modelcontextprotocol · raashish1601 · Mar 28, 2026 · Mar 28, 2026 · Mar 28, 2026
diff --git a/tests/issues/test_2328_stdio_invalid_utf8.py b/tests/issues/test_2328_stdio_invalid_utf8.py
@@ -0,0 +1,127 @@
+"""Regression test for issue #2328: raw invalid UTF-8 over stdio must not crash the server."""
+
+import io
+from io import TextIOWrapper
+from typing import cast
+
+import anyio
+import pytest
+from pydantic import AnyHttpUrl, AnyUrl, TypeAdapter
+
+from mcp import types
+from mcp.server import ServerRequestContext
+from mcp.server.lowlevel.server import Server
+from mcp.server.mcpserver import MCPServer
+from mcp.server.stdio import stdio_server
+from mcp.types import JSONRPCError, JSONRPCResponse, jsonrpc_message_adapter
+
+
+@pytest.mark.anyio
+async def test_stdio_server_returns_error_for_raw_invalid_utf8_tool_arguments():
+    """Invalid UTF-8 bytes in a request body should become a JSON-RPC error, not a crash."""
+
+    url_adapter = TypeAdapter(AnyUrl)
+
+    async def handle_list_tools(
+        ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        return types.ListToolsResult(
+            tools=[
+                types.Tool(
+                    name="fetch",
+                    description="Fetch a URL",
+                    input_schema={
+                        "type": "object",
+                        "required": ["url"],
+                        "properties": {"url": {"type": "string"}},
+                    },
+                )
+            ]
+        )
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: types.CallToolRequestParams) -> types.CallToolResult:
+        arguments = params.arguments or {}
+        url_adapter.validate_python(arguments["url"])
+        return types.CallToolResult(content=[types.TextContent(type="text", text="ok")])
+
+    ctx = cast(ServerRequestContext, None)
+    list_tools_result = await handle_list_tools(ctx, None)
+    assert list_tools_result.tools[0].name == "fetch"
+
+    valid_tool_call_result = await handle_call_tool(
+        ctx,
+        types.CallToolRequestParams(name="fetch", arguments={"url": "https://example.com"}),
+    )
+    assert valid_tool_call_result.content == [types.TextContent(type="text", text="ok")]
+
+    server = Server("test-server", on_list_tools=handle_list_tools, on_call_tool=handle_call_tool)
+
+    raw_stdin = io.BytesIO(
+        b'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}\n'
+        b'{"jsonrpc":"2.0","method":"notifications/initialized"}\n'
+        b'{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"fetch","arguments":{"url":"http://x\xff\xfe"}}}\n'
+    )
+    raw_stdout = io.BytesIO()
+    stdout = TextIOWrapper(raw_stdout, encoding="utf-8")
+
+    async with stdio_server(
+        stdin=anyio.wrap_file(TextIOWrapper(raw_stdin, encoding="utf-8", errors="replace")),
+        stdout=anyio.wrap_file(stdout),
+    ) as (read_stream, write_stream):
+        await server.run(read_stream, write_stream, server.create_initialization_options())
+
+    stdout.flush()
+    responses = [
+        jsonrpc_message_adapter.validate_json(line) for line in raw_stdout.getvalue().decode("utf-8").splitlines()
+    ]
+
+    assert len(responses) == 2
+    assert isinstance(responses[0], JSONRPCResponse)
+    assert responses[0].id == 1
+    assert isinstance(responses[1], JSONRPCError)
+    assert responses[1].id == 3
+    assert responses[1].error.message
+
+
+@pytest.mark.anyio
+async def test_stdio_server_stays_alive_when_tool_validation_finishes_after_stdin_eof():
+    """The MCPServer tool path should not crash if validation loses the response race."""
+
+    mcp = MCPServer("test")
+
+    @mcp.tool()
+    async def fetch(url: str) -> str:
+        # Delay validation so stdin can hit EOF and close the session write
+        # stream before the tool returns its validation failure.
+        await anyio.sleep(0.1)
+        return str(TypeAdapter(AnyHttpUrl).validate_python(url))
+
+    assert await fetch("https://example.com") == "https://example.com/"
+
+    raw_stdin = io.BytesIO(
+        b'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}\n'
+        b'{"jsonrpc":"2.0","method":"notifications/initialized"}\n'
+        b'{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"fetch","arguments":{"url":"http://x\xff\xfe"}}}\n'
+    )
+    raw_stdout = io.BytesIO()
+    stdout = TextIOWrapper(raw_stdout, encoding="utf-8")
+
+    async with stdio_server(
+        stdin=anyio.wrap_file(TextIOWrapper(raw_stdin, encoding="utf-8", errors="replace")),
+        stdout=anyio.wrap_file(stdout),
+    ) as (read_stream, write_stream):
+        with anyio.fail_after(5):
+            await mcp._lowlevel_server.run(
+                read_stream,
+                write_stream,
+                mcp._lowlevel_server.create_initialization_options(),
+            )
+
+    stdout.flush()
+    responses = [
+        jsonrpc_message_adapter.validate_json(line) for line in raw_stdout.getvalue().decode("utf-8").splitlines()
+    ]
+
+    assert responses
+    assert isinstance(responses[0], JSONRPCResponse)
+    assert responses[0].id == 1