diff --git a/nixos/gitea-runner.nix b/nixos/gitea-runner.nix
index 4c010fb6..4a47534f 100644
--- a/nixos/gitea-runner.nix
+++ b/nixos/gitea-runner.nix
@@ -58,9 +58,21 @@ makeEnable config "myModules.gitea-runner" false {
     in {
       XDG_CONFIG_HOME = gitea-runner-directory;
       XDG_CACHE_HOME = "${gitea-runner-directory}/.cache";
+      # Despite PrivateTmp=false below, the upstream module's DynamicUser=true
+      # still gives the service a private /tmp: a tmpfs capped at 10% of RAM
+      # (~3.2G here) with 400k inodes. CI jobs inherit it — `nix develop`
+      # shell dirs (leaked on cancelled jobs) and pytest's ephemeral postgres
+      # (tempfile.mkdtemp) slowly fill it until tests die with
+      # DiskFull/ENOSPC while `df /` shows hundreds of GB free. Point job
+      # temp at the disk-backed state dir instead; tmpfiles rule below prunes
+      # leaked entries.
+      TMPDIR = "${gitea-runner-directory}/tmp";
     };
     serviceConfig.PrivateTmp = false;
   };
+  systemd.tmpfiles.rules = [
+    "d /var/lib/private/gitea-runner/tmp 0750 gitea-runner gitea-runner 2d"
+  ];
   users.groups.gitea-runner = {};
   users.users.gitea-runner = {
     isSystemUser = true;
diff --git a/nixos/machines/railbird-sf.nix b/nixos/machines/railbird-sf.nix
index ad85d7c2..011dcaf7 100644
--- a/nixos/machines/railbird-sf.nix
+++ b/nixos/machines/railbird-sf.nix
@@ -55,6 +55,23 @@
   myModules.postgres.enable = true;
   features.full.enable = true;
 
+  # CI on this box grows /nix unboundedly (per-commit `nix develop
+  # '.?submodules=1'` source copies + railbird-full CUDA image builds); the
+  # store had 20k dead paths / 40G+ reclaimable when this was added. Daily GC
+  # keeps the baseline down; min-free makes nix GC mid-build when free space
+  # drops below 50G instead of running into ENOSPC. (The CI postgres DiskFull
+  # failures were a separate issue — the runner's private 3.2G tmpfs /tmp;
+  # see gitea-runner.nix TMPDIR.)
+  nix.gc = {
+    automatic = true;
+    dates = "daily";
+    options = "--delete-older-than 7d";
+  };
+  nix.settings = {
+    min-free = 50 * 1024 * 1024 * 1024; # trigger GC below 50G free
+    max-free = 150 * 1024 * 1024 * 1024; # GC until 150G free
+  };
+
   hardware.nvidia = {
     powerManagement.enable = false;
     # Fine-grained power management. Turns off GPU when not in use.