diff --git a/cli/bunfig.toml b/cli/bunfig.toml
new file mode 100644
index 00000000..3fb4604a
--- /dev/null
+++ b/cli/bunfig.toml
@@ -0,0 +1,2 @@
+[test]
+preload = ["./src/__tests__/preload.ts"]
diff --git a/cli/src/__tests__/preload.ts b/cli/src/__tests__/preload.ts
new file mode 100644
index 00000000..059a73be
--- /dev/null
+++ b/cli/src/__tests__/preload.ts
@@ -0,0 +1,41 @@
+/**
+ * Test preload script — filesystem isolation for CLI tests.
+ *
+ * Loaded before every test file via bunfig.toml `preload`.
+ * Redirects HOME and XDG dirs to a temp directory so no test
+ * can accidentally write to the real user's home directory
+ * (e.g. ~/.claude/settings.json, ~/.zshrc).
+ *
+ * This prevents the class of bugs where a test (or the code under test)
+ * overwrites real config files on the developer's machine.
+ */
+
+import { mkdirSync, rmSync, mkdtempSync } from "fs";
+import { join } from "path";
+import { tmpdir } from "os";
+
+// ── Create isolated HOME ────────────────────────────────────────────────────
+
+const TEST_HOME = mkdtempSync(join(tmpdir(), "spawn-test-home-"));
+
+// Redirect all user-directory env vars to the isolated temp
+process.env.HOME = TEST_HOME;
+process.env.XDG_CACHE_HOME = join(TEST_HOME, ".cache");
+process.env.XDG_CONFIG_HOME = join(TEST_HOME, ".config");
+process.env.XDG_DATA_HOME = join(TEST_HOME, ".local", "share");
+
+// Pre-create common directories tests might expect
+mkdirSync(join(TEST_HOME, ".cache"), { recursive: true });
+mkdirSync(join(TEST_HOME, ".config"), { recursive: true });
+mkdirSync(join(TEST_HOME, ".claude"), { recursive: true });
+mkdirSync(join(TEST_HOME, ".local", "share"), { recursive: true });
+
+// ── Cleanup on exit ─────────────────────────────────────────────────────────
+
+process.on("exit", () => {
+  try {
+    rmSync(TEST_HOME, { recursive: true, force: true });
+  } catch {
+    // Best-effort cleanup
+  }
+});