diff --git a/start-vs-VisualFSharpSln.ps1 b/start-vs-VisualFSharpSln.ps1
index 9ba9aec4e8a..6ab83f91c6d 100644
--- a/start-vs-VisualFSharpSln.ps1
+++ b/start-vs-VisualFSharpSln.ps1
@@ -8,9 +8,12 @@
 # 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.
+# -RoslynRepo is the Roslyn repository whose packages a locally built hive Roslyn comes from (default:
+# a roslyn folder next to this repository).
 [CmdletBinding()]
 param(
     [string]$RoslynVersion,
+    [string]$RoslynRepo,
     [string]$DevEnv,
     [string]$RootSuffix = 'RoslynDev',
     [string]$Solution = 'VisualFSharp.slnx',
@@ -18,12 +21,48 @@ param(
 )
 Set-StrictMode -Version Latest; $ErrorActionPreference = 'Stop'; $root = $PSScriptRoot
 
+# The package folders of a locally built Roslyn, checked to actually hold that version: a hive can carry
+# a dev build whose packages were cleaned or never packed, and NU1101 halfway through a restore says far
+# less about that than a message here does.
+function Get-RoslynDevFeeds([string]$repo, [string]$version) {
+    if (-not $repo) { $repo = Join-Path (Split-Path $PSScriptRoot) 'roslyn' }
+    # NuGet resolves a relative source against each project's directory, not this one.
+    $repo = $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($repo)
+    if (-not (Test-Path -LiteralPath $repo)) {
+        throw "$RootSuffix runs a locally built Roslyn $version, which only its own packages provide, but there is no repository at $repo; pass -RoslynRepo."
+    }
+
+    # NuGet fails restore (NU1301) on a local source that does not exist, so only the folders present are feeds.
+    $feeds = @('Shipping', 'NonShipping' | ForEach-Object { Join-Path $repo "artifacts\packages\Release\$_" } |
+        Where-Object { Test-Path -LiteralPath $_ })
+    $probe = "Microsoft.VisualStudio.LanguageServices.ExternalAccess.$version.nupkg"
+
+    if (-not ($feeds | Where-Object { Test-Path -LiteralPath (Join-Path $_ $probe) })) {
+        throw "$repo has no $probe; pack that Roslyn (.\Build.cmd -restore -pack -c Release) or pass -RoslynVersion to build against a published one instead."
+    }
+
+    $feeds
+}
+
+# A repack keeps the -dev version, and restore would serve the previous pack from the global packages
+# folder, so the extracted copy of every package these feeds hold at that version is dropped first.
+function Get-StaleRoslynDevPackages([string[]]$feeds, [string]$version) {
+    $cache = if ($env:NUGET_PACKAGES) { $env:NUGET_PACKAGES } else { Join-Path $env:USERPROFILE '.nuget\packages' }
+    $suffix = ".$version.nupkg"
+    foreach ($feed in $feeds) {
+        Get-ChildItem -LiteralPath $feed -Filter '*.nupkg' |
+            Where-Object { $_.Name.EndsWith($suffix, [StringComparison]::OrdinalIgnoreCase) } |
+            ForEach-Object { Join-Path $cache "$($_.Name.Substring(0, $_.Name.Length - $suffix.Length).ToLowerInvariant())\$($version.ToLowerInvariant())" } |
+            Where-Object { Test-Path -LiteralPath $_ }
+    }
+}
+
 if (-not $DevEnv) {
     $vswhere = Join-Path ${env:ProgramFiles(x86)} 'Microsoft Visual Studio\Installer\vswhere.exe'
     $DevEnv = if ($env:DevEnvDir) { Join-Path $env:DevEnvDir 'devenv.exe' }
-              elseif (Test-Path $vswhere) { & $vswhere -latest -prerelease -property productPath 2>$null }
+              elseif (Test-Path -LiteralPath $vswhere) { & $vswhere -latest -prerelease -property productPath 2>$null }
 }
-if (-not ($DevEnv -and (Test-Path $DevEnv))) { throw 'devenv.exe not found; run from a VS Developer prompt or pass -DevEnv.' }
+if (-not ($DevEnv -and (Test-Path -LiteralPath $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)
@@ -36,7 +75,7 @@ if (-not $RoslynVersion) {
     if ($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 |
+    $hiveRoslyn = Get-ChildItem -LiteralPath (Join-Path $hive 'Extensions') -Recurse -Filter 'Microsoft.CodeAnalysis.dll' -ErrorAction SilentlyContinue |
         Where-Object { $_.DirectoryName -like '*Roslyn Language Services*' } | Select-Object -First 1
 
     if ($hiveRoslyn) {
@@ -45,31 +84,39 @@ if (-not $RoslynVersion) {
     }
     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." }
+        if (-not (Test-Path -LiteralPath $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."
+    # A locally built Roslyn is deployed precisely because its API surface differs from the flowed
+    # package - it is where an ExternalAccess contract lives before it flows - so a matching minor says
+    # nothing and the override belongs here whatever the repo flows. Its own packages are the only ones
+    # that carry the difference, so the feeds they sit in come along; without them restore cannot find
+    # the version at all.
+    if ($target -match '-dev$') { $RoslynVersion = $target }
+    else {
+        # A shipped Roslyn is a real package version, and within a minor any 5.Y.* binds identically,
+        # so the flowed packages are the match when the minors agree.
+        $targetMinor = [version](($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."
+        }
+        else { $RoslynVersion = $target }
     }
-    else { $RoslynVersion = $target }
 }
 
+$feeds = $null
 if ($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)."
+    if ($RoslynVersion -match '-dev$') { $feeds = Get-RoslynDevFeeds $RoslynRepo $RoslynVersion }
+    Write-Host "Building the F# extension against Roslyn $RoslynVersion ($DevEnv)$(if ($feeds) { ' from its own packages' })."
 }
 
 # Repoint every Roslyn package (versions set in eng/Version.Details.props) via a props file MSBuild
@@ -81,8 +128,14 @@ $names = 'MicrosoftCodeAnalysis', 'MicrosoftCodeAnalysisCompilers', 'MicrosoftCo
 # whichever ran last decides what a long-lived VS session restores against.
 $override = Join-Path $root 'artifacts\RoslynOverride.start-vs.props'
 if ($RoslynVersion) {
+    # RestoreAdditionalProjectSources has to arrive through the props import rather than on the command
+    # line: Microsoft.FSharp.NetSdk.targets declares it TreatAsLocalProperty and appends to it.
+    $sources =
+        if ($feeds) { "<RestoreAdditionalProjectSources>`$(RestoreAdditionalProjectSources);$([Security.SecurityElement]::Escape($feeds -join ';'))</RestoreAdditionalProjectSources>" }
+        else { '' }
+
     New-Item -ItemType Directory -Force (Split-Path $override) | Out-Null
-    "<Project><PropertyGroup>$(-join ($names | ForEach-Object { "<${_}Version>$RoslynVersion</${_}Version>" }))</PropertyGroup></Project>" |
+    "<Project><PropertyGroup>$(-join ($names | ForEach-Object { "<${_}Version>$RoslynVersion</${_}Version>" }))$sources</PropertyGroup></Project>" |
         Set-Content -LiteralPath $override -Encoding UTF8
 }
 
@@ -102,11 +155,14 @@ 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] }
+    $stale = if ($feeds) { @(Get-StaleRoslynDevPackages $feeds $RoslynVersion) } else { @() }
     if ($DryRun) {
         if ($RoslynVersion) { Write-Host "DryRun: wrote $override" } else { Write-Host 'DryRun: no override needed' }
+        $stale | ForEach-Object { Write-Host "DryRun: would remove $_" }
         Write-Host "Would restore, then open $Solution in $DevEnv; F5 deploys into and launches $RootSuffix"
         return
     }
+    $stale | ForEach-Object { Remove-Item -LiteralPath $_ -Recurse -Force }
     & (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)`""