fix(runtime): treat cross-realm Error as an error on exit + thrown paths (Ink parity) (#86)

app.exit(crossRealmError) resolved waitUntilExit() with the error as a result value,
and a component-thrown cross-realm Error was re-wrapped (losing the original) -- both
because `instanceof Error` fails across VM realms. Adds Ink's isErrorInput check
(instanceof Error || Object.prototype.toString.call(v) === "[object Error]", the
realm-independent [object Error] brand) and uses it at the 3 exit() classification
sites and the 2 error-boundary normalization sites. A cross-realm Error now rejects
(exit) / is preserved (throw), matching Ink; genuine non-Error values still resolve /
still wrap.

Flips the previously-locked "cross-realm resolves" test to Ink's reject (the behavior
was never in ink-divergences.md, so per the repo rule it was an unverified gap, not a
sanctioned divergence). Adds component-throw cross-realm + a non-Error-still-wraps guard.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime-tests/integration/lifecycle/error-handling.test.tsx

@@ -103,6 +103,64 @@ test("error in component triggered after mount routes through exit", async () =>
   await expect(waitUntilExit()).rejects.toThrow("post-mount boom");
 });
 
+test("component-thrown cross-realm Error preserves the original (not re-wrapped)", async () => {
+  // A cross-realm Error (created in a different VM context) is a genuine Error
+  // but fails `instanceof Error` because its prototype comes from the other
+  // realm. The error-boundary path must NOT re-wrap it into
+  // `new Error(String(foreignError))` (which would yield "Error: boom" and lose
+  // the original identity). It uses the same isErrorInput brand check as exit(),
+  // so the ORIGINAL foreign Error rejects waitUntilExit() — matching Ink's
+  // ErrorBoundary, which rejects with the thrown value itself.
+  const vm = await import("node:vm");
+  const foreignError = vm.runInNewContext("new Error('boom')") as Error;
+
+  const trigger = shallowRef(false);
+  const App = defineComponent(() => {
+    return () => {
+      if (trigger.value) throw foreignError;
+      return <Text>ok</Text>;
+    };
+  });
+
+  const { waitUntilExit, lastFrame } = await render(App);
+  expect(lastFrame()).toContain("ok");
+
+  trigger.value = true;
+  await nextTick();
+  await nextTick();
+  await Promise.resolve();
+
+  // Same identity (not a re-wrapped copy) and the original message "boom"
+  // (NOT "Error: boom" that re-wrapping would produce).
+  await expect(waitUntilExit()).rejects.toBe(foreignError);
+  await expect(waitUntilExit()).rejects.toMatchObject({ message: "boom" });
+});
+
+test("component-thrown non-Error value is still wrapped into an Error", async () => {
+  // Guard: a true non-Error throw must still be wrapped into a real Error so the
+  // exit/ErrorOverview machinery always receives an Error. Only the cross-realm
+  // Error case is preserved; this case is unchanged.
+  const trigger = shallowRef(false);
+  const App = defineComponent(() => {
+    return () => {
+      // eslint-disable-next-line no-throw-literal -- exercising a non-Error throw on purpose
+      if (trigger.value) throw "plain";
+      return <Text>ok</Text>;
+    };
+  });
+
+  const { waitUntilExit, lastFrame } = await render(App);
+  expect(lastFrame()).toContain("ok");
+
+  trigger.value = true;
+  await nextTick();
+  await nextTick();
+  await Promise.resolve();
+
+  await expect(waitUntilExit()).rejects.toBeInstanceOf(Error);
+  await expect(waitUntilExit()).rejects.toMatchObject({ message: "plain" });
+});
+
 // --- Ink error validation tests ---
 
 test("fail when Box nested inside Text", async () => {

packages/runtime-tests/integration/lifecycle/exit.test.tsx

@@ -408,12 +408,13 @@ test("waitUntilExit resolves FIRST exit value when exit is re-entered during unm
   expect(result).toBe("first");
 });
 
-test("exit with cross-realm Error resolves after stdout write callback", async () => {
-  // vue-tui uses `instanceof Error` to distinguish errors from result values.
-  // A cross-realm Error (created in a different VM context) fails the
-  // instanceof check, so it is treated as a result value and resolves
-  // rather than rejecting. This differs from Ink which rejects. The test
-  // verifies the write-callback timing: resolution waits for the barrier.
+test("exit with cross-realm Error rejects after stdout write callback", async () => {
+  // A cross-realm Error (created in a different VM context) is a genuine Error
+  // but fails `instanceof Error` because its prototype comes from the other
+  // realm. We classify exit() input with Ink's isErrorInput (instanceof OR the
+  // [object Error] brand check), so a cross-realm Error REJECTS waitUntilExit()
+  // — matching Ink. The test also verifies the write-callback timing:
+  // resolution/rejection waits for the stdout write barrier to flush.
   const vm = await import("node:vm");
   let writeCallbackFired = false;
   let barrierWriteCallbackFired = false;
@@ -450,10 +451,11 @@ test("exit with cross-realm Error resolves after stdout write callback", async (
   const { stream: stdin } = makeFakeStdin();
   app.mount({ stdout, stdin, stderr, exitOnCtrlC: false });
 
-  // Cross-realm Error fails instanceof check, so exit resolves with the
-  // error object as a value instead of rejecting.
-  const result = await app.waitUntilExit();
-  expect(result).toBe(foreignError);
+  // Cross-realm Error is detected via the [object Error] brand check, so exit
+  // rejects with the foreign error (matching Ink) rather than resolving it as
+  // a value.
+  await expect(app.waitUntilExit()).rejects.toBe(foreignError);
+  await expect(app.waitUntilExit()).rejects.toMatchObject({ message: "boom" });
   expect(writeCallbackFired).toBe(true);
   expect(barrierWriteCallbackFired).toBe(true);
 });

packages/runtime/src/render.ts

@@ -161,6 +161,20 @@ function shouldClearTerminalForFrame(opts: {
 // the entry and removes it on teardown; a "no-op" second mount never touches it.
 const liveInstances = new WeakMap<NodeJS.WriteStream, TuiApp>();
 
+// Classify an exit() input as error-vs-result, matching Ink's isErrorInput
+// (ink.tsx:154-159 @ v7.0.4). The plain `instanceof Error` check fails for a
+// cross-realm Error — one created in a different VM context (e.g.
+// `vm.runInNewContext("new Error()")`) has a prototype from the OTHER realm, so
+// it isn't an instance of THIS realm's Error even though it is a genuine Error.
+// The `[object Error]` brand (Symbol.toStringTag, not prototype-based) crosses
+// realms, so it catches those foreign Errors and they REJECT waitUntilExit()
+// instead of being silently swallowed as a resolved result value. Non-Error
+// result values (string/number/plain object) brand as e.g. `[object String]`,
+// so they still RESOLVE — exactly Ink's contract.
+function isErrorInput(value: unknown): value is Error {
+  return value instanceof Error || Object.prototype.toString.call(value) === "[object Error]";
+}
+
 export function createApp(root: Component, rootProps?: RootProps | null): TuiApp {
   // exit promise — created at createApp time so waitUntilExit() works even
   // before mount (it just hangs until mount + exit).
@@ -229,7 +243,7 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
     // Skipped mount: no stream was ever wired; resolve the exit promise directly
     // without any write-barrier so the owner's stdout is never touched.
     if (skippedMount) {
-      if (pendingExitError instanceof Error) {
+      if (isErrorInput(pendingExitError)) {
         exitReject(pendingExitError);
       } else {
         exitResolve(pendingExitResult);
@@ -241,7 +255,7 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
     const hasWritableState = (stdout as any)._writableState !== undefined;
 
     const finish = () => {
-      if (pendingExitError instanceof Error) {
+      if (isErrorInput(pendingExitError)) {
         exitReject(pendingExitError);
       } else {
         exitResolve(pendingExitResult);
@@ -409,7 +423,12 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
       const errored = shallowRef(false);
 
       onErrorCaptured((err) => {
-        const e = err instanceof Error ? err : new Error(String(err));
+        // Preserve a genuine Error — including a cross-realm one (fails
+        // `instanceof Error`, passes the `[object Error]` brand check) — so the
+        // ORIGINAL thrown error reaches exit()/waitUntilExit() unchanged,
+        // matching Ink's ErrorBoundary (rejects with the thrown value itself).
+        // A true non-Error throw (`throw "x"`, `throw 0`) is still wrapped.
+        const e = isErrorInput(err) ? err : new Error(String(err));
         caught.value = err;
         errored.value = true;
         // Flush the ErrorOverview frame, then exit
@@ -567,7 +586,7 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
         // Record the FIRST value/error synchronously (before the deferred
         // teardown microtask) so a re-entrant exit() — which is blocked above
         // anyway — and the eventual resolveExit() always settle on this value.
-        if (errorOrResult instanceof Error) {
+        if (isErrorInput(errorOrResult)) {
           pendingExitError = errorOrResult;
         } else {
           pendingExitResult = errorOrResult;
@@ -980,7 +999,9 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
     // async errors in Vue's internal scheduler). The error boundary returns
     // false to stop propagation, so caught errors won't reach here.
     baseApp.config.errorHandler = (err) => {
-      appContext.exit(err instanceof Error ? err : new Error(String(err)));
+      // Preserve a genuine (incl. cross-realm) Error so the original survives to
+      // exit(); only wrap a true non-Error. See isErrorInput / onErrorCaptured.
+      appContext.exit(isErrorInput(err) ? err : new Error(String(err)));
     };
 
     // Only listen for resize in interactive mode (matching Ink).