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fix: return HTTP 404 for METHOD_NOT_FOUND in stateless transport (#1072) #1078

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fix: return HTTP 404 for METHOD_NOT_FOUND in stateless transport (#1072) #1078

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22 changes: 21 additions & 1 deletion ...in/java/io/modelcontextprotocol/server/transport/HttpServletStatelessServerTransport.java
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Original file line number Diff line number Diff line change
Expand Up @@ -172,7 +172,12 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)

response.setContentType(APPLICATION_JSON);
response.setCharacterEncoding(UTF_8);
response.setStatus(HttpServletResponse.SC_OK);

int httpStatus = HttpServletResponse.SC_OK;
if (jsonrpcResponse.error() != null) {
httpStatus = mapJsonRpcErrorToHttpStatus(jsonrpcResponse.error().code());
}
response.setStatus(httpStatus);

String jsonResponseText = jsonMapper.writeValueAsString(jsonrpcResponse);
PrintWriter writer = response.getWriter();
Expand Down Expand Up @@ -223,6 +228,21 @@ else if (message instanceof McpSchema.JSONRPCNotification jsonrpcNotification) {
}
}

/**
* Maps a JSON-RPC error code to an appropriate HTTP status code per the MCP
* Streamable HTTP specification (2026-07-28). Only METHOD_NOT_FOUND is mapped to a
* non-200 status (HTTP 404) as specified by the protocol. All other JSON-RPC errors
* are returned with HTTP 200 per standard JSON-RPC conventions.
* @param jsonRpcErrorCode The JSON-RPC error code
* @return The corresponding HTTP status code
*/
private static int mapJsonRpcErrorToHttpStatus(int jsonRpcErrorCode) {
if (jsonRpcErrorCode == McpSchema.ErrorCodes.METHOD_NOT_FOUND) {
return HttpServletResponse.SC_NOT_FOUND;
}
return HttpServletResponse.SC_OK;
}

/**
* Sends an error response to the client.
* @param response The HTTP servlet response
Expand Down
88 changes: 59 additions & 29 deletions ...st/src/test/java/io/modelcontextprotocol/server/HttpServletStatelessIntegrationTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,7 @@
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

import jakarta.servlet.http.HttpServletResponse;
import org.springframework.mock.web.MockHttpServletRequest;
import org.springframework.mock.web.MockHttpServletResponse;
import org.springframework.web.client.RestClient;
Expand Down Expand Up @@ -773,45 +774,74 @@ void testThrownMcpErrorAndJsonRpcError() throws Exception {
}

@Test
void testMissingHandlerReturnsMethodNotFoundError() {
void testMissingHandlerReturnsMethodNotFoundError() throws Exception {
var mcpServer = McpServer.sync(mcpStatelessServerTransport)
.serverInfo("test-server", "1.0.0")
.capabilities(ServerCapabilities.builder().build())
.build();
var clientTransport = HttpClientStreamableHttpTransport.builder("http://localhost:" + PORT)
.endpoint(CUSTOM_MESSAGE_ENDPOINT)

// Use MockHttpServletRequest/Response to directly verify the HTTP 404 status
// and JSON-RPC METHOD_NOT_FOUND error, since a real HTTP client treats 404
// as a transport-level error per the spec.
McpSchema.JSONRPCRequest jsonrpcRequest = new McpSchema.JSONRPCRequest("foo/bar", "test-request-123");

MockHttpServletRequest request = new MockHttpServletRequest("POST", CUSTOM_MESSAGE_ENDPOINT);
MockHttpServletResponse response = new MockHttpServletResponse();

byte[] content = JSON_MAPPER.writeValueAsBytes(jsonrpcRequest);
request.setContent(content);
request.addHeader("Content-Type", APPLICATION_JSON);
request.addHeader("Content-Length", Integer.toString(content.length));
request.addHeader("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM);
request.addHeader(HttpHeaders.PROTOCOL_VERSION, ProtocolVersions.MCP_2025_03_26);

mcpStatelessServerTransport.service(request, response);

assertThat(response.getStatus()).isEqualTo(HttpServletResponse.SC_NOT_FOUND);

McpSchema.JSONRPCResponse jsonrpcResponse = JSON_MAPPER.readValue(response.getContentAsByteArray(),
McpSchema.JSONRPCResponse.class);

assertThat(jsonrpcResponse).isNotNull();
assertThat(jsonrpcResponse.error()).isNotNull();
assertThat(jsonrpcResponse.error().code()).isEqualTo(McpSchema.ErrorCodes.METHOD_NOT_FOUND);
assertThat(jsonrpcResponse.error().message()).isEqualTo("Method not found: foo/bar");

mcpServer.closeGracefully();
}

@Test
void testUnknownMethodReturnsHttp404WithMethodNotFoundError() throws Exception {
var mcpServer = McpServer.sync(mcpStatelessServerTransport)
.serverInfo("test-server", "1.0.0")
.capabilities(ServerCapabilities.builder().build())
.build();

try (var mcpClient = McpClient.sync(clientTransport).build()) {
// Create a session using an MCP client
McpSchema.InitializeResult initResult = mcpClient.initialize();
assertThat(initResult).isNotNull();
McpSchema.JSONRPCRequest jsonrpcRequest = new McpSchema.JSONRPCRequest("server/discover", "discover-1");

// Override the response handler in the client to capture responses
AtomicReference<McpSchema.JSONRPCResponse> response = new AtomicReference<>();
var handler = (Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>>) (
message) -> message.doOnNext(r -> {
if (r instanceof McpSchema.JSONRPCResponse resp) {
response.set(resp);
}
});
StepVerifier.create(clientTransport.connect(handler)).verifyComplete();
MockHttpServletRequest request = new MockHttpServletRequest("POST", CUSTOM_MESSAGE_ENDPOINT);
MockHttpServletResponse response = new MockHttpServletResponse();

// Send a request for a non-existent method through the transport, bypassing
// the client's capability checks
StepVerifier
.create(clientTransport.sendMessage(new McpSchema.JSONRPCRequest("foo/bar", "test-request-123")))
.verifyComplete();
byte[] content = JSON_MAPPER.writeValueAsBytes(jsonrpcRequest);
request.setContent(content);
request.addHeader("Content-Type", APPLICATION_JSON);
request.addHeader("Content-Length", Integer.toString(content.length));
request.addHeader("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM);
request.addHeader(HttpHeaders.PROTOCOL_VERSION, ProtocolVersions.MCP_2025_03_26);

// Wait until we've received the response
await().atMost(Duration.ofSeconds(1)).until(() -> response.get() != null);
mcpStatelessServerTransport.service(request, response);

assertThat(response.get().error().code()).isEqualTo(McpSchema.ErrorCodes.METHOD_NOT_FOUND);
assertThat(response.get().error().message()).isEqualTo("Method not found: foo/bar");
}
finally {
mcpServer.closeGracefully();
}
assertThat(response.getStatus()).isEqualTo(HttpServletResponse.SC_NOT_FOUND);

McpSchema.JSONRPCResponse jsonrpcResponse = JSON_MAPPER.readValue(response.getContentAsByteArray(),
McpSchema.JSONRPCResponse.class);

assertThat(jsonrpcResponse).isNotNull();
assertThat(jsonrpcResponse.error()).isNotNull();
assertThat(jsonrpcResponse.error().code()).isEqualTo(ErrorCodes.METHOD_NOT_FOUND);
assertThat(jsonrpcResponse.error().message()).isEqualTo("Method not found: server/discover");

mcpServer.close();
}

@Test
Expand Down