Use explicit UTF-8 charset in StdioServerTransportProvider (#826)

Also add a test simulating a different locale in an isolated process.

Resolves #295

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java

@@ -214,7 +214,7 @@ public class StdioServerTransportProvider implements McpServerTransportProvider
 					inboundReady.tryEmitValue(null);
 					BufferedReader reader = null;
 					try {
-						reader = new BufferedReader(new InputStreamReader(inputStream));
+						reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8));
 						while (!isClosing.get()) {
 							try {
 								String line = reader.readLine();

mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java

@@ -4,9 +4,11 @@
 
 package io.modelcontextprotocol.server.transport;
 
+import java.io.BufferedReader;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.InputStream;
+import java.io.InputStreamReader;
 import java.io.PrintStream;
 import java.nio.charset.StandardCharsets;
 import java.util.Map;
@@ -135,6 +137,42 @@ class StdioServerTransportProviderTests {
 		}).verifyComplete();
 	}
 
+	@Test
+	void shouldHandleUtf8MessagesWithNonUtf8DefaultCharset() throws Exception {
+		String utf8Content = "한글 漢字 café 🎉";
+		String jsonMessage = "{\"jsonrpc\":\"2.0\",\"method\":\"test\"," + "\"params\":{\"message\":\"" + utf8Content
+				+ "\"},\"id\":1}\n";
+
+		// Start a subprocess with non-UTF-8 default charset
+		String javaHome = System.getProperty("java.home");
+		String classpath = System.getProperty("java.class.path");
+		ProcessBuilder pb = new ProcessBuilder(javaHome + "/bin/java", "-Dfile.encoding=ISO-8859-1", "-cp", classpath,
+				StdioUtf8TestServer.class.getName());
+		pb.redirectErrorStream(false);
+		Process process = pb.start();
+
+		try {
+			// Write UTF-8 encoded JSON-RPC message to the subprocess stdin
+			process.getOutputStream().write(jsonMessage.getBytes(StandardCharsets.UTF_8));
+			process.getOutputStream().flush();
+			process.getOutputStream().close();
+
+			// Read the echoed message from subprocess stdout
+			String result;
+			try (BufferedReader reader = new BufferedReader(
+					new InputStreamReader(process.getInputStream(), StandardCharsets.UTF_8))) {
+				result = reader.readLine();
+			}
+
+			// Verify that multi-byte UTF-8 characters survived the round trip
+			assertThat(result).isEqualTo(utf8Content);
+		}
+		finally {
+			process.destroyForcibly();
+			process.waitFor(10, TimeUnit.SECONDS);
+		}
+	}
+
 	@Test
 	void shouldNotifyClients() {
 		// Set session factory

mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioUtf8TestServer.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server.transport;
+
+import java.io.OutputStream;
+import java.io.PrintStream;
+import java.nio.charset.StandardCharsets;
+import java.util.Map;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import io.modelcontextprotocol.json.McpJsonDefaults;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpServerSession;
+import reactor.core.publisher.Mono;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Minimal STDIO server process for testing UTF-8 encoding behavior.
+ *
+ * <p>
+ * This class is spawned as a subprocess with {@code -Dfile.encoding=ISO-8859-1} to
+ * simulate a non-UTF-8 default charset environment. It uses
+ * {@link StdioServerTransportProvider} to read a JSON-RPC message from stdin and echoes
+ * the received {@code params.message} value back to stdout, allowing the parent test to
+ * verify that multi-byte UTF-8 characters are preserved regardless of the JVM default
+ * charset.
+ *
+ * @see StdioServerTransportProviderTests#shouldHandleUtf8MessagesWithNonUtf8DefaultCharset
+ */
+public class StdioUtf8TestServer {
+
+	@SuppressWarnings("unchecked")
+	public static void main(String[] args) throws Exception {
+		// Capture the original stdout for echoing the result later
+		PrintStream originalOut = System.out;
+
+		// Redirect System.out to stderr so that logger output does not
+		// interfere with the test result written to stdout
+		System.setOut(new PrintStream(System.err, true));
+
+		CountDownLatch messageLatch = new CountDownLatch(1);
+		StringBuilder receivedMessage = new StringBuilder();
+
+		StdioServerTransportProvider transportProvider = new StdioServerTransportProvider(McpJsonDefaults.getMapper(),
+				System.in, OutputStream.nullOutputStream());
+
+		McpServerSession.Factory sessionFactory = transport -> {
+			McpServerSession session = mock(McpServerSession.class);
+			when(session.handle(any())).thenAnswer(invocation -> {
+				McpSchema.JSONRPCMessage msg = invocation.getArgument(0);
+				if (msg instanceof McpSchema.JSONRPCRequest request) {
+					Map<String, Object> params = (Map<String, Object>) request.params();
+					receivedMessage.append(params.get("message"));
+				}
+				messageLatch.countDown();
+				return Mono.empty();
+			});
+			when(session.closeGracefully()).thenReturn(Mono.empty());
+			return session;
+		};
+
+		// Start processing stdin
+		transportProvider.setSessionFactory(sessionFactory);
+
+		// Wait for the message to be processed
+		if (messageLatch.await(10, TimeUnit.SECONDS)) {
+			// Write the received message to the original stdout in UTF-8
+			originalOut.write(receivedMessage.toString().getBytes(StandardCharsets.UTF_8));
+			originalOut.write('\n');
+			originalOut.flush();
+		}
+
+		transportProvider.closeGracefully().block(java.time.Duration.ofSeconds(5));
+	}
+
+}