arxiv: 2605.12620 · v1 · submitted 2026-05-12 · 💻 cs.AI

Recognition: 2 theorem links

· Lean Theorem

Think Twice, Act Once: Verifier-Guided Action Selection For Embodied Agents

Anna Rohrbach, Christian Bialas, Georgia Chalvatzaki, Marcus Rohrbach, Nishad Singhi, Snehal Jauhri, Vignesh Prasad

Authors on Pith no claims yet

Pith reviewed 2026-05-14 21:00 UTC · model grok-4.3

classification 💻 cs.AI

keywords embodied agentsmultimodal large language modelsaction selectionverificationgeneralizationchain-of-thoughtHabitatALFRED

0 comments

The pith

A test-time verifier trained on synthesized failures helps MLLM agents pick reliable actions from multiple candidates.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that multimodal language models used for embodied tasks often fail on unfamiliar situations even with chain-of-thought reasoning. VeGAS fixes this at inference time by generating several possible actions and routing them through a separate generative verifier that picks the best one. The verifier itself is trained on a wide range of failure examples that an LLM creates automatically, creating a curriculum of mistakes to learn from. Experiments across Habitat and ALFRED show consistent gains, especially on long sequences with many objects. The method leaves the original policy unchanged while adding this verification layer.

Core claim

VeGAS is a test-time framework that samples an ensemble of candidate actions from an MLLM-based embodied agent and uses a generative verifier, trained on LLM-synthesized failure cases, to select the most reliable action without altering the base policy, yielding up to 36 percent relative improvement over strong CoT baselines on challenging multi-object long-horizon tasks in Habitat and ALFRED.

What carries the argument

Generative verifier trained on an LLM-driven curriculum of synthesized failure cases that ranks sampled candidate actions for reliability.

If this is right

Consistent gains on multi-object long-horizon tasks in Habitat and ALFRED environments.
Up to 36 percent relative improvement over chain-of-thought baselines on the hardest cases.
No need to retrain or modify the underlying MLLM policy.
Explicit verification step addresses brittleness in out-of-distribution scenarios.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same sampling-plus-verifier pattern could be applied to other decision-making settings where base models produce varied outputs.
Synthetic failure data may lower the cost of building robust agents compared with collecting real error traces.
Test-time verification offers a way to improve agents without scaling up the original training compute.

Load-bearing premise

A verifier trained only on automatically generated failure examples will still recognize good actions when the base model encounters truly novel real-world errors.

What would settle it

Performance on a held-out embodied benchmark with new object interactions and failure modes not present in the synthesized training data shows no gain over the CoT baseline.

Figures

Figures reproduced from arXiv: 2605.12620 by Anna Rohrbach, Christian Bialas, Georgia Chalvatzaki, Marcus Rohrbach, Nishad Singhi, Snehal Jauhri, Vignesh Prasad.

**Figure 2.** Figure 2: Example of a synthetic mistake and verification gen [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: Synthetic data generation and training workflow for the verifier. [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 4.** Figure 4: Verification example on the LangR benchmark. [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗

**Figure 5.** Figure 5: Verification example on the ALFRED benchmark. [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗

**Figure 6.** Figure 6: Scaling candidate actions on EB-ALFRED. Average success rate as the number of candidate actions N increases. Both methods use the same total number of LLM calls. VEGAS scales better with compute than Self-Consistency. yields the best performance, a much cheaper teacher still provides meaningful gains, making our pipeline accessible without requiring expensive frontier models. Approach Avg. Success Rate w/ … view at source ↗

**Figure 7.** Figure 7: Verification example on the LangR benchmark. Here, the task is to move a “purple fruit”, but the agent tries to pick up a strawberry. The verifier correctly identifies that the strawberry is not the right object, classifying the action as incorrect. Instruction: Move the rubriks cube to the brown table, the lego to the brown table, and the bowl to the brown table. Sampled Action: <task>Remaining: locate an… view at source ↗

**Figure 8.** Figure 8: Verification example on the LangR benchmark. Here, the agent erroneously attempts to place the bowl without successfully picking it up first. The verifier identifies this mistake, classifying the action as incorrect. 12.2. ALFRED Instruction: Place washed leafy green vegetable in a receptacle that can keep it fresh for several days Sampled Action: <task>The fridge is open and I (mistakenly) see another let… view at source ↗

**Figure 9.** Figure 9: Verification example on the ALFRED benchmark. Here, the agent incorrectly attempts to close the fridge before putting the lettuce inside because it mistakenly detects a second lettuce in the fridge. The verifier identifies this mistake, classifying the action as incorrect [PITH_FULL_IMAGE:figures/full_fig_p024_9.png] view at source ↗

**Figure 10.** Figure 10: Verification example on the ALFRED benchmark. Here, the task is to move a spoon, but the agent proposes to move a ladle, which is not in the scene. The verifier correctly identifies that the correct object is a spoon, classifying the action as incorrect. Instruction: During a cooking session, keep a clean flipping tool on a wooden table. Sampled Action: <task>Entire task still outstanding: locate a suitab… view at source ↗

**Figure 11.** Figure 11: Verification example on the ALFRED benchmark. Here, the task is to move a spatula, but the agent proposes to move a knife. The verifier fails to identify this mistake and incorrectly classifies the action as correct [PITH_FULL_IMAGE:figures/full_fig_p025_11.png] view at source ↗

read the original abstract

Building generalist embodied agents capable of solving complex real-world tasks remains a fundamental challenge in AI. Multimodal Large Language Models (MLLMs) have significantly advanced the reasoning capabilities of such agents through strong vision-language knowledge and chain-of-thought (CoT) reasoning, yet remain brittle when faced with challenging out-of-distribution scenarios. To address this, we propose Verifier-Guided Action Selection (VegAS), a test-time framework designed to improve the robustness of MLLM-based embodied agents through an explicit verification step. At inference time, rather than committing to a single decoded action, VeGAS samples an ensemble of candidate actions and uses a generative verifier to identify the most reliable choice, without modifying the underlying policy. Crucially, we find that using an MLLM off-the-shelf as a verifier yields no improvement, motivating our LLM-driven data synthesis strategy, which automatically constructs a diverse curriculum of failure cases to expose the verifier to a rich distribution of potential errors at training time. Across embodied reasoning benchmarks spanning the Habitat and ALFRED environments, VeGAS consistently improves generalization, achieving up to a 36% relative performance gain over strong CoT baselines on the most challenging multi-object, long-horizon tasks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

VeGAS gets its reported gains from training a verifier on LLM-synthesized failures, but those gains may not hold if the synthetic errors miss real embodied failure modes.

read the letter

Hi colleague, the main point is that VeGAS samples an ensemble of actions from an MLLM at test time and routes them through a generative verifier trained on automatically generated failure cases. This produces up to 36% relative gains over CoT baselines on the harder multi-object, long-horizon tasks in Habitat and ALFRED. The synthesis step is the actual novelty: an off-the-shelf MLLM verifier adds nothing, so they build a curriculum of potential mistakes with another LLM to expose the verifier during training. That is a practical move for improving selection without touching the base policy, and the paper shows the gains are consistent across the two environments. The approach is clean in that it stays test-time only. The soft spot is exactly the one the stress test flags. The whole claim rests on the synthetic error distribution covering the actual mistakes the agent makes in out-of-distribution visual grounding or state tracking. If it does not, the verifier adds little and the ensemble sampling alone could explain most of the lift. The abstract gives the headline number but no statistical significance, exact baseline numbers, or ablation breakdowns, so it is hard to judge how robust the 36% figure really is. This is for people working on reliable MLLM-based embodied agents. Readers who care about test-time methods or automatic data generation for verifiers will get concrete experiments to look at. I would send it to peer review so the full results and error analysis can be checked.

Referee Report

2 major / 2 minor

Summary. The manuscript introduces Verifier-Guided Action Selection (VeGAS), a test-time framework for MLLM-based embodied agents. Rather than committing to a single CoT-decoded action, the method samples an ensemble of candidate actions and employs a generative verifier—trained exclusively on LLM-synthesized failure cases—to rank and select the most reliable action without modifying the underlying policy. Evaluations on Habitat and ALFRED benchmarks report consistent gains, with up to 36% relative improvement over strong CoT baselines on the most challenging multi-object, long-horizon tasks.

Significance. If the empirical results hold under rigorous controls, the work would offer a practical contribution to robust embodied reasoning by showing that test-time verification with synthetic curricula can mitigate OOD brittleness in MLLM agents. The separation of policy and verifier, together with the emphasis on automatically generated training data, provides a scalable template that could be adopted in other agentic settings where full retraining is costly.

major comments (2)

[Abstract] Abstract: the headline claim of a 36% relative gain on multi-object long-horizon tasks is presented without accompanying statistical significance tests, run-to-run variance, exact baseline configurations, or ablation controls that isolate the verifier’s contribution from simple ensemble sampling effects.
[Method] Method section (data synthesis paragraph): the central assumption that LLM-synthesized failure cases produce a verifier capable of identifying correct actions on real OOD errors is load-bearing for the generalization claim, yet no quantitative analysis or coverage metrics are supplied to show that the synthetic error distribution matches the actual failure modes observed in Habitat and ALFRED.

minor comments (2)

[Method] The notation for the verifier scoring function should be introduced with an explicit equation rather than inline prose to improve readability.
[Experiments] Table captions would benefit from explicit statements of the number of evaluation episodes and random seeds used for each environment.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed feedback. The comments highlight important aspects of statistical rigor and validation of our data synthesis approach. We address each point below and will revise the manuscript to strengthen these elements.

read point-by-point responses

Referee: [Abstract] Abstract: the headline claim of a 36% relative gain on multi-object long-horizon tasks is presented without accompanying statistical significance tests, run-to-run variance, exact baseline configurations, or ablation controls that isolate the verifier’s contribution from simple ensemble sampling effects.

Authors: We agree that the abstract claim would benefit from supporting statistical details. In the revised version, we will report run-to-run variance across multiple random seeds, include statistical significance tests (e.g., paired t-tests against baselines), explicitly specify the exact baseline configurations (including prompt templates and decoding parameters), and add ablation studies that compare full VeGAS against ensemble sampling without the verifier. These results will be summarized in the abstract and detailed in the Experiments section. revision: yes
Referee: [Method] Method section (data synthesis paragraph): the central assumption that LLM-synthesized failure cases produce a verifier capable of identifying correct actions on real OOD errors is load-bearing for the generalization claim, yet no quantitative analysis or coverage metrics are supplied to show that the synthetic error distribution matches the actual failure modes observed in Habitat and ALFRED.

Authors: We acknowledge that quantitative validation of the synthetic error distribution against real failures would better support the generalization argument. Our empirical gains on the target benchmarks provide indirect evidence, but we did not include explicit coverage metrics or distributional comparisons in the original submission. In revision, we will expand the data synthesis section with additional analysis, including categorized examples of synthetic versus observed failures and any feasible quantitative metrics (such as error-type histograms) to characterize the match. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical test-time method with external benchmark validation

full rationale

The paper describes VeGAS as a test-time sampling and verification procedure for MLLM-based agents. Performance gains (up to 36% relative) are measured directly on external embodied benchmarks (Habitat, ALFRED) rather than derived from internal equations or fitted parameters. No self-definitional loops, fitted-input predictions, or load-bearing self-citations appear in the derivation chain; the verifier training uses synthesized data but its effectiveness is evaluated out-of-distribution on held-out tasks. The approach is self-contained against external benchmarks with no reduction of claims to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions about MLLM capabilities for action generation and the utility of synthesized error data for training verifiers; no free parameters or invented entities are introduced.

axioms (1)

domain assumption Off-the-shelf MLLMs are insufficient as verifiers but can be improved via targeted training on failure cases
Stated directly in the abstract as motivation for the data synthesis strategy.

pith-pipeline@v0.9.0 · 5545 in / 1203 out tokens · 46332 ms · 2026-05-14T21:00:48.159839+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear
VeGAS samples an ensemble of candidate actions and uses a generative verifier to identify the most reliable choice... LLM-driven data synthesis strategy, which automatically constructs a diverse curriculum of failure cases
IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear
using an MLLM off-the-shelf as a verifier yields no improvement, motivating our LLM-driven data synthesis strategy

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Qi Sun, Pengfei Hong, Tej Deep Pala, Vernon Toh, U-Xuan Tan, Deepanway Ghosal, and Soujanya Poria. Emma-x: An embodied multimodal action model with grounded chain of thought and look-ahead spatial reasoning. InAnnual Meeting of the Association for Computational Linguistics (ACL), 2025. 2

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Habitat 2.0: Training home assistants to rearrange their habitat

Andrew Szot, Alex Clegg, Eric Undersander, Erik Wijmans, Yili Zhao, John Turner, Noah Maestre, Mustafa Mukadam, Devendra Chaplot, Oleksandr Maksymets, Aaron Gokaslan, Vladimir V ondrus, Sameer Dharur, Franziska Meier, Wojciech Galuba, Angel Chang, Zsolt Kira, Vladlen Koltun, Jitendra Malik, Manolis Savva, and Dhruv Batra. Habitat 2.0: Training home assist...

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Large language models as generalizable policies for embodied tasks

Andrew Szot, Max Schwarzer, Harsh Agrawal, Bogdan Ma- zoure, Rin Metcalf, Walter Talbott, Natalie Mackraz, R Devon Hjelm, and Alexander T Toshev. Large language models as generalizable policies for embodied tasks. InInternational Conference on Learning Representations (ICLR), 2023. 1, 2, 3, 5, 6

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Grounding multi- modal large language models in actions.Advances in Neural Information Processing Systems, 37:20198–20224, 2024

Andrew Szot, Bogdan Mazoure, Harsh Agrawal, R Devon Hjelm, Zsolt Kira, and Alexander Toshev. Grounding multi- modal large language models in actions.Advances in Neural Information Processing Systems, 37:20198–20224, 2024. 1, 3, 5, 6

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Self-Consistency Improves Chain of Thought Reasoning in Language Models

Xuezhi Wang, Jason Wei, Dale Schuurmans, Quoc Le, Ed Chi, Sharan Narang, Aakanksha Chowdhery, and Denny Zhou. Self-consistency improves chain of thought reasoning in lan- guage models.arXiv preprint arXiv:2203.11171, 2022. 2, 6

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Chain-of- thought prompting elicits reasoning in large language mod- els.Advances in Neural Information Processing Systems (NeurIPS), 2022

Jason Wei, Xuezhi Wang, Dale Schuurmans, Maarten Bosma, Fei Xia, Ed Chi, Quoc V Le, Denny Zhou, et al. Chain-of- thought prompting elicits reasoning in large language mod- els.Advances in Neural Information Processing Systems (NeurIPS), 2022. 2, 4

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Efficient rein- forcement learning with large language model priors

Xue Yan, Yan Song, Xidong Feng, Mengyue Yang, Haifeng Zhang, Haitham Bou Ammar, and Jun Wang. Efficient rein- forcement learning with large language model priors. InIn- ternational Conference on Learning Representations (ICLR),
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Embodiedbench: Comprehensive benchmarking multi-modal large language models for vision-driven embodied agents

Rui Yang, Hanyang Chen, Junyu Zhang, Mark Zhao, Cheng Qian, Kangrui Wang, Qineng Wang, Teja Venkat Koripella, Marziyeh Movahedi, Manling Li, et al. Embodiedbench: Comprehensive benchmarking multi-modal large language models for vision-driven embodied agents. InInternational Conference on Machine Learning (ICML), 2025. 1, 2, 3, 5, 6, 7, 8

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Embodied multi-modal agent trained by an llm from a parallel textworld

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Generative verifiers: Reward modeling as next-token prediction

Lunjun Zhang, Arian Hosseini, Hritik Bansal, Mehran Kazemi, Aviral Kumar, and Rishabh Agarwal. Generative verifiers: Reward modeling as next-token prediction. InIn- ternational Conference on Learning Representations (ICLR),
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Llamafac- tory: Unified efficient fine-tuning of 100+ language models

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Collaborative tree search for enhancing embodied multi-agent collaboration

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Details about Benchmarks 8.1. LangR The LangR benchmark [ 44], built on the Habitat 2.0 [ 43] simulator, is designed to evaluate the generalization capability of embodied agents in household rearrangement scenarios. Agents receive high-level instructions and must execute tasks that involve manipulating objects (pick, open, place), searching for target ite...
[59]

Full finetuning is applied while keeping the vision encoder and projection module fixed

Training Details We train both the policy and verifier using the LLaMAFactory framework [56]. Full finetuning is applied while keeping the vision encoder and projection module fixed. All training runs are conducted on 8×NVIDIA L40 GPUs. Training data is formatted as multi turn dialogues us- ing thesharegptformat provided byLLaMAFactory. The inputs to the ...
[60]

{instruction}

Prompts 10.1. Prompt for CoT data generation Prompt to Generate Synthetic Chain-of-Thought (adapted and modified from [53]) 1You’re an expert reinforcement learning researcher. You’ve trained a policy for controlling a robot with an arm to move around and perform tasks in a household environment. 2The robot successfully completed a task specified by the i...
[61]

possible actions are: pick(object), place_on_recep(receptacle), navigate(receptacle), open_fridge(), close_fridge(), open_cabinet(cabinet)
[62]

Possible objects are: ball, clamp, hammer, screwdriver, padlock, scissors, block, drill, spatula, knife, spoon, plate, sponse, cleanser, plum, pear, peach, apple, lemon, can, box, banana, strawberry, lego, rubrik’s cube, book, bowl, cup, mug, orange, lid, toy airplane, wrench
[63]

I choose action XYZ because

When exploring, select from possible receptacles: [cabinet, drawer 7, cabinet drawer 6, fridge, chair, black table, brown table, TV stand, sink, right counter, left counter] 16 17## Your objective 18 19I want you to annotate the given trajectory with reasoning. That is, for each step, I need to know not only which action should be chosen, 20but importantl...
[64]

5 6verification:

Examples of Synthetic Incorrect Actions Here, we show some examples of trajectories with synthetic mistakes generated by the LLM to train the verifier. 11.1. LangR Example 1 (LangR): The agent erroneously picks up the bowl even though the instruction specifies a cleaner. 1Instruction: Shift a cleanser and place it into the specified black table. 2 3 4acti...
[65]

purple fruit

Additional Qualitative Examples of Verification During Inference 12.1. LangR Instruction:Procure a purple fruit and deliver it to the left counter. Sampled Action: <task>Find and pick up the purple fruit and then deliver it to the left counter.</task> <plan>[Search TV stand, search sofa, search table2, pick fruit, navigate to left counter, place fruit]</p...