From 8a36bdedc0970c9a864b33c5cb6d7b6c93680937 Mon Sep 17 00:00:00 2001
From: Piyush Jagadish Bag <piyushbag4@gmail.com>
Date: Thu, 23 Jul 2026 08:56:36 -0700
Subject: [PATCH] fix(stdio): release ReadBuffer backing after final byte

Clear _buffer when readMessage consumes through the trailing newline so
idle stdio transports do not retain the previous allocation and the next
append can assign directly instead of Buffer.concat.
---
 packages/core-internal/src/shared/stdio.ts       |  3 ++-
 packages/core-internal/test/shared/stdio.test.ts | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/packages/core-internal/src/shared/stdio.ts b/packages/core-internal/src/shared/stdio.ts
index 8bd794b87b..e90bd4cab2 100644
--- a/packages/core-internal/src/shared/stdio.ts
+++ b/packages/core-internal/src/shared/stdio.ts
@@ -31,7 +31,8 @@ export class ReadBuffer {
             }
 
             const line = this._buffer.toString('utf8', 0, index).replace(/\r$/, '');
-            this._buffer = this._buffer.subarray(index + 1);
+            const remainder = this._buffer.subarray(index + 1);
+            this._buffer = remainder.length === 0 ? undefined : remainder;
 
             try {
                 return deserializeMessage(line);
diff --git a/packages/core-internal/test/shared/stdio.test.ts b/packages/core-internal/test/shared/stdio.test.ts
index f8d27a4c1f..114912a6d7 100644
--- a/packages/core-internal/test/shared/stdio.test.ts
+++ b/packages/core-internal/test/shared/stdio.test.ts
@@ -1,3 +1,4 @@
+import { vi } from 'vitest';
 import { ReadBuffer, STDIO_DEFAULT_MAX_BUFFER_SIZE } from '../../src/shared/stdio';
 import type { JSONRPCMessage } from '../../src/types/index';
 
@@ -156,3 +157,17 @@ describe('buffer size limit', () => {
         expect(readBuffer.readMessage()).not.toBeNull();
     });
 });
+
+test('should clear backing allocation when final byte is consumed', () => {
+    const readBuffer = new ReadBuffer();
+    const largePayload = 'x'.repeat(1024);
+    readBuffer.append(Buffer.from(JSON.stringify({ jsonrpc: '2.0', method: largePayload }) + '\n'));
+    expect(readBuffer.readMessage()).toEqual({ jsonrpc: '2.0', method: largePayload });
+
+    const appendSpy = vi.spyOn(Buffer, 'concat');
+    const nextChunk = Buffer.from(JSON.stringify(testMessage) + '\n');
+    readBuffer.append(nextChunk);
+    expect(appendSpy).not.toHaveBeenCalled();
+    expect(readBuffer.readMessage()).toEqual(testMessage);
+    appendSpy.mockRestore();
+});