diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index 0befbfa..f61cfbf 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -36,17 +36,27 @@
 $(call add_soong_config_var,ANDROID,BOARD_BUILD_SYSTEM_ROOT_IMAGE)
 $(call add_soong_config_var,ANDROID,PRODUCT_INSTALL_DEBUG_POLICY_TO_SYSTEM_EXT)
 
-ifneq (,$(filter sdk win_sdk sdk_addon,$(MAKECMDGOALS)))
-  # The artifacts in the SDK zip are OK to build with prebuilt stubs enabled,
-  # even if prebuilt apexes are not enabled, because the system images in the
-  # SDK stub are not currently used (and will be removed: b/205008975).
-  MODULE_BUILD_FROM_SOURCE ?= false
-else ifeq (,$(findstring com.google.android.conscrypt,$(PRODUCT_PACKAGES)))
+# Default behavior for the tree wrt building modules or using prebuilts. This
+# can always be overridden by setting the environment variable
+# MODULE_BUILD_FROM_SOURCE.
+BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE := true
+
+ifneq (,$(MODULE_BUILD_FROM_SOURCE))
+  # Keep an explicit setting.
+else ifeq (,$(filter sdk win_sdk sdk_addon,$(MAKECMDGOALS))$(findstring com.google.android.conscrypt,$(PRODUCT_PACKAGES)))
   # Prebuilt module SDKs require prebuilt modules to work, and currently
   # prebuilt modules are only provided for com.google.android.xxx. If we can't
   # find one of them in PRODUCT_PACKAGES then assume com.android.xxx are in use,
   # and disable prebuilt SDKs. In particular this applies to AOSP builds.
+  #
+  # However, sdk/win_sdk/sdk_addon builds might not include com.google.android.xxx
+  # packages, so for those we respect the default behavior.
   MODULE_BUILD_FROM_SOURCE := true
+else ifeq (,$(filter-out modules_% mainline_modules_%,$(TARGET_PRODUCT)))
+  # Always build from source in unbundled builds using the module targets.
+  MODULE_BUILD_FROM_SOURCE := true
+else
+  MODULE_BUILD_FROM_SOURCE := $(BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE)
 endif
 
 # TODO(b/172480615): Remove when platform uses ART Module prebuilts by default.