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Send Mcp-Method header on Streamable HTTP client requests (SEP-2243) #1026

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Send Mcp-Method header on Streamable HTTP client requests (SEP-2243) #1026

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7 changes: 7 additions & 0 deletions ...main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
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Original file line number Diff line number Diff line change
Expand Up @@ -503,6 +503,13 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
transportSession.sessionId().get());
}

if (sentMessage instanceof McpSchema.JSONRPCRequest request) {
requestBuilder = requestBuilder.header(HttpHeaders.MCP_METHOD, request.method());
}
else if (sentMessage instanceof McpSchema.JSONRPCNotification notification) {
requestBuilder = requestBuilder.header(HttpHeaders.MCP_METHOD, notification.method());
}

var builder = requestBuilder.uri(uri)
.header(HttpHeaders.ACCEPT, APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON_UTF8)
Expand Down
7 changes: 7 additions & 0 deletions mcp-core/src/main/java/io/modelcontextprotocol/spec/HttpHeaders.java
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,13 @@ public interface HttpHeaders {
*/
String PROTOCOL_VERSION = "MCP-Protocol-Version";

/**
* Mirrors the JSON-RPC method of the request or notification carried in the body.
* @see <a href=
* "https://modelcontextprotocol.io/seps/2243-http-standardization">SEP-2243</a>
*/
String MCP_METHOD = "Mcp-Method";

/**
* The HTTP Content-Length header.
* @see <a href=
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99 changes: 99 additions & 0 deletions .../java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -7,23 +7,27 @@
import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientRequestCustomizer;
import io.modelcontextprotocol.client.transport.customizer.McpSyncHttpClientRequestCustomizer;
import io.modelcontextprotocol.common.McpTransportContext;
import io.modelcontextprotocol.spec.HttpHeaders;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpTransportSessionClosedException;
import io.modelcontextprotocol.spec.ProtocolVersions;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.http.HttpRequest;
import java.util.Map;
import java.util.function.Consumer;
import java.util.function.Function;

import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
import org.mockito.ArgumentCaptor;
import org.testcontainers.containers.GenericContainer;
import org.testcontainers.containers.wait.strategy.Wait;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.ArgumentMatchers.any;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.atLeastOnce;
Expand Down Expand Up @@ -166,4 +170,99 @@ void testCloseInitialized() {
.verify();
}

@Test
void testMcpMethodHeaderOnRequest() throws URISyntaxException {
var uri = new URI(host + "/mcp");
var mockRequestCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);

var transport = HttpClientStreamableHttpTransport.builder(host)
.httpRequestCustomizer(mockRequestCustomizer)
.build();

withTransport(transport, (t) -> {
var initializeRequest = McpSchema.InitializeRequest
.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().roots(true).build(),
McpSchema.Implementation.builder("MCP Client", "0.3.1").build())
.build();
var testMessage = new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "test-id", initializeRequest);

StepVerifier
.create(t.sendMessage(testMessage).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, context)))
.verifyComplete();

var requestCaptor = ArgumentCaptor.forClass(HttpRequest.Builder.class);
verify(mockRequestCustomizer, atLeastOnce()).customize(requestCaptor.capture(), eq("POST"), eq(uri), any(),
eq(context));
assertThat(requestCaptor.getValue().build().headers().firstValue(HttpHeaders.MCP_METHOD))
.hasValue(McpSchema.METHOD_INITIALIZE);
});
}

@Test
void testMcpMethodHeaderOnNotification() throws URISyntaxException {
var uri = new URI(host + "/mcp");
var mockRequestCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);

var transport = HttpClientStreamableHttpTransport.builder(host)
.httpRequestCustomizer(mockRequestCustomizer)
.build();

withTransport(transport, (t) -> {
var initializeRequest = McpSchema.InitializeRequest
.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().roots(true).build(),
McpSchema.Implementation.builder("MCP Client", "0.3.1").build())
.build();
var initializeMessage = new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "test-id",
initializeRequest);
StepVerifier
.create(t.sendMessage(initializeMessage).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, context)))
.verifyComplete();

var testMessage = new McpSchema.JSONRPCNotification(McpSchema.METHOD_NOTIFICATION_INITIALIZED);

StepVerifier
.create(t.sendMessage(testMessage).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, context)))
.verifyComplete();

var requestCaptor = ArgumentCaptor.forClass(HttpRequest.Builder.class);
verify(mockRequestCustomizer, atLeastOnce()).customize(requestCaptor.capture(), eq("POST"), eq(uri), any(),
eq(context));
assertThat(requestCaptor.getValue().build().headers().firstValue(HttpHeaders.MCP_METHOD))
.hasValue(McpSchema.METHOD_NOTIFICATION_INITIALIZED);
});
}

@Test
void testNoMcpMethodHeaderOnResponse() throws URISyntaxException {
var uri = new URI(host + "/mcp");
var mockRequestCustomizer = mock(McpSyncHttpClientRequestCustomizer.class);

var transport = HttpClientStreamableHttpTransport.builder(host)
.httpRequestCustomizer(mockRequestCustomizer)
.build();

withTransport(transport, (t) -> {
var initializeRequest = McpSchema.InitializeRequest
.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().roots(true).build(),
McpSchema.Implementation.builder("MCP Client", "0.3.1").build())
.build();
var initializeMessage = new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "test-id",
initializeRequest);
StepVerifier
.create(t.sendMessage(initializeMessage).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, context)))
.verifyComplete();

var testMessage = McpSchema.JSONRPCResponse.result("test-id-2", Map.of());

StepVerifier
.create(t.sendMessage(testMessage).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, context)))
.verifyComplete();

var requestCaptor = ArgumentCaptor.forClass(HttpRequest.Builder.class);
verify(mockRequestCustomizer, atLeastOnce()).customize(requestCaptor.capture(), eq("POST"), eq(uri), any(),
eq(context));
assertThat(requestCaptor.getValue().build().headers().firstValue(HttpHeaders.MCP_METHOD)).isEmpty();
});
}

}