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fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding #2907

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fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding #2907

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2 changes: 1 addition & 1 deletion src/mcp/client/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -106,7 +106,7 @@ 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: Literal["strict", "ignore", "replace"] = "replace"
"""Encoding error handler; see https://docs.python.org/3/library/codecs.html#error-handlers."""


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24 changes: 24 additions & 0 deletions tests/client/test_stdio.py
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Expand Up @@ -307,6 +307,30 @@ 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_from_the_server_surfaces_as_an_in_stream_exception(
monkeypatch: pytest.MonkeyPatch,
) -> None:
"""A line containing invalid UTF-8 is replaced and delivered as an Exception on the read stream.

The transport stays alive so subsequent valid messages still come through.
Regression for #2454.
"""
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()
# The transport surfaces parse failures as the underlying validation error.
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,
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