{
  "approach": "Locate the `replace_block_with_op` method in `qiskit/dagcircuit/dagcircuit.py` and add `return new_node` at the end of the method body, after the block replacement logic but before the method ends.",
  "files_to_edit": [
    "qiskit/dagcircuit/dagcircuit.py"
  ],
  "finish_reason": "stop",
  "hypothesis": "The issue describes that `DAGCircuit.replace_block_with_op()` should return the newly created node (`new_node`) but currently does not. The failing test expects the function to return a node whose `.op` attribute is an `XGate`, but instead receives `None`. Adding `return new_node` at the end of the method will fix this.",
  "retries": {},
  "usage": {
    "completion": 159,
    "prompt": 16837,
    "reasoning": 0
  }
}