diff --git a/cli-tools/nddev_codex.py b/cli-tools/nddev_codex.py
index 2cd0576..47ea79a 100644
--- a/cli-tools/nddev_codex.py
+++ b/cli-tools/nddev_codex.py
@@ -3486,15 +3486,25 @@ def run_builder_plugin_command(
     process: subprocess.Popen[bytes] | None = None
     captured = {"stdout": bytearray(), "stderr": bytearray()}
     try:
-        process = subprocess.Popen(
-            [str(installation.executable), *arguments],
-            cwd=target,
-            env=environment,
-            stdin=subprocess.DEVNULL,
-            stdout=subprocess.PIPE,
-            stderr=subprocess.PIPE,
-            start_new_session=True,
-        )
+        # Normalize the child's umask so the directories and files the Codex CLI
+        # creates (plugins/, cache/, marketplace/, the cached plugin manifest and
+        # tree) are owner-only. The builder cache validators reject group- or
+        # other-writable entries, and an ambient umask of 0002 (the Ubuntu
+        # default, unlike macOS 022) would create 0775 dirs that the validators
+        # then fail closed on. Mirrors run_verified_installer.
+        prior_umask = os.umask(0o077)
+        try:
+            process = subprocess.Popen(
+                [str(installation.executable), *arguments],
+                cwd=target,
+                env=environment,
+                stdin=subprocess.DEVNULL,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
+                start_new_session=True,
+            )
+        finally:
+            os.umask(prior_umask)
         if process.stdout is None or process.stderr is None:
             fail("official Codex plugin command output pipes are unavailable")
         selector = selectors.DefaultSelector()
diff --git a/tests/test_umask_guard.py b/tests/test_umask_guard.py
new file mode 100644
index 0000000..ab0c637
--- /dev/null
+++ b/tests/test_umask_guard.py
@@ -0,0 +1,102 @@
+# Regression tests for the builder-plugin-command umask guard.
+#
+# Background: run_builder_plugin_command spawns the official Codex CLI to run
+# `plugin marketplace add` / `plugin add`, and the CLI creates directories
+# (plugins/, cache/, marketplace/, version, manifest) and files that the module's
+# own validators reject when they are group- or other-writable (mode & 0o022).
+# On hosts where the ambient umask is 0002 (the Ubuntu default, unlike macOS
+# 022), an unguarded child inherits that umask and produces 0775/0664 entries,
+# which then fail closed. The fix wraps the Popen in os.umask(0o077).
+#
+# These tests do not invoke the Codex CLI. They assert the invariants the guard
+# must uphold: (1) the process umask around the Popen is 0o077, regardless of
+# the ambient umask, and (2) a child that mimics the Codex CLI's directory
+# creation cannot produce group-writable entries while the guard is active.
+
+import os
+import stat
+import subprocess
+import sys
+from pathlib import Path
+
+CODEX_MODULE = Path(__file__).resolve().parents[1] / "cli-tools" / "nddev_codex.py"
+
+
+def _child_that_makes_dirs(target_dir: str) -> int:
+    """A tiny Python program that creates a dir/file like the Codex CLI would."""
+    code = (
+        "import os, sys; "
+        f"d = os.path.join({target_dir!r}, 'plugins', 'cache'); "
+        "os.makedirs(d); "
+        f"open(os.path.join(d, 'plugin.json'), 'w').close(); "
+        "sys.exit(0)"
+    )
+    return subprocess.Popen([sys.executable, "-c", code]).wait()
+
+
+def test_guard_restores_prior_umask(tmp_path):
+    """The umask guard must restore the ambient umask after the Popen, even
+    when the ambient umask is the Ubuntu default 0002."""
+    ambient = 0o002
+    prior = os.umask(ambient)
+    try:
+        captured = {}
+
+        # Reproduce the guard exactly as run_builder_plugin_command applies it.
+        # os.umask() returns the PREVIOUS mask, so the pattern is:
+        #   prior_umask = os.umask(0o077)   # now effective = 0o077, returns old
+        #   ... child runs under 0o077 ...
+        #   os.umask(prior_umask)           # restores old
+        prior_umask = os.umask(0o077)
+        try:
+            # The effective mask is now 0o077; verify by setting it again.
+            effective = os.umask(0o077)
+            os.umask(0o077)  # keep it at 0o077 for the child
+            captured["effective"] = effective
+            _child_that_makes_dirs(str(tmp_path))
+        finally:
+            os.umask(prior_umask)
+        captured["restored"] = os.umask(ambient)
+        os.umask(ambient)  # leave it where the outer block expects it
+    finally:
+        os.umask(prior)
+
+    # The effective umask inside the guarded region is 0o077.
+    assert captured["effective"] == 0o077
+    # After the guard, the ambient umask is restored to the caller's value.
+    assert captured["restored"] == 0o002
+
+
+def test_child_dirs_are_not_group_writable_under_guard(tmp_path):
+    """A child process creating dirs/files under the 0o077 guard must not
+    produce any group- or other-writable entry (the validator's invariant)."""
+    ambient = 0o002
+    prior = os.umask(ambient)
+    try:
+        inner = os.umask(0o077)
+        try:
+            rc = _child_that_makes_dirs(str(tmp_path))
+        finally:
+            os.umask(inner)
+    finally:
+        os.umask(prior)
+
+    assert rc == 0
+    offending = []
+    for path in tmp_path.rglob("*"):
+        mode = stat.S_IMODE(path.stat().st_mode)
+        if mode & 0o022:
+            offending.append(f"{path} mode={oct(mode)}")
+    assert not offending, f"group/other-writable entries created: {offending}"
+
+
+def test_source_has_the_guard():
+    """Static guard: the Popen in run_builder_plugin_command is wrapped by the
+    umask guard, and the sibling run_verified_installer already has it."""
+    source = CODEX_MODULE.read_text()
+    # The guard pair must appear in the builder-plugin-command region too.
+    assert "prior_umask = os.umask(0o077)" in source
+    assert source.count("prior_umask = os.umask(0o077)") >= 2, (
+        "expected the umask guard in both run_verified_installer and "
+        "run_builder_plugin_command"
+    )