Here is an article that explains what the “optional-script-verify-flag (Witness hash mismatch)” error means and why you might see it when trying to send a raw signed transaction via Bitcoinlib:

Error Information: Optional Script Verification Flag

When sending raw Bitcoin transactions, you must ensure that all required flags are included. One of these required flags is the script-verify flag, which allows you to verify the transaction script (the code that performs various operations on the input). However, there is a problem: some scripts rely on the presence of specific hashes from the witness program.

What causes the error?

The “optional-script-verify-flag (Witness hash mismatch)” error usually occurs when the script-verify flag is not included in the transaction script. This can happen if your code is not handling the witness programs correctly or if there are dependencies between scripts that are not met.

What does this mean?

When a transaction is sent, Bitcoin checks several flags to make sure everything is valid and working. The script-verify flag is critical to this process because some transactions rely on hash values ​​specific to the witnesses in the script. If these hash values ​​don’t match, the transaction may be rejected.

Simplified code example:

Here is a simplified example of how you can create a transaction using Bitcoinlib:

from bitcoinlib.transactions import Output, Key

Create a key for the sender
sender_key = Key.from_str("my-sender-key", "hex")

Define an output script (simple example)
output_script = "0c6f1d2c6e8b76e42f8..."

Create an input script that relies on a specific hash from the witness program
input_script = "0b95fa7f3e4daeb5d34..."
def main():

Create input and output scripts
output_output = Output.from_str("0c6f1d2c6e8b76e42f8...", sender_key, "hex", None) 
No script validation flags
input_input = Input.from_str(input_script, sender_key, "hex")

Submit transaction
tx_hash = bitcoinlib.utils.hash(output_output)
print(f"Transaction hash: {tx_hash}")
if __name__ == "__main__":
main()

Conclusion

In this example, we created a simple transaction with multiple outputs and inputs. However, when we submitted this transaction via Bitcoinlib, we encountered a “non-mandatory-script-verify-flag (Witness hash mismatch)” error because one of the input scripts was missing the “script-verify” flag.

Best practices:

To avoid similar issues in the future:

• Always include the script-verify flag in your transaction script.

• Make sure all witness hashes are present and correct.

• Consider adding dependencies between scripts to minimize the risk of errors.

By following these best practices, you can ensure that your transactions pass verification and avoid error messages such as “optional-script-verify-flag (Witness hash mismatch)”.