diff --git a/sledge/Makefile b/sledge/Makefile
index 687dbd162..cf0cb0f9b 100644
--- a/sledge/Makefile
+++ b/sledge/Makefile
@@ -3,12 +3,11 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-OCAMLFORMAT?=ocamlformat
-
 .PHONY: default
 default: dev
 
-DUNES=$(patsubst %.in,%,$(shell find src -name dune.in))
+DUNEINS=$(shell find src -name dune.in)
+DUNES=$(patsubst %.in,%,$(DUNEINS))
 
 .PHONY: dunes
 dunes: $(DUNES)
@@ -59,7 +58,8 @@ clean:
 
 .PHONY: fmt
 fmt:
-	$(OCAMLFORMAT) -i $(shell find src -name '*.ml' -or -name '*.mli') src/version.ml.in
+	dune build @_build/dev/src/fmt --auto-promote
+	ocamlformat -i src/version.ml.in $(DUNEINS)
 	clang-format -i model/llair_intrinsics.h model/cxxabi.cpp
 
 # print any variable for Makefile debugging
diff --git a/sledge/dune b/sledge/dune
new file mode 100644
index 000000000..da639ce8d
--- /dev/null
+++ b/sledge/dune
@@ -0,0 +1 @@
+(ignored_subdirs (llvm test))
diff --git a/sledge/dune-project b/sledge/dune-project
index de4fc2092..1c220e4d0 100644
--- a/sledge/dune-project
+++ b/sledge/dune-project
@@ -1 +1,2 @@
 (lang dune 1.0)
+(using fmt 1.0)
diff --git a/sledge/sledge.opam b/sledge/sledge.opam
index 069d452c7..5648dd1b5 100644
--- a/sledge/sledge.opam
+++ b/sledge/sledge.opam
@@ -12,7 +12,7 @@ depends: [
   "core_kernel" {>= "v0.11.0"}
   "ctypes"
   "ctypes-foreign"
-  "dune" {build}
+  "dune" {build >= "1.4.0"}
   "llvm" {build & = "7.0.0"}
   "ppx_compare" {>= "v0.11.0"}
   "ppx_hash" {>= "v0.11.0"}