[mcp] fix some edge cases for streamable HTTP

lib/srv/mcp/http.go

@@ -143,9 +143,13 @@ func (t *streamableHTTPTransport) RoundTrip(r *http.Request) (*http.Response, er
 
 	default:
 		t.emitInvalidHTTPRequest(t.parentCtx, r)
+
+		statusText := http.StatusText(http.StatusMethodNotAllowed)
 		return &http.Response{
 			Request:    r,
+			Status:     statusText,
 			StatusCode: http.StatusMethodNotAllowed,
+			Body:       io.NopCloser(bytes.NewReader(nil)), // Body must not be nil.
 		}, nil
 	}
 }

lib/srv/mcp/http_test.go

@@ -168,6 +168,18 @@ func Test_handleStreamableHTTP(t *testing.T) {
 		require.False(t, lastEvent.Success)
 		require.Equal(t, "HTTP 404 Not Found", lastEvent.Error)
 	})
+
+	t.Run("unsupported method", func(t *testing.T) {
+		emitter.Reset()
+		httpClient := listener.MakeHTTPClient()
+		request, err := http.NewRequestWithContext(t.Context(), http.MethodOptions, "http://localhost/", nil)
+		require.NoError(t, err)
+		response, err := httpClient.Do(request)
+		require.NoError(t, err)
+		defer response.Body.Close()
+		require.Equal(t, http.StatusMethodNotAllowed, response.StatusCode)
+		require.Equal(t, libevents.MCPSessionInvalidHTTPRequest, emitter.LastEvent().GetType())
+	})
 }
 
 func Test_handleAuthErrHTTP(t *testing.T) {

lib/utils/mcputils/http.go

@@ -26,6 +26,7 @@ import (
 	"log/slog"
 	"mime"
 	"net/http"
+	"strconv"
 
 	"github.com/gravitational/trace"
 	mcpclienttransport "github.com/mark3labs/mcp-go/client/transport"
@@ -76,7 +77,11 @@ func ReplaceHTTPResponse(ctx context.Context, resp *http.Response, processor Ser
 			return trace.Wrap(err)
 		}
 		resp.Body = io.NopCloser(bytes.NewReader(respToClientAsBody))
+
+		// Make sure content length in both the response field and the header
+		// are updated.
 		resp.ContentLength = int64(len(respToClientAsBody))
+		resp.Header.Set("Content-Length", strconv.FormatInt(int64(len(respToClientAsBody)), 10))
 		return nil
 
 	case "text/event-stream":
@@ -88,6 +93,11 @@ func ReplaceHTTPResponse(ctx context.Context, resp *http.Response, processor Ser
 			SSEResponseReader: NewSSEResponseReader(resp.Body),
 			processor:         processor,
 		}
+
+		// Content-Length should be -1 from server for streams. Force to -1 again just to
+		// be sure.
+		resp.ContentLength = -1
+		resp.Header.Del("Content-Length")
 		return nil
 	default:
 		return trace.BadParameter("unsupported response type %s", mediaType)

lib/utils/mcputils/http_test.go

@@ -19,7 +19,9 @@
 package mcputils
 
 import (
+	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"log/slog"
 	"maps"
@@ -49,7 +51,7 @@ func TestReplaceHTTPResponse(t *testing.T) {
 		ctx := t.Context()
 
 		// Set up a server. Use InMemoryListener for synctest.
-		mcpServer := mcptest.NewServer()
+		mcpServer := mcptest.NewServerWithVersion("11.22.33")
 		listener := listenerutils.NewInMemoryListener()
 		httpServer := http.Server{
 			Handler: mcpserver.NewStreamableHTTPServer(mcpServer),
@@ -77,7 +79,8 @@ func TestReplaceHTTPResponse(t *testing.T) {
 		require.NoError(t, client.Start(ctx))
 
 		// Initialize client and call a tool.
-		mcptest.MustInitializeClient(t, client)
+		result := mcptest.MustInitializeClient(t, client)
+		require.Equal(t, "111.222.333", result.ServerInfo.Version)
 		mcptest.MustCallServerTool(t, client)
 		require.Equal(t, uint32(2), httpClientTransport.countMCPResponse.Load())
 
@@ -136,10 +139,27 @@ func (t *testReplaceHTTPResponseTransport) RoundTrip(r *http.Request) (*http.Res
 	if err := ReplaceHTTPResponse(r.Context(), resp, t); err != nil {
 		return nil, trace.Wrap(err)
 	}
+
+	if resp != nil {
+		switch {
+		case resp.ContentLength >= 0:
+			if resp.Header.Get("Content-Length") != fmt.Sprintf("%d", resp.ContentLength) {
+				return nil, trace.CompareFailed("Content-Length does not match Content-Length header")
+			}
+
+		default:
+			if resp.Header.Get("Content-Length") != "" {
+				return nil, trace.CompareFailed("Content-Length does not match Content-Length header")
+			}
+		}
+	}
+
 	return resp, nil
 }
 
 func (t *testReplaceHTTPResponseTransport) ProcessResponse(_ context.Context, response *JSONRPCResponse) mcp.JSONRPCMessage {
+	// Replace server version.
+	response.Result = bytes.ReplaceAll(response.Result, []byte("11.22.33"), []byte("111.222.333"))
 	t.countMCPResponse.Add(1)
 	return response
 }