Heterogeneous Front-Door Effects: Debiased Estimation with Quasi-Oracle Guarantees

In many applications, treatment and outcome are confounded by unobservables, yet mediators remain unconfounded. The front‑door (FD) adjustment identifies causal effects through mediators even with unmeasured confounding. However, most estimators focus on *average treatment effects*, and work on *heterogeneous treatment effect* (HTE) estimation remain scarce. We address this gap with two *debiased* learners for heterogeneous FD effects: *FD‑DR‑Learner* and *FD‑R‑Learner*. Both attain fast, quasi-oracle rates (i.e., performance comparable to an oracle that knows the nuisances) even when nuisance functions converge as slowly as $n^{-1/4}$. Beyond theory, we demonstrate fast convergence and debiasedness in synthetic and real-world evaluations. Our results show that the proposed learners deliver robust and debiased HTE estimates under the FD scenario.