🩹 [Patch]: Ensure only the selected module version is loaded

src/Invoke-Pester.Helpers.psm1

@@ -1283,14 +1283,14 @@ function Invoke-ProcessTestDirectory {
 function Install-PSResourceWithRetry {
  <#
  .SYNOPSIS
- Installs a PowerShell module with a retry mechanism, then imports the installed version.
+ Installs a PowerShell module with a retry mechanism, then imports the installed version as the only loaded version.
 
  .DESCRIPTION
  Attempts to install a PowerShell module multiple times in case of failure. When a version
- constraint is supplied, the newest version satisfying it is installed; that exact version is
-then imported so the loaded module is deterministic even when other versions (for example the
- runner's preinstalled copy) are present on the machine. When no version is supplied, the latest
- available version is installed.
+ constraint is supplied, the newest version satisfying it is installed. Any other versions already
+loaded in the session are then removed and that exact version is imported, so the loaded module is
+deterministic even when other versions (for example the runner's preinstalled copy) are present on
+the machine. When no version is supplied, the latest available version is installed.
  #>
  [CmdletBinding()]
  param (
@@ -1363,28 +1363,43 @@ function Install-PSResourceWithRetry {
 
  # Resolve the exact version to import. Prefer what was just installed; if the resource was
  # already present, Install-PSResource returns nothing, so fall back to the newest installed
- # version that satisfies the requested constraint.
- $resolved = $installed | Where-Object { $_.Name -eq $Name } | Sort-Object Version -Descending | Select-Object -First 1
- if (-not $resolved) {
+ # version that satisfies the requested constraint. When prerelease versions are not requested,
+ # never resolve to one; when a stable and a prerelease share a base version, the stable one wins.
+ # PSResourceGet exposes prerelease state as the 'Prerelease' string (empty for stable), present on
+ # both Install-PSResource and Get-InstalledPSResource output.
+ $sortOrder = @(
+ @{ Expression = 'Version'; Descending = $true }
+ @{ Expression = { -not [string]::IsNullOrWhiteSpace($_.Prerelease) }; Descending = $false }
+ )
+ $candidates = @($installed | Where-Object { $_.Name -eq $Name })
+ if ($candidates.Count -eq 0) {
  $getParams = @{ Name = $Name; Verbose = $false; ErrorAction = 'SilentlyContinue' }
  if (-not [string]::IsNullOrWhiteSpace($Version)) {
  $getParams['Version'] = $Version
  }
- $resolved = Get-InstalledPSResource @getParams | Sort-Object Version -Descending | Select-Object -First 1
+ $candidates = @(Get-InstalledPSResource @getParams)
  }
+ if (-not $Prerelease) {
+ $candidates = @($candidates | Where-Object { [string]::IsNullOrWhiteSpace($_.Prerelease) })
+ }
+ $resolved = $candidates | Sort-Object -Property $sortOrder | Select-Object -First 1
 
- # Import into the global session state so the resolved version is the one every subsequent
- # command (for example Invoke-Pester in exec.ps1) uses, instead of PowerShell auto-loading the
- # highest version available on PSModulePath.
+ # A version constraint was requested but no satisfying installed version could be resolved. Fail fast
+ # before mutating session state; importing unconstrained would silently load an arbitrary copy from
+ # PSModulePath (for example the runner's preinstalled module), defeating the deterministic,
+ # constraint-driven selection this function exists to guarantee.
+ if (-not $resolved -and -not [string]::IsNullOrWhiteSpace($Version)) {
+ throw "No installed '$Name' version satisfies constraint '$Version'; refusing to import an unconstrained version."
+ }
+
+ # Remove every already-loaded instance (all versions, including nested) from the session so the
+ # chosen version is the only one loaded, then import into the global session state so the resolved
+ # version is the one every subsequent command (for example Invoke-Pester in exec.ps1) uses, instead
+ # of PowerShell auto-loading the highest version available on PSModulePath.
+ Get-Module -Name $Name -All | Remove-Module -Force -ErrorAction SilentlyContinue
  if ($resolved) {
  Write-Output "Importing module: $Name $($resolved.Version)"
  $imported = Import-Module -Name $Name -RequiredVersion $resolved.Version -Force -Global -PassThru -ErrorAction Stop
- } elseif (-not [string]::IsNullOrWhiteSpace($Version)) {
- # A version constraint was requested but no satisfying installed version could be resolved. Importing the
- # module unconstrained here would silently load an arbitrary copy from PSModulePath (for example the
- # runner's preinstalled module), defeating the deterministic, constraint-driven selection this function
- # exists to guarantee. Fail fast instead.
- throw "No installed '$Name' version satisfies constraint '$Version'; refusing to import an unconstrained version."
  } else {
  $imported = Import-Module -Name $Name -Force -Global -PassThru -ErrorAction Stop
  }