I want an unsigned payload to sign a transaction later with a ECDSA Private key.
This is a MPC wallet so I don’t have the private key directly. The process is:

• I send the unsigned transaction payload like below:

web3 = Web3(Web3.HTTPProvider(rpc_url))
        nonce = web3.eth.get_transaction_count(web3.to_checksum_address(send_signature.to_address.lower()))

        transaction = {
            "nonce": nonce,
            "to": web3.to_checksum_address(send_signature.to_address.lower()),
            "value": web3.to_wei('0.002, "ether"),
            "gas": 21000,
            "maxPriorityFeePerGas": web3.to_wei("5", "gwei"),
            "maxFeePerGas": web3.to_wei("15", "gwei"),
            "chainId": 56,
            "type": 2,
        }

        # Encode the transaction for hashing
        tx_message = [
            transaction["nonce"],
            transaction["maxPriorityFeePerGas"],
            transaction["maxFeePerGas"],
            transaction["gas"],
            bytes.fromhex(transaction["to"][2:]), 
            transaction["value"],
            b"",  # data field
            [],  # access list
            transaction["chainId"],
            transaction["type"],
        ]

    encoded_tx = rlp.encode(tx_message)
    serialized_tx = keccak(encoded_tx).hex()

    data = {"kind": "Hash", "hash": serialized_tx}

• After the mpc wallet signs the message and returns the data as signature as follows:

{'id': 'sig-98667-gl6f8', 'walletId': 'wa-3q6ek-if17t', 'network': 'KeyECDSA', 'requester': {'userId': 'us-1r68b', 'appId': 'ap'}, 'requestBody': {'kind': 'Hash', 'hash': 'e3a4ca418a03227e9f6db95ded00acef0b86457a71b39818aebf7a3b4dee040c'}, 'status': 'Signed', 'signature': {'r': '0x5f654363ecd0620b53927c5e9ee000870ce009612f9864844xxxxxx', 's': '0x51bf7042f9dfdbd698ee06e659bed7f93f3d902ad8eb16065c1d6f611430f06e', 'recid': 0}, 'dateRequested': '2024-07-28T07:57:46.313Z', 'dateSigned': '2024-07-28T07:57:48.071Z'}

• When I try to publish this transaction:

chain_id = 56
        signature = {
            "r": int(broadcast_data["signature"]["r"], 16),
            "s": int(broadcast_data["signature"]["s"], 16),
            "v": int(broadcast_data["signature"]["recid"]) + 35 + 2 * chain_id,
        }


        signed_tx_message = txn_message + [
            signature["v"],
            to_bytes(signature["r"]),
            to_bytes(signature["s"]),
        ]

        encoded_signed_tx = rlp.encode(signed_tx_message)
        tx_hash = web3.eth.send_raw_transaction(encoded_signed_tx)

I get the following error:

ValueError: {'code': -32000, 'message': 'unmarshal transaction failed'}

References:

• https://docs.dfns.co/d/api-docs/wallets/generate-signature-from-wallet/pseudo-networks-all-other-chains-generate-signature