7 @@ jobs:
 
       - name: Install and authenticate
         id: setup
-        uses: ./
+        uses: $/
         with:
           api-key: ${{ secrets.CLOUDSMITH_API_KEY }}
           export-auth-token: ${{ matrix.export-auth-token }}
@@ -324,7 +351,7 @@ jobs:
 
       - name: Install and authenticate
         id: setup
-        uses: ./
+        uses: $/
         with:
           oidc-namespace: ${{ vars.CLOUDSMITH_NAMESPACE }}
           oidc-service-slug: ${{ vars.CLOUDSMITH_SERVICE_SLUG }}
diff --git a/CONTRIBUTION.md b/CONTRIBUTION.md
index 6b2005f..2aa13b7 100644
--- a/CONTRIBUTION.md
+++ b/CONTRIBUTION.md
@@ -21,6 +21,8 @@ Thank you for considering contributing to the Cloudsmith CLI Setup action!
    bash -n scripts/setup.sh
    shellcheck --severity=style scripts/setup.sh
    pwsh -Command "Invoke-ScriptAnalyzer -Path scripts/setup.ps1 -Settings PSGallery -EnableExit"
+   bash scripts/test-setup.sh
+   pwsh -NoProfile -File scripts/test-setup.ps1
    ```
 
 4. Commit, push to your fork, and open a pull request.
diff --git a/scripts/setup.ps1 b/scripts/setup.ps1
index d255d50..4398981 100644
--- a/scripts/setup.ps1
+++ b/scripts/setup.ps1
@@ -165,8 +165,18 @@ $credentialUsername = ''
 if ($exportAuthToken -eq 'true') {
   # Resolve once: the helper performs the OIDC exchange when OIDC is the
   # effective source and emits a versioned JSON credential document.
-  $credentialJsonLines = & $executable credential-helper generic
-  $credentialStatus = $LASTEXITCODE
+  $previousApiKey = $env:CLOUDSMITH_API_KEY
+  try {
+    # Ignore tokens exported by earlier invocations, but keep explicit API keys.
+    if ($hasOidcNamespace -and -not $hasApiKey) {
+      $env:CLOUDSMITH_API_KEY = $null
+    }
+    $credentialJsonLines = & $executable credential-helper generic
+    $credentialStatus = $LASTEXITCODE
+  }
+  finally {
+    $env:CLOUDSMITH_API_KEY = $previousApiKey
+  }
   if ($credentialStatus -ne 0) {
     throw "Failed to resolve credentials. 'export-auth-token' requires Cloudsmith CLI 1.21.0 or later and valid credentials."
   }
diff --git a/scripts/setup.sh b/scripts/setup.sh
index eaacae4..7c3ad7f 100755
--- a/scripts/setup.sh
+++ b/scripts/setup.sh
@@ -156,7 +156,13 @@ if [[ "$export_auth_token" == "true" ]]; then
 
   # Resolve once: the helper performs the OIDC exchange when OIDC is the
   # effective source and emits a versioned JSON credential document.
-  if ! credential_document="$("$executable" credential-helper generic)"; then
+  if ! credential_document="$(
+    # Ignore tokens exported by earlier invocations, but keep explicit API keys.
+    if [[ "$has_oidc_namespace" == "yes" && -z "$INPUT_API_KEY" ]]; then
+      unset CLOUDSMITH_API_KEY
+    fi
+    "$executable" credential-helper generic
+  )"; then
     fail "Failed to resolve credentials. 'export-auth-token' requires Cloudsmith CLI 1.21.0 or later and valid credentials."
   fi
 
diff --git a/scripts/test-setup.ps1 b/scripts/test-setup.ps1
new file mode 100644
index 0000000..0358df2
--- /dev/null
+++ b/scripts/test-setup.ps1
@@ -0,0 +1,118 @@
+$ErrorActionPreference = 'Stop'
+$setupScript = Join-Path $PSScriptRoot 'setup.ps1'
+$testRoot = Join-Path ([IO.Path]::GetTempPath()) ("cloudsmith-setup-test-" + [Guid]::NewGuid().ToString('N'))
+
+function Assert-Equal {
+  param($Actual, $Expected)
+  if ($Actual -cne $Expected) {
+    throw "Expected '$Expected', got '$Actual'"
+  }
+}
+
+function Invoke-SetupTest {
+  param([string]$Expected)
+  Set-Content -LiteralPath $env:GITHUB_ENV -Value ''
+  Set-Content -LiteralPath $env:GITHUB_OUTPUT -Value ''
+  # Use a fresh process, just like a separate action step.
+  & (Join-Path $PSHOME 'pwsh') -NoProfile -File $setupScript
+  Assert-Equal $LASTEXITCODE 0
+  foreach ($line in Get-Content -LiteralPath $env:GITHUB_ENV) {
+    if ($line -match '^([^=]+)=(.*)$') {
+      Set-Item -Path "Env:$($matches[1])" -Value $matches[2]
+    }
+  }
+  Assert-Equal $env:CLOUDSMITH_API_KEY $Expected
+  Assert-Equal (Get-Content (Join-Path $testRoot 'verified.log') | Select-Object -Last 1) $Expected
+  Assert-Equal ((Get-Content $env:GITHUB_ENV) -contains "CLOUDSMITH_API_KEY=$Expected") $true
+  if ($env:INPUT_EXPORT_AUTH_TOKEN -eq 'true' -or $env:INPUT_OIDC_AUTH_ONLY -eq 'true') {
+    Assert-Equal $env:CLOUDSMITH_USERNAME 'token'
+    Assert-Equal ((Get-Content $env:GITHUB_OUTPUT) -contains "oidc-token=$Expected") $true
+  }
+}
+
+try {
+  New-Item -ItemType Directory -Path "$testRoot/installer", "$testRoot/bin" | Out-Null
+  @'
+param($Version, $InstallRoot, $OutputFile)
+$binDirectory = Join-Path $env:GITHUB_ACTION_PATH 'bin'
+$executable = Join-Path $binDirectory 'cloudsmith.ps1'
+Set-Content -LiteralPath $OutputFile -Value @(
+  'version=1.21.0', 'target=test', "bin_dir=$binDirectory", "executable=$executable"
+)
+'@ | Set-Content -LiteralPath "$testRoot/installer/install.ps1"
+  @'
+$global:LASTEXITCODE = 0
+switch ($args -join ' ') {
+  'credential-helper generic' {
+    Add-Content "$env:GITHUB_ACTION_PATH/helper.log" 'helper'
+    # Match CLI precedence: an API key wins over OIDC.
+    $token = $env:CLOUDSMITH_API_KEY
+    if ([string]::IsNullOrEmpty($token)) {
+      Add-Content "$env:GITHUB_ACTION_PATH/exchanges.log" $env:CLOUDSMITH_SERVICE_SLUG
+      $token = "token-$env:CLOUDSMITH_SERVICE_SLUG"
+    }
+    @{ version = 1; username = 'token'; password = $token } | ConvertTo-Json -Compress
+  }
+  'whoami' { Add-Content "$env:GITHUB_ACTION_PATH/verified.log" $env:CLOUDSMITH_API_KEY }
+  default { throw "Unexpected CLI arguments: $args" }
+}
+'@ | Set-Content -LiteralPath "$testRoot/bin/cloudsmith.ps1"
+
+  $env:GITHUB_ACTION_PATH = $testRoot
+  $env:RUNNER_TEMP = $testRoot
+  $env:GITHUB_ENV = Join-Path $testRoot 'env'
+  $env:GITHUB_OUTPUT = Join-Path $testRoot 'output'
+  $env:GITHUB_PATH = Join-Path $testRoot 'path'
+  $env:GITHUB_REPOSITORY_OWNER = 'test-owner'
+  $env:ACTIONS_ID_TOKEN_REQUEST_URL = 'https://example.invalid/oidc'
+  $env:INPUT_CLI_VERSION = 'latest'
+  $env:INPUT_INSTALL_DIRECTORY = ''
+  $env:INPUT_API_KEY = ''
+  $env:INPUT_OIDC_NAMESPACE = 'test-org'
+  $env:INPUT_OIDC_SERVICE_SLUG = 'pull-only'
+  $env:INPUT_OIDC_AUDIENCE = ''
+  $env:INPUT_API_HOST = ''
+  $env:INPUT_API_PROXY = ''
+  $env:INPUT_API_SSL_VERIFY = ''
+  $env:INPUT_API_USER_AGENT = ''
+  $env:INPUT_VERIFY_AUTH = 'true'
+  $env:INPUT_EXPORT_AUTH_TOKEN = 'true'
+  $env:INPUT_OIDC_AUTH_ONLY = 'false'
+  $env:CLOUDSMITH_API_KEY = $null
+
+  Invoke-SetupTest 'token-pull-only'
+  $env:INPUT_OIDC_SERVICE_SLUG = 'push-capable'
+  Invoke-SetupTest 'token-push-capable'
+  Assert-Equal ((Get-Content "$testRoot/exchanges.log") -join ',') 'pull-only,push-capable'
+  Assert-Equal @(Get-Content "$testRoot/helper.log").Count 2
+
+  # Explicit API keys still win, even with OIDC inputs and an inherited token.
+  $env:INPUT_API_KEY = 'explicit-key'
+  Invoke-SetupTest 'explicit-key'
+  $env:INPUT_OIDC_NAMESPACE = ''
+  $env:INPUT_OIDC_SERVICE_SLUG = ''
+  $env:INPUT_API_KEY = 'api-only-key'
+  Invoke-SetupTest 'api-only-key'
+  Assert-Equal ((Get-Content "$testRoot/exchanges.log") -join ',') 'pull-only,push-capable'
+  Assert-Equal @(Get-Content "$testRoot/helper.log").Count 4
+
+  $env:INPUT_EXPORT_AUTH_TOKEN = 'false'
+  $env:INPUT_API_KEY = 'unexported-key'
+  Invoke-SetupTest 'unexported-key'
+  Assert-Equal @(Get-Content "$testRoot/helper.log").Count 4
+  Assert-Equal ([bool]((Get-Content $env:GITHUB_OUTPUT) -match '^oidc-token=')) $false
+
+  # The deprecated alias must also ignore the previously exported API key.
+  $env:INPUT_API_KEY = ''
+  $env:INPUT_OIDC_NAMESPACE = 'test-org'
+  $env:INPUT_OIDC_SERVICE_SLUG = 'alias-service'
+  $env:INPUT_OIDC_AUTH_ONLY = 'true'
+  Invoke-SetupTest 'token-alias-service'
+  Assert-Equal ((Get-Content "$testRoot/exchanges.log") -join ',') 'pull-only,push-capable,alias-service'
+  Write-Output 'PowerShell setup tests passed'
+}
+finally {
+  if (Test-Path -LiteralPath $testRoot) {
+    Remove-Item -LiteralPath $testRoot -Recurse -Force
+  }
+}
diff --git a/scripts/test-setup.sh b/scripts/test-setup.sh
new file mode 100644
index 0000000..dc034af
--- /dev/null
+++ b/scripts/test-setup.sh
@@ -0,0 +1,90 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+setup_script="$(cd "$(dirname "$0")" && pwd)/setup.sh"
+test_root="$(mktemp -d)"
+trap 'rm -rf "$test_root"' EXIT
+mkdir -p "$test_root/installer" "$test_root/bin"
+
+cat > "$test_root/installer/install.sh" <<'EOF'
+#!/bin/sh
+while [ "$1" != "--output-file" ]; do shift; done
+printf 'version=1.21.0\ntarget=test\nbin_dir=%s/bin\nexecutable=%s/bin/cloudsmith\n' \
+  "$GITHUB_ACTION_PATH" "$GITHUB_ACTION_PATH" > "$2"
+EOF
+
+cat > "$test_root/bin/cloudsmith" <<'EOF'
+#!/usr/bin/env bash
+set -euo pipefail
+case "$*" in
+  "credential-helper generic")
+    echo helper >> "$GITHUB_ACTION_PATH/helper.log"
+    # Match CLI precedence: an API key wins over OIDC.
+    token="${CLOUDSMITH_API_KEY:-}"
+    if [[ -z "$token" ]]; then
+      echo "$CLOUDSMITH_SERVICE_SLUG" >> "$GITHUB_ACTION_PATH/exchanges.log"
+      token="token-$CLOUDSMITH_SERVICE_SLUG"
+    fi
+    printf '{"version":1,"username":"token","password":"%s"}\n' "$token"
+    ;;
+  whoami) echo "${CLOUDSMITH_API_KEY:-}" >> "$GITHUB_ACTION_PATH/verified.log" ;;
+  *) exit 1 ;;
+esac
+EOF
+chmod +x "$test_root/bin/cloudsmith"
+
+export GITHUB_ACTION_PATH="$test_root" RUNNER_TEMP="$test_root"
+export GITHUB_ENV="$test_root/env" GITHUB_OUTPUT="$test_root/output" GITHUB_PATH="$test_root/path"
+export GITHUB_REPOSITORY_OWNER=test-owner ACTIONS_ID_TOKEN_REQUEST_URL=https://example.invalid/oidc
+export INPUT_CLI_VERSION=latest INPUT_INSTALL_DIRECTORY="" INPUT_API_KEY=""
+export INPUT_OIDC_NAMESPACE=test-org INPUT_OIDC_SERVICE_SLUG=pull-only INPUT_OIDC_AUDIENCE=""
+export INPUT_API_HOST="" INPUT_API_PROXY="" INPUT_API_SSL_VERIFY="" INPUT_API_USER_AGENT=""
+export INPUT_VERIFY_AUTH=true INPUT_EXPORT_AUTH_TOKEN=true INPUT_OIDC_AUTH_ONLY=false
+unset CLOUDSMITH_API_KEY
+
+run_setup() {
+  local expected="$1" line
+  : > "$GITHUB_ENV"
+  : > "$GITHUB_OUTPUT"
+  bash "$setup_script"
+  # Apply GITHUB_ENV as the runner would before the next action invocation.
+  while IFS= read -r line; do
+    export "${line?}"
+  done < "$GITHUB_ENV"
+  test "$CLOUDSMITH_API_KEY" = "$expected"
+  test "$(tail -n 1 "$test_root/verified.log")" = "$expected"
+  grep -Fx "CLOUDSMITH_API_KEY=$expected" "$GITHUB_ENV"
+  if [[ "$INPUT_EXPORT_AUTH_TOKEN" == true || "$INPUT_OIDC_AUTH_ONLY" == true ]]; then
+    test "$CLOUDSMITH_USERNAME" = token
+    grep -Fx "oidc-token=$expected" "$GITHUB_OUTPUT"
+  fi
+}
+
+run_setup token-pull-only
+export INPUT_OIDC_SERVICE_SLUG=push-capable
+run_setup token-push-capable
+test "$(cat "$test_root/exchanges.log")" = $'pull-only\npush-capable'
+test "$(wc -l < "$test_root/helper.log" | tr -d ' ')" = 2
+
+# Explicit API keys still win, even with OIDC inputs and an inherited token.
+export INPUT_API_KEY=explicit-key
+run_setup explicit-key
+export INPUT_OIDC_NAMESPACE="" INPUT_OIDC_SERVICE_SLUG="" INPUT_API_KEY=api-only-key
+run_setup api-only-key
+test "$(cat "$test_root/exchanges.log")" = $'pull-only\npush-capable'
+test "$(wc -l < "$test_root/helper.log" | tr -d ' ')" = 4
+
+export INPUT_EXPORT_AUTH_TOKEN=false INPUT_API_KEY=unexported-key
+run_setup unexported-key
+test "$(wc -l < "$test_root/helper.log" | tr -d ' ')" = 4
+if grep -q '^oidc-token=' "$GITHUB_OUTPUT"; then
+  echo "Unexpected oidc-token output" >&2
+  exit 1
+fi
+
+# The deprecated alias must also ignore the previously exported API key.
+export INPUT_API_KEY="" INPUT_OIDC_NAMESPACE=test-org INPUT_OIDC_SERVICE_SLUG=alias-service
+export INPUT_OIDC_AUTH_ONLY=true
+run_setup token-alias-service
+test "$(cat "$test_root/exchanges.log")" = $'pull-only\npush-capable\nalias-service'
+echo "Bash setup tests passed"