diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index 3e0cddb527..0cda8e00bb 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -219,6 +219,9 @@ test_packages="./tests/csapi ./tests ./tests/msc3874 ./tests/msc3890 ./tests/msc
 # All environment variables starting with PASS_ will be shared.
 # (The prefix is stripped off before reaching the container.)
 export COMPLEMENT_SHARE_ENV_PREFIX=PASS_
+# Enable dirty runs, so tests will reuse the same container where possible.
+# This significantly speeds up tests, but increases the possibility of test pollution.
+export COMPLEMENT_ENABLE_DIRTY_RUNS=1
 
 # It takes longer than 10m to run the whole suite.
 extra_test_args+=("-timeout=60m")