Bound STDIO client reads

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Kehrlann

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

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+/**
+ * Thrown when reading an inbound message would exceed the configured maximum size.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+class MaxSizeExceededException extends Exception {
+
+	MaxSizeExceededException(String message) {
+		super(message);
+	}
+
+}

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

@@ -44,6 +44,8 @@ public class StdioClientTransport implements McpClientTransport {
 
 	private static final Logger logger = LoggerFactory.getLogger(StdioClientTransport.class);
 
+	private static final int DEFAULT_INPUT_MAX_SIZE = 16 * 1024 * 1024; // 16MB
+
 	// @formatter:off
 	private static final Set<Integer> EXIT_SUCCESS_CODES = Set.of(
 			0,   // success
@@ -76,6 +78,8 @@ public class StdioClientTransport implements McpClientTransport {
 
 	private final Sinks.Many<String> errorSink;
 
+	private final int inputMaxSize;
+
 	private volatile boolean isClosing = false;
 
 	// visible for tests
@@ -87,8 +91,21 @@ public class StdioClientTransport implements McpClientTransport {
 	 * @param jsonMapper The JsonMapper to use for JSON serialization/deserialization
 	 */
 	public StdioClientTransport(ServerParameters params, McpJsonMapper jsonMapper) {
+		this(params, jsonMapper, DEFAULT_INPUT_MAX_SIZE);
+	}
+
+	/**
+	 * Creates a new StdioClientTransport with the specified parameters and JsonMapper.
+	 * @param params The parameters for configuring the server process
+	 * @param jsonMapper The JsonMapper to use for JSON serialization/deserialization
+	 * @param inputMaxSize The maximum number of characters read for a single inbound
+	 * message. A peer that sends a longer message (or never terminates a line) has its
+	 * message rejected instead of forcing the transport to buffer it in memory.
+	 */
+	public StdioClientTransport(ServerParameters params, McpJsonMapper jsonMapper, int inputMaxSize) {
 		Assert.notNull(params, "The params can not be null");
 		Assert.notNull(jsonMapper, "The JsonMapper can not be null");
+		Assert.isTrue(inputMaxSize > 0, "inputMaxSize must be positive");
 
 		this.inboundSink = Sinks.many().unicast().onBackpressureBuffer();
 		this.outboundSink = Sinks.many().unicast().onBackpressureBuffer();
@@ -97,6 +114,8 @@ public StdioClientTransport(ServerParameters params, McpJsonMapper jsonMapper) {
 
 		this.jsonMapper = jsonMapper;
 
+		this.inputMaxSize = inputMaxSize;
+
 		this.errorSink = Sinks.many().unicast().onBackpressureBuffer();
 
 		// Start threads
@@ -260,7 +279,7 @@ private void startInboundProcessing() {
 		this.inboundScheduler.schedule(() -> {
 			try (BufferedReader processReader = new BufferedReader(new InputStreamReader(process.getInputStream()))) {
 				String line;
-				while (!isClosing && (line = processReader.readLine()) != null) {
+				while (!isClosing && (line = readLine(processReader, inputMaxSize)) != null) {
 					try {
 						JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(this.jsonMapper, line);
 						if (!this.inboundSink.tryEmitNext(message).isSuccess()) {
@@ -278,6 +297,11 @@ private void startInboundProcessing() {
 					}
 				}
 			}
+			catch (MaxSizeExceededException e) {
+				if (!isClosing) {
+					logger.error("Inbound message exceeds the maximum allowed size", e);
+				}
+			}
 			catch (IOException e) {
 				if (!isClosing) {
 					logger.error("Error reading from input stream", e);
@@ -290,6 +314,37 @@ private void startInboundProcessing() {
 		});
 	}
 
+	/**
+	 * Reads a single line, mirroring {@link BufferedReader#readLine()}, but aborting once
+	 * more than {@code maxSize} characters have been read without encountering a line
+	 * terminator. This bounds how much memory a single inbound message can occupy.
+	 */
+	static String readLine(BufferedReader reader, int maxSize) throws IOException, MaxSizeExceededException {
+		StringBuilder sb = new StringBuilder();
+		int c;
+		while ((c = reader.read()) != -1) {
+			if (c == '\n') {
+				return sb.toString();
+			}
+			if (c == '\r') {
+				// Consume an optional trailing '\n' so that "\r\n" is treated as a
+				// single terminator, mirroring BufferedReader#readLine().
+				reader.mark(1);
+				int next = reader.read();
+				if (next != '\n' && next != -1) {
+					reader.reset();
+				}
+				return sb.toString();
+			}
+			if (sb.length() >= maxSize) {
+				throw new MaxSizeExceededException(
+						"Inbound message exceeds the maximum allowed size of " + maxSize + " characters");
+			}
+			sb.append((char) c);
+		}
+		return sb.isEmpty() ? null : sb.toString();
+	}
+
 	/**
 	 * Starts the outbound processing thread that writes JSON-RPC messages to the
 	 * process's output stream. Messages are serialized to JSON and written with a newline

mcp-test/src/test/java/io/modelcontextprotocol/client/transport/StdioClientTransportTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.time.Duration;
+
+import org.awaitility.Awaitility;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import reactor.test.StepVerifier;
+
+import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests for {@link StdioClientTransport}.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+class StdioClientTransportTests {
+
+	private final PrintStream originalOut = System.out;
+
+	private final PrintStream originalErr = System.err;
+
+	private ByteArrayOutputStream testErr;
+
+	@BeforeEach
+	void setUp() {
+		testErr = new ByteArrayOutputStream();
+		PrintStream testOutPrintStream = new PrintStream(testErr, true);
+		System.setOut(testOutPrintStream);
+		System.setErr(testOutPrintStream);
+	}
+
+	@AfterEach
+	void tearDown() {
+		System.setOut(originalOut);
+		System.setErr(originalErr);
+	}
+
+	@Test
+	void shouldRejectInboundMessageExceedingMaxSize() throws Exception {
+		// A server process that emits an endless line with no newline terminator. A
+		// plain BufferedReader#readLine would buffer it all; the bounded reader must
+		// abort instead of exhausting memory.
+		int maxSize = 1024;
+		ServerParameters params = ServerParameters.builder("sh").args("-c", "while :; do printf a; done").build();
+
+		StdioClientTransport transport = new StdioClientTransport(params, JSON_MAPPER, maxSize);
+		try {
+			StepVerifier.create(transport.connect(msg -> msg)).verifyComplete();
+
+			Awaitility.await()
+				.atMost(Duration.ofSeconds(5))
+				.pollInterval(Duration.ofMillis(100))
+				.untilAsserted(() -> assertThat(testErr.toString())
+					.contains("Inbound message exceeds the maximum allowed size"));
+		}
+		finally {
+			StepVerifier.create(transport.closeGracefully()).verifyComplete();
+		}
+	}
+
+}