diff --git a/scripts/devServerAuth.test.ts b/scripts/devServerAuth.test.ts
index d6a7f707fb3..c10b1fa6431 100644
--- a/scripts/devServerAuth.test.ts
+++ b/scripts/devServerAuth.test.ts
@@ -1,10 +1,20 @@
-import { afterEach, beforeAll, beforeEach, describe, expect, test } from "bun:test";
+import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, test } from "bun:test";
+import * as fsSync from "node:fs";
 import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import * as path from "node:path";
 
 const REPO_ROOT = path.resolve(import.meta.dir, "..");
 
+// make claims the default socket folder at parse time, in `$XDG_RUNTIME_DIR` or /tmp. A dry run must
+// not leave one on the test host, so every make call here gets a runtime dir of its own.
+//
+// The base is under /tmp, not os.tmpdir(): macOS's os.tmpdir() is a long /var/folders/... path, and
+// `<base>/xum-dev-<uid>/b3111-192.168.1.5.sock` must fit the 103 bytes of a macOS socket path.
+const SHORT_TMP = process.platform === "win32" ? os.tmpdir() : "/tmp";
+const RUNTIME_BASE = fsSync.mkdtempSync(path.join(SHORT_TMP, "xum-make-base-"));
+afterAll(() => fsSync.rmSync(RUNTIME_BASE, { recursive: true, force: true }));
+
 interface DryRun {
   /** XUM_BACKEND_SOCKET as make exports it to the recipe's processes (env, not secret). */
   socket: string | undefined;
@@ -23,7 +33,12 @@ async function dryRun(
 ): Promise<DryRun> {
   const proc = Bun.spawn(["make", "-pn", ...makeFlags, goal], {
     cwd: REPO_ROOT,
-    env: { PATH: process.env.PATH ?? "", HOME: process.env.HOME ?? "", ...env },
+    env: {
+      PATH: process.env.PATH ?? "",
+      HOME: process.env.HOME ?? "",
+      XDG_RUNTIME_DIR: RUNTIME_BASE,
+      ...env,
+    },
     stdout: "pipe",
     stderr: "pipe",
   });
@@ -250,7 +265,7 @@ describe.skipIf(process.platform === "win32")("make dev-server backend socket",
 
   test("the backend listens on the exported path, and the path fits sun_path", async () => {
     const socket = exportedSocket(socketRun);
-    expect(socket.startsWith("/")).toBe(true);
+    expect(socket.startsWith(`${RUNTIME_BASE}${path.sep}`)).toBe(true);
     expect(socket).toMatch(/b3111-127\.0\.0\.1\.sock$/);
     expect(Buffer.byteLength(socket)).toBeLessThanOrEqual(100);
     expect(await backendSocketArgument(socketRun)).toBe(socket);
@@ -296,7 +311,7 @@ describe.skipIf(process.platform === "win32")("make dev-server backend socket",
     let runtimeDir: string;
 
     beforeEach(async () => {
-      runtimeDir = await fs.mkdtemp(path.join(os.tmpdir(), "xum-make-test-"));
+      runtimeDir = await fs.mkdtemp(path.join(SHORT_TMP, "xum-make-test-"));
     });
 
     afterEach(async () => {
@@ -344,26 +359,44 @@ describe.skipIf(process.platform === "win32")("make dev-server backend socket",
       expect(await backendSocketArgument(run)).toBe(socket);
     });
 
-    test("make stops with a message when the parent cannot be trusted", async () => {
-      const shared = path.join(runtimeDir, "shared");
-      await fs.mkdir(shared);
-      await fs.chmod(shared, 0o777);
+    /** `make -pn dev-server` that is expected to stop: its exit code and stderr. */
+    async function makeStops(xdgRuntimeDir: string) {
       const proc = Bun.spawn(["make", "-pn", "dev-server"], {
         cwd: REPO_ROOT,
         env: {
           PATH: process.env.PATH ?? "",
           HOME: process.env.HOME ?? "",
-          XDG_RUNTIME_DIR: shared,
+          XDG_RUNTIME_DIR: xdgRuntimeDir,
         },
         stdout: "pipe",
         stderr: "pipe",
       });
       const [exitCode, stderr] = await Promise.all([proc.exited, new Response(proc.stderr).text()]);
+      return { exitCode, stderr };
+    }
+
+    test("make stops with a message when the parent cannot be trusted", async () => {
+      const shared = path.join(runtimeDir, "shared");
+      await fs.mkdir(shared);
+      await fs.chmod(shared, 0o777);
+      const { exitCode, stderr } = await makeStops(shared);
       expect(exitCode).not.toBe(0);
       expect(stderr).toContain("XUM_BACKEND_SOCKET");
       expect(stderr).toContain("XDG_RUNTIME_DIR");
       expect(await fs.readdir(shared)).toEqual([]);
     });
+
+    // The backend refuses a path that does not fit sun_path, so a folder made for it would be an
+    // empty folder for nothing: make stops first, and creates nothing.
+    test("make stops with a message when the default path is too long, and creates nothing", async () => {
+      const deep = path.join(runtimeDir, "d".repeat(100));
+      await fs.mkdir(deep, { mode: 0o700 });
+      const { exitCode, stderr } = await makeStops(deep);
+      expect(exitCode).not.toBe(0);
+      expect(stderr).toMatch(/too long/);
+      expect(stderr).toContain("XUM_BACKEND_SOCKET");
+      expect(await fs.readdir(deep)).toEqual([]);
+    });
   });
 
   test("Windows ignores an ambient XUM_BACKEND_SOCKET", () => {
diff --git a/src/node/utils/privateUnixSocket.test.ts b/src/node/utils/privateUnixSocket.test.ts
index b4dae5afbd4..ceaf69fe5b5 100644
--- a/src/node/utils/privateUnixSocket.test.ts
+++ b/src/node/utils/privateUnixSocket.test.ts
@@ -5,7 +5,7 @@ import * as net from "node:net";
 import * as os from "node:os";
 import * as path from "node:path";
 import { acquireProcessFileLock } from "@/node/utils/concurrency/fileLock";
-import { withPrivateUnixSocketPath } from "./privateUnixSocket";
+import { assertLockPathUsable, withPrivateUnixSocketPath } from "./privateUnixSocket";
 import { MAX_UNIX_SOCKET_PATH_BYTES } from "./privateUnixSocketFolder";
 
 const posix = process.platform !== "win32";
@@ -337,12 +337,22 @@ describe.skipIf(!posix)("withPrivateUnixSocketPath", () => {
       expect((await fs.lstat(`${socketPath}.lock`)).isSymbolicLink()).toBe(true);
     });
 
+    // Called on its own: through withPrivateUnixSocketPath a foreign uid fails the folder check first,
+    // and only root could make a file of another user inside our private folder. The file here is
+    // ours, and the expected owner is not.
     test("refuses a lock file that another user owns", async () => {
       await fs.mkdir(dir, { mode: 0o700 });
-      await fs.writeFile(`${socketPath}.lock`, "someone else", { mode: 0o600 });
-      await expect(prepare(socketPath, process.getuid!() + 1)).rejects.toThrow(
+      const lockPath = `${socketPath}.lock`;
+      await fs.writeFile(lockPath, "someone else", { mode: 0o600 });
+      await expect(assertLockPathUsable(lockPath, process.getuid!() + 1)).rejects.toThrow(
         /owned by another user/
       );
+      await assertLockPathUsable(lockPath, process.getuid!());
+    });
+
+    test("accepts a missing lock file", async () => {
+      await fs.mkdir(dir, { mode: 0o700 });
+      await assertLockPathUsable(`${socketPath}.lock`, process.getuid!());
     });
 
     test("refuses a lock path that is a folder", async () => {
diff --git a/src/node/utils/privateUnixSocket.ts b/src/node/utils/privateUnixSocket.ts
index b46e344bba6..5ed19204768 100644
--- a/src/node/utils/privateUnixSocket.ts
+++ b/src/node/utils/privateUnixSocket.ts
@@ -56,7 +56,7 @@ async function removeStaleSocket(socketPath: string, ownerUid: number): Promise<
 }
 
 /** The lock file must be a regular file of ours: a symlink would send the lock somewhere else. */
-async function assertLockPathUsable(lockPath: string, ownerUid: number): Promise<void> {
+export async function assertLockPathUsable(lockPath: string, ownerUid: number): Promise<void> {
   let entry;
   try {
     entry = await fs.lstat(lockPath);
diff --git a/src/node/utils/privateUnixSocketFolder.test.ts b/src/node/utils/privateUnixSocketFolder.test.ts
index 346daec5805..b33a9e8fec4 100644
--- a/src/node/utils/privateUnixSocketFolder.test.ts
+++ b/src/node/utils/privateUnixSocketFolder.test.ts
@@ -47,6 +47,20 @@ describe.skipIf(!posix)("chooseDevSocketPath", () => {
     expect(stat.mode & 0o777).toBe(0o700);
   });
 
+  test("a default path that cannot fit is refused before the folder is created", async () => {
+    const error = await chooseDevSocketPath(
+      preferred,
+      "s".repeat(MAX_UNIX_SOCKET_PATH_BYTES),
+      uid
+    ).then(
+      () => null,
+      (e: unknown) => e
+    );
+    expect(error).toBeInstanceOf(Error);
+    expect((error as Error).message).toMatch(/too long/);
+    expect(await fs.readdir(base)).toEqual([]);
+  });
+
   test("a private folder of ours is the answer", async () => {
     await fs.mkdir(preferred, { mode: 0o700 });
     const choice = await chooseDevSocketPath(preferred, "b1.sock", uid);
diff --git a/src/node/utils/privateUnixSocketFolder.ts b/src/node/utils/privateUnixSocketFolder.ts
index 7c5c16d93fc..1c085b40127 100644
--- a/src/node/utils/privateUnixSocketFolder.ts
+++ b/src/node/utils/privateUnixSocketFolder.ts
@@ -130,6 +130,7 @@ const MKDTEMP_SUFFIX = ".XXXXXX";
  * caller to tell the user. The Vite hop never falls back to TCP: the squat window that the socket
  * closes would open again.
  *
+ * Throws, before it creates anything, when the default path does not fit a socket path.
  * Throws when a parent of the folder is unsafe: a fresh folder next to it would be just as open to
  * another account, so only the user can fix it (XUM_BACKEND_SOCKET or XDG_RUNTIME_DIR). It also
  * throws, before it creates the fresh folder, when the longer fallback path would not fit.
@@ -140,6 +141,9 @@ export async function chooseDevSocketPath(
   ownerUid: number
 ): Promise<DevSocketChoice> {
   const preferred = path.join(folder, fileName);
+  // Before anything is created: the backend refuses a path that does not fit, and the folder made
+  // for it would stay empty.
+  assertSocketPathFits(preferred);
   await assertParentsTrusted(folder, ownerUid);
   try {
     await fs.mkdir(folder, { mode: 0o700 });