diff --git a/api/client/builder/client.go b/api/client/builder/client.go
index e740dcea1..b9709abcd 100644
--- a/api/client/builder/client.go
+++ b/api/client/builder/client.go
@@ -304,6 +304,8 @@ func (c *Client) SubmitBlindedBlock(ctx context.Context, sb interfaces.ReadOnlyS
 		}
 		versionOpt := func(r *http.Request) {
 			r.Header.Add("Eth-Consensus-Version", version.String(version.Bellatrix))
+			r.Header.Set("Content-Type", "application/json")
+			r.Header.Set("Accept", "application/json")
 		}
 		rb, err := c.do(ctx, http.MethodPost, postBlindedBeaconBlockPath, bytes.NewBuffer(body), versionOpt)
 
@@ -341,6 +343,8 @@ func (c *Client) SubmitBlindedBlock(ctx context.Context, sb interfaces.ReadOnlyS
 		}
 		versionOpt := func(r *http.Request) {
 			r.Header.Add("Eth-Consensus-Version", version.String(version.Capella))
+			r.Header.Set("Content-Type", "application/json")
+			r.Header.Set("Accept", "application/json")
 		}
 		rb, err := c.do(ctx, http.MethodPost, postBlindedBeaconBlockPath, bytes.NewBuffer(body), versionOpt)
 
@@ -379,6 +383,8 @@ func (c *Client) SubmitBlindedBlock(ctx context.Context, sb interfaces.ReadOnlyS
 
 		versionOpt := func(r *http.Request) {
 			r.Header.Add("Eth-Consensus-Version", version.String(version.Deneb))
+			r.Header.Set("Content-Type", "application/json")
+			r.Header.Set("Accept", "application/json")
 		}
 		rb, err := c.do(ctx, http.MethodPost, postBlindedBeaconBlockPath, bytes.NewBuffer(body), versionOpt)
 		if err != nil {
diff --git a/api/client/builder/client_test.go b/api/client/builder/client_test.go
index a5dae2648..5131ec716 100644
--- a/api/client/builder/client_test.go
+++ b/api/client/builder/client_test.go
@@ -321,6 +321,8 @@ func TestSubmitBlindedBlock(t *testing.T) {
 			Transport: roundtrip(func(r *http.Request) (*http.Response, error) {
 				require.Equal(t, postBlindedBeaconBlockPath, r.URL.Path)
 				require.Equal(t, "bellatrix", r.Header.Get("Eth-Consensus-Version"))
+				require.Equal(t, "application/json", r.Header.Get("Content-Type"))
+				require.Equal(t, "application/json", r.Header.Get("Accept"))
 				return &http.Response{
 					StatusCode: http.StatusOK,
 					Body:       io.NopCloser(bytes.NewBufferString(testExampleExecutionPayload)),
@@ -347,6 +349,8 @@ func TestSubmitBlindedBlock(t *testing.T) {
 			Transport: roundtrip(func(r *http.Request) (*http.Response, error) {
 				require.Equal(t, postBlindedBeaconBlockPath, r.URL.Path)
 				require.Equal(t, "capella", r.Header.Get("Eth-Consensus-Version"))
+				require.Equal(t, "application/json", r.Header.Get("Content-Type"))
+				require.Equal(t, "application/json", r.Header.Get("Accept"))
 				return &http.Response{
 					StatusCode: http.StatusOK,
 					Body:       io.NopCloser(bytes.NewBufferString(testExampleExecutionPayloadCapella)),
@@ -376,6 +380,8 @@ func TestSubmitBlindedBlock(t *testing.T) {
 			Transport: roundtrip(func(r *http.Request) (*http.Response, error) {
 				require.Equal(t, postBlindedBeaconBlockPath, r.URL.Path)
 				require.Equal(t, "deneb", r.Header.Get("Eth-Consensus-Version"))
+				require.Equal(t, "application/json", r.Header.Get("Content-Type"))
+				require.Equal(t, "application/json", r.Header.Get("Accept"))
 				var req structs.SignedBlindedBeaconBlockDeneb
 				err := json.NewDecoder(r.Body).Decode(&req)
 				require.NoError(t, err)