diff --git a/ide/basebuildmanager.pas b/ide/basebuildmanager.pas
index 0fa733c73d..69c760a332 100644
--- a/ide/basebuildmanager.pas
+++ b/ide/basebuildmanager.pas
@@ -66,6 +66,7 @@ type
 
     procedure RescanCompilerDefines(ResetBuildTarget, ClearCaches,
                                     WaitTillDone, Quiet: boolean); virtual; abstract;
+    function CompilerOnDiskChanged: boolean; virtual; abstract;
 
     function CheckAmbiguousSources(const AFilename: string;
                                    Compiling: boolean): TModalResult; virtual; abstract;
diff --git a/ide/buildmanager.pas b/ide/buildmanager.pas
index ee83edf71e..ba789119df 100644
--- a/ide/buildmanager.pas
+++ b/ide/buildmanager.pas
@@ -165,6 +165,7 @@ type
     procedure UpdateEnglishErrorMsgFilename;
     procedure RescanCompilerDefines(ResetBuildTarget, ClearCaches,
                                     WaitTillDone, Quiet: boolean); override;
+    function CompilerOnDiskChanged: boolean; override;
     procedure LoadFPCDefinesCaches;
     procedure SaveFPCDefinesCaches;
     property UnitSetCache: TFPCUnitSetCache read FUnitSetCache write SetUnitSetCache;
@@ -750,6 +751,17 @@ begin
   end;
 end;
 
+function TBuildManager.CompilerOnDiskChanged: boolean;
+var
+  CfgCache: TFPCTargetConfigCache;
+begin
+  Result:=false;
+  if UnitSetCache=nil then exit;
+  CfgCache:=UnitSetCache.GetConfigCache(false);
+  if CfgCache=nil then exit;
+  Result:=CfgCache.NeedsUpdate;
+end;
+
 procedure TBuildManager.LoadFPCDefinesCaches;
 var
   aFilename: String;
diff --git a/ide/main.pp b/ide/main.pp
index eb101a3129..339876b9ac 100644
--- a/ide/main.pp
+++ b/ide/main.pp
@@ -12402,6 +12402,9 @@ begin
     Result:=DebugBoss.DoStopProject;
     if Result<>mrOk then exit;
   end;
+
+  if MainBuildBoss.CompilerOnDiskChanged then
+    MainBuildBoss.RescanCompilerDefines(false,false,false,false);
 end;
 
 function TMainIDE.OnRunExternalTool(Tool: TIDEExternalToolOptions): TModalResult;