diff --git a/packages/bcode-browser/src/browser-execute.ts b/packages/bcode-browser/src/browser-execute.ts
index 944673dff..e5bc164d2 100644
--- a/packages/bcode-browser/src/browser-execute.ts
+++ b/packages/bcode-browser/src/browser-execute.ts
@@ -92,6 +92,13 @@ export const make = Effect.fn("BrowserExecute.make")(function* () {
   const execute = (args: Parameters, ctx: ExecuteContext) =>
     Effect.gen(function* () {
       const harnessDir = yield* Effect.promise(() => resolveHarnessDir())
+      // Pre-flight check on harnessDir: spawn ENOENT on a missing cwd surfaces
+      // with `path: "uv"` on Bun/Windows, which is indistinguishable from a
+      // truly-missing uv. Catch it here so the user gets the real cause
+      // instead of a misleading "uv not on PATH" hint.
+      if (!(yield* Effect.promise(() => fs.access(harnessDir).then(() => true, () => false)))) {
+        return yield* Effect.fail(new Error(`harness directory not found at ${harnessDir} — bcode build is broken; please reinstall`))
+      }
       yield* Effect.promise(() => fs.mkdir(ctx.bhTmpDir, { recursive: true }))
       const uv = yield* locate
       const proc = ChildProcess.make(
diff --git a/packages/bcode-browser/src/harness.ts b/packages/bcode-browser/src/harness.ts
index f75d1d07e..8d3b03e6d 100644
--- a/packages/bcode-browser/src/harness.ts
+++ b/packages/bcode-browser/src/harness.ts
@@ -37,7 +37,15 @@ import path from "path"
 import { fileURLToPath } from "url"
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
-const isCompiled = __dirname.startsWith("/$bunfs/") || __dirname.startsWith("B:/~BUN/")
+// Bun's bunfs root is `/$bunfs/` on POSIX and `B:\~BUN\` on Windows (native
+// separators). Normalize before comparing so the compiled-mode check works on
+// both platforms — without this, the Windows compiled binary falls through to
+// DEV_HARNESS_DIR (which doesn't exist on the user's machine) and every
+// subsequent spawn fails with a misleading uv-missing error.
+const isCompiled = (() => {
+  const d = __dirname.replaceAll("\\", "/")
+  return d.startsWith("/$bunfs/") || d.startsWith("B:/~BUN/")
+})()
 const DEV_HARNESS_DIR = path.resolve(__dirname, "..", "harness")
 const cachedHarnessDir = path.join(os.homedir(), ".cache", "bcode", "harness")