From c2e75b3cffaaaa63e5db9b7a11d4fa4858779780 Mon Sep 17 00:00:00 2001
From: Jakub Jares <jajares@microsoft.com>
Date: Wed, 17 Jun 2026 15:47:01 +0200
Subject: [PATCH] Fix #2585: Prevent $WhatIfPreference from breaking Pester

Set $WhatIfPreference = $false at the start of Invoke-Pester's begin
block. When a caller sets $WhatIfPreference = $true, it leaks into
Pester's internal commands (New-Item, Remove-Item, etc.) and causes
test execution to fail with cryptic errors or excessive 'What if:'
messages. The local assignment scopes it to Invoke-Pester and its
children without affecting the caller's preference.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 src/Main.ps1                         |  4 ++++
 tst/Pester.RSpec.InNewProcess.ts.ps1 | 26 ++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/src/Main.ps1 b/src/Main.ps1
index 4430e6ed6..0ff407707 100644
--- a/src/Main.ps1
+++ b/src/Main.ps1
@@ -454,6 +454,10 @@ function Invoke-Pester {
         [PesterConfiguration] $Configuration
     )
     begin {
+        # Prevent $WhatIfPreference from leaking into Pester internals (#2585).
+        # When the caller sets $WhatIfPreference = $true, it propagates to child
+        # scopes and breaks commands that Pester relies on (New-Item, Remove-Item, etc.).
+        $WhatIfPreference = $false
         $start = [DateTime]::Now
         # this will inherit to child scopes and allow Describe / Context to run directly from a file or command line
         $invokedViaInvokePester = $true
diff --git a/tst/Pester.RSpec.InNewProcess.ts.ps1 b/tst/Pester.RSpec.InNewProcess.ts.ps1
index a1e869d9e..b66329305 100644
--- a/tst/Pester.RSpec.InNewProcess.ts.ps1
+++ b/tst/Pester.RSpec.InNewProcess.ts.ps1
@@ -391,4 +391,30 @@ i -PassThru:$PassThru {
             }
         }
     }
+
+    b '$WhatIfPreference does not break Pester' {
+        t 'Invoke-Pester succeeds when $WhatIfPreference is $true' {
+                $sb = {
+                    $WhatIfPreference = $true
+
+                    $PesterPreference = [PesterConfiguration]::Default
+                    $PesterPreference.Output.Verbosity = 'None'
+
+                    $container = New-PesterContainer -ScriptBlock {
+                        Describe 'd1' {
+                            It 'i1' {
+                                1 | Should -Be 1
+                            }
+                        }
+                    }
+                    $r = Invoke-Pester -Container $container -PassThru
+                    if ($r.FailedCount -ne 0) { throw "Expected 0 failures, got $($r.FailedCount)" }
+                    if ($r.PassedCount -ne 1) { throw "Expected 1 passed, got $($r.PassedCount)" }
+                }
+
+                $output = Invoke-InNewProcess $sb
+                $whatIfLines = $output | Select-String -Pattern 'What if:'
+                @($whatIfLines).Count | Verify-Equal 0
+        }
+    }
 }