diff --git a/internal/mcp/network_client.go b/internal/mcp/network_client.go
index b8b829af2..ce0b41162 100644
--- a/internal/mcp/network_client.go
+++ b/internal/mcp/network_client.go
@@ -216,6 +216,9 @@ func (client *networkClient) request(ctx context.Context, method string, params
 	if err != nil {
 		return err
 	}
+	if message.isRequestOrNotification() {
+		return fmt.Errorf("MCP %s expected a response from server %s, got method %q instead", method, client.server.Name, message.Method)
+	}
 	if !rpcIDMatches(message.ID, id) {
 		return fmt.Errorf("MCP %s response id mismatch for server %s", method, client.server.Name)
 	}
diff --git a/internal/mcp/network_client_test.go b/internal/mcp/network_client_test.go
index e4b406447..8cb0d2f13 100644
--- a/internal/mcp/network_client_test.go
+++ b/internal/mcp/network_client_test.go
@@ -316,6 +316,50 @@ func TestNonOAuthServerIsUnaffected(t *testing.T) {
 	}
 }
 
+func TestNetworkClientRejectsServerInitiatedRequestAsResponse(t *testing.T) {
+	// A streamable-HTTP server that answers a tools/call POST with a
+	// method-bearing frame (a server-initiated request) must surface a protocol
+	// error rather than an empty, error-free result.
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		message := readHTTPRPCMessage(t, r)
+		switch message.Method {
+		case "initialize":
+			writeHTTPRPCResponse(t, w, message.ID, map[string]any{"protocolVersion": "2024-11-05"})
+		case "notifications/initialized":
+			w.WriteHeader(http.StatusAccepted)
+		case "tools/call":
+			w.Header().Set("Content-Type", "application/json")
+			if _, err := w.Write([]byte(`{"jsonrpc":"2.0","id":` + string(mustRaw(message.ID)) + `,"method":"roots/list","params":{}}`)); err != nil {
+				t.Errorf("write server request: %v", err)
+			}
+		default:
+			writeHTTPRPCResponse(t, w, message.ID, map[string]any{})
+		}
+	}))
+	defer upstream.Close()
+
+	client, err := Connect(ctx, Server{Name: "plain", Type: ServerTypeHTTP, URL: upstream.URL})
+	if err != nil {
+		t.Fatalf("Connect() error = %v", err)
+	}
+	defer func() {
+		if err := client.Close(); err != nil {
+			t.Fatalf("Close() error = %v", err)
+		}
+	}()
+
+	result, err := client.CallTool(ctx, "lookup", map[string]any{"query": "zero"})
+	if err == nil {
+		t.Fatalf("CallTool() error = nil, result = %#v, want protocol error", result)
+	}
+	if !strings.Contains(err.Error(), `"roots/list"`) {
+		t.Fatalf("CallTool() error = %q, want it to name the unexpected method", err)
+	}
+}
+
 func TestDecodeSSERPCMessageSkipsNotifications(t *testing.T) {
 	// A leading server notification (has a method) on the POST's event stream must
 	// be skipped so the actual response (no method) is returned, instead of the