diff --git a/start-vs-VisualFSharpSln.ps1 b/start-vs-VisualFSharpSln.ps1
index 7dc9cecb955..9ba9aec4e8a 100644
--- a/start-vs-VisualFSharpSln.ps1
+++ b/start-vs-VisualFSharpSln.ps1
@@ -1,10 +1,18 @@
-# Launch VS on VisualFSharp.slnx, building the F# VS extension against the Roslyn your installed VS
-# ships (not the newer one flowed into the repo) so F5 loads it. See DEVGUIDE.md. Needs VS Roslyn >= 5.10.
-# -RoslynVersion forces a version (e.g. if the exact VS build isn't on a feed); any 5.Y.* binds identically.
+# Launch VS on VisualFSharp.slnx so that F5 builds the F# VS extension against the Roslyn it will
+# actually run against, and deploys it to $RootSuffix. See DEVGUIDE.md.
+#
+# What F5 runs against is the hive's Roslyn, not the installed VS's: deploying a locally built Roslyn
+# into the hive stamps it 42.42.42.42 and redirects every reference to itself, so the hive wins. Only
+# when the hive has no Roslyn of its own does the installation's version apply. This picks whichever
+# of the two is in force and matches the package versions to it - or leaves the repo's flowed versions
+# alone when they already match, which is the common case for a hive built from this same source.
+#
+# -RoslynVersion forces a package version; any 5.Y.* binds identically within a minor.
 [CmdletBinding()]
 param(
     [string]$RoslynVersion,
     [string]$DevEnv,
+    [string]$RootSuffix = 'RoslynDev',
     [string]$Solution = 'VisualFSharp.slnx',
     [switch]$DryRun
 )
@@ -17,45 +25,92 @@ if (-not $DevEnv) {
 }
 if (-not ($DevEnv -and (Test-Path $DevEnv))) { throw 'devenv.exe not found; run from a VS Developer prompt or pass -DevEnv.' }
 
+$ideDir = Split-Path $DevEnv
+$flowed = ([xml](Get-Content -LiteralPath (Join-Path $root 'eng\Version.Details.props') -Raw)
+    ).GetElementsByTagName('MicrosoftCodeAnalysisPackageVersion')[0].InnerText.Trim()
+
 if (-not $RoslynVersion) {
-    $dll = "$(Split-Path $DevEnv)\CommonExtensions\Microsoft\VBCSharp\LanguageServices\Microsoft.CodeAnalysis.dll"
-    if (-not (Test-Path $dll)) { throw "Can't detect your VS Roslyn version; pass -RoslynVersion." }
-    $RoslynVersion = ([System.Diagnostics.FileVersionInfo]::GetVersionInfo($dll).ProductVersion -split '\+')[0]
+    $ini = Get-Content -LiteralPath (Join-Path $ideDir 'devenv.isolation.ini') -Raw
+    if ($ini -notmatch '(?m)^InstallationID=(?<id>\S+)') { throw 'No InstallationID in devenv.isolation.ini.' }
+    $installationId = $Matches.id
+    if ($ini -notmatch '(?m)^InstallationVersion=(?<v>\d+)') { throw 'No InstallationVersion in devenv.isolation.ini.' }
+    $hive = Join-Path $env:LOCALAPPDATA ('Microsoft\VisualStudio\{0}.0_{1}{2}' -f $Matches.v, $installationId, $RootSuffix)
+
+    $hiveRoslyn = Get-ChildItem (Join-Path $hive 'Extensions') -Recurse -Filter 'Microsoft.CodeAnalysis.dll' -ErrorAction SilentlyContinue |
+        Where-Object { $_.DirectoryName -like '*Roslyn Language Services*' } | Select-Object -First 1
+
+    if ($hiveRoslyn) {
+        $target = [System.Diagnostics.FileVersionInfo]::GetVersionInfo($hiveRoslyn.FullName).ProductVersion
+        $source = "deployed in $RootSuffix"
+    }
+    else {
+        $dll = Join-Path $ideDir 'CommonExtensions\Microsoft\VBCSharp\LanguageServices\Microsoft.CodeAnalysis.dll'
+        if (-not (Test-Path $dll)) { throw "Can't detect the Roslyn version for $RootSuffix; pass -RoslynVersion." }
+        $target = ([System.Diagnostics.FileVersionInfo]::GetVersionInfo($dll).ProductVersion -split '\+')[0]
+        $source = 'shipped with the installed VS'
+    }
+    Write-Host "Roslyn in force: $target ($source)."
+
+    # A dev build's "5.12.0-dev" is no package version, so only its minor is usable - and when that is
+    # the minor the repo already flows, the flowed packages are the match and no override belongs here.
+    $targetMinor = [version](($target -split '-')[0])
+    $flowedMinor = [version](($flowed -split '-')[0])
+    if ($targetMinor.Major -eq $flowedMinor.Major -and $targetMinor.Minor -eq $flowedMinor.Minor) {
+        Write-Host "Repo already flows Roslyn $flowed - building against it, no override."
+    }
+    elseif ($target -match '-dev$') {
+        throw "$RootSuffix has a locally built Roslyn $target but the repo flows $flowed; pass -RoslynVersion with a $($targetMinor.Major).$($targetMinor.Minor).* package version to build against that minor."
+    }
+    else { $RoslynVersion = $target }
 }
-$minor = [version](($RoslynVersion -split '-')[0])
-if ($minor -lt [version]'5.10.0') {
-    throw "Your VS ships Roslyn $RoslynVersion but the repo references packages from >= 5.10 (unified ExternalAccess, #20099). Update VS or deploy a local Roslyn."
+
+if ($RoslynVersion) {
+    $minor = [version](($RoslynVersion -split '-')[0])
+    if ($minor -lt [version]'5.10.0') {
+        throw "Roslyn $RoslynVersion is older than the 5.10 the repo's sources expect (unified ExternalAccess, #20099). Update VS or deploy a local Roslyn."
+    }
+    Write-Host "Building the F# extension against Roslyn $RoslynVersion ($DevEnv)."
 }
-Write-Host "Building the F# extension against Roslyn $RoslynVersion ($DevEnv)."
 
 # Repoint every Roslyn package (versions set in eng/Version.Details.props) via a props file MSBuild
 # imports after it through the CustomAfterMicrosoftCommonProps hook the launched VS inherits.
 $names = 'MicrosoftCodeAnalysis', 'MicrosoftCodeAnalysisCompilers', 'MicrosoftCodeAnalysisCSharp',
     'MicrosoftCodeAnalysisEditorFeatures', 'MicrosoftCodeAnalysisEditorFeaturesText', 'MicrosoftCodeAnalysisFeatures',
     'MicrosoftVisualStudioLanguageServices', 'MicrosoftVisualStudioLanguageServicesExternalAccess'
-$override = Join-Path $root 'artifacts\RoslynOverride.props'
-New-Item -ItemType Directory -Force (Split-Path $override) | Out-Null
-"<Project><PropertyGroup>$(-join ($names | ForEach-Object { "<${_}Version>$RoslynVersion</${_}Version>" }))</PropertyGroup></Project>" |
-    Set-Content -LiteralPath $override -Encoding UTF8
+# Its own file: build-vs-VisualFSharpSln.ps1 writes an override too, and one path for both means
+# whichever ran last decides what a long-lived VS session restores against.
+$override = Join-Path $root 'artifacts\RoslynOverride.start-vs.props'
+if ($RoslynVersion) {
+    New-Item -ItemType Directory -Force (Split-Path $override) | Out-Null
+    "<Project><PropertyGroup>$(-join ($names | ForEach-Object { "<${_}Version>$RoslynVersion</${_}Version>" }))</PropertyGroup></Project>" |
+        Set-Content -LiteralPath $override -Encoding UTF8
+}
 
 # Apply to THIS process only, restoring in finally so it can't leak into a later build.cmd/CI run
 # (which must keep the flowed Roslyn); the launched VS snapshots the env for its F5/restore builds.
+# VSRootSuffix is what the VSIX projects deploy into and what F5 passes as /rootsuffix, so the hive
+# probed above is also the one the extension lands in.
 $vars = @{
-    CustomAfterMicrosoftCommonProps = $override
     DOTNET_ROOT                     = Join-Path $root '.dotnet'
     'DOTNET_ROOT(x86)'              = Join-Path $root '.dotnet\x86'
     PATH                            = "$(Join-Path $root '.dotnet');$env:PATH"
     RunNetFrameworkApiCompat        = 'false'
     RunRefApiCompat                 = 'false'
+    VSRootSuffix                    = $RootSuffix
 }
+if ($RoslynVersion) { $vars.CustomAfterMicrosoftCommonProps = $override }
 $saved = @{}; foreach ($k in $vars.Keys) { $saved[$k] = [Environment]::GetEnvironmentVariable($k) }
 try {
     foreach ($k in $vars.Keys) { Set-Item -LiteralPath "Env:\$k" -Value $vars[$k] }
-    if ($DryRun) { Write-Host "DryRun: $override"; return }
+    if ($DryRun) {
+        if ($RoslynVersion) { Write-Host "DryRun: wrote $override" } else { Write-Host 'DryRun: no override needed' }
+        Write-Host "Would restore, then open $Solution in $DevEnv; F5 deploys into and launches $RootSuffix"
+        return
+    }
     & (Join-Path $root 'Restore.cmd')
     if ($LASTEXITCODE) { throw "Restore failed for Roslyn $RoslynVersion; try another 5.$($minor.Minor).* build via -RoslynVersion." }
     Start-Process $DevEnv "`"$(Join-Path $root $Solution)`""
-    Write-Host 'Launched VS. Set VisualFSharpDebug as the startup project, then F5 / Ctrl+F5.'
+    Write-Host "Launched VS. Set VisualFSharpDebug as the startup project, then F5 / Ctrl+F5 to run in $RootSuffix."
 }
 finally {
     foreach ($k in $saved.Keys) {
diff --git a/vsintegration/Vsix/Directory.Build.props b/vsintegration/Vsix/Directory.Build.props
index 66b95cf6e1a..d81bbf2613f 100644
--- a/vsintegration/Vsix/Directory.Build.props
+++ b/vsintegration/Vsix/Directory.Build.props
@@ -4,7 +4,7 @@
 
   <PropertyGroup>
     <VisualStudioInsertionComponent>Microsoft.FSharp</VisualStudioInsertionComponent>
-    <VSRootSuffix>RoslynDev</VSRootSuffix>
+    <VSRootSuffix Condition="'$(VSRootSuffix)' == ''">RoslynDev</VSRootSuffix>
     <VSSDKTargetPlatformRegRootSuffix>$(VSRootSuffix)</VSSDKTargetPlatformRegRootSuffix>
     <!-- Path to the locally built F# compiler for use during VSIX debugging -->
     <FSharpCompilerPathForDebuggingLocally>$(ArtifactsDir)bin\fscAnyCpu\$(Configuration)\net472\</FSharpCompilerPathForDebuggingLocally>