From 45ad3950be6491f39fac9a6bdfc038d064150cef Mon Sep 17 00:00:00 2001
From: Lars Grammel <lars.grammel@gmail.com>
Date: Thu, 9 Jul 2026 17:05:49 +0200
Subject: [PATCH] reproduction (reproduced)

---
 .../ui-message-chunk-forward-compatibility.ts | 133 ++++++++++++++++++
 1 file changed, 133 insertions(+)
 create mode 100644 examples/ai-functions/src/reproduction/ui-message-chunk-forward-compatibility.ts

diff --git a/examples/ai-functions/src/reproduction/ui-message-chunk-forward-compatibility.ts b/examples/ai-functions/src/reproduction/ui-message-chunk-forward-compatibility.ts
new file mode 100644
index 000000000000..4ae9a9521958
--- /dev/null
+++ b/examples/ai-functions/src/reproduction/ui-message-chunk-forward-compatibility.ts
@@ -0,0 +1,133 @@
+import assert from 'node:assert/strict';
+import {
+  parseJsonEventStream,
+  TypeValidationError,
+  uiMessageChunkSchema,
+  type UIMessageChunk,
+} from 'ai';
+
+function createEventStream(chunk: unknown): ReadableStream<Uint8Array> {
+  const encoder = new TextEncoder();
+
+  return new ReadableStream({
+    start(controller) {
+      controller.enqueue(encoder.encode(`data: ${JSON.stringify(chunk)}\n\n`));
+      controller.close();
+    },
+  });
+}
+
+async function readUiMessageChunks(chunk: unknown): Promise<UIMessageChunk[]> {
+  const reader = parseJsonEventStream<UIMessageChunk>({
+    stream: createEventStream(chunk),
+    schema: uiMessageChunkSchema,
+  }).getReader();
+  const chunks: UIMessageChunk[] = [];
+
+  while (true) {
+    const result = await reader.read();
+
+    if (result.done) {
+      break;
+    }
+
+    if (!result.value.success) {
+      throw result.value.error;
+    }
+
+    chunks.push(result.value.value);
+  }
+
+  return chunks;
+}
+
+function findUnrecognizedKeys(error: unknown): string[] | undefined {
+  const stack = [error];
+  const seen = new Set<unknown>();
+  const matches: string[][] = [];
+
+  while (stack.length > 0) {
+    const value = stack.pop();
+
+    if (value == null || typeof value !== 'object' || seen.has(value)) {
+      continue;
+    }
+
+    seen.add(value);
+
+    if (
+      'code' in value &&
+      value.code === 'unrecognized_keys' &&
+      'keys' in value &&
+      Array.isArray(value.keys)
+    ) {
+      matches.push(value.keys.map(String));
+    }
+
+    if (Array.isArray(value)) {
+      stack.push(...value);
+      continue;
+    }
+
+    const record = value as Record<string, unknown>;
+
+    for (const key of ['issues', 'errors', 'cause'] as const) {
+      if (key in record) {
+        stack.push(record[key]);
+      }
+    }
+
+    stack.push(...Object.values(record));
+  }
+
+  return matches.sort((left, right) => left.length - right.length)[0];
+}
+
+async function main() {
+  const baseChunk = {
+    type: 'tool-output-available',
+    toolCallId: 'call-123',
+    output: { ok: true },
+  } satisfies UIMessageChunk;
+
+  const parsedBaseChunks = await readUiMessageChunks(baseChunk);
+  assert.equal(parsedBaseChunks.length, 1);
+
+  const newerServerChunk = {
+    ...baseChunk,
+    providerMetadata: {
+      openai: {
+        itemId: 'item-123',
+      },
+    },
+    optionalFieldFromNewerServer: {
+      addedIn: 'future-ai-sdk-patch',
+    },
+  };
+
+  try {
+    const parsedNewerServerChunks = await readUiMessageChunks(newerServerChunk);
+    assert.equal(parsedNewerServerChunks.length, 1);
+    assert.equal(parsedNewerServerChunks[0]?.type, baseChunk.type);
+  } catch (error) {
+    console.error(
+      'Expected uiMessageChunkSchema to ignore unrecognized optional stream-event keys for forward compatibility, but parsing failed.',
+    );
+
+    if (TypeValidationError.isInstance(error)) {
+      console.error(`Observed error: ${error.name}`);
+      console.error(
+        `Unrecognized keys: ${findUnrecognizedKeys(error.cause)?.join(', ')}`,
+      );
+    }
+
+    throw new Error(
+      'Reproduced: a newer-server UI message chunk with an extra optional key is rejected by the client schema.',
+    );
+  }
+}
+
+main().catch(error => {
+  console.error(error);
+  process.exit(1);
+});