diff --git a/tests/setup/isolateDataDir.js b/tests/setup/isolateDataDir.js
new file mode 100644
index 00000000..06e11bbd
--- /dev/null
+++ b/tests/setup/isolateDataDir.js
@@ -0,0 +1,26 @@
+// Global test isolation: never let the unit/translator suite write to the user's
+// real database (~/.9router). Some tests (e.g. zed-live-models, zed-native-auth)
+// exercise real route handlers that call createProviderConnection — without this
+// they append test rows ("zed-live-*@example.com", "guard-*@example.com",
+// "Account N") straight into the live DB.
+//
+// DATA_DIR must be set before src/lib/dataDir.js is imported (it reads the env
+// at module-eval time), which is exactly what a vitest setupFile guarantees.
+//
+// Escape hatches:
+//   - RUN_REAL=1        → keep the real DATA_DIR so *.real.test.js can read it.
+//   - DATA_DIR=<path>   → respect an explicit override (CI / manual isolation).
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const RUN_REAL = process.env.RUN_REAL === "1";
+const explicit = process.env.DATA_DIR;
+
+if (!RUN_REAL && !explicit) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "9router-test-"));
+  process.env.DATA_DIR = dir;
+  process.on("exit", () => {
+    try { fs.rmSync(dir, { recursive: true, force: true }); } catch { /* best effort */ }
+  });
+}
diff --git a/tests/unit/test-data-dir-isolation.test.js b/tests/unit/test-data-dir-isolation.test.js
new file mode 100644
index 00000000..7b0dbeda
--- /dev/null
+++ b/tests/unit/test-data-dir-isolation.test.js
@@ -0,0 +1,27 @@
+// Guard: the test harness must never write into the user's real DB.
+//
+// Root cause this locks down: route-level tests (zed-live-models,
+// zed-native-auth) call createProviderConnection, which persists to
+// $DATA_DIR/db/data.sqlite. With no DATA_DIR set, that resolved to ~/.9router —
+// polluting the live DB with "zed-live-*@example.com", "guard-*@example.com"
+// and "Account N" rows on every `npx vitest run`.
+//
+// tests/setup/isolateDataDir.js redirects DATA_DIR to a temp dir unless the
+// caller opts out via RUN_REAL=1 or an explicit DATA_DIR.
+import { describe, it, expect } from "vitest";
+import os from "node:os";
+import path from "node:path";
+
+// When the caller opts into the real DB (RUN_REAL=1) or supplies DATA_DIR,
+// isolation is intentionally disabled — this guard only applies to the default.
+const ISOLATED = !process.env.RUN_REAL && !process.env.EXPECT_REAL_DATA_DIR;
+
+describe.skipIf(!ISOLATED)("test DATA_DIR isolation", () => {
+  it("points DATA_DIR at a temp dir, not ~/.9router", () => {
+    const dir = process.env.DATA_DIR;
+    expect(dir).toBeTruthy();
+    const home = path.join(os.homedir(), ".9router");
+    expect(path.resolve(dir)).not.toBe(path.resolve(home));
+    expect(dir.startsWith(os.tmpdir())).toBe(true);
+  });
+});
diff --git a/tests/vitest.config.js b/tests/vitest.config.js
index d53d85e0..3db8a4ea 100644
--- a/tests/vitest.config.js
+++ b/tests/vitest.config.js
@@ -9,6 +9,10 @@ export default defineConfig({
     environment: "node",
     globals: true,
     include: ["**/*.test.js"],
+    // Redirect DATA_DIR to a throwaway temp dir so route-level tests that call
+    // createProviderConnection never touch the user's real ~/.9router DB.
+    // RUN_REAL=1 or an explicit DATA_DIR opts out (see setup/isolateDataDir.js).
+    setupFiles: ["./setup/isolateDataDir.js"],
     // Don't scan into git worktrees nested under .claude/ — they carry their
     // own copies of the test files but lack an installed node_modules (open-sse,
     // etc.), which makes provider imports fail during collection.