From 991926b6856180681b63e7c87ef2647128f6267a Mon Sep 17 00:00:00 2001
From: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Date: Wed, 5 Aug 2026 22:01:57 +0000
Subject: [PATCH] reproduction (reproduced)

Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>
---
 .ai-sdk-factory/reproduction-replay.json      |  13 ++
 examples/ai-functions/package.json            |  11 ++
 ...sume-stream-duplicates-previous-message.ts | 128 ++++++++++++++++++
 3 files changed, 152 insertions(+)
 create mode 100644 .ai-sdk-factory/reproduction-replay.json
 create mode 100644 examples/ai-functions/package.json
 create mode 100644 examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts

diff --git a/.ai-sdk-factory/reproduction-replay.json b/.ai-sdk-factory/reproduction-replay.json
new file mode 100644
index 000000000000..45688ca8e6e1
--- /dev/null
+++ b/.ai-sdk-factory/reproduction-replay.json
@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "issueId": 18457,
+  "command": "pnpm -C examples/ai-functions exec tsx src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts",
+  "artifactPaths": [
+    "examples/ai-functions/package.json",
+    "examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts"
+  ],
+  "expectedFailure": {
+    "exitCode": 1,
+    "signal": "ISSUE_18457_REPRODUCED: resumeStream duplicated the previous assistant message"
+  }
+}
diff --git a/examples/ai-functions/package.json b/examples/ai-functions/package.json
new file mode 100644
index 000000000000..084fcda5c199
--- /dev/null
+++ b/examples/ai-functions/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "@example/ai-functions",
+  "version": "0.0.0",
+  "private": true,
+  "dependencies": {
+    "ai": "link:../../packages/ai"
+  },
+  "devDependencies": {
+    "tsx": "link:../ai-core/node_modules/tsx"
+  }
+}
diff --git a/examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts b/examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts
new file mode 100644
index 000000000000..219080a7da97
--- /dev/null
+++ b/examples/ai-functions/src/reproduction/issue-18457-resume-stream-duplicates-previous-message.ts
@@ -0,0 +1,128 @@
+import {
+  AbstractChat,
+  type ChatState,
+  type ChatStatus,
+  type ChatTransport,
+  type UIMessage,
+  type UIMessageChunk,
+} from 'ai';
+
+class State implements ChatState<UIMessage> {
+  status: ChatStatus = 'ready';
+  error: Error | undefined;
+
+  constructor(public messages: UIMessage[]) {}
+
+  pushMessage = (message: UIMessage) => {
+    this.messages = [...this.messages, message];
+  };
+
+  popMessage = () => {
+    this.messages = this.messages.slice(0, -1);
+  };
+
+  replaceMessage = (index: number, message: UIMessage) => {
+    this.messages = this.messages.map((current, currentIndex) =>
+      currentIndex === index ? message : current,
+    );
+  };
+
+  snapshot = <T>(value: T): T => structuredClone(value);
+}
+
+class Chat extends AbstractChat<UIMessage> {}
+
+function chunkStream(chunks: UIMessageChunk[]) {
+  let index = 0;
+
+  return new ReadableStream<UIMessageChunk>({
+    pull(controller) {
+      if (index >= chunks.length) {
+        controller.close();
+      } else {
+        controller.enqueue(chunks[index++]);
+      }
+    },
+  });
+}
+
+function text(message: UIMessage | undefined) {
+  return (
+    message?.parts
+      .filter(part => part.type === 'text')
+      .map(part => part.text)
+      .join(' | ') ?? ''
+  );
+}
+
+async function main() {
+  const previousAnswer = 'The capital of France is Paris.';
+  const resumedAnswer = 'It has been the capital since 987 AD.';
+  const state = new State([
+    {
+      id: 'u1',
+      role: 'user',
+      parts: [{ type: 'text', text: 'What is the capital of France?' }],
+    },
+    {
+      id: 'a1',
+      role: 'assistant',
+      parts: [{ type: 'text', text: previousAnswer }],
+    },
+  ]);
+
+  const transport: ChatTransport<UIMessage> = {
+    sendMessages: async () => chunkStream([]),
+    reconnectToStream: async () =>
+      chunkStream([
+        { type: 'start', messageId: 'server-msg-2' },
+        { type: 'text-start', id: 't1' },
+        { type: 'text-delta', id: 't1', delta: resumedAnswer },
+        { type: 'text-end', id: 't1' },
+        { type: 'finish' },
+      ]),
+  };
+
+  const chat = new Chat({
+    id: 'c1',
+    state,
+    transport,
+    generateId: () => 'client-generated',
+  });
+
+  await chat.resumeStream();
+
+  const previousAnswerOccurrences = state.messages.filter(message =>
+    text(message).includes(previousAnswer),
+  ).length;
+  const resumedMessage = state.messages.find(
+    message => message.id === 'server-msg-2',
+  );
+  const resumedText = text(resumedMessage);
+
+  if (previousAnswerOccurrences !== 1 || resumedText.includes(previousAnswer)) {
+    console.error(
+      'ISSUE_18457_REPRODUCED: resumeStream duplicated the previous assistant message',
+    );
+    console.error(JSON.stringify(state.messages, null, 2));
+    process.exitCode = 1;
+    return;
+  }
+
+  if (
+    state.messages.length !== 3 ||
+    resumedText !== resumedAnswer ||
+    text(state.messages[1]) !== previousAnswer
+  ) {
+    throw new Error(
+      `Unexpected resumeStream result: ${JSON.stringify(state.messages)}`,
+    );
+  }
+
+  console.log('Issue #18457 did not reproduce.');
+}
+
+main().catch(error => {
+  console.error(error);
+  process.exitCode = 1;
+});