REVIEW 2 major objections 6 minor 3 cited by
RoboSSM claims that replacing the Transformer backbone with a state-space model lets an in-context imitation learning policy maintain or improve success as test-time demonstrations grow to 16 times the training count, while a Transformer ba
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 →
2026-08-04 15:20 UTC pith:B6IDQXZ3
load-bearing objection The paper shows a real and useful result — SSM-based ICIL extrapolates to much longer prompts than a Transformer baseline — but the unspecified test-time beta-scaling hyperparameter muddies the exact source of the advantage. the 2 major comments →
RoboSSM: Scalable In-context Imitation Learning via State-Space Models
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
On the paper's terms, the central discovery is that Longhorn's recurrent key-value memory, derived from an online regression objective, gives RoboSSM prompt-length extrapolation in in-context imitation learning. Trained with N_train=2 demonstrations, RoboSSM keeps or improves success as N_test grows to 32, attaining its best LIBERO-Object performance at N_test=32. ICRT, a Transformer method with a LLaMA2 backbone, degrades sharply once N_test exceeds N_train and collapses at N_test=32. The same pattern holds when the prompt is stretched by repeating frames (time dilation up to factor 16) rather than adding demonstrations. The paper interprets these results as evidence that SSMs are a scalabl
What carries the argument
Longhorn, a state-space model whose hidden state s_t is a d×m memory updated by s_t = A_t ⊙ s_{t-1} + B_t, with A_t and B_t functions of the input key-value pair. The update is the solution to an online convex programming problem balancing old-state retention against fitting the new observation; a sigmoid-gated vector β_t sets the trade-off. At test time RoboSSM scales β_t by γ ∈ (0,1], which the paper uses to bias the model toward attending to accumulated prompt context. This linear recurrence gives O(L+T_q) inference and lets the memory integrate many demonstrations without the quadratic cost and length-mismatch failure of attention.
Load-bearing premise
The central extrapolation claim assumes that the reported RoboSSM success rates, including the best-at-32 result, do not depend on the test-time gamma multiplier being tuned per task suite on the test tasks; the paper sweeps gamma but does not state which value produced the headline curves.
What would settle it
Run the Figure 3 extrapolation experiment with the test-time beta-scaling multiplier fixed to γ=1 across all suites and report success rates for N_test=2..32; if RoboSSM's success rate no longer rises beyond N_train and instead declines like ICRT, the claimed advantage is an artifact of test-set gamma selection rather than an architectural extrapolation property.
If this is right
- If the central claim holds, deploying RoboSSM with more demonstrations than used in training should not hurt, and may help, success on unseen tasks.
- Long-horizon or variable-speed demonstration prompts can be used at test time without retraining, since time dilation up to 16x preserves performance.
- In-context imitation learning with SSMs can scale to long prompts at linear-time inference cost, making it practical for embodied agents with limited compute.
- Prompting a trained RoboSSM policy with demonstrations of a new task becomes a way to adapt without parameter updates, supporting continual adaptation as new tasks arrive.
Where Pith is reading between the lines
- The paper does not state whether the headline extrapolation curves use γ=1 or a test-selected γ from the sweep; a decisive test is to fix γ=1 and see whether the monotonic improvement at N_test=32 remains. If it does not, part of the claimed advantage is fitted to the evaluation set.
- The β-scaling dependence suggests a simpler alternative design: a learned or schedule-based forgetting rate that lets the state integrate longer prompts, which could be tested by varying the training objective to expose the model to longer prompt lengths.
- Because the paper's core input excludes language instructions, the mechanism is purely visual-proprioceptive. An extension would be to combine language with long prompts; if the advantage persists, the two conditioning signals may yield even better unseen-task generalization.
- The LIBERO results are simulated; transferring the same setup to a physical robot with noisy teleoperated demonstrations would test whether the 16x extrapolation survives real-world distribution shift.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. RoboSSM replaces the Transformer backbone of in-context imitation learning (ICIL) with the Longhorn state-space model, aiming to handle prompts substantially longer than those seen in training while maintaining linear-time inference. The paper evaluates on multiple LIBERO suites with disjoint train/test task splits, comparing against ICRT and multi-task baselines. The central claims are that RoboSSM maintains or improves success as the number of test-time demonstrations grows beyond the training prompt length (up to N_test=32 with N_train=2, i.e., 16x longer), that it remains robust under temporal dilation of demonstrations, and that it achieves competitive or better in-distribution performance than Transformer-based ICRT. The method includes a test-time beta-scaling multiplier gamma for the Longhorn state update.
Significance. If the central length-extrapolation result holds, the paper provides a valuable empirical demonstration that SSMs can be a scalable backbone for ICIL, addressing a known weakness of Transformer-based policies. The paper's strengths include multi-suite LIBERO evaluation with six seeds, released code, a runtime analysis confirming nearly linear inference scaling, and an explicit study of the beta-scaling mechanism. The gamma-disclosure issue described below is the main load-bearing concern; the rest is largely presentation and statistical reporting. The result is not circular: it is an empirical comparison, and Longhorn is evaluated in a new domain rather than assumed to work.
major comments (2)
- [Sec. III.A.3, Sec. IV-E, Figs. 3-6] The test-time beta-scaling multiplier gamma in (0,1] is introduced in Sec. III.A.3, but the value used for the headline extrapolation results is never stated. Sec. IV-E sweeps gamma in [0.4, 1.0] only for N_test=8, N_train=2, while Figs. 3-6 report the main 'prompts 16x longer' claim without disclosing gamma. If a test-selected gamma was used, part of the claimed advantage over ICRT at N_test=32 could be attributable to oracle tuning rather than the SSM's intrinsic length extrapolation. Please state the gamma value used in every reported experiment; if gamma=1 was used in the headline curves, say so explicitly; if gamma<1 was used, provide the gamma=1 results and describe how gamma was selected without using the test tasks.
- [Table I vs. Sec. IV-C] In Table I, ICRT w/o lang achieves 66.3±3.8 on LIBERO-Object while RoboSSM w/o lang achieves 57.9±3.5, yet Sec. IV-C states that RoboSSM 'consistently achieves higher success rates than ICRT across most scenarios.' Clarify whether Table I and Fig. 7 use identical protocols (N_train, N_test=4 and the same task split), and explain the apparent discrepancy. If the 'consistently higher' claim is meant only for the Study and Living Room scenes, the text should be revised to avoid overclaiming.
minor comments (6)
- [Figs. 3-6] The main comparison figures do not show error bars or confidence intervals, despite success rates being averaged over 6 seeds. Please add shaded regions or error bars, or report standard deviations in the captions, so the significance of the N_test > N_train trend can be assessed.
- [Fig. 6] Temporal dilation by repeating observation embeddings is an artificial proxy for long-horizon tasks. The caption should state explicitly that this simulates temporal variability rather than demonstrating generalization to genuinely novel long-horizon task structures.
- [Sec. IV-A.2] The text says the two backbones have 'a similar number of parameters' but no parameter counts are reported. Please include the actual numbers for ICRT and RoboSSM.
- [Sec. IV-G] The runtime analysis reports only LIBERO-Object and does not describe how many runs or which statistic (mean/median) is shown. Specify the hardware and the variance across runs.
- [Sec. IV-F / Ref. [37]] The t-SNE visualization cites reference [37], which is 'Gradients are not all you need' rather than the original t-SNE paper (van der Maaten & Hinton, 2008). The citation should be corrected.
- [Fig. 1 caption] The caption contains an obvious duplication: 'Longhorn Longhorn'. Also, the introduction refers to 'beta-scaling ablations' before the beta-scaling mechanism is defined; a forward pointer to Sec. IV-E would help.
Circularity Check
No significant circularity; central claim is empirical and self-contained, with one minor overlapping-author citation and an unspecified test-time beta-scaling knob that is a correctness risk, not a circular reduction.
full rationale
The paper's central claim—that RoboSSM maintains or improves success as the number of test-time demonstrations grows beyond training length—is an empirical comparison on LIBERO, not a derivation from an equation that reduces to its inputs. The main potential circularity is the self-citation of Longhorn [23], which shares authors with this paper. However, RoboSSM evaluates Longhorn on a new domain (ICIL on LIBERO) and does not assume the conclusion; the extrapolation is demonstrated experimentally in Figures 3–6. This is therefore a non-load-bearing self-citation. A second concern is the test-time beta-scaling multiplier γ introduced in Sec. III.A.3 and swept in Sec. IV-E (γ∈[0.4,1.0] for N_test=8, N_train=2). The paper never states which γ produced the headline Figures 3–6, including the peak at N_test=32 after training with N_train=2. If a per-suite, test-selected γ was used, part of the claimed long-prompt advantage would be fitted to the evaluation set. But this is a missing-specification / external-validity issue, not demonstrated circularity: no equation equates the prediction to the fitted parameter, and the paper does not rename the fit as a prediction in its derivation. No self-definitional, uniqueness-imported, ansatz-smuggled, or renaming patterns are present. Overall score reflects the minor self-citation and the unresolved gamma-specification risk while affirming that the central empirical result is not circular by construction.
Axiom & Free-Parameter Ledger
free parameters (1)
- gamma (test-time beta-scaling multiplier) =
not reported; swept in [0.4, 1.0] on test suites
axioms (4)
- domain assumption Longhorn recurrence (Eqs. 2-5) is a suitable in-context learner for robot demonstrations.
- ad hoc to paper Excluding actions and language from the input forces the policy to infer the task from observation prompts.
- ad hoc to paper Frame repetition creates valid long-horizon test scenarios.
- domain assumption LIBERO success rate on unseen tasks is a meaningful proxy for ICIL generalization.
Cite this review
Pith. "Pith review of RoboSSM: Scalable In-context Imitation Learning via State-Space Models." pith.science (2026). https://pith.science/paper/B6IDQXZ3
@misc{pith2026250919658,
author = {Pith},
title = {Pith review of: RoboSSM: Scalable In-context Imitation Learning via State-Space Models},
year = {2026},
howpublished = {\url{https://pith.science/paper/B6IDQXZ3}},
note = {Machine review of arXiv:2509.19658}
}
read the original abstract
In-context imitation learning (ICIL) enables robots to learn tasks from prompts consisting of just a handful of demonstrations. By eliminating the need for parameter updates at deployment time, this paradigm supports few-shot adaptation to novel tasks. However, recent ICIL methods rely on Transformers, which have computational limitations and tend to underperform when handling longer prompts than those seen during training. In this work, we introduce RoboSSM, a scalable recipe for in-context imitation learning based on state-space models (SSM). Specifically, RoboSSM replaces Transformers with Longhorn -- a state-of-the-art SSM that provides linear-time inference and strong extrapolation capabilities, making it well-suited for long-context prompts. Through diverse experiments on the LIBERO benchmark, we demonstrate the effectiveness of applying SSMs to ICIL, achieving improved generalization to both unseen and long-horizon tasks than Transformer-based ICIL methods by handling longer contexts at test-time. These results show for the first time that SSMs are an efficient and scalable backbone for ICIL. Our code is available at https://github.com/youngjuY/RoboSSM.
Figures
Forward citations
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Reference graph
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This paper was first reviewed by deepseek-v4-flash on August 4, 2026.
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