diff --git a/src/mcp/client/stdio.py b/src/mcp/client/stdio.py
index 6a3ad12111..66867eca05 100644
--- a/src/mcp/client/stdio.py
+++ b/src/mcp/client/stdio.py
@@ -147,10 +147,16 @@ async def stdout_reader() -> None:
             async with read_stream_writer:
                 try:
                     # One line at a time; no read-ahead while a delivery is blocked.
-                    buffer = ""
+                    # An unfinished line is kept as pieces and joined once a newline
+                    # arrives. Rebuilding the whole buffer on every chunk is quadratic
+                    # in the size of one large JSON line.
+                    parts: list[str] = []
                     async for chunk in stdout:
-                        lines = (buffer + chunk).split("\n")
-                        buffer = lines.pop()
+                        if "\n" not in chunk:
+                            parts.append(chunk)
+                            continue
+                        lines = ("".join(parts) + chunk).split("\n")
+                        parts = [lines.pop()]
                         for line in lines:
                             try:
                                 await read_stream_writer.send(_parse_line(line))
diff --git a/tests/client/test_stdio.py b/tests/client/test_stdio.py
index b64eb01902..dacdb2ea00 100644
--- a/tests/client/test_stdio.py
+++ b/tests/client/test_stdio.py
@@ -277,6 +277,35 @@ async def test_messages_split_and_packed_across_chunks_are_reframed(monkeypatch:
                 await read_stream.receive()
 
 
+@pytest.mark.anyio
+async def test_a_large_line_split_across_many_chunks_is_delivered_intact(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A single JSON-RPC line split into many chunks is delivered as that one message.
+
+    Pins SDK stdout framing for a multi-megabyte line (issue #3699): the pieces are
+    joined when the newline arrives, and the following line still frames normally.
+    """
+    text = "x" * (8 * 1024 * 1024)
+    response = JSONRPCResponse(jsonrpc="2.0", id=1, result={"text": text})
+    following = JSONRPCRequest(jsonrpc="2.0", id=2, method="ping")
+    wire = response.model_dump_json(by_alias=True, exclude_unset=True).encode()
+    process = FakeProcess(on_stdin_close=lambda: process.exit(0))
+
+    install_fake_process(monkeypatch, process)
+
+    # 8 MiB in 1 KiB pieces: recopying the buffer on every chunk cannot finish
+    # inside the fail_after bound, so a quadratic reader fails this test.
+    with anyio.fail_after(5):
+        async with stdio_client(FAKE_PARAMS) as (read_stream, _):
+            for start in range(0, len(wire), 1024):
+                await process.feed(wire[start : start + 1024])
+            await process.feed(b"\n" + _line(following))
+
+            assert await _next_message(read_stream) == response
+            assert await _next_message(read_stream) == following
+
+
 @pytest.mark.anyio
 async def test_each_outgoing_message_is_written_as_exactly_one_line(monkeypatch: pytest.MonkeyPatch) -> None:
     """Client -> server framing writes one line per message.