fix(client): tolerate invalid UTF-8 from server stdout in stdio_client

src/mcp/client/stdio.py

@@ -106,8 +106,13 @@ class StdioServerParameters(BaseModel):
     encoding: str = "utf-8"
     """Text encoding for messages to and from the server."""
 
-    encoding_error_handler: Literal["strict", "ignore", "replace"] = "strict"
-    """Encoding error handler; see https://docs.python.org/3/library/codecs.html#error-handlers."""
+    encoding_error_handler: Literal["strict", "ignore", "replace"] = "replace"
+    """Encoding error handler; see https://docs.python.org/3/library/codecs.html#error-handlers.
+
+    Defaults to ``"replace"`` so malformed bytes from a buggy server become U+FFFD and surface
+    as an in-stream JSON parse error (kept alive for subsequent valid messages) instead of
+    crashing the transport task group. This mirrors the server-side stdin hardening.
+    """
 
 
 @asynccontextmanager

tests/client/test_stdio.py

@@ -307,6 +307,31 @@ async def test_invalid_json_from_the_server_surfaces_as_an_in_stream_exception(
             assert await _next_message(read_stream) == ping
 
 
+@pytest.mark.anyio
+async def test_invalid_utf8_mid_session_surfaces_as_an_in_stream_exception(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A line with non-UTF-8 bytes is surfaced as a parse error, not a transport crash.
+
+    Regression test for a buggy server emitting malformed bytes mid-session: the default
+    ``encoding_error_handler="replace"`` turns the bad bytes into U+FFFD so the line fails
+    JSON validation and is delivered as an Exception, keeping the transport alive for the
+    valid messages that follow (instead of a UnicodeDecodeError tearing down the task group).
+    """
+    ping = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
+    process = FakeProcess(on_stdin_close=lambda: process.exit(0))
+
+    install_fake_process(monkeypatch, process)
+
+    with anyio.fail_after(5):
+        async with stdio_client(FAKE_PARAMS) as (read_stream, _):
+            await process.feed(b"\xff\xfe\n" + _line(ping))
+
+            error = await read_stream.receive()
+            assert isinstance(error, ValueError)
+            assert await _next_message(read_stream) == ping
+
+
 @pytest.mark.anyio
 async def test_a_server_that_dies_before_responding_fails_initialize_with_connection_closed(
     monkeypatch: pytest.MonkeyPatch,