diff --git a/packages/api/src/beacon/routes/validator.ts b/packages/api/src/beacon/routes/validator.ts
index ef946d5f7352..fb2bfcc4dbcc 100644
--- a/packages/api/src/beacon/routes/validator.ts
+++ b/packages/api/src/beacon/routes/validator.ts
@@ -1,6 +1,6 @@
 import {ContainerType, Type, ValueOf} from "@chainsafe/ssz";
 import {ChainForkConfig} from "@lodestar/config";
-import {isForkPostDeneb, isForkPostElectra} from "@lodestar/params";
+import {VALIDATOR_REGISTRY_LIMIT, isForkPostDeneb, isForkPostElectra} from "@lodestar/params";
 import {
   Attestation,
   BLSSignature,
@@ -44,6 +44,7 @@ import {
   VersionType,
 } from "../../utils/metadata.js";
 import {fromGraffitiHex, toBoolean, toGraffitiHex} from "../../utils/serdes.js";
+import {WireFormat} from "../../utils/wireFormat.js";
 
 export enum BuilderSelection {
   Default = "default",
@@ -209,7 +210,10 @@ export const ProposerPreparationDataListType = ArrayOf(ProposerPreparationDataTy
 export const BeaconCommitteeSelectionListType = ArrayOf(BeaconCommitteeSelectionType);
 export const SyncCommitteeSelectionListType = ArrayOf(SyncCommitteeSelectionType);
 export const LivenessResponseDataListType = ArrayOf(LivenessResponseDataType);
-export const SignedValidatorRegistrationV1ListType = ArrayOf(ssz.bellatrix.SignedValidatorRegistrationV1);
+export const SignedValidatorRegistrationV1ListType = ArrayOf(
+  ssz.bellatrix.SignedValidatorRegistrationV1,
+  VALIDATOR_REGISTRY_LIMIT
+);
 
 export type ValidatorIndices = ValueOf<typeof ValidatorIndicesType>;
 export type AttesterDuty = ValueOf<typeof AttesterDutyType>;
@@ -955,6 +959,9 @@ export function getDefinitions(config: ChainForkConfig): RouteDefinitions<Endpoi
         },
       },
       resp: EmptyResponseCodec,
+      init: {
+        requestWireFormat: WireFormat.ssz,
+      },
     },
   };
 }
diff --git a/packages/api/src/builder/routes.ts b/packages/api/src/builder/routes.ts
index a056eeb38aa2..5d2cfeddc8e1 100644
--- a/packages/api/src/builder/routes.ts
+++ b/packages/api/src/builder/routes.ts
@@ -1,5 +1,5 @@
 import {ChainForkConfig} from "@lodestar/config";
-import {ForkName, isForkPostDeneb} from "@lodestar/params";
+import {ForkName, VALIDATOR_REGISTRY_LIMIT, isForkPostDeneb} from "@lodestar/params";
 import {
   BLSPubkey,
   ExecutionPayload,
@@ -22,13 +22,13 @@ import {
   EmptyRequestCodec,
   EmptyResponseCodec,
   EmptyResponseData,
-  JsonOnlyReq,
   WithVersion,
 } from "../utils/codecs.js";
 import {getPostBellatrixForkTypes, getPostDenebForkTypes, toForkName} from "../utils/fork.js";
 import {fromHeaders} from "../utils/headers.js";
 import {Endpoint, RouteDefinitions, Schema} from "../utils/index.js";
 import {MetaHeader, VersionCodec, VersionMeta} from "../utils/metadata.js";
+import {WireFormat} from "../utils/wireFormat.js";
 
 // Mev-boost might not return any data if there are no bids from builders or min-bid threshold was not reached.
 // In this case, we receive a success response (204) which is not handled as an error. The generic response
@@ -36,7 +36,7 @@ import {MetaHeader, VersionCodec, VersionMeta} from "../utils/metadata.js";
 // It is important that this type indicates that there might be no value to ensure it is properly handled downstream.
 export type MaybeSignedBuilderBid = SignedBuilderBid | undefined;
 
-const RegistrationsType = ArrayOf(ssz.bellatrix.SignedValidatorRegistrationV1);
+const RegistrationsType = ArrayOf(ssz.bellatrix.SignedValidatorRegistrationV1, VALIDATOR_REGISTRY_LIMIT);
 
 export type Endpoints = {
   status: Endpoint<
@@ -88,12 +88,17 @@ export function getDefinitions(config: ChainForkConfig): RouteDefinitions<Endpoi
     registerValidator: {
       url: "/eth/v1/builder/validators",
       method: "POST",
-      req: JsonOnlyReq({
+      req: {
         writeReqJson: ({registrations}) => ({body: RegistrationsType.toJson(registrations)}),
         parseReqJson: ({body}) => ({registrations: RegistrationsType.fromJson(body)}),
+        writeReqSsz: ({registrations}) => ({body: RegistrationsType.serialize(registrations)}),
+        parseReqSsz: ({body}) => ({registrations: RegistrationsType.deserialize(body)}),
         schema: {body: Schema.ObjectArray},
-      }),
+      },
       resp: EmptyResponseCodec,
+      init: {
+        requestWireFormat: WireFormat.ssz,
+      },
     },
     getHeader: {
       url: "/eth/v1/builder/header/{slot}/{parent_hash}/{pubkey}",