diff --git a/execution/execution.proto b/execution/execution.proto
index 5371d4c..e1d8477 100644
--- a/execution/execution.proto
+++ b/execution/execution.proto
@@ -107,6 +107,34 @@ message ValidationRequest {
     uint64 number = 2;
 }
 
+message AssembleBlockRequest {
+	types.H256 parentHash = 1;
+    uint64 timestamp = 2;
+    types.H256 mixDigest = 3;
+    types.H160 suggestedFeeRecipent = 4;
+    repeated types.Withdrawal withdrawals = 5;
+}
+
+message AssembleBlockResponse {
+    uint64 id = 1;
+    bool busy = 2;
+}
+
+message GetAssembledBlockRequest {
+    uint64 id = 1;
+}
+
+message AssembledBlockData {
+    types.ExecutionPayload execution_payload = 1;
+    types.H256 block_value = 2;
+    types.BlobsBundleV1 blobs_bundle = 3;
+}
+
+message GetAssembledBlockResponse {
+    optional AssembledBlockData data = 1;
+    bool busy = 2;
+}
+
 service Execution {
     // Chain Putters.
     rpc InsertHeaders(InsertHeadersRequest) returns(InsertionResult);
@@ -114,7 +142,10 @@ service Execution {
     // Chain Validation and ForkChoice.
     rpc ValidateChain(ValidationRequest) returns(ValidationReceipt);
     rpc UpdateForkChoice(ForkChoice) returns(ForkChoiceReceipt);
-    rpc AssembleBlock(google.protobuf.Empty) returns(types.ExecutionPayload); // Builds on top of current head.
+    // Block Assembly
+    // EAGAIN design here, AssembleBlock initiates the asynchronous request, and GetAssembleBlock just return it if ready.
+    rpc AssembleBlock(AssembleBlockRequest) returns(AssembleBlockResponse); 
+    rpc GetAssembledBlock(GetAssembledBlockRequest) returns(GetAssembledBlockResponse);
     // Chain Getters.
     rpc GetHeader(GetSegmentRequest) returns(GetHeaderResponse);
     rpc GetBody(GetSegmentRequest) returns(GetBodyResponse);