fix(bin): report unsupported Windows ARM64 fallback clearly

.changeset/windows-arm64-launcher-error.md

@@ -0,0 +1,5 @@
+---
+"hunkdiff": patch
+---
+
+Report that Windows ARM64 is not supported by the current bundled runtime instead of crashing with a low-context Bun FFI error.

packages/hunk/bin/hunk.cjs

@@ -102,6 +102,21 @@ function bundledBunRuntime() {
   }
 }
 
+// Exits on Windows ARM64 before bundled Bun starts OpenTUI. Bun disables TinyCC there
+// (oven-sh/bun#28055). Callers honor HUNK_BIN_PATH and any installed prebuilt first.
+function rejectUnsupportedWindowsArm64Runtime() {
+  if (os.platform() !== "win32" || os.arch() !== "arm64") {
+    return;
+  }
+
+  console.error(
+    "hunk: Windows ARM64 is not supported by the current bundled runtime. " +
+      "Bun disables TinyCC on this platform, so OpenTUI cannot load. " +
+      "Tracking: oven-sh/bun#28055",
+  );
+  process.exit(1);
+}
+
 const forwardedArgs = process.argv.slice(2);
 if (forwardedArgs.length === 2 && forwardedArgs[0] === "skill" && forwardedArgs[1] === "path") {
   const skillPath = bundledSkillPath();
@@ -125,6 +140,8 @@ if (prebuiltBinary) {
   run(prebuiltBinary, forwardedArgs);
 }
 
+rejectUnsupportedWindowsArm64Runtime();
+
 const bunBinary = bundledBunRuntime();
 if (bunBinary) {
   const entrypoint = path.join(__dirname, "..", "dist", "npm", "main.js");

test/cli/entrypoint.test.ts

@@ -272,6 +272,62 @@ describe("CLI entrypoint contracts", () => {
     }
   });
 
+  test("bin wrapper reports Windows ARM64 before the bundled Bun fallback", () => {
+    const tempDir = mkdtempSync(join(tmpdir(), "hunk-wrapper-win-arm64-"));
+    const packageRoot = join(tempDir, "package");
+    const tempWrapperPath = join(packageRoot, "bin", "hunk.cjs");
+    const preloadPath = join(tempDir, "os-preload.cjs");
+    const bunMarkerPath = join(tempDir, "bun-invoked");
+    const fakeBunDir = join(packageRoot, "node_modules", "bun", "bin");
+    const fakeBunPath = join(fakeBunDir, "bun.exe");
+
+    try {
+      mkdirSync(join(packageRoot, "bin"), { recursive: true });
+      mkdirSync(fakeBunDir, { recursive: true });
+      copyFileSync(join(process.cwd(), "packages", "hunk", "bin", "hunk.cjs"), tempWrapperPath);
+      writeFileSync(
+        join(packageRoot, "node_modules", "bun", "package.json"),
+        JSON.stringify({ name: "bun", version: "0.0.0" }),
+      );
+      // Resolvable bundled runtime. Records a marker only if the wrapper actually spawns it.
+      writeFileSync(
+        fakeBunPath,
+        `#!/usr/bin/env node\nrequire("node:fs").writeFileSync(${JSON.stringify(bunMarkerPath)}, "invoked\\n");\n`,
+        { mode: 0o755 },
+      );
+      writeFileSync(
+        preloadPath,
+        'const os = require("node:os");\nos.platform = () => "win32";\nos.arch = () => "arm64";\n',
+      );
+
+      const env = { ...process.env };
+      delete env.HUNK_BIN_PATH;
+
+      const proc = Bun.spawnSync(["node", "-r", preloadPath, tempWrapperPath, "diff"], {
+        cwd: packageRoot,
+        stdin: "ignore",
+        stdout: "pipe",
+        stderr: "pipe",
+        env,
+      });
+
+      const stdout = Buffer.from(proc.stdout).toString("utf8");
+      const stderr = Buffer.from(proc.stderr).toString("utf8");
+
+      expect(proc.exitCode).toBe(1);
+      expect(stdout).toBe("");
+      expect(stderr).toContain("Windows ARM64 is not supported by the current bundled runtime");
+      expect(stderr).toContain("oven-sh/bun#28055");
+      expect(stderr).not.toContain("bun:ffi");
+      expect(stderr).not.toContain("dlopen");
+      expect(stderr).not.toContain("Bun v");
+      expect(stderr).not.toContain("Unsupported platform for prebuilt Hunk binaries");
+      expect(existsSync(bunMarkerPath)).toBe(false);
+    } finally {
+      rmSync(tempDir, { recursive: true, force: true });
+    }
+  });
+
   test("general pager mode falls back to plain text for non-diff stdin", () => {
     const proc = Bun.spawnSync(["bun", "run", "packages/hunk/src/main.tsx", "pager"], {
       cwd: process.cwd(),