diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1afc351..60d6ab4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -78,6 +78,9 @@ jobs:
     runs-on: macos-latest
     continue-on-error: true
     timeout-minutes: 15
+    # The per-user TMPDIR leaves a HotCell::TestCell socket path a few bytes from macOS's 104-byte limit.
+    env:
+      TMPDIR: /tmp
     permissions:
       contents: read
     steps:
diff --git a/hotcell-server/lib/hot_cell/test_cell.rb b/hotcell-server/lib/hot_cell/test_cell.rb
index 09baf09..bfb31d8 100644
--- a/hotcell-server/lib/hot_cell/test_cell.rb
+++ b/hotcell-server/lib/hot_cell/test_cell.rb
@@ -26,6 +26,9 @@ module HotCell
   # which is the only place such a library may be loaded.
   #
   #   HotCell::TestCell.boot(operations: -> { require "active_storage/hot_cell/server" })
+  #
+  # The cell's sockets go beneath `Dir.tmpdir`, and a Unix socket path holds 104 bytes on macOS, whose per-user
+  # `TMPDIR` spends about half of that. A suite on macOS should set a short `TMPDIR`, such as `/tmp`.
   class TestCell
     READY = "up"