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Flush headers for idle Streamable HTTP GET requests #1159

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Flush headers for idle Streamable HTTP GET requests #1159

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6124bd7
Flush streamable HTTP GET headers
karthiksenv Oct 3, 2026
980e6a2
Add required AI contribution disclosure
karthiksenv Oct 3, 2026
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1 change: 1 addition & 0 deletions disclosure.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
This change was submitted despite me reading the rules and understanding AI contribution guidelines.
1 change: 1 addition & 0 deletions ...o/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java
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Expand Up @@ -452,6 +452,7 @@ protected void doGet(HttpServletRequest request, HttpServletResponse response)
.listeningStream(sessionTransport);

registerAsyncLifecycle(asyncContext, sessionId, listeningStream::releaseTransport);
response.flushBuffer();
}
catch (Exception e) {
logger.error("Failed to handle GET request for session {}: {}", sessionId, e.getMessage());
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21 changes: 21 additions & 0 deletions ...t/src/test/java/io/modelcontextprotocol/server/HttpServletStreamableIntegrationTests.java
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Expand Up @@ -18,6 +18,7 @@
import java.util.Queue;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;
import java.util.stream.Stream;
Expand Down Expand Up @@ -450,6 +451,26 @@ void sessionIsNotEvictedWithoutSweepInterval() {
}
}

@Test
void getStreamSendsHeadersBeforeFirstEvent() throws Exception {
withTransportProvider(null, null);
String sessionId = initializeSession(httpClient);

var get = HttpRequest.newBuilder()
.uri(URI.create("http://localhost:" + PORT + MESSAGE_ENDPOINT))
.header("Accept", "text/event-stream")
.header(HttpHeaders.MCP_SESSION_ID, sessionId)
.GET()
.build();

HttpResponse<InputStream> response = httpClient.sendAsync(get, HttpResponse.BodyHandlers.ofInputStream())
.get(5, TimeUnit.SECONDS);

assertThat(response.statusCode()).isEqualTo(HttpServletResponse.SC_OK);
assertThat(response.headers().firstValue("Content-Type").orElse("")).startsWith("text/event-stream");
response.body().close();
}

/**
* A client which aborts a tool call must not leave its session behind.
*/
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