diff --git a/lib/bin-context.ts b/lib/bin-context.ts
index faa1c65a2..d7edaa5a9 100644
--- a/lib/bin-context.ts
+++ b/lib/bin-context.ts
@@ -6,10 +6,37 @@
  */
 
 import { spawnSync } from "child_process";
+import type { SpawnSyncOptionsWithStringEncoding, SpawnSyncReturns } from "child_process";
+
+/**
+ * Spawn one of gstack's own `#!/usr/bin/env bash` bins.
+ *
+ * Windows has no shebang support, so `spawnSync(bin, ...)` on an extensionless
+ * bash script fails with ENOENT. `shell: true` does not help either: that routes
+ * through cmd.exe, which reports "is not recognized as an internal or external
+ * command". Only spawning `bash` with the script as argv[0] actually starts it.
+ *
+ * (`shell: true` IS correct for `gbrain` — see NEEDS_SHELL_ON_WINDOWS in
+ * lib/gbrain-exec.ts — but only because gbrain ships a `.cmd` shim on Windows,
+ * which cmd.exe can run. gstack's own bins have no such shim.)
+ *
+ * Tries the direct spawn first so POSIX keeps its existing behaviour untouched,
+ * and only falls back to bash when the direct spawn produced nothing.
+ */
+export function spawnBashBin(
+  binPath: string,
+  args: string[] = [],
+  opts: Partial<SpawnSyncOptionsWithStringEncoding> = {},
+): SpawnSyncReturns<string> {
+  const options = { encoding: "utf-8" as const, ...opts };
+  const direct = spawnSync(binPath, args, options);
+  if (!direct.error && direct.stdout != null) return direct;
+  return spawnSync("bash", [binPath, ...args], options);
+}
 
 /** Resolve the project slug via the `gstack-slug` helper (parses `SLUG=...`). */
 export function resolveSlug(slugBinPath: string): string {
-  const r = spawnSync(slugBinPath, { encoding: "utf-8" });
+  const r = spawnBashBin(slugBinPath);
   const m = (r.stdout || "").match(/^SLUG=(.+)$/m);
   return m ? m[1].trim() : "unknown";
 }