diff --git a/build.ps1 b/build.ps1
index f793e9f..aa8f931 100644
--- a/build.ps1
+++ b/build.ps1
@@ -155,8 +155,22 @@ function Build-Protocol([hashtable]$sql) {
 function Build-Core([hashtable]$sql) {
     Write-Step 'core: building NucleicCore (full Windows manifest)'
     Write-Note 'Expected to fail until the §12 item 7-9 shims land — the errors ARE the M0 gap list.'
-    if ($Test) { return Invoke-Swift @('test', '--filter', 'NucleicCarbonTests') $sql }
-    return Invoke-Swift @('build', '--target', 'NucleicCore') $sql
+    # NucleicCore only EXISTS when NUCLEIC_WINDOWS_PROTOCOL_ONLY is absent: with it set,
+    # Package.swift narrows the manifest to [NucleicProtocol, NucleicProtocolTests] and this
+    # fails with "no target named 'NucleicCore'". CI never sets it for this job, but a local
+    # shell that ran the protocol leg by hand keeps it — so clear it just for this build and
+    # hand it back afterwards.
+    $inherited = $env:NUCLEIC_WINDOWS_PROTOCOL_ONLY
+    if ($inherited) {
+        Write-Note 'Ignoring inherited NUCLEIC_WINDOWS_PROTOCOL_ONLY — core needs the full manifest.'
+        Remove-Item Env:\NUCLEIC_WINDOWS_PROTOCOL_ONLY
+    }
+    try {
+        if ($Test) { return Invoke-Swift @('test', '--filter', 'NucleicCarbonTests') $sql }
+        return Invoke-Swift @('build', '--target', 'NucleicCore') $sql
+    } finally {
+        if ($inherited) { $env:NUCLEIC_WINDOWS_PROTOCOL_ONLY = $inherited }
+    }
 }
 
 function Build-Broker {