diff --git a/autoninja b/autoninja
index d6e065f12..408f49547 100755
--- a/autoninja
+++ b/autoninja
@@ -4,6 +4,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# Set unique build ID.
+export AUTONINJA_BUILD_ID="$(python -c "import uuid; print uuid.uuid4()")"
 
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
diff --git a/autoninja.bat b/autoninja.bat
index a071fb96e..20564e81f 100755
--- a/autoninja.bat
+++ b/autoninja.bat
@@ -3,6 +3,9 @@
 :: Use of this source code is governed by a BSD-style license that can be
 :: found in the LICENSE file.
 
+REM Set unique build ID.
+FOR /f "usebackq tokens=*" %%a in (`python -c "import uuid; print uuid.uuid4()"`) do set AUTONINJA_BUILD_ID=%%a
+
 REM Execute whatever is printed by autoninja.py.
 REM Also print it to reassure that the right settings are being used.
 FOR /f "usebackq tokens=*" %%a in (`python %~dp0autoninja.py "%*"`) do echo %%a & %%a