Set UTF-8 character encoding in server transport

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

@@ -387,6 +387,10 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
+			// Servlet containers (e.g. Tomcat, Undertow) default to ISO-8859-1 when no
+			// charset is specified in the Content-Type header. JSON is always UTF-8 per
+			// RFC 8259, so we must explicitly set the encoding before reading the body.
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;

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

@@ -154,6 +154,10 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
+			// Servlet containers (e.g. Tomcat, Undertow) default to ISO-8859-1 when no
+			// charset is specified in the Content-Type header. JSON is always UTF-8 per
+			// RFC 8259, so we must explicitly set the encoding before reading the body.
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;

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

@@ -429,6 +429,10 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		try {
+			// Servlet containers (e.g. Tomcat, Undertow) default to ISO-8859-1 when no
+			// charset is specified in the Content-Type header. JSON is always UTF-8 per
+			// RFC 8259, so we must explicitly set the encoding before reading the body.
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;

mcp-test/src/main/java/io/modelcontextprotocol/AbstractMcpClientServerIntegrationTests.java

@@ -818,6 +818,44 @@ void testToolCallSuccess(String clientType) {
 		}
 	}
 
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@MethodSource("clientsForTesting")
+	void testToolCallWithUnicodeArguments(String clientType) {
+
+		var clientBuilder = clientBuilders.get(clientType);
+
+		// String containing multi-byte UTF-8 characters: em dash, Chinese, emoji
+		String unicodeInput = "Test \u2014 em dash, \u5929\u6c14\u9884\u62a5, \ud83d\ude00";
+
+		McpServerFeatures.SyncToolSpecification echoTool = McpServerFeatures.SyncToolSpecification.builder()
+			.tool(Tool.builder().name("echo").description("Echoes input").inputSchema(EMPTY_JSON_SCHEMA).build())
+			.callHandler((exchange, request) -> {
+				String text = (String) request.arguments().get("text");
+				return CallToolResult.builder().addContent(new TextContent(text)).build();
+			})
+			.build();
+
+		var mcpServer = prepareSyncServerBuilder().capabilities(ServerCapabilities.builder().tools(true).build())
+			.tools(echoTool)
+			.build();
+
+		try (var mcpClient = clientBuilder.build()) {
+			InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			CallToolResult response = mcpClient
+				.callTool(new McpSchema.CallToolRequest("echo", Map.of("text", unicodeInput)));
+
+			assertThat(response).isNotNull();
+			assertThat(response.content()).hasSize(1);
+			assertThat(response.content().get(0)).isInstanceOf(TextContent.class);
+			assertThat(((TextContent) response.content().get(0)).text()).isEqualTo(unicodeInput);
+		}
+		finally {
+			mcpServer.closeGracefully();
+		}
+	}
+
 	@ParameterizedTest(name = "{0} : {displayName} ")
 	@MethodSource("clientsForTesting")
 	void testThrowingToolCallIsCaughtBeforeTimeout(String clientType) {

mcp-test/src/test/java/io/modelcontextprotocol/server/HttpServletStatelessIntegrationTests.java

@@ -639,6 +639,44 @@ void testThrownMcpErrorAndJsonRpcError() throws Exception {
 		mcpServer.close();
 	}
 
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@ValueSource(strings = { "httpclient" })
+	void testToolCallWithUnicodeArguments(String clientType) {
+
+		var clientBuilder = clientBuilders.get(clientType);
+
+		// String containing multi-byte UTF-8 characters: em dash, Chinese, emoji
+		String unicodeInput = "Test \u2014 em dash, \u5929\u6c14\u9884\u62a5, \ud83d\ude00";
+
+		var echoTool = new McpStatelessServerFeatures.SyncToolSpecification(
+				Tool.builder().name("echo").description("Echoes input").inputSchema(EMPTY_JSON_SCHEMA).build(),
+				(transportContext, request) -> {
+					String text = (String) request.arguments().get("text");
+					return CallToolResult.builder().content(List.of(new TextContent(text))).build();
+				});
+
+		var mcpServer = McpServer.sync(mcpStatelessServerTransport)
+			.capabilities(ServerCapabilities.builder().tools(true).build())
+			.tools(echoTool)
+			.build();
+
+		try (var mcpClient = clientBuilder.build()) {
+			InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			CallToolResult response = mcpClient
+				.callTool(new McpSchema.CallToolRequest("echo", Map.of("text", unicodeInput)));
+
+			assertThat(response).isNotNull();
+			assertThat(response.content()).hasSize(1);
+			assertThat(response.content().get(0)).isInstanceOf(TextContent.class);
+			assertThat(((TextContent) response.content().get(0)).text()).isEqualTo(unicodeInput);
+		}
+		finally {
+			mcpServer.close();
+		}
+	}
+
 	private double evaluateExpression(String expression) {
 		// Simple expression evaluator for testing
 		return switch (expression) {