diff --git a/.clean_build_epoch b/.clean_build_epoch
new file mode 100644
--- /dev/null
+++ b/.clean_build_epoch
@@ -0,0 +1,7 @@
+# Add a new line containing a larger number than the last (e.g. YYYYMMDD) for
+# every change that breaks ABI or requires a full rebuild for some other reason.
+# Only the last entry is required, but having the full list may be useful for
+# looking at frequency of ABI breakage, etc.
+
+# 4757b351ea9: openssl: Import version 3.5.1
+20250807	# All OpenSSL-using bits need rebuilt
diff --git a/Makefile.inc1 b/Makefile.inc1
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -1083,7 +1083,7 @@
 	@echo ">>> Deleting stale dependencies...";
 	MACHINE=${MACHINE} MACHINE_ARCH=${MACHINE_ARCH} \
 	    ALL_libcompats=${_ALL_libcompats:Q} \
-	    sh ${.CURDIR}/tools/build/depend-cleanup.sh ${OBJTOP}
+	    sh ${.CURDIR}/tools/build/depend-cleanup.sh ${OBJTOP} ${SRCTOP}
 
 _cleanworldtmp: .PHONY
 .if ${MK_CLEAN} == "yes"
@@ -1176,7 +1176,8 @@
 	${_+_}cd ${.CURDIR}; ${LIB${LIBCOMPAT}WMAKE} _NO_INCLUDE_COMPILERMK=t -f Makefile.inc1 ${CLEANDIR}
 .endfor
 .else
-	${_+_}cd ${.CURDIR}; ${WMAKE} _NO_INCLUDE_COMPILERMK=t _cleanobj_fast_depend_hack
+	${_+_}cd ${.CURDIR}; env CLEANMK="_NO_INCLUDE_COMPILERMK=t ${CLEANDIR}" \
+	    MAKE=${MAKE} ${WMAKE} _cleanobj_fast_depend_hack
 .endif	# ${MK_CLEAN} == "yes"
 _obj:
 	@echo
diff --git a/tools/build/depend-cleanup.sh b/tools/build/depend-cleanup.sh
--- a/tools/build/depend-cleanup.sh
+++ b/tools/build/depend-cleanup.sh
@@ -63,6 +63,10 @@
 #   if [ "$MACHINE_ARCH" = "amd64" ]; then
 #           clean_dep lib/libc bcmp c
 #   fi
+#
+# We also have a big hammer at the top of the tree, .clean_build_epoch, to be
+# used in severe cases where we can't surgically remove just the parts that
+# need rebuilt.  This should be used sparingly.
 
 set -e
 set -u
@@ -80,7 +84,7 @@
 
 usage()
 {
-	echo "usage: $(basename $0) [-v] [-n] objtop" >&2
+	echo "usage: $(basename $0) [-v] [-n] objtop srctop" >&2
 }
 
 VERBOSE=
@@ -101,17 +105,31 @@
 done
 shift $((OPTIND-1))
 
-if [ $# -ne 1 ]; then
+if [ $# -ne 2 ]; then
 	usage
 	exit 1
 fi
 
 OBJTOP=$1
 shift
+SRCTOP=$1
+shift
+
 if [ ! -d "$OBJTOP" ]; then
 	err "$OBJTOP: Not a directory"
 fi
 
+if [ ! -d "$SRCTOP" ]; then
+	err "$SRCTOP: Not a directory"
+fi
+
+: ${CLEANMK=""}
+if [ -n "$CLEANMK" ]; then
+	if [ -z "${MAKE+set}" ]; then
+		err "MAKE not set"
+	fi
+fi
+
 if [ -z "${MACHINE+set}" ]; then
 	err "MACHINE not set"
 fi
@@ -151,6 +169,84 @@
 	done
 }
 
+clean_world()
+{
+	local buildepoch="$1"
+
+	# The caller may set CLEANMK in the environment to make target(s) that
+	# should be invoked instead of just destroying everything.  This is
+	# generally used after legacy/bootstrap tools to avoid over-cleansing
+	# since we're generally in the temporary tree's ancestor.
+	if [ -n "$CLEANMK" ]; then
+		echo "Cleaning up the object tree"
+		run $MAKE -C "$SRCTOP" -f "$SRCTOP"/Makefile.inc1 $CLEANMK
+	else
+		echo "Cleaning up the temporary build tree"
+		run rm -rf "$OBJTOP"
+	fi
+
+	# Preemptively stamp the new objdir with the epoch at time of cleaning,
+	# so that we can pick up a subsequent need to clean under the assumption
+	# that this build will complete.  We don't assume that all callers will
+	# have grabbed the build epoch, so we'll do it here as needed.
+	if  [ -z "$buildepoch" ]; then
+		buildepoch=$(extract_epoch "$SRCTOP"/.clean_build_epoch)
+	fi
+
+	mkdir -p "$OBJTOP"
+	echo "$buildepoch" > "$OBJTOP"/.clean_build_epoch
+
+	exit 0
+}
+
+extract_epoch()
+{
+	local epochfile="$1"
+
+	grep -ve '^#' -e '^$' "$epochfile" | tail -1 | cut -f1 | egrep '^[0-9]+$'
+}
+
+check_epoch()
+{
+	local srcepoch objepoch
+
+	srcepoch=$(extract_epoch "$SRCTOP"/.clean_build_epoch)
+	if [ -z "$srcepoch" ]; then
+		err "Malformed .clean_build_epoch; please validate the last line"
+	fi
+
+	if [ -s "$OBJTOP"/.clean_build_epoch ]; then
+		objepoch=$(extract_epoch "$OBJTOP"/.clean_build_epoch)
+
+		if [ "$VERBOSE" ]; then
+			echo "src epoch: $srcepoch, objdir epoch: $objepoch"
+		fi
+	else
+		# If we don't have an epoch, we can't really tell anything about
+		# the current objdir.  We have no choice but to just stamp it
+		# with the current build and hope it works.
+		if [ "$VERBOSE" ]; then
+			echo "Stamping fresh .clean_build_epoch"
+		fi
+
+		echo "$srcepoch" > "$OBJTOP"/.clean_build_epoch
+		return
+	fi
+
+	if [ "$srcepoch" -gt "$objepoch" ]; then
+		if [ "$VERBOSE" ]; then
+			echo "objdir epoch is older than src; cleaning"
+		fi
+
+		clean_world "$srcepoch"
+		# NORETURN
+	elif [ "$srcepoch" -lt "$objepoch" ]; then
+		err ".clean_build_epoch appears to have gone backwards; please validate the last line"
+	fi
+}
+
+check_epoch
+
 # Date      Rev      Description
 
 # 20220326  fbc002cb72d2    move from bcmp.c to bcmp.S