REVIEW 3 major objections 6 minor 40 references
ABPT: Amended Backpropagation through Time with Partially Differentiable Rewards
T0 review · 3 major / 6 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read ABPT combines the N-step return with a 0-step value gradient so that non-differentiable reward components no longer bias backpropagation-through-time, yielding faster convergence and higher final rewards on quadrotor flight tasks.
desk verdict A simple and plausibly useful gradient-mixing trick for differentiable simulators, but the empirical claim is weakened by missing the two closest baselines and by the absence of released code. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the 0-step return $J_0^\theta = V_\phi(s_t)$, defined through the action-value function with an entropy bonus: $V_\phi(s) = \mathbb{E}_{a\sim\pi}[Q_\phi(s,a)] + \kappa H(\pi_\theta(\cdot|s))$. Unlike a state-only value $V_\phi(s)$, this form depends on $\theta$ through the policy, so its gradient with respect to $\theta$ is nonzero and can be backpropagated. The N-step return $J_N^\theta = \sum_{k=0}^{N-1}\gamma^k R(s_{t+k}) + (1-d)\gamma^N V_\phi(s_{t+N})$ supplies the direct first-order reward path. Averaging the two yields a gradient whose value-gradient half is immune to non-differentiable rewards; a target critic, TD-$\lambda$ return estimation, entropy regularization, and a state replay buffer stabilize the learned value and exploration.
What would settle it
Compare ABPT with plain BPTT on the racing task while scaling up the non-differentiable gate-passing bonus: if the bonus dominates and ABPT's converged gate-crossing rate or reward falls to BPTT's level, the claimed compensation is bounded. A sharper test is to replace the learned critic with a deliberately poor one at initialization and measure the angle between ABPT's parameter update and the exact analytical gradient of a fully differentiable surrogate reward; if the update direction is no closer than BPTT's, the value-gradient correction is not carrying the argument.
Extended reading notes
Core claim
The central claim is that the biased first-order gradient caused by partially differentiable rewards can be corrected by averaging in the value gradient. With the objective $J_\theta = \frac{1}{2|B|}\sum_i \left(J_N^\theta + J_0^\theta\right)$, where $J_0^\theta = V_\phi(s_t)$ is the state value computed from the learned action-value function $Q_\phi(s, \pi_\theta(s))$, backpropagation produces the average of the value gradient $\nabla^{[q]}_\theta G_{t:t+N}$ and the first-order gradient plus the terminal value term. Because $Q_\phi$ is a differentiable function of $\theta$ regardless of reward differentiability, the 0-step term supplies gradient information about the non-differentiable part of the reward (for example, gate-passing bonuses) that the direct reward path drops. The paper states that this combination 'remains effective in guiding the parameter updates toward the correct direction, when the first-order gradient is biased,' and supports it with a parameter-residual experiment in Appendix A.
Load-bearing premise
The method's correction depends on the learned value function being accurate enough in the states actually visited; if the critic is wrong early in training, the added value-gradient term can push the policy in the wrong direction, and the paper provides no guarantee that the equal-weight average stays unbiased.
Editorial extensions
If this is right
- Where rewards are partially differentiable, ABPT's update should stay closer to the true policy gradient than plain BPTT because the 0-step value gradient carries information about non-differentiable reward components.
- The equal-weight average, rather than a variance-based adaptive mixture, keeps the per-step overhead small, making the method practical on GPU-parallel differentiable simulators.
- On fully differentiable tasks, ABPT should retain BPTT-level convergence speed while gaining exploration from entropy and state replay, as demonstrated for hovering and tracking.
- When non-differentiable rewards dominate the total reward, performance may still degrade; the paper's own limitation statement says the bias may not be fully eliminated in that regime.
Reading between the lines
- If the bias-correction story holds generally, the same 0-step/value-gradient averaging should transfer to other differentiable simulators with contact-rich or hybrid rewards, such as legged locomotion with foot-contact bonuses, not just quadrotors.
- The method depends on the critic's accuracy early in training; a testable extension would adaptively weight the value-gradient term by critic uncertainty instead of a fixed one-half, which the paper explicitly leaves open.
- Because ABPT stores only states rather than transitions, its replay buffer could be augmented with goal-conditioned or adversarially sampled states to stress-test generalization to corner cases.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes ABPT (Amended Backpropagation through Time), an on-policy actor-critic method for training quadrotor policies when the reward function is only partially differentiable. The core idea is to combine a 0-step return (a value term computed through a learned Q-function) with the N-step return used by first-order gradient methods such as SHAC, and to update the actor by differentiating the averaged objective in Eq. (9). Entropy regularization and a state-only replay buffer are added to improve exploration and sample efficiency. The method is evaluated on four quadrotor tasks (hovering, tracking, landing, racing) in the VisFly simulator, with qualitative real-world demonstrations, and is compared against PPO, BPTT, and SHAC. The paper claims that ABPT converges faster and reaches higher final rewards than these baselines, especially when rewards contain non-differentiable components.
Significance. If the empirical claims hold, ABPT is a useful and simple modification of the SHAC/BPTT pipeline for differentiable-simulation policy learning: it is easy to implement, preserves first-order gradient efficiency, and appears to mitigate the bias caused by non-differentiable reward terms. The paper includes four tasks with varying reward differentiability, ablations of the three proposed components, reward-robustness and learning-rate robustness experiments, and real-world video demonstrations. These are genuine strengths. The significance is moderated, however, by the absence of comparisons with the closest prior gradient-mixing methods (AGPO and AOBG), by an algebraic inconsistency in the central derivation in Appendix A, and by the lack of released code or complete implementation details, which makes the empirical claims hard to verify independently.
major comments (3)
- [§6.2, Appendix E] The central empirical claim that ABPT converges significantly faster and achieves higher ultimate rewards than existing learning algorithms is not tested against AGPO [36] and AOBG [11], the two prior methods explicitly designed for biased or high-variance first-order gradients in differentiable simulation. The manuscript only asserts in §6.2 and Appendix E that these methods 'require extremely long training times due to the need for variance estimation at each step' without reporting any experiment or wall-clock measurement. Since ABPT's update is a fixed equal-weight average of a value gradient and a first-order gradient, and AGPO already mixes Q-gradients with first-order gradients using an adaptive ratio, the claimed superiority over existing gradient-mixing methods is currently unverified. The authors should either add these baselines or explicitly restrict the headline claim to the evaluated baselines.
- [Appendix A, Eqs. (16) and (19)] The bias-correction argument hinges on the unproven approximation G_{t:t+N} ≈ V_t - (1-d)γ^N V_{t+N+1} in Eq. (16). The paper gives no formal statement of the approximation error, no condition under which the critic is accurate enough for the value-gradient term to be reliable, and no bound on the residual bias after the equal-weight average. Furthermore, Eq. (19) is algebraically inconsistent: the first line writes ∇J as \bar∇G + (1-d)γ^N ∇V_{t+N+1} with \bar∇G = (∇[q]G + ∇G)/2, while the second line writes it as (∇[q]G + ∇G + (1-d)γ^N ∇V_{t+N+1})/2; these two expressions differ by a factor of two on the terminal-value term. This does not prevent direct implementation of Eq. (9), but it invalidates the derivation as written and should be corrected, together with a precise statement of what gradient is actually being averaged.
- [§6.2, Appendix F] The empirical evaluation is not reproducible from the manuscript as submitted. The abstract states that code will be released at a GitHub URL, while the main text refers to a placeholder 'Anonymous' link, and no code or supplementary material is provided. Network architectures, critic update schedules, the exact adaptive entropy coefficient computation, the TD-λ target details beyond Eq. (13), and the state-buffer sampling mechanism are not specified. For a paper whose contributions are primarily empirical, these details are necessary for verification; please provide complete implementation details or the actual code.
minor comments (6)
- [§1] The sentence 'Though reinforcement learning (RL) can tackle such limitation by self-exploration of agents, it relies on RL relies on zero-order gradient (ZOG) approximations' contains a duplicated phrase 'it relies on RL relies on' and should be reworded.
- [Eq. (4)] The notation '∇[0]θ Jθ' is nonstandard and the definition of the advantage A^{πθ}(s_t, a_t) is not given; please define it explicitly and use a consistent gradient notation throughout.
- [Eqs. (6) and (7)] The first-order gradient formulas omit the expectation over trajectories and the minibatch index; this makes it unclear whether the gradient is computed per trajectory or averaged over the batch.
- [Table 1] In the Racing row, the observation is described as 'state & next 2 gates' but the task text in Appendix C describes four gates; please clarify whether the policy observes only the next two gates and, if so, how this is reconciled with the task description.
- [§6.3 and Figure 1] The real-world validation is qualitative only; no quantitative metrics such as success rate, tracking error, or number of trials are reported. If real-world performance is part of the claimed contribution, please add quantitative results.
- [Abstract and §6.2] The abstract and main text are inconsistent about code availability: the abstract gives a GitHub URL, while the body says the code is 'to be released' at an anonymous placeholder, and the acknowledgments section contains boilerplate text that appears to be from a template; please clean this up.
Circularity Check
No significant circularity: the gradient-averaging identity is a direct differentiation of the paper's own objective, and the empirical superiority claim is validated against external baselines and real-world flight.
full rationale
The derivation in Appendix A is not circular. The objective (9) is explicitly defined as an average of the N-step return and the 0-step return, so the 'proof' that its gradient equals the average of the corresponding gradients is simply the chain rule applied to the definition; no conclusion is imported from the definition. The only nontrivial ingredient is the approximate Bellman consistency (16), which the paper states as an assumption ('Supposed the value function Qϕ is well trained...'), and which is a standard critic-bootstrap condition rather than a restatement of the paper's performance claim. The 0-step value gradient is obtained from a learned Q-function trained with TD-λ targets (Eqs. 12-14) and the reparameterized Gaussian policy (Eq. 11); it is a fitted critic, but it is not relabeled as a held-out prediction, and the claim that the combination reduces bias is tested empirically against PPO, BPTT, and SHAC, with real-world validation. The paper even states a limitation (Appendix B) that ABPT 'may still fail to fully eliminate extreme bias,' which confirms the claim is not true by construction. The main self-citation, VisFly [29], is used as the evaluation simulator, but it is a separate released environment, the training curves are only one part of the evidence, and the real-world trials make the comparison externally falsifiable; this does not make the central claim reduce to a self-citation. The absence of AGPO/AOBG from the headline comparisons is a benchmarking and completeness concern, not a circularity issue.
Assumptions & free parameters
free parameters (4)
- Per-task learning rate =
0.01 / 0.002 / 0.005 / 0.0002 depending on task and algorithm
- Horizon length H =
96 for ABPT and SHAC, 256 or 512 for BPTT
- Entropy temperature κ =
Adaptive, following SAC
- Replay buffer size =
1,000,000 for hovering, tracking, landing; 50,000 for racing
assumptions (4)
- domain assumption VisFly provides accurate differentiable quadrotor dynamics with correct gradients.
- domain assumption The critic Qϕ is accurate enough for Eq. (16) to hold.
- domain assumption The Gaussian policy with tanh normalization can represent the needed control distributions.
- standard math Standard RL and TD learning results are accepted.
Cite this review
Pith. "Pith review of ABPT: Amended Backpropagation through Time with Partially Differentiable Rewards." pith.science (2026). https://pith.science/paper/Q5I5NJUI
@misc{pith2026250114513,
author = {Pith},
title = {Pith review of: ABPT: Amended Backpropagation through Time with Partially Differentiable Rewards},
year = {2026},
howpublished = {\url{https://pith.science/paper/Q5I5NJUI}},
note = {Machine review of arXiv:2501.14513}
}
read the original abstract
Quadrotor control policies can be trained with high performance using the exact gradients of the rewards to directly optimize policy parameters via backpropagation-through-time (BPTT). However, designing a fully differentiable reward architecture is often challenging. Partially differentiable rewards will result in biased gradient propagation that degrades training performance. To overcome this limitation, we propose Amended Backpropagation-through-Time (ABPT), a novel approach that mitigates gradient bias while preserving the training efficiency of BPTT. ABPT combines 0-step and N-step returns, effectively reducing the bias by leveraging value gradients from the learned Q-value function. Additionally, it adopts entropy regularization and state initialization mechanisms to encourage exploration during training. We evaluate ABPT on four representative quadrotor flight tasks \li{in both real world and simulation}. Experimental results demonstrate that ABPT converges significantly faster and achieves higher ultimate rewards than existing learning algorithms, particularly in tasks involving partially differentiable rewards. The code will be released at http://github.com/Fanxing-LI/ABPT.
Figures
Figures from the paper (6 more)
Reference graph
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