diff --git a/src/lib/dataPaths.ts b/src/lib/dataPaths.ts
index 3152265828..5ad61ddbee 100644
--- a/src/lib/dataPaths.ts
+++ b/src/lib/dataPaths.ts
@@ -70,6 +70,45 @@ export function resolveDataDir({ isCloud = false }: { isCloud?: boolean } = {}):
   return getDefaultDataDir();
 }
 
+/**
+ * Resolve the data directory and guarantee it is writable.
+ *
+ * Unlike {@link resolveDataDir} (a pure, side-effect-free path resolver used by
+ * many callers), this variant probes the resolved directory by attempting to
+ * create it. When a configured `DATA_DIR` is not writable (`EACCES`/`EPERM`),
+ * it falls back to the default user directory so the app keeps working instead
+ * of crashing on an unwritable, operator-supplied path. Any other error (e.g.
+ * `ENOTDIR`, `ENOSPC`) still propagates.
+ *
+ * Use this only at the single startup site that owns directory creation
+ * (currently `db/core.ts`); everywhere else keep using the pure resolver.
+ */
+export function resolveWritableDataDir({ isCloud = false }: { isCloud?: boolean } = {}): string {
+  const resolved = resolveDataDir({ isCloud });
+
+  // Cloud/serverless never owns a writable home dir; leave its sentinel alone.
+  if (isCloud) return resolved;
+
+  // No explicit override → already the default user dir; nothing to fall back to.
+  const configured = normalizeConfiguredPath(process.env.DATA_DIR);
+  if (!configured) return resolved;
+
+  try {
+    fs.mkdirSync(resolved, { recursive: true });
+    return resolved;
+  } catch (err: unknown) {
+    const code = (err as NodeJS.ErrnoException | null)?.code;
+    if (code === "EACCES" || code === "EPERM") {
+      const fallback = getDefaultDataDir();
+      console.warn(
+        `[DATA_DIR] '${resolved}' is not writable (${code}) → falling back to '${fallback}'`
+      );
+      return fallback;
+    }
+    throw err;
+  }
+}
+
 export function isSamePath(a: string | null | undefined, b: string | null | undefined): boolean {
   if (!a || !b) return false;
   const normalizedA = path.resolve(a);
diff --git a/src/lib/db/core.ts b/src/lib/db/core.ts
index 99a04cda7f..261749a814 100644
--- a/src/lib/db/core.ts
+++ b/src/lib/db/core.ts
@@ -13,7 +13,7 @@ import {
 } from "./adapters/driverFactory";
 import path from "path";
 import fs from "fs";
-import { resolveDataDir, getLegacyDotDataDir } from "../dataPaths";
+import { resolveWritableDataDir, getLegacyDotDataDir } from "../dataPaths";
 import { runMigrations } from "./migrationRunner";
 import { runDbHealthCheck } from "./healthCheck";
 import { resetAllDbModuleState } from "./stateReset";
@@ -83,7 +83,7 @@ export const isBuildPhase = process.env.NEXT_PHASE === "phase-production-build";
 
 // ──────────────── Paths ────────────────
 
-export const DATA_DIR = resolveDataDir({ isCloud });
+export const DATA_DIR = resolveWritableDataDir({ isCloud });
 const LEGACY_DATA_DIR = isCloud ? null : getLegacyDotDataDir();
 export const SQLITE_FILE = isCloud ? null : path.join(DATA_DIR, "storage.sqlite");
 const JSON_DB_FILE = isCloud ? null : path.join(DATA_DIR, "db.json");
diff --git a/tests/unit/data-dir-writable-fallback.test.ts b/tests/unit/data-dir-writable-fallback.test.ts
new file mode 100644
index 0000000000..77661b530c
--- /dev/null
+++ b/tests/unit/data-dir-writable-fallback.test.ts
@@ -0,0 +1,115 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+import {
+  resolveWritableDataDir,
+  getDefaultDataDir,
+  resolveDataDir,
+} from "../../src/lib/dataPaths.ts";
+
+// Running as root bypasses POSIX permission bits, so a chmod-based "unwritable"
+// directory would still be writable and the EACCES/EPERM branch never triggers.
+const IS_ROOT = typeof process.getuid === "function" && process.getuid() === 0;
+const IS_WINDOWS = process.platform === "win32";
+
+async function withTempEnv(
+  fn: (paths: { root: string; home: string }) => void | Promise<void>
+) {
+  const originalEnv = { ...process.env };
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), "omni-datadir-"));
+  const home = path.join(root, "home");
+  fs.mkdirSync(home, { recursive: true });
+
+  delete process.env.DATA_DIR;
+  delete process.env.XDG_CONFIG_HOME;
+  delete process.env.APPDATA;
+  process.env.HOME = home;
+  process.env.USERPROFILE = home;
+
+  try {
+    await fn({ root, home });
+  } finally {
+    for (const key of Object.keys(process.env)) {
+      if (!(key in originalEnv)) delete process.env[key];
+    }
+    for (const [key, value] of Object.entries(originalEnv)) {
+      process.env[key] = value;
+    }
+    // Restore perms before cleanup so rmSync can delete read-only parents.
+    try {
+      fs.chmodSync(root, 0o755);
+    } catch {
+      // ignore
+    }
+    fs.rmSync(root, { recursive: true, force: true });
+  }
+}
+
+test("resolveWritableDataDir returns the configured DATA_DIR when it is writable", async () => {
+  await withTempEnv(({ root }) => {
+    const configured = path.join(root, "writable-data");
+    process.env.DATA_DIR = configured;
+
+    const resolved = resolveWritableDataDir();
+    assert.equal(resolved, path.resolve(configured));
+    // The probe creates the directory as a side effect.
+    assert.ok(fs.existsSync(configured));
+  });
+});
+
+test("resolveWritableDataDir falls back to the default dir when DATA_DIR is not writable (EACCES/EPERM)", { skip: IS_ROOT || IS_WINDOWS }, async () => {
+  await withTempEnv(({ root, home }) => {
+    // A read-only parent makes mkdir of the child fail with EACCES/EPERM.
+    const lockedParent = path.join(root, "locked");
+    fs.mkdirSync(lockedParent, { recursive: true });
+    fs.chmodSync(lockedParent, 0o555);
+
+    const configured = path.join(lockedParent, "data");
+    process.env.DATA_DIR = configured;
+
+    const resolved = resolveWritableDataDir();
+    const expectedFallback = getDefaultDataDir();
+
+    // It must NOT return the unwritable configured dir...
+    assert.notEqual(resolved, path.resolve(configured));
+    // ...and instead fall back to the default user dir (~/.omniroute under HOME).
+    assert.equal(resolved, expectedFallback);
+    assert.ok(resolved.startsWith(path.resolve(home)));
+  });
+});
+
+test("resolveWritableDataDir returns the default dir (no probe) when DATA_DIR is unset", async () => {
+  await withTempEnv(() => {
+    delete process.env.DATA_DIR;
+    const resolved = resolveWritableDataDir();
+    assert.equal(resolved, getDefaultDataDir());
+    // Matches the pure resolver when no override is present.
+    assert.equal(resolved, resolveDataDir());
+  });
+});
+
+test("resolveWritableDataDir rethrows non-permission errors", { skip: IS_WINDOWS }, async () => {
+  await withTempEnv(({ root }) => {
+    // Point DATA_DIR at a path whose parent is a regular file → ENOTDIR, not EACCES.
+    const fileParent = path.join(root, "iam-a-file");
+    fs.writeFileSync(fileParent, "x");
+
+    const configured = path.join(fileParent, "data");
+    process.env.DATA_DIR = configured;
+
+    assert.throws(() => resolveWritableDataDir(), (err: NodeJS.ErrnoException) => {
+      return err.code !== "EACCES" && err.code !== "EPERM";
+    });
+  });
+});
+
+test("resolveWritableDataDir leaves the cloud sentinel untouched", async () => {
+  await withTempEnv(() => {
+    process.env.DATA_DIR = "/some/configured/path";
+    const resolved = resolveWritableDataDir({ isCloud: true });
+    assert.equal(resolved, "/tmp");
+  });
+});