DRAFT - Fix StdioClientTransport missing charset

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/StdioClientTransport.java

@@ -180,7 +180,7 @@ public void awaitForExit() {
 	private void startErrorProcessing() {
 		this.errorScheduler.schedule(() -> {
 			try (BufferedReader processErrorReader = new BufferedReader(
-					new InputStreamReader(process.getErrorStream()))) {
+					new InputStreamReader(process.getErrorStream(), StandardCharsets.UTF_8))) {
 				String line;
 				while (!isClosing && (line = processErrorReader.readLine()) != null) {
 					try {
@@ -246,7 +246,8 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 	 */
 	private void startInboundProcessing() {
 		this.inboundScheduler.schedule(() -> {
-			try (BufferedReader processReader = new BufferedReader(new InputStreamReader(process.getInputStream()))) {
+			try (BufferedReader processReader = new BufferedReader(
+					new InputStreamReader(process.getInputStream(), StandardCharsets.UTF_8))) {
 				String line;
 				while (!isClosing && (line = processReader.readLine()) != null) {
 					try {

mcp-core/src/test/java/io/modelcontextprotocol/client/transport/StdioClientTransportCharsetTests.java

@@ -0,0 +1,128 @@
+package io.modelcontextprotocol.client.transport;
+
+import io.modelcontextprotocol.json.McpJsonMapper;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.json.gson.GsonMcpJsonMapper;
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link StdioClientTransport} to ensure it correctly handles character
+ * encodings.
+ */
+class StdioClientTransportCharsetTests {
+
+	@Test
+	void testUtf8DecodingWithNonUtf8DefaultCharset() throws Exception {
+		String javaHome = System.getProperty("java.home");
+		String javaBin = javaHome + File.separator + "bin" + File.separator + "java";
+		String classpath = System.getProperty("java.class.path");
+
+		// We MUST run a separate JVM to test this because the bug only manifests
+		// when the JVM's default charset is not UTF-8. Charset.defaultCharset() is
+		// cached at JVM startup and cannot be changed at runtime for the background
+		// threads spawned by StdioClientTransport.
+		// We use -Xmx32m to ensure the child JVM uses minimal memory on CI.
+		ProcessBuilder pb = new ProcessBuilder(javaBin, "-Xmx32m", "-Dfile.encoding=ISO-8859-1", "-cp", classpath,
+				ClientMain.class.getName());
+
+		pb.redirectErrorStream(true);
+		Process process = pb.start();
+
+		try {
+			// Wait with a timeout to prevent the test from hanging indefinitely
+			boolean finished = process.waitFor(15, TimeUnit.SECONDS);
+			String output = new String(process.getInputStream().readAllBytes());
+
+			assertThat(finished).as("Client process timed out. Output:\n" + output).isTrue();
+			assertThat(process.exitValue()).as("Client process failed with output:\n" + output).isEqualTo(0);
+			assertThat(output).contains("SUCCESS_MATCH");
+		}
+		finally {
+			// Guarantee the child JVM is killed even if the test fails or is aborted
+			if (process.isAlive()) {
+				process.destroyForcibly();
+			}
+		}
+	}
+
+	public static class ClientMain {
+
+		public static void main(String[] args) {
+			try {
+				McpJsonMapper jsonMapper = new GsonMcpJsonMapper();
+
+				String javaHome = System.getProperty("java.home");
+				String javaBin = javaHome + File.separator + "bin" + File.separator + "java";
+				String classpath = System.getProperty("java.class.path");
+
+				// We use -Xmx32m to ensure the child JVM uses minimal memory on CI.
+				ServerParameters params = ServerParameters.builder(javaBin)
+					.args("-Xmx32m", "-cp", classpath, ServerMain.class.getName())
+					.build();
+
+				StdioClientTransport transport = new StdioClientTransport(params, jsonMapper);
+
+				CountDownLatch latch = new CountDownLatch(1);
+				AtomicReference<McpSchema.JSONRPCMessage> receivedMessage = new AtomicReference<>();
+
+				transport.connect(msgMono -> msgMono.doOnNext(msg -> {
+					receivedMessage.set(msg);
+					latch.countDown();
+				})).block();
+
+				boolean received = latch.await(10, TimeUnit.SECONDS);
+				if (!received) {
+					System.err.println("Did not receive message in time");
+					System.exit(1);
+				}
+
+				transport.closeGracefully().block();
+
+				McpSchema.JSONRPCMessage msg = receivedMessage.get();
+				if (msg instanceof McpSchema.JSONRPCNotification notif) {
+					if ("こんにちは".equals(notif.method())) {
+						System.out.println("SUCCESS_MATCH");
+						System.exit(0);
+					}
+					else {
+						System.err.println("Method mismatch: " + notif.method());
+						System.exit(1);
+					}
+				}
+				else {
+					System.err.println("Wrong message type: " + msg);
+					System.exit(1);
+				}
+			}
+			catch (Exception e) {
+				// Catch any unexpected errors to ensure the JVM doesn't hang
+				e.printStackTrace();
+				System.exit(1);
+			}
+		}
+
+	}
+
+	public static class ServerMain {
+
+		public static void main(String[] args) throws Exception {
+			// Write a UTF-8 JSON-RPC message containing Japanese characters
+			String json = "{\"jsonrpc\":\"2.0\",\"method\":\"こんにちは\"}\n";
+			System.out.write(json.getBytes(java.nio.charset.StandardCharsets.UTF_8));
+			System.out.flush();
+			// Sleep to keep the process alive long enough for the client to read the
+			// message.
+			// It will automatically exit after 5 seconds, preventing any permanent leaks.
+			Thread.sleep(5000);
+		}
+
+	}
+
+}