diff --git a/build/config.mk b/build/config.mk
index 77e116caa0..552e6225c3 100644
--- a/build/config.mk
+++ b/build/config.mk
@@ -2,7 +2,7 @@
 #── vi: set noet ft=make ts=8 sw=8 fenc=utf-8 :vi ────────────────────┘
 
 PREFIX = /usr/local
-COSMOCC = .cosmocc/3.9.7
+COSMOCC = .cosmocc/4.0.0
 TOOLCHAIN = $(COSMOCC)/bin/cosmo
 
 CC = $(TOOLCHAIN)cc
@@ -52,5 +52,5 @@ clean:; rm -rf o
 .PHONY: distclean
 distclean:; rm -rf o .cosmocc
 
-.cosmocc/3.9.7:
-	build/download-cosmocc.sh $@ 3.9.7 3f559555d08ece35bab1a66293a2101f359ac9841d563419756efa9c79f7a150
+.cosmocc/4.0.0:
+	build/download-cosmocc.sh $@ 4.0.0 15d8ab4442c94ce925f1d59884c772ab817af5e2889549c21ce5fa11c5d773bc