diff --git a/MDK-ARM/build.bat b/MDK-ARM/build.bat
index 98d86c7d..938c1e56 100644
--- a/MDK-ARM/build.bat
+++ b/MDK-ARM/build.bat
@@ -6,14 +6,38 @@ set PROJ=%~dp0STM32F407-Demo.uvprojx
 set LOG=%~dp0build_log.txt
 set TARGET=STM32F407-Demo
 
+:: UV4 is single-instance: if an IDE process is alive, this -b call may be
+:: forwarded to it (exit code is lost, GUI may open). Warn only; never kill.
+tasklist /fi "imagename eq UV4.exe" /nh | find /i "UV4.exe" >nul
+if not errorlevel 1 echo WARNING: UV4 IDE is running - this build may be forwarded to the IDE. Close it for a deterministic CLI build.
+
+:: Remove stale log first: UV4 -o writes the log only when a build actually
+:: runs. A leftover log would otherwise be misreported as this build's result.
+if exist "%LOG%" del "%LOG%"
+
 "%UV4%" -b "%PROJ%" -t%TARGET% -j0 -o "%LOG%"
 
-findstr /C:"0 Error(s), 0 Warning(s)" "%LOG%" >nul
-if %ERRORLEVEL% equ 0 (
-    echo BUILD_OK: 0 errors, 0 warnings
-    exit /b 0
+if not exist "%LOG%" (
+    echo BUILD_FAIL: no build log produced - UV4 did not run or hung
+    exit /b 1
 )
 
-echo BUILD_FAIL: no success marker in log
-type "%LOG%"
-exit /b 1
+:: Error diagnostics in the log: ARMCC V5 "file(line): error:  #NN" and
+:: "no source: Error:  #5" both contain "error:"; a 0E0W log never does.
+findstr /I /C:"error:" "%LOG%" >nul
+if not errorlevel 1 (
+    echo BUILD_FAIL: error diagnostics in build log
+    type "%LOG%"
+    exit /b 1
+)
+
+:: Success proof: exact 0E0W summary line (also rejects warning-only builds).
+findstr /C:"0 Error(s), 0 Warning(s)" "%LOG%" >nul
+if errorlevel 1 (
+    echo BUILD_FAIL: no 0E0W marker in build log
+    type "%LOG%"
+    exit /b 1
+)
+
+echo BUILD_OK: 0 errors, 0 warnings
+exit /b 0