fix: preserve UTF-8 in SageMaker streaming decode

core/llm/llms/SageMaker.ts

@@ -50,9 +50,10 @@ class SageMaker extends BaseLLM {
     const response = await client.send(command, { abortSignal: signal });
     if (response.Body) {
       let buffer = "";
+      const decoder = new TextDecoder();
       for await (const rawValue of response.Body) {
         const binaryChunk = rawValue.PayloadPart?.Bytes;
-        let value = new TextDecoder().decode(binaryChunk);
+        let value = decoder.decode(binaryChunk, { stream: true });
         buffer += value;
         let position;
         while ((position = buffer.indexOf("\n")) >= 0) {
@@ -99,9 +100,10 @@ class SageMaker extends BaseLLM {
     const response = await client.send(command, { abortSignal: signal });
     if (response.Body) {
       let buffer = "";
+      const decoder = new TextDecoder();
       for await (const rawValue of response.Body) {
         const binaryChunk = rawValue.PayloadPart?.Bytes;
-        let value = new TextDecoder().decode(binaryChunk);
+        let value = decoder.decode(binaryChunk, { stream: true });
         buffer += value;
         let position;
         while ((position = buffer.indexOf("\n")) >= 0) {

core/llm/llms/SageMaker.vitest.ts

@@ -0,0 +1,90 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { ChatMessage } from "../../index.js";
+
+// Simulated streaming body returned by the SageMaker runtime client.
+// A multibyte UTF-8 character is split across two network chunks.
+let mockBody: AsyncIterable<{ PayloadPart?: { Bytes?: Uint8Array } }>;
+
+vi.mock("@aws-sdk/client-sagemaker-runtime", () => ({
+  SageMakerRuntimeClient: class {
+    async send() {
+      return { Body: mockBody };
+    }
+  },
+  InvokeEndpointCommand: class {
+    constructor(public input: any) {}
+  },
+  InvokeEndpointWithResponseStreamCommand: class {
+    constructor(public input: any) {}
+  },
+}));
+
+vi.mock("@aws-sdk/credential-providers", () => ({
+  fromNodeProviderChain: () => async () => ({
+    accessKeyId: "test",
+    secretAccessKey: "test",
+    sessionToken: "test",
+  }),
+}));
+
+import SageMaker from "./SageMaker.js";
+
+function bodyFromChunks(chunks: Uint8Array[]) {
+  return (async function* () {
+    for (const c of chunks) {
+      yield { PayloadPart: { Bytes: c } };
+    }
+  })();
+}
+
+describe("SageMaker streaming multibyte handling", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("does not corrupt a multibyte character split across chunks (_streamChat)", async () => {
+    const line = 'data:{"choices":[{"delta":{"content":"\u{1F600}"}}]}\n';
+    const bytes = new TextEncoder().encode(line);
+    // Split in the middle of the 4-byte emoji sequence (starts with 0xF0).
+    const idx = bytes.indexOf(0xf0);
+    const chunk1 = bytes.slice(0, idx + 2);
+    const chunk2 = bytes.slice(idx + 2);
+    mockBody = bodyFromChunks([chunk1, chunk2]);
+
+    const sm = new SageMaker({ model: "m", region: "us-west-2" } as any);
+    const messages: ChatMessage[] = [{ role: "user", content: "hi" }];
+    const out: string[] = [];
+    for await (const msg of (sm as any)._streamChat(
+      messages,
+      new AbortController().signal,
+      {},
+    )) {
+      out.push(msg.content as string);
+    }
+
+    expect(out.join("")).toBe("\u{1F600}");
+  });
+
+  it("does not corrupt a multibyte character split across chunks (_streamComplete)", async () => {
+    const line = 'data:{"choices":[{"text":"café"}]}\n';
+    const bytes = new TextEncoder().encode(line);
+    // Split in the middle of the 2-byte é sequence (starts with 0xC3).
+    const idx = bytes.indexOf(0xc3);
+    const chunk1 = bytes.slice(0, idx + 1);
+    const chunk2 = bytes.slice(idx + 1);
+    mockBody = bodyFromChunks([chunk1, chunk2]);
+
+    const sm = new SageMaker({ model: "m", region: "us-west-2" } as any);
+    const out: string[] = [];
+    for await (const chunk of (sm as any)._streamComplete(
+      "hi",
+      new AbortController().signal,
+      {},
+    )) {
+      out.push(chunk as string);
+    }
+
+    expect(out.join("")).toBe("café");
+  });
+});