diff --git a/powershell/Dotfiles/Dotfiles.psd1 b/powershell/Dotfiles/Dotfiles.psd1
index 752e234..c17ec81 100644
--- a/powershell/Dotfiles/Dotfiles.psd1
+++ b/powershell/Dotfiles/Dotfiles.psd1
@@ -41,6 +41,7 @@
         'Get-DotToolNudge'
         'Test-DotNonInteractiveArg'
         'Test-InteractiveShell'
+        'Test-InMux'
         'Get-DotfilesLinkPlan'
         'ConvertTo-WslPath'
         # doctor: result model, aggregation + pure logic (host probes stay in the fragment)
diff --git a/powershell/core/05-lib.ps1 b/powershell/core/05-lib.ps1
index 35091fe..166b585 100644
--- a/powershell/core/05-lib.ps1
+++ b/powershell/core/05-lib.ps1
@@ -17,7 +17,7 @@
 # ============================================================================
 
 # --- load contract (checked by tests/LoadContract.Tests.ps1) ------------------
-# provides: Test-SensitiveHistoryLine, Get-DotConfirmAnswer, Test-DotGum, Read-DotConfirm, Get-DotStringSha256, Get-DotSpinnerFrame, Invoke-DotSpinner, Test-DotEmailish, Get-DotToolNudge, Test-DotNonInteractiveArg, Test-InteractiveShell, Get-DotfilesLinkPlan, Test-DotColor, Test-DotUnicode, Get-DotGlyph, Write-DotHost, Write-DotBanner, Get-DotConsoleWidth, Format-DotWrap, Write-DotRule, Write-DotErr, Write-DotOk, Write-DotWarn
+# provides: Test-SensitiveHistoryLine, Get-DotConfirmAnswer, Test-DotGum, Read-DotConfirm, Get-DotStringSha256, Get-DotSpinnerFrame, Invoke-DotSpinner, Test-DotEmailish, Get-DotToolNudge, Test-DotNonInteractiveArg, Test-InteractiveShell, Test-InMux, Get-DotfilesLinkPlan, Test-DotColor, Test-DotUnicode, Get-DotGlyph, Write-DotHost, Write-DotBanner, Get-DotConsoleWidth, Format-DotWrap, Write-DotRule, Write-DotErr, Write-DotOk, Write-DotWarn
 # requires: (none)
 
 # --- Test-SensitiveHistoryLine ------------------------------------------------
@@ -357,6 +357,19 @@ function Test-InteractiveShell {
     return -not (Test-DotNonInteractiveArg ([Environment]::GetCommandLineArgs()))
 }
 
+# --- Test-InMux ---------------------------------------------------------------
+# Are we running inside a psmux/tmux pane (i.e. the status bar is showing)? psmux
+# exports TMUX + PSMUX_SESSION into pane shells (verified against psmux src/pane.rs);
+# either marker means "in a pane". THE single source of truth for pane detection —
+# shared by the psmux auto-attach guard (os/30-windows.ps1, so it never nests a
+# session inside an existing pane) and the pill's auto-arm/status (os/33-psmux-pill.ps1),
+# which previously each hand-maintained their own marker list and had already drifted.
+function Test-InMux {
+    [OutputType([bool])]
+    param()
+    [bool]($env:TMUX -or $env:PSMUX_SESSION)
+}
+
 # --- Get-DotfilesLinkPlan -----------------------------------------------------
 # THE single source of truth for every symlink this repo wires: one ordered list
 # that install.ps1 creates, uninstall.ps1 removes, and dotfiles-doctor verifies.
diff --git a/powershell/os/30-windows.ps1 b/powershell/os/30-windows.ps1
index 2c6bd05..9991606 100644
--- a/powershell/os/30-windows.ps1
+++ b/powershell/os/30-windows.ps1
@@ -4,7 +4,7 @@
 
 # --- load contract (checked by tests/LoadContract.Tests.ps1) ------------------
 # provides: scu, scs, sci, scl, sccl, wgu, wgs, wgi, update-host, path, open, admin, setenv, getenv, modules-localize
-# requires: Get-DotModulePrunePlan, Test-Cmd, Test-InteractiveShell, up, Write-DotErr, Write-DotHost, Write-DotWarn
+# requires: Get-DotModulePrunePlan, Test-Cmd, Test-InMux, Test-InteractiveShell, up, Write-DotErr, Write-DotHost, Write-DotWarn
 
 # --- scoop (your primary CLI package manager on the host) ---------------------
 if (Test-Cmd scoop) {
@@ -136,13 +136,11 @@ function modules-localize {
 }
 
 # --- Start psmux session (top-level interactive shell only) -------------------
-# $InMux must list every marker psmux sets inside a pane, or a shell that IS in a
-# pane won't be recognised and we'd auto-launch a NESTED session. The authoritative
-# pair — verified against psmux src/pane.rs and used by os/33-psmux-pill.ps1's
-# Test-InMux — is TMUX + PSMUX_SESSION. (The old list checked PSMUX / PSMUX_PANE,
-# which psmux does NOT export, and omitted PSMUX_SESSION; it only worked because
-# TMUX happens to be set.) Keep TMUX_PANE too as a harmless belt-and-braces marker.
-$InMux = $env:TMUX -or $env:PSMUX_SESSION -or $env:TMUX_PANE
+# Pane detection is Test-InMux (core/05-lib.ps1) — the single source of truth,
+# shared with os/33-psmux-pill.ps1. If a shell that IS in a pane weren't
+# recognised we'd auto-launch a NESTED session, so the marker set (TMUX +
+# PSMUX_SESSION, verified against psmux src/pane.rs) lives in exactly one place.
+$InMux = Test-InMux
 
 # Only auto-launch for a *top-level interactive* shell. The profile is ALSO
 # loaded for `pwsh -Command ...` / `pwsh -File ...` (VS Code tasks, git hooks,
diff --git a/powershell/os/33-psmux-pill.ps1 b/powershell/os/33-psmux-pill.ps1
index 8600875..902cbcd 100644
--- a/powershell/os/33-psmux-pill.ps1
+++ b/powershell/os/33-psmux-pill.ps1
@@ -33,16 +33,15 @@
 
 # --- load contract (checked by tests/LoadContract.Tests.ps1) ------------------
 # provides: psmux-pill-now, psmux-pill-enable, psmux-pill-disable, psmux-pill-status
-# requires: Test-Cmd, Write-DotErr, Write-DotHost, Write-DotOk
+# requires: Test-Cmd, Test-InMux, Write-DotErr, Write-DotHost, Write-DotOk
 
 if (-not (Test-Cmd psmux)) { return }
 
 $script:PillCache  = Join-Path $env:LOCALAPPDATA 'dotfiles\psmux-netinfo.pill'
 $script:PillSource = 'PsmuxPillRefresh'   # Register-ObjectEvent SourceIdentifier
 
-# Inside a psmux pane? psmux exports TMUX + PSMUX_SESSION into pane shells
-# (verified in psmux src/pane.rs). Either marker means "the bar is showing".
-function script:Test-InMux { [bool]($env:TMUX -or $env:PSMUX_SESSION) }
+# Pane detection (Test-InMux — "is the bar showing?") is shared from
+# core/05-lib.ps1 now, so it can't drift from the psmux auto-attach guard's copy.
 
 function script:Get-PillScript {
     $p = if ($global:DOTFILES) { Join-Path $global:DOTFILES 'psmux\scripts\psmux-netinfo.ps1' } else { $null }
diff --git a/tests/Lib.Tests.ps1 b/tests/Lib.Tests.ps1
index 646081f..5e9889a 100644
--- a/tests/Lib.Tests.ps1
+++ b/tests/Lib.Tests.ps1
@@ -58,6 +58,24 @@ Describe 'Test-DotNonInteractiveArg' {
     }
 }
 
+Describe 'Test-InMux' {
+    BeforeAll {
+        $script:savedTmux = $env:TMUX
+        $script:savedSess = $env:PSMUX_SESSION
+    }
+    AfterAll {
+        if ($null -eq $script:savedTmux) { Remove-Item Env:TMUX -ErrorAction SilentlyContinue } else { $env:TMUX = $script:savedTmux }
+        if ($null -eq $script:savedSess) { Remove-Item Env:PSMUX_SESSION -ErrorAction SilentlyContinue } else { $env:PSMUX_SESSION = $script:savedSess }
+    }
+    BeforeEach {
+        Remove-Item Env:TMUX -ErrorAction SilentlyContinue
+        Remove-Item Env:PSMUX_SESSION -ErrorAction SilentlyContinue
+    }
+    It 'is $false outside a pane (no markers)' { Test-InMux | Should -BeFalse }
+    It 'is $true when TMUX is set'             { $env:TMUX = 'default,1,0'; Test-InMux | Should -BeTrue }
+    It 'is $true when PSMUX_SESSION is set'    { $env:PSMUX_SESSION = 'main'; Test-InMux | Should -BeTrue }
+}
+
 Describe 'Test-DotColor' {
     It 'enables colour by default'        { Test-DotColor -NoColor '' -Term 'xterm' | Should -BeTrue }
     It 'disables colour when NO_COLOR set' { Test-DotColor -NoColor '1' -Term 'xterm' | Should -BeFalse }
diff --git a/tests/Module.Tests.ps1 b/tests/Module.Tests.ps1
index d2a4638..c32676e 100644
--- a/tests/Module.Tests.ps1
+++ b/tests/Module.Tests.ps1
@@ -43,7 +43,7 @@ Describe 'Dotfiles module exports' {
             'Test-DotGum', 'Test-DotEmailish',
             'Get-DotSpinnerFrame', 'Format-DotSpinnerLine', 'Invoke-DotSpinner', 'Test-SensitiveHistoryLine',
             'Get-DotStringSha256', 'Get-DotToolNudge', 'Get-DotfilesLinkPlan',
-            'Test-DotNonInteractiveArg', 'Test-InteractiveShell',
+            'Test-DotNonInteractiveArg', 'Test-InteractiveShell', 'Test-InMux',
             'ConvertTo-WslPath',
             'New-DoctorResult', 'Get-DoctorSummary', 'Get-DoctorGroup',
             'Get-FragmentHealthResult', 'Get-DotRepoVersionDetail',