Preserve whitespace in SSE field values

Author: NewPeople-star

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

@@ -498,7 +498,7 @@ Optional<SseEvent> feed(String line) {
 				// valueless `data:` line still marks the event as carrying data and gets
 				// dispatched with empty data. Servers send such an event to prime a
 				// stream, and dropping it leaves the request it answers hanging.
-				String value = line.substring(5).trim();
+				String value = fieldValue(line, 5);
 				// Measured before appending, so that an event carrying exactly
 				// maxSize of data is accepted: the trailing separator below is
 				// stripped again before the event is emitted.
@@ -509,15 +509,15 @@ Optional<SseEvent> feed(String line) {
 				data.append(value).append('\n');
 			}
 			else if (line.startsWith("id:")) {
-				String value = line.substring(3).trim();
+				String value = fieldValue(line, 3);
 				// The spec ignores an id carrying a NULL, and an empty id resets the last
 				// event ID, which leaves nothing to resume from.
 				if (value.indexOf('\0') == -1) {
 					id = value.isEmpty() ? null : value;
 				}
 			}
 			else if (line.startsWith("event:")) {
-				String value = line.substring(6).trim();
+				String value = fieldValue(line, 6);
 				event = value.isEmpty() ? null : value;
 			}
 			else if (line.startsWith(":")) {
@@ -531,6 +531,15 @@ else if (line.startsWith(":")) {
 			return Optional.empty();
 		}
 
+		private static String fieldValue(String line, int offset) {
+			// SSE removes only one optional U+0020 after the colon; all other
+			// leading and trailing whitespace belongs to the field value.
+			if (line.length() > offset && line.charAt(offset) == ' ') {
+				offset++;
+			}
+			return line.substring(offset);
+		}
+
 		/**
 		 * Emits the pending event, if a {@code data:} field was seen, and resets the
 		 * per-event state. The event type is reset even when nothing is dispatched, as
@@ -543,7 +552,9 @@ Optional<SseEvent> flush() {
 			if (data.isEmpty()) {
 				return Optional.empty();
 			}
-			SseEvent result = new SseEvent(id, type != null ? type : DEFAULT_EVENT_TYPE, data.toString().trim());
+			// Remove only the final LF appended by the parser, preserving the payload.
+			SseEvent result = new SseEvent(id, type != null ? type : DEFAULT_EVENT_TYPE,
+					data.substring(0, data.length() - 1));
 			data.setLength(0);
 			return Optional.of(result);
 		}

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

@@ -5,13 +5,20 @@
 package io.modelcontextprotocol.client.transport;
 
 import java.util.Optional;
+import java.util.stream.Stream;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import io.modelcontextprotocol.client.transport.ResponseBodyHandlers.SseEvent;
 import io.modelcontextprotocol.client.transport.ResponseBodyHandlers.SseEventParser;
+import io.modelcontextprotocol.spec.McpTransportException;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 class SseEventParserTests {
 
@@ -27,13 +34,56 @@ void simpleDataEvent() {
 	}
 
 	@Test
-	void multiLineDataAccumulatesWithNewlineSeparatorAndTrims() {
+	void multiLineDataAccumulatesWithNewlineSeparatorAndPreservesWhitespace() {
 		SseEventParser p = new SseEventParser(Integer.MAX_VALUE);
-		assertThat(p.feed("data: first")).isEmpty();
-		assertThat(p.feed("data: second")).isEmpty();
+		assertThat(p.feed("data:  first ")).isEmpty();
+		assertThat(p.feed("data: \tsecond\t")).isEmpty();
 		Optional<SseEvent> event = p.feed("");
 		assertThat(event).isPresent();
-		assertThat(event.get().data()).isEqualTo("first\nsecond");
+		assertThat(event.get().data()).isEqualTo(" first \n\tsecond\t");
+	}
+
+	@ParameterizedTest
+	@MethodSource("fieldValues")
+	void fieldValuesRemoveAtMostOneLeadingSpace(String input, String expected) {
+		SseEventParser p = new SseEventParser(Integer.MAX_VALUE);
+		p.feed("id:" + input);
+		p.feed("event:" + input);
+		p.feed("data:" + input);
+		SseEvent first = p.feed("").orElseThrow();
+		assertThat(first.id()).isEqualTo(expected);
+		assertThat(first.event()).isEqualTo(expected);
+		assertThat(first.data()).isEqualTo(expected);
+
+		p.feed("data: next");
+		SseEvent second = p.feed("").orElseThrow();
+		assertThat(second.id()).isEqualTo(expected);
+		assertThat(second.event()).isEqualTo("message");
+		assertThat(second.data()).isEqualTo("next");
+	}
+
+	static Stream<Arguments> fieldValues() {
+		return Stream.of(Arguments.of("token", "token"), Arguments.of(" token", "token"),
+				Arguments.of(" token ", "token "), Arguments.of("  token  ", " token  "),
+				Arguments.of("\ttoken\t", "\ttoken\t"), Arguments.of(" \ttoken\t", "\ttoken\t"),
+				Arguments.of("  ", " "), Arguments.of("\t", "\t"));
+	}
+
+	@Test
+	void emptyDataLinesPreserveLeadingAndTrailingNewlines() {
+		SseEventParser p = new SseEventParser(Integer.MAX_VALUE);
+		p.feed("data:");
+		p.feed("data: payload");
+		p.feed("data:");
+		assertThat(p.feed("").orElseThrow().data()).isEqualTo("\npayload\n");
+	}
+
+	@Test
+	void preservedWhitespaceCountsTowardsSizeLimit() {
+		SseEventParser p = new SseEventParser(3);
+		p.feed("data:  a ");
+		assertThat(p.feed("").orElseThrow().data()).isEqualTo(" a ");
+		assertThatThrownBy(() -> p.feed("data:  a  ")).isInstanceOf(McpTransportException.class);
 	}
 
 	@Test
@@ -90,11 +140,12 @@ void emptyIdClearsLastEventId() {
 		assertThat(p.feed("").orElseThrow().id()).isNull();
 	}
 
-	@Test
-	void idContainingNullIsIgnored() {
+	@ParameterizedTest
+	@ValueSource(strings = { "\0token", "token\0", "to\0ken" })
+	void idContainingNullIsIgnored(String id) {
 		SseEventParser p = new SseEventParser(Integer.MAX_VALUE);
 		p.feed("id: 1");
-		p.feed("id: 2\u0000" + "3");
+		p.feed("id: " + id);
 		p.feed("data: payload");
 		assertThat(p.feed("").orElseThrow().id()).isEqualTo("1");
 	}