diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index fe12bb4c..437d7440 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -1442,6 +1442,19 @@ in
 
     boot.isNspawnContainer = true;
 
+    # Use a disk-backed /tmp instead of the default tmpfs so large scratch
+    # writes (nix-develop shells, cargo build dirs, multi-GB downloads) land
+    # on disk rather than eating container RAM.  The tmpfs default mounts
+    # ~3.2 GB of RAM per container; disk-backed /tmp is effectively unlimited
+    # and cheaper for agents that do heavy build work.
+    #
+    # cleanOnBoot defaults to false in nixpkgs — set it explicitly so /tmp is
+    # cleared on each container start (D! tmpfiles rule), preserving the same
+    # ephemeral-per-boot semantics agents already expect from the old tmpfs,
+    # just without the RAM cost.
+    boot.tmp.useTmpfs = false;
+    boot.tmp.cleanOnBoot = true;
+
     # Every agent gets flakes + the modern `nix` CLI out of the box.
     # Equivalent to passing `--extra-experimental-features 'nix-command
     # flakes'` on every invocation. Agents shell out to `nix build` /