mcp: properly handle missing session ID header (#1366)

**Description**

Return a 400 Bad Request when the client does not send the session ID
header, according to the spec [1].

**Related Issues/PRs (if applicable)**

Fixes #1364

**Special notes for reviewers (if applicable)**

N/A

1:
https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#session-management

---------

Signed-off-by: Ignasi Barrera <[email protected]>

Author: nacx

internal/mcpproxy/handlers.go

@@ -154,12 +154,20 @@ func (m *MCPProxy) servePOST(w http.ResponseWriter, r *http.Request) {
 		onErrorResponse(w, http.StatusBadRequest, fmt.Sprintf("invalid JSON-RPC message: %v", err))
 		return
 	}
+
 	switch msg := rawMsg.(type) {
 	case *jsonrpc.Response:
 		if str, ok := msg.ID.Raw().(string); ok && strings.HasPrefix(str, envoyAIGatewayServerToClientPingRequestIDPrefix) {
 			w.Header().Set(sessionIDHeader, string(s.clientGatewaySessionID()))
 			w.WriteHeader(http.StatusAccepted)
 		} else {
+			// We do require a Session ID. If it is not present, a 400 Bad Request response should be returned:
+			// https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#session-management
+			if s == nil {
+				errType = metrics.MCPErrorInvalidSessionID
+				onErrorResponse(w, http.StatusBadRequest, "missing session ID")
+				return
+			}
 			m.l.Debug("Decoded MCP response", slog.Any("response", msg))
 			err = m.handleClientToServerResponse(ctx, s, w, msg)
 		}
@@ -169,6 +177,16 @@ func (m *MCPProxy) servePOST(w http.ResponseWriter, r *http.Request) {
 			m.l.Debug("Decoded MCP request",
 				slog.Any("id", msg.ID), slog.String("method", msg.Method), slog.String("params", string(msg.Params)))
 		}
+
+		// We do require a Session ID. If it is not present for requests other than initialize,
+		// a 400 Bad Request response should be returned:
+		// https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#session-management
+		if s == nil && msg.Method != "initialize" {
+			errType = metrics.MCPErrorInvalidSessionID
+			onErrorResponse(w, http.StatusBadRequest, "missing session ID")
+			return
+		}
+
 		switch msg.Method {
 		case "notifications/roots/list_changed":
 			p := &mcp.RootsListChangedParams{}

internal/mcpproxy/handlers_test.go

@@ -32,7 +32,7 @@ import (
 	"github.com/envoyproxy/ai-gateway/internal/internalapi"
 	"github.com/envoyproxy/ai-gateway/internal/metrics"
 	"github.com/envoyproxy/ai-gateway/internal/testing/testotel"
-	tracing "github.com/envoyproxy/ai-gateway/internal/tracing"
+	"github.com/envoyproxy/ai-gateway/internal/tracing"
 	tracingapi "github.com/envoyproxy/ai-gateway/internal/tracing/api"
 )
 
@@ -164,6 +164,15 @@ func TestServePOST_InvalidSessionID(t *testing.T) {
 	require.Contains(t, rr.Body.String(), "invalid session ID")
 }
 
+func TestServePOST_MissingSessionID(t *testing.T) {
+	proxy := newTestMCPProxy()
+	req := httptest.NewRequest(http.MethodPost, "/mcp", strings.NewReader(`{"jsonrpc":"2.0","method":"tools/call","params":{"name":"test-tool"},"id":"1"}`))
+	rr := httptest.NewRecorder()
+	proxy.servePOST(rr, req)
+	require.Equal(t, http.StatusBadRequest, rr.Code)
+	require.Contains(t, rr.Body.String(), "missing session ID")
+}
+
 func TestServePOST_InitializeRequest(t *testing.T) {
 	// Create a test server to simulate the mcp backend listener.
 	testServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -611,6 +620,7 @@ func TestServePOST_UnsupportedMethod(t *testing.T) {
 	require.NoError(t, err)
 
 	httpReq := httptest.NewRequest(http.MethodPost, "/mcp", bytes.NewReader(body))
+	httpReq.Header.Set(sessionIDHeader, secureID(t, proxy, "test-route@@backend1:dGVzdC1zZXNzaW9u")) // "test-session" base64 encoded.
 	rr := httptest.NewRecorder()
 
 	proxy.servePOST(rr, httpReq)
@@ -803,6 +813,7 @@ func TestServePOST_NotificationsInitialized(t *testing.T) {
 	require.NoError(t, err)
 
 	httpReq := httptest.NewRequest(http.MethodPost, "/mcp", bytes.NewReader(body))
+	httpReq.Header.Set(sessionIDHeader, secureID(t, proxy, "test-route@@backend1:dGVzdC1zZXNzaW9u")) // "test-session" base64 encoded.
 	rr := httptest.NewRecorder()
 
 	proxy.servePOST(rr, httpReq)
@@ -883,6 +894,7 @@ func TestServePOST_InvalidPromptsGetParams(t *testing.T) {
 	require.NoError(t, err)
 
 	httpReq := httptest.NewRequest(http.MethodPost, "/mcp", bytes.NewReader(body))
+	httpReq.Header.Set(sessionIDHeader, secureID(t, proxy, "test-route@@backend1:dGVzdC1zZXNzaW9u")) // "test-session" base64 encoded.
 	rr := httptest.NewRecorder()
 
 	proxy.servePOST(rr, httpReq)