diff --git a/packages/codev-sandbox/script/build-vzhelper.ts b/packages/codev-sandbox/script/build-vzhelper.ts
index 95a1ace699..273f9cedd8 100644
--- a/packages/codev-sandbox/script/build-vzhelper.ts
+++ b/packages/codev-sandbox/script/build-vzhelper.ts
@@ -14,12 +14,12 @@ const universal = process.argv.includes("--universal")
 
 const slices = await (async () => {
   if (!universal) {
-    await $`swift build -c release --package-path ${root}`
+    await swiftBuild([])
     return [`${root}.build/release/codev-vmm`]
   }
   const triples = ["arm64-apple-macosx", "x86_64-apple-macosx"]
   for (const triple of triples) {
-    await $`swift build -c release --package-path ${root} --triple ${triple}`
+    await swiftBuild(["--triple", triple])
   }
   return triples.map((triple) => `${root}.build/${triple}/release/codev-vmm`)
 })()
@@ -31,3 +31,17 @@ else await $`lipo -create ${slices} -output ${root}dist/codev-vmm`
 // fat Mach-O, and lipo would strip a per-slice signature anyway.
 await $`codesign --force --sign - --entitlements ${root}vzhelper.entitlements ${root}dist/codev-vmm`
 console.log(`built ${root}dist/codev-vmm${universal ? " (universal)" : ""}`)
+
+// Swift 6.4 made the swiftbuild backend the default, and on Command Line
+// Tools setups it fails before compiling anything ("Could not initialize
+// build system ... Unknown error parsing property list"). The native backend
+// still builds this package, so a failed build retries with it: toolchains
+// whose default works never see the retry, and the flag stays out of the
+// first attempt in case a later Swift drops it.
+async function swiftBuild(extra: string[]) {
+  const args = ["build", "-c", "release", "--package-path", root, ...extra]
+  const first = await $`swift ${args}`.nothrow()
+  if (first.exitCode === 0) return
+  console.warn("swift build failed with the default build system; retrying with --build-system native")
+  await $`swift ${args} --build-system native`
+}
diff --git a/packages/codev-sandbox/src/vz/provider.test.ts b/packages/codev-sandbox/src/vz/provider.test.ts
index 3b7b686743..503e8a1ec5 100644
--- a/packages/codev-sandbox/src/vz/provider.test.ts
+++ b/packages/codev-sandbox/src/vz/provider.test.ts
@@ -211,6 +211,18 @@ test("surfaces helper error events with the stderr tail", async () => {
   await rm(dir, { recursive: true, force: true })
 })
 
+test("fails the start, instead of throwing, when the helper binary is missing", async () => {
+  const dir = await mkdtemp(join(tmpdir(), "codev-vz-"))
+  const helperPath = join(dir, "codev-vmm")
+  // The real spawn: a missing binary emits "error" and never "exit".
+  const { options, projectDir } = await preparedOptions(dir, FAKE_HELPER, { helperPath, spawnImpl: spawn })
+  const provider = createVzProvider(options)
+  await expect(provider.start({ ...config, projectDir })).rejects.toThrow(
+    `sandbox VM helper is not installed (expected at ${helperPath})`,
+  )
+  await rm(dir, { recursive: true, force: true })
+})
+
 test("times out when the helper never reaches running", async () => {
   const dir = await mkdtemp(join(tmpdir(), "codev-vz-"))
   const prepared = await preparedOptions(dir, SILENT_HELPER, { startTimeoutMs: 300 })
diff --git a/packages/codev-sandbox/src/vz/provider.ts b/packages/codev-sandbox/src/vz/provider.ts
index e3cd349819..27b355c601 100644
--- a/packages/codev-sandbox/src/vz/provider.ts
+++ b/packages/codev-sandbox/src/vz/provider.ts
@@ -213,6 +213,17 @@ export function createVzProvider(options: VzProviderOptions): SandboxProvider {
       const running = new Promise<"running">((resolve) => {
         handlers.running = () => resolve("running")
       })
+      // A helper that cannot be spawned (not built, deleted, not executable)
+      // emits "error" and never "exit". Unhandled, that event is an uncaught
+      // exception — in the desktop's main process, a crash dialog at launch
+      // when a sandbox autostarts — so it fails this start instead.
+      child.once("error", (err: NodeJS.ErrnoException) => {
+        handlers.error?.(
+          err.code === "ENOENT"
+            ? `sandbox VM helper is not installed (expected at ${options.helperPath})`
+            : `could not run the sandbox VM helper: ${err.message}`,
+        )
+      })
       child.stdout?.on(
         "data",
         createVzHelperEventReader((event: VzHelperEvent) => {