diff --git a/package.json b/package.json
index bf8873c..762d2e0 100644
--- a/package.json
+++ b/package.json
@@ -50,8 +50,8 @@
     "install:codex": "node scripts/installer-cli.mjs install",
     "prepack": "npm run check:version-sync && npm run check:changelog",
     "setup:git-hooks": "node scripts/setup-git-hooks.mjs",
-    "test": "node --test tests/*.test.mjs",
-    "test:cross-platform": "node --test tests/args.test.mjs tests/changelog.test.mjs tests/claude-cli.test.mjs tests/fs.test.mjs tests/prompts.test.mjs tests/render.test.mjs tests/sandbox-modes.test.mjs tests/skills-contracts.test.mjs tests/structured-output.test.mjs tests/version-sync.test.mjs",
+    "test": "node --import ./tests/support/isolated-codex-home.mjs --test tests/*.test.mjs",
+    "test:cross-platform": "node --import ./tests/support/isolated-codex-home.mjs --test tests/args.test.mjs tests/changelog.test.mjs tests/claude-cli.test.mjs tests/fs.test.mjs tests/prompts.test.mjs tests/render.test.mjs tests/sandbox-modes.test.mjs tests/skills-contracts.test.mjs tests/structured-output.test.mjs tests/version-sync.test.mjs",
     "test:integration": "node --test tests/integration/*.test.mjs",
     "test:e2e": "node --test tests/e2e/*.test.mjs",
     "uninstall:codex": "node scripts/installer-cli.mjs uninstall",
diff --git a/tests/support/isolated-codex-home.mjs b/tests/support/isolated-codex-home.mjs
new file mode 100644
index 0000000..8da145a
--- /dev/null
+++ b/tests/support/isolated-codex-home.mjs
@@ -0,0 +1,21 @@
+/**
+ * Copyright 2026 Sendbird, Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+// Preloaded by the unit test scripts with `node --import`, which also applies it
+// to every test file's process. Plugin state defaults to ~/.codex, so without
+// this the tests that create jobs write into the developer's real Codex plugin
+// data. Each process gets its own throwaway home. Tests that need a specific
+// layout still pass their own HOME or CODEX_HOME to the processes they spawn.
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import process from "node:process";
+
+const home = fs.mkdtempSync(path.join(os.tmpdir(), "cc-plugin-codex-test-home-"));
+process.env.HOME = home;
+process.env.USERPROFILE = home;
+delete process.env.CODEX_HOME;
+process.on("exit", () => {
+  fs.rmSync(home, { recursive: true, force: true });
+});