REVIEW 3 major objections 4 minor 2 cited by
This paper shows that exact forward-backward representations cannot exist in continuous control with finite-dimensional representations, and that a simplified one-step variant converges to far smaller errors and improves zero-shot performan
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 00:09 UTC pith:UEFUG46N
load-bearing objection Useful paper for the zero-shot RL crowd: solid discrete analysis and a simpler method that works, but the headline impossibility claim for continuous domains is asserted rather than proved. the 3 major comments →
Can We Really Learn One Representation to Optimize All Rewards?
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that forward-backward (FB) representation learning—which pre-trains F and B so that M^π/ρ = F(s,a,z)ᵀB(s',a') and then sets z = E[B r] to recover optimal policies—cannot in general achieve its promised fixed point. In any discrete CMP the successor measure matrix is full rank, so an exact factorization forces the representation dimension d to be at least the number of state-action pairs; taking the state-action space to be infinite, finite-dimensional ground-truth representations are impossible. The paper also rederives the FB objective as a temporal-difference variant of least-squares importance fitting and shows that the associated FB Bellman operator is not a γ-contra
What carries the argument
The load-bearing object is the successor measure ratio M^π(s', a' | s,a)/ρ(s',a'), the discounted occupancy of future state-action pairs under a policy, approximated as a bilinear factorization F(s,a,z)ᵀB(s',a'). In discrete CMPs the successor measure matrix is full rank, forcing any exact factorization to have dimension d ≥ |S × A| (Proposition 1). One-step FB makes the target static by fixing a behavioral policy π_β, so the factorization reduces to an SVD of a fixed matrix rather than a self-referential equation. The FB Bellman operator—the contraction-style map underlying the original algorithm—is shown not to be a γ-contraction, which is why the standard fixed-point proof fails.
Load-bearing premise
The impossibility result for continuous domains rests on carrying the finite-state rank bound d ≥ |S × A| over to infinite state-action spaces, and the didactic failure experiments substitute the Monte-Carlo loss for the actual temporal-difference objective.
What would settle it
Run the exact temporal-difference FB loss (Eq. 9), not the Monte-Carlo proxy, on the three-state and five-state didactic CMPs; if FB converges to a fixed point whose Q predictions obey the affine-equivariance property, the paper's central empirical failure claim is refuted.
If this is right
- Universal zero-shot optimality with finite representations is out of reach for large or continuous state-action spaces; exact FB needs one dimension per state-action pair.
- The FB training objective is a temporal-difference version of least-squares importance fitting, so it is a fitted Q-evaluation for the successor-measure ratio rather than a standard value iteration; its Bellman operator lacks a contraction guarantee.
- The one-step FB variant—fitting a fixed behavioral policy's successor measure and then taking one policy improvement step—converges to errors about 1e5 times smaller in didactic settings, and achieves +24% average zero-shot return over FB on 10 benchmark domains.
- One-step FB's zero-shot policies serve as efficient initializations for further online fine-tuning, improving sample efficiency relative to learning from scratch.
Where Pith is reading between the lines
- Inference: If exact universal representations are impossible, the field's objective for behavioral foundation models should be reframed from 'optimal for any reward' to 'one policy-improvement step from a behavioral prior', which is a strictly weaker and achievable target.
- Inference: The non-contraction result likely generalizes to any representation-learning method whose policy and value target are coupled; such objectives should be checked for multiple fixed points (e.g., rotational symmetries of F and B) before assuming convergence.
- Inference: The SVD view suggests one-step FB's quality is governed by the spectrum of the behavioral successor measure ratio, pointing to exploration strategies that shape that spectrum rather than try to cover all possible rewards.
- Inference: A natural extension is to test one-step FB under limited coverage and stochastic or partially observed dynamics; the paper itself flags that performance may degrade when the behavioral policy covers the downstream task poorly.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes forward-backward (FB) representation learning for zero-shot RL. It claims: (i) exact ground-truth FB representations require representation dimension at least |S×A| in finite CMPs, and are impossible in continuous CMPs with finite d (Proposition 1, Corollary 1); (ii) the FB objective can be derived as a temporal-difference least-squares importance fitting loss, and the associated FB Bellman operator is not a γ-contraction (Proposition 3); (iii) a simplified method, one-step FB, fits the successor measure ratio of a fixed behavioral policy and performs one step of policy improvement; didactic and continuous-control experiments show better convergence and zero-shot performance than FB. The paper includes proofs of the finite-state results, an implementation, and ablations.
Significance. If the finite-CMP results are accepted, the paper makes a useful conceptual contribution: it connects FB to density-ratio estimation and fitted Q-evaluation, shows a rank obstruction to exact FB representations, and identifies a circular-dependency issue in FB's Bellman operator. The proposed one-step FB method is simple, reproducible, and empirically competitive, with a clear interpretation as fitting a static successor-measure ratio. These positive aspects are partially offset by the fact that the headline negative answer to the title question rests on an unproven infinite-dimensional limit, and by a gap in the proof of Proposition 2. The empirical and algorithmic parts are stronger than the theoretical parts.
major comments (3)
- [§3.1, Corollary 1; Appendix C.1; §6 Limitations] Corollary 1 is the linchpin of the negative answer to the title question, but it is not established by Proposition 1. Proposition 1 proves d >= |S×A| for finite CMPs; Corollary 1 then lets |S×A| → ∞ and concludes that finite d is impossible for continuous CMPs. This limit is not a defined mathematical operation: no operator-theoretic definition of the successor-measure ratio on infinite-dimensional spaces is given, no rank theorem for infinite-dimensional kernels is proved, and rank is not continuous under such limits. Infinite-dimensional operators can have finite rank even when the index set is infinite, and the delta term in Eq. (1) may make the continuous object a distribution rather than a Hilbert–Schmidt kernel. The paper's own Limitations (Sec. 6) state that extending the analysis to continuous settings 'relies on approximation,' which contradicts the categorical wording of Coroll
- [§5.1, Eq. (7) vs Eq. (9); Appendix D.1] The didactic experiment that claims 'the FB algorithm fails to converge' optimizes the Monte-Carlo FB loss (Eq. 7), not the TD FB objective (Eq. 9) that the practical FB algorithm actually uses. The MC loss is a supervised regression to an analytically computed successor measure; the TD loss involves target networks and on-policy next-action sampling, and the two can have different fixed points and optimization dynamics. The paper calls Eq. (7) an 'analytical analogy,' but no formal relationship is given that would let the MC-loss behavior transfer to the TD objective. Since the practical convergence claim is central to the motivation for one-step FB, the experiment should be run with the TD objective (or with a precise equivalence theorem).
- [Appendix C.3, Proposition 2] The proof of Proposition 2 is incomplete. From the affine-equivariance of Q-values the authors derive, for every reward r, an equality F(s,a,z_{νr+ξ})ᵀ z_r = F(s,a,z_r)ᵀ z_r. This only shows equality of inner products with z_r. By Proposition 1, z_r = B(r ⊙ ρ) lies in the column space of B, which has dimension |S×A|; when d > |S×A|, these vectors do not span R^d, so the conclusion F(s,a,z_{νr+ξ}) = F(s,a,z_r) does not follow. The proposition as stated is therefore not proven unless d = |S×A| or an additional spanning assumption is imposed. This matters because Proposition 2 is the theoretical basis for the equivariance-error criterion used in the didactic experiments (Secs. 5.1 and 5.2).
minor comments (4)
- [Abstract] The phrase 'errors 105 smaller' should read '10^5 times smaller' (typesetting issue).
- [Table 1 / Table 2] The visual-domain rows in Tables 1 and 2 appear to contain fewer numeric entries than the six method columns in the header. This makes it hard to verify the claimed +24% average improvement over FB on image-based domains. Please reformat so each row has one entry per method.
- [§3.3, Definition 3] The FB Bellman operator TFB is defined on the inner product f(s,a,z)ᵀ b(sf,af) but its value depends on the policy induced by f, not only on the inner-product function. This should be stated explicitly to avoid ambiguity about what 'applying TFB to an inner product' means.
- [Appendix E.2] There are typos such as 'proprobability' and 'convergence transits'; please proofread the appendix.
Circularity Check
No significant circularity: central results derive from external rank/linear-algebra facts and held-out benchmarks; the only flagged issue is an underproved continuous limit, which is a rigor gap, not a circular reduction.
full rationale
The paper's central impossibility result (Proposition 1 and Corollary 1) is derived from the externally cited full-rank successor-measure lemma (Agarwal et al. 2019, Lemma 1.6/Corollary 1.5) plus standard linear algebra. The FB definitions are imported from Touati & Ollivier (2021), not from the authors' own prior work, and no load-bearing conclusion is justified by a self-citation. Proposition 3's non-contraction proof uses a rotation construction from Touati et al. 2022, which is external and does not assume the result. The didactic 'convergence' of one-step FB is transparently a supervised fit to a static target: Appendix D.2 states 'the target density ratio ... is fixed given πβ, the learning one-step FB reduces to solving a supervised learning problem,' and the reported errors are MSEs to that same target. This is a self-consistency check, not an external prediction, so it is not circular. The main benchmark claims are tested against held-out ExORL/OGBench tasks, so they do not reduce to the training objective. The one genuine weakness is Corollary 1: the finite matrix rank bound d >= |S x A| is extended to continuous CMPs by letting |S x A| -> infinity without defining the infinite-dimensional successor-measure operator or proving a rank theorem there. The paper's own Limitations (Sec. 6) concede 'extending it to continuous settings relies on approximation.' That is an unsupported extrapolation / correctness risk, not a circular definitional reduction; per the stated rules it does not raise the circularity score.
Axiom & Free-Parameter Ledger
free parameters (5)
- lambda_BC (behavioral-cloning regularization coefficient) =
0 to 0.3 per domain (Table 4)
- lambda_ortho (orthonormalization coefficient) =
0 to 1 per domain (Table 4)
- tau_reward (reward softmax temperature) =
3 to 300 per domain (Table 4)
- d (representation dimension) =
50 for ExORL; 128 or 512 for OGBench
- tau_policy (didactic softmax temperature) =
5e-3 during training, 1 during evaluation
axioms (4)
- standard math The successor measure matrix M^pi is full rank with rank |S x A| (Lemma 1 of Agarwal et al. 2019).
- domain assumption The marginal measure rho has full support on S x A.
- ad hoc to paper Finite-CMP rank results extend to continuous CMPs by letting |S x A| -> infinity.
- ad hoc to paper In didactic experiments the Monte-Carlo FB loss (Eq. 7) is a faithful proxy for the TD FB objective (Eq. 9).
read the original abstract
As unsupervised pretraining becomes increasingly ubiquitous in reinforcement learning, a more thorough theoretical understanding of these methods becomes of equal importance to their empirical success. We focus on the setting of unsupervised learning via interaction, where the forward-backward (FB) representation learning serves as a prototypical and popular example. In this paper, we shed light on FB by formally contextualizing the method within a broader class of recent methods that use regression to obtain a low-rank approximation of a successor measure ratio. Our analysis clarifies when FB representations can exist and how the low-rank approximation converges in practice. Building upon the theory, we propose a variant of FB that is both more amenable to theoretical understanding and simpler to optimize in practice. Experiments in didactic settings, as well as in $10$ state-based and image-based continuous control domains, demonstrate that our method converges to desired representations with $10^5 \times$ smaller errors than FB, achieving $+24\%$ improved zero-shot performance on average. We also demonstrate that zero-shot policies inferred by our algorithm provide an efficient initialization if the user prefers further fine-tuning on downstream tasks. Our project website is available at https://chongyi-zheng.github.io/onestep-fb.
Figures
Forward citations
Cited by 2 Pith papers
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Reference graph
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discussion (0)
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