diff --git a/Changes.md b/Changes.md
index 4aa7ea2..2d5cd97 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,5 +1,22 @@
 # Build System Changes for Android.mk Writers
 
+## `BUILD_NUMBER` removal from Android.mk  {#BUILD_NUMBER}
+
+`BUILD_NUMBER` should not be used directly in Android.mk files, as it would
+trigger them to be re-read every time the `BUILD_NUMBER` changes (which it does
+on every build server build). If possible, just remove the use so that your
+builds are more reproducible. If you do need it, use `BUILD_NUMBER_FROM_FILE`:
+
+``` make
+$(LOCAL_BUILT_MODULE):
+	mytool --build_number $(BUILD_NUMBER_FROM_FILE) -o $@
+```
+
+That will expand out to a subshell that will read the current `BUILD_NUMBER`
+whenever it's run. It will not re-run your command if the build number has
+changed, so incremental builds will have the build number from the last time
+the particular output was rebuilt.
+
 ## `DIST_DIR`, `dist_goal`, and `dist-for-goals`  {#dist}
 
 `DIST_DIR` and `dist_goal` are no longer available when reading Android.mk
diff --git a/core/main.mk b/core/main.mk
index 6ff5f93..461c3b4 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -66,12 +66,15 @@
 $(shell mkdir -p $(OUT_DIR) && \
     echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)
 BUILD_NUMBER_FILE := $(OUT_DIR)/build_number.txt
+.KATI_READONLY := BUILD_NUMBER_FILE
+$(KATI_obsolete_var BUILD_NUMBER,See https://android.googlesource.com/platform/build/+/master/Changes.md#BUILD_NUMBER)
 
 ifeq ($(HOST_OS),darwin)
 DATE_FROM_FILE := date -r $(BUILD_DATETIME_FROM_FILE)
 else
 DATE_FROM_FILE := date -d @$(BUILD_DATETIME_FROM_FILE)
 endif
+.KATI_READONLY := DATE_FROM_FILE
 
 # Pick a reasonable string to use to identify files.
 ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
@@ -81,6 +84,7 @@
 else
   FILE_NAME_TAG := $(file <$(BUILD_NUMBER_FILE))
 endif
+.KATI_READONLY := FILE_NAME_TAG
 
 # Make an empty directory, which can be used to make empty jars
 EMPTY_DIRECTORY := $(OUT_DIR)/empty