diff --git a/beacon/types/committee.go b/beacon/types/committee.go
index 5f89c27554..9d3b9ff9e9 100644
--- a/beacon/types/committee.go
+++ b/beacon/types/committee.go
@@ -164,21 +164,44 @@ func (sc *SyncCommittee) VerifySignature(signingRoot common.Hash, signature *Syn
 	return bls.FastAggregateVerify(keys, signingRoot[:], &sig)
 }
 
-//go:generate go run github.com/fjl/gencodec -type SyncAggregate -field-override syncAggregateMarshaling -out gen_syncaggregate_json.go
-
 // SyncAggregate represents an aggregated BLS signature with Signers referring
 // to a subset of the corresponding sync committee.
 //
 // See data structure definition here:
 // https://github.com/ethereum/consensus-specs/blob/dev/specs/altair/beacon-chain.md#syncaggregate
 type SyncAggregate struct {
-	Signers   [params.SyncCommitteeBitmaskSize]byte `gencodec:"required" json:"sync_committee_bits"`
-	Signature [params.BLSSignatureSize]byte         `gencodec:"required" json:"sync_committee_signature"`
+	Signers   [params.SyncCommitteeBitmaskSize]byte
+	Signature [params.BLSSignatureSize]byte
 }
 
-type syncAggregateMarshaling struct {
-	Signers   hexutil.Bytes
-	Signature hexutil.Bytes
+type jsonSyncAggregate struct {
+	Signers   hexutil.Bytes `json:"sync_committee_bits"`
+	Signature hexutil.Bytes `json:"sync_committee_signature"`
+}
+
+// MarshalJSON implements json.Marshaler.
+func (s *SyncAggregate) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&jsonSyncAggregate{
+		Signers:   s.Signers[:],
+		Signature: s.Signature[:],
+	})
+}
+
+// UnmarshalJSON implements json.Marshaler.
+func (s *SyncAggregate) UnmarshalJSON(input []byte) error {
+	var sc jsonSyncAggregate
+	if err := json.Unmarshal(input, &sc); err != nil {
+		return err
+	}
+	if len(sc.Signers) != params.SyncCommitteeBitmaskSize {
+		return fmt.Errorf("invalid signature bitmask size %d", len(sc.Signers))
+	}
+	if len(sc.Signature) != params.BLSSignatureSize {
+		return fmt.Errorf("invalid signature size %d", len(sc.Signature))
+	}
+	copy(s.Signers[:], sc.Signers)
+	copy(s.Signature[:], sc.Signature)
+	return nil
 }
 
 // SignerCount returns the number of signers in the aggregate signature.
diff --git a/beacon/types/gen_syncaggregate_json.go b/beacon/types/gen_syncaggregate_json.go
deleted file mode 100644
index 1547ec5f01..0000000000
--- a/beacon/types/gen_syncaggregate_json.go
+++ /dev/null
@@ -1,51 +0,0 @@
-// Code generated by github.com/fjl/gencodec. DO NOT EDIT.
-
-package types
-
-import (
-	"encoding/json"
-	"errors"
-
-	"github.com/ethereum/go-ethereum/common/hexutil"
-)
-
-var _ = (*syncAggregateMarshaling)(nil)
-
-// MarshalJSON marshals as JSON.
-func (s SyncAggregate) MarshalJSON() ([]byte, error) {
-	type SyncAggregate struct {
-		Signers   hexutil.Bytes `gencodec:"required" json:"sync_committee_bits"`
-		Signature hexutil.Bytes `gencodec:"required" json:"sync_committee_signature"`
-	}
-	var enc SyncAggregate
-	enc.Signers = s.Signers[:]
-	enc.Signature = s.Signature[:]
-	return json.Marshal(&enc)
-}
-
-// UnmarshalJSON unmarshals from JSON.
-func (s *SyncAggregate) UnmarshalJSON(input []byte) error {
-	type SyncAggregate struct {
-		Signers   *hexutil.Bytes `gencodec:"required" json:"sync_committee_bits"`
-		Signature *hexutil.Bytes `gencodec:"required" json:"sync_committee_signature"`
-	}
-	var dec SyncAggregate
-	if err := json.Unmarshal(input, &dec); err != nil {
-		return err
-	}
-	if dec.Signers == nil {
-		return errors.New("missing required field 'sync_committee_bits' for SyncAggregate")
-	}
-	if len(*dec.Signers) != len(s.Signers) {
-		return errors.New("field 'sync_committee_bits' has wrong length, need 64 items")
-	}
-	copy(s.Signers[:], *dec.Signers)
-	if dec.Signature == nil {
-		return errors.New("missing required field 'sync_committee_signature' for SyncAggregate")
-	}
-	if len(*dec.Signature) != len(s.Signature) {
-		return errors.New("field 'sync_committee_signature' has wrong length, need 96 items")
-	}
-	copy(s.Signature[:], *dec.Signature)
-	return nil
-}