diff --git a/scripts/dev-runner.test.ts b/scripts/dev-runner.test.ts
index 11b702e33705..9383d2c3abe7 100644
--- a/scripts/dev-runner.test.ts
+++ b/scripts/dev-runner.test.ts
@@ -3,6 +3,7 @@ import * as NodeServices from "@effect/platform-node/NodeServices";
 import * as NodeFS from "node:fs";
 import * as NodeOS from "node:os";
 import * as NodePath from "node:path";
+import * as NodeSqlite from "node:sqlite";
 import * as NetService from "@t3tools/shared/Net";
 import * as HostProcess from "@t3tools/shared/HostProcess";
 import { assert, describe, it } from "@effect/vitest";
@@ -395,6 +396,46 @@ it.layer(NodeServices.layer)("dev-runner", (it) => {
       });
     });
 
+    // The server's `node --watch` parent outlives a refused start, and its next
+    // restart races the running owner for the lock, so refusal must happen
+    // before anything is spawned.
+    it.effect("refuses to start when another server owns the state directory", () => {
+      let spawned = false;
+      const spawnerLayer = Layer.succeed(
+        ChildProcessSpawner.ChildProcessSpawner,
+        ChildProcessSpawner.make(() => {
+          spawned = true;
+          return Effect.succeed(mockProcess(0));
+        }),
+      );
+
+      return Effect.gen(function* () {
+        const t3Home = NodeFS.mkdtempSync(NodePath.join(NodeOS.tmpdir(), "t3code-dev-owner-"));
+        const stateDir = NodePath.join(t3Home, "userdata");
+        NodeFS.mkdirSync(stateDir, { recursive: true });
+        const owner = new NodeSqlite.DatabaseSync(NodePath.join(stateDir, "server-owner.sqlite"));
+        owner.exec("BEGIN EXCLUSIVE;");
+
+        const error = yield* runDevRunnerWithInput({ ...devServerInput, t3Home }).pipe(
+          Effect.provide(Layer.mergeAll(emptyConfigLayer, netServiceLayer, spawnerLayer)),
+          Effect.provideService(HostProcess.Platform, "linux"),
+          Effect.flip,
+          Effect.ensuring(
+            Effect.sync(() => {
+              owner.close();
+              NodeFS.rmSync(t3Home, { recursive: true, force: true });
+            }),
+          ),
+        );
+
+        if (error._tag !== "DevRunnerStateDirOwnedError") {
+          assert.fail(`Unexpected error: ${error._tag}`);
+        }
+        assert.equal(error.stateDir, stateDir);
+        assert.isFalse(spawned);
+      });
+    });
+
     // `tailscale serve` config outlives the process, so a dry run that shared
     // would replace and then tear down whatever mapping the port already had.
     it.effect("ignores an ambient T3CODE_HOME when no home is resolved", () =>
diff --git a/scripts/dev-runner.ts b/scripts/dev-runner.ts
index 256064d85553..6bf9957aabee 100644
--- a/scripts/dev-runner.ts
+++ b/scripts/dev-runner.ts
@@ -1,6 +1,7 @@
 #!/usr/bin/env node
 
 import * as NodeOS from "node:os";
+import * as NodeSqlite from "node:sqlite";
 
 import * as NodeRuntime from "@effect/platform-node/NodeRuntime";
 import * as NodeServices from "@effect/platform-node/NodeServices";
@@ -182,6 +183,15 @@ export class DevRunnerProcessExitError extends Schema.TaggedError<DevRunnerProce
   }
 }
 
+export class DevRunnerStateDirOwnedError extends Schema.TaggedError<DevRunnerStateDirOwnedError>()(
+  "DevRunnerStateDirOwnedError",
+  { stateDir: Schema.String },
+) {
+  override get message(): string {
+    return `A T3 Code server already owns ${this.stateDir}. Stop that server, or pass --home-dir to use a separate T3 home, then retry. Nothing was started.`;
+  }
+}
+
 export class DevRunnerHostNotProxiableError extends Schema.TaggedError<DevRunnerHostNotProxiableError>()(
   "DevRunnerHostNotProxiableError",
   {
@@ -680,6 +690,31 @@ const prepareFirstStart = Effect.fn("dev-runner.prepareFirstStart")(function* (i
   }
 });
 
+/**
+ * Mirrors the server's ownership lock (apps/server/src/serverOwnership.ts).
+ * A refused server under `node --watch` leaves its watcher alive, and the
+ * next file change restarts it into a race with the running owner.
+ */
+const isStateDirOwned = Effect.fn("dev-runner.isStateDirOwned")(function* (stateDir: string) {
+  const path = yield* Path.Path;
+  const fs = yield* FileSystem.FileSystem;
+  const lockPath = path.join(stateDir, "server-owner.sqlite");
+  if (!(yield* fs.exists(lockPath).pipe(Effect.orElseSucceed(() => false)))) return false;
+  return yield* Effect.sync(() => {
+    const db = new NodeSqlite.DatabaseSync(lockPath);
+    try {
+      db.exec("PRAGMA busy_timeout = 0; BEGIN EXCLUSIVE;");
+      db.exec("ROLLBACK;");
+      return false;
+    } catch (cause) {
+      if (cause instanceof Error && "errcode" in cause && cause.errcode === 5) return true;
+      throw cause;
+    } finally {
+      db.close();
+    }
+  });
+});
+
 interface DevRunnerCliInput {
   readonly mode: DevMode;
   readonly t3Home: string | undefined;
@@ -772,6 +807,19 @@ export const runDevRunnerWithInput = Effect.fn("dev-runner.runDevRunnerWithInput
     `[dev-runner] mode=${input.mode} source=${source}${selectionSuffix} serverPort=${String(env.T3CODE_PORT)} webPort=${String(env.PORT)} baseDir=${baseDir}`,
   );
 
+  // Mirrors the server's deriveServerPaths: an explicit home uses userdata.
+  const stateDir = (yield* Path.Path).join(
+    baseDir,
+    env.T3CODE_HOME === undefined ? "dev" : "userdata",
+  );
+  if (
+    !input.dryRun &&
+    (input.mode === "dev" || input.mode === "dev:server") &&
+    (yield* isStateDirOwned(stateDir))
+  ) {
+    return yield* new DevRunnerStateDirOwnedError({ stateDir });
+  }
+
   // Before the share block: --dry-run only resolves and prints. Sharing would
   // replace, then tear down, whatever mapping the port already had — a
   // surprising side effect from a command documented as inert.