diff --git a/src/StdUtils.ts b/src/StdUtils.ts
index 3d9e2159..25ead637 100644
--- a/src/StdUtils.ts
+++ b/src/StdUtils.ts
@@ -1,4 +1,5 @@
 import {Readable, Writable} from "node:stream";
+import {StringDecoder} from "node:string_decoder";
 import {Emitter} from "vscode-jsonrpc/node";
 import type {DataCallback, Disposable, Message, MessageReader, MessageWriter, PartialMessageInfo} from "vscode-jsonrpc/node";
 import * as acp from "@agentclientprotocol/sdk";
@@ -33,8 +34,9 @@ export function createJSONRPCReader(readable: Readable): MessageReader {
     return {
         listen(callback: DataCallback): Disposable {
             let buf = '';
+            const decoder = new StringDecoder('utf8');
             const onData = (chunk: Buffer) => {
-                buf += chunk.toString();
+                buf += decoder.write(chunk);
                 for (;;) {
                     const i = buf.indexOf('\n');
                     if (i < 0) break;
diff --git a/src/__tests__/StdUtils.test.ts b/src/__tests__/StdUtils.test.ts
new file mode 100644
index 00000000..5c151365
--- /dev/null
+++ b/src/__tests__/StdUtils.test.ts
@@ -0,0 +1,24 @@
+import {PassThrough} from "node:stream";
+import {describe, expect, it, vi} from "vitest";
+
+import {createJSONRPCReader} from "../StdUtils";
+
+describe("createJSONRPCReader", () => {
+    it("preserves UTF-8 characters split across stdout chunks", () => {
+        const stdout = new PassThrough();
+        const onMessage = vi.fn();
+        createJSONRPCReader(stdout).listen(onMessage);
+
+        const prefix = Buffer.from('{"method":"test","params":{"title":"');
+        const firstCharacter = Buffer.from("\u4F60");
+        stdout.write(Buffer.concat([prefix, firstCharacter.subarray(0, 2)]));
+        stdout.write(Buffer.concat([firstCharacter.subarray(2), Buffer.from('\u597D"}}\n')]));
+
+        expect(onMessage).toHaveBeenCalledOnce();
+        expect(onMessage).toHaveBeenCalledWith({
+            jsonrpc: "2.0",
+            method: "test",
+            params: {title: "\u4F60\u597D"},
+        });
+    });
+});