Borrowing-dependent estimands in external-control trials: Identification, efficiency, and sensitivity in survival analysis

-consistency; (iv) introduce a sensitivity-analysis framework yielding closed-form bias bounds, E-values, and tipping points; and (v) characterise the sensitivity of the delivered estimand to the pre-specified borrowing rule via Fréchet differentiability. Simulations across 12 factorial scenarios confirm up to 49% RMSE reduction under compatibility, show that the estimators remain unbiased for the borrowing-dependent estimand when compatibility fails, and validate the sensitivity bounds. A worked example in metastatic colorectal cancer illustrates the complete analytical workflow.