diff --git a/build.ps1 b/build.ps1
index 9a8f5dd..df4917a 100644
--- a/build.ps1
+++ b/build.ps1
@@ -170,11 +170,6 @@ function Invoke-Swift([string[]]$SwiftArgs, [hashtable]$Sql) {
     # carries the prefix — it is global, so it also downgrades any FUTURE duplicate to a
     # warning. Read the warnings.
     $full = @($SwiftArgs) + @(
-        # Index-while-building costs real per-file work (`-index-store-path` +
-        # `-index-system-modules` on every frontend job) and nothing consumes the store from a
-        # command-line build — it exists for editors. Off here; CI leaves it at the default,
-        # where a cold index nothing reads costs nothing either way.
-        '--disable-index-store',
         '-Xcc', "-I$($Sql.Include)",
         # `/LIBPATH:`, NOT `-LIBPATH:`. A leading `-L` is claimed by the driver's GNU-style
         # library-path flag, so `-LIBPATH:C:\…\lib` is read as `-L` + `IBPATH:C:\…\lib` and
@@ -196,7 +191,13 @@ function Build-Protocol([hashtable]$sql) {
     try {
         Write-Step 'protocol: building NucleicProtocol'
         $code = if ($Test) {
-            Invoke-Swift @('test', '--filter', 'NucleicProtocolTests') $sql
+            # SwiftPM 6.3.3 on Windows still schedules index units for test discovery even with
+            # `--disable-index-store`, then a clean build dies opening units it never emitted.
+            # With indexing enabled, parallel frontends contend for shared SDK/module unit files
+            # and Windows reports sharing violations as "permission denied". Serialize test
+            # builds until SwiftPM fixes that pair of index-store bugs; ordinary builds stay
+            # parallel.
+            Invoke-Swift @('test', '-j', '1', '--filter', 'NucleicProtocolTests') $sql
         } else {
             Invoke-Swift @('build', '--target', 'NucleicProtocol') $sql
         }
@@ -222,7 +223,10 @@ function Build-Core([hashtable]$sql) {
         Remove-Item Env:\NUCLEIC_WINDOWS_PROTOCOL_ONLY
     }
     try {
-        if ($Test) { return Invoke-Swift @('test', '--filter', 'NucleicCarbonTests') $sql }
+        if ($Test) {
+            # See Build-Protocol: clean Windows test builds require indexing + one frontend job.
+            return Invoke-Swift @('test', '-j', '1', '--filter', 'NucleicCarbonTests') $sql
+        }
         return Invoke-Swift @('build', '--target', 'NucleicCore') $sql
     } finally {
         if ($inherited) { $env:NUCLEIC_WINDOWS_PROTOCOL_ONLY = $inherited }