fix ProduceSyncCommitteeContribution error on invalid index (#15770)

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

beacon-chain/rpc/eth/validator/handlers.go

@@ -690,6 +690,10 @@ func (s *Server) ProduceSyncCommitteeContribution(w http.ResponseWriter, r *http
 	if !ok {
 		return
 	}
+	if index >= params.BeaconConfig().SyncCommitteeSubnetCount {
+		httputil.HandleError(w, fmt.Sprintf("Subcommittee index needs to be between 0 and %d, %d is outside of this range.", params.BeaconConfig().SyncCommitteeSubnetCount-1, index), http.StatusBadRequest)
+		return
+	}
 	_, slot, ok := shared.UintFromQuery(w, r, "slot", true)
 	if !ok {
 		return

beacon-chain/rpc/eth/validator/handlers_test.go

@@ -2117,6 +2117,27 @@ func TestProduceSyncCommitteeContribution(t *testing.T) {
 		server.ProduceSyncCommitteeContribution(writer, request)
 		assert.Equal(t, http.StatusServiceUnavailable, writer.Code)
 	})
+	t.Run("invalid subcommittee_index", func(t *testing.T) {
+		url := "http://example.com?slot=1&subcommittee_index=10&beacon_block_root=0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2"
+		request := httptest.NewRequest(http.MethodGet, url, nil)
+		writer := httptest.NewRecorder()
+		writer.Body = &bytes.Buffer{}
+
+		// Use non-optimistic server for this test
+		server := Server{
+			CoreService: &core.Service{
+				HeadFetcher: &mockChain.ChainService{
+					SyncCommitteeIndices: []primitives.CommitteeIndex{0},
+				},
+			},
+			SyncCommitteePool:     syncCommitteePool,
+			OptimisticModeFetcher: &mockChain.ChainService{}, // Optimistic: false by default
+		}
+
+		server.ProduceSyncCommitteeContribution(writer, request)
+		assert.Equal(t, http.StatusBadRequest, writer.Code)
+		require.ErrorContains(t, "Subcommittee index needs to be between 0 and 3, 10 is outside of this range.", errors.New(writer.Body.String()))
+	})
 }
 
 func TestServer_RegisterValidator(t *testing.T) {

changelog/muzry_fix_produce_sync_committee.md

@@ -0,0 +1,4 @@
+### Fixed
+
+- Fix ProduceSyncCommitteeContribution not returning error when committee index is out of range
+