Cited by 10Advanced Bandit Algorithms ResearchAI-based Problem Solving and PlanningData Stream Mining Techniques

The DOI matches its Crossref record; 23 of the 23 references it cites were found in a registry or an index; none of the 11 sources its claims cite is checked for support yet.

AEL: Evolving Agent Harness in Open-Ended Environments

Open access
Xu, Wujiang, Han, Jiaojiao, Guo, Minghao, Mei, Kai, Zhu, Xi, Zhang, Han, Metaxas, Dimitris N.·2026·Findings of the Association for Computational Linguistics: EMNLP 2026

Abstract

LLM Agents Harnesses are hand-designed and stay fixed, so agents accumulate experience but never learn how to use it: which memories to retrieve, when retrieved evidence is misleading, and when the retrieval strategy itself should change.We introduce Agent Evolving Learning (AEL), a two-timescale framework that evolves the harness, recasting memory use as online policy selection.A fast Thompson Sampling bandit selects among memory-retrieval policies episode by episode, while slow LLM reflection follows a diagnose-before-prescribe principle: it first explains why performance degraded, then injects a targeted new retrieval policy as a bandit arm when the current pool plateaus.AEL outperforms ten self-improving and non-LLM baselines on a sequential portfolio benchmark, lifting Sharpe by 27% over the strongest memory-only variant with the lowest variance among all stochastic methods, and generalizes to a support-ticket routing stream, where it improves accuracy by 18% over reflectionfree Thompson Sampling and by 51% over the best prior baseline.Mechanism studies further show that the gains are causal: reflection helps precisely when regimes demand different retrieval behavior, and is provably no-harm/nogain when the best policy is stable.Code is available at 1 .

Overview

AI snapshot

Written from the paper’s full text.

TL;DR

AEL is a two-timescale agent framework that uses a Thompson Sampling bandit to select memory retrieval policies at the fast timescale and LLM-driven reflection to diagnose failures and inject causal insights at the slow timescale. On a sequential portfolio benchmark, AEL achieves a Sharpe ratio of 2.13±0.47, outperforming five prior self-improving methods and all non-LLM baselines while maintaining the lowest variance among LLM-based approaches.

Key findings

  • AEL achieves the highest Sharpe (2.13), Sortino (4.08), and Calmar (10.40) on the D-full benchmark, with the tightest MaxDD (-1.53%) and lowest Sharpe variance (±0.47) among all stochastic/LLM methods.
  • Incremental build: Stateless 1.35 → +Memory 1.68 (+24%) → AEL 2.13 (+27%), indicating memory enables cross-episode learning and reflection adds a further diagnostic gain.
  • A nine-variant ablation shows every added mechanism degrades performance: removing warm-up Δ-0.88, removing reflection Δ-0.45; adding planner evolution Δ-1.72, per-tool selection Δ-1.70, cold-start Δ-1.31, skills Δ-1.11, FCC Δ-1.09, and LLM-FCC Δ-0.64.
  • Uniform credit outperforms sophisticated credit assignment; prior methods show high seed variance, e.g., EvoTool 1.37±1.74, ExpeL 0.76±1.93, Reflexion -0.59±1.33, while the deterministic momentum-weighted baseline reaches 1.44.

Methods

  • Sequential portfolio allocation benchmark D-full: 10 sector-diverse tickers across 7 GICS sectors, 1-hour bars, 208 episodes (140 train / 40 val / 28 test), 5 random seeds; test set contains a bear-to-bull transition, and all learning is frozen at test time.
  • Baselines include four non-LLM methods, five prior self-improving methods (Reflexion, ExpeL, FactorMiner, Meta-Reflexion, EvoTool), Hyper-Agent, and an AEL incremental build; all methods share the same 12 tools, backbone LLM (Claude Haiku 4.5), and data split.
  • Main analysis reports frozen test-phase Sharpe, Sortino, Calmar, cumulative Return%, MaxDD%, win rate, and tail ratio, with component ablations and credit-assignment comparisons.

Limitations

  • The paper states AEL is domain-agnostic but evaluated only on a single financial domain with a 2-week test period; cross-domain validation on coding and web benchmarks is described as the key next step.
  • The paper notes cost estimates at 5, 10, and 20 bp are approximate, based on average turnover statistics from logged portfolio histories.
  • The paper reports that pre-fix credit results were partially driven by buggy interactions; after credit-system bug fixes, FCC and LLM-FCC collapsed (-0.07 and -0.53 Sharpe), while uniform-credit AEL remained at 2.13. Evident from the text: the benchmark is short-horizon (208 episodes, 28 test bars) and high-noise, which the authors themselves emphasize as the setting where adaptive mechanisms incur exploration overhead.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.18653/2026.findings-emnlp.1316

Cites

23 of 23 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Verification

Claims the paper makes

47 Own result · 38 Own interpretation · 11 Own method · 6 No basis given · 3 Sources not found · 2 Not checked yet

Verify all 2 references — free account

Show all 52 claims

  • The quote only partly states this claim. 2
  • It may state two separate claims as one. 38
  • Probably not a substantial claim. 25
55 of 55
  • The paper relies on this without support.

    The sequential portfolio benchmark (10 sector-diverse tickers, 208 episodes) is a valid and representative testbed for open-ended environments, such that results generalize to other domains.

    “While AEL is domain-agnostic, financial prediction provides objective ground truth and regime diversity that stress-tests adaptation”
  • The paper relies on this without support.

    The 208-episode horizon constitutes a 'short-horizon, high-noise' domain where exploration overhead outweighs benefit.

    “in short-horizon, high-noise domains, the exploration overhead of adaptive mechanisms outweighs their benefit.”
  • The paper relies on this without support.

    The test set contains a bear-to-bull transition, testing whether the learned architecture generalizes to unseen conditions.

    “the test set contains a bear-to-bull transition, testing whether the learned architecture generalizes to unseen conditions.”
  • The paper relies on this without support.

    The reward signal s_t ∈ [-1, 1] converted to uniform reward r_t = clip((s_t + 1)/2, 0, 1) is an appropriate learning signal for the bandit.

    “each episode produces a scalar outcome s t ∈ [-1, 1] that is converted into a uniform reward rt = clip((s t + 1)/2, 0, 1) and used directly to update the memory-policy bandit posterior”
  • The paper relies on this without support.

    The 12-tool finance registry is sufficient to represent the relevant information for portfolio allocation decisions.

    “All methods share the same 12-tool finance registry for fair comparison”
  • The paper relies on this without support.

    Different planners may rely on different levels of abstraction.

    “different planners may rely on different levels of abstraction”
  • The paper relies on this without support.

    The credit-assignment mechanism must be able to inspect both low-level evidence and high-level recommendations.

    “the credit-assignment mechanism must be able to inspect both low-level evidence and high-level recommendations.”
  • The paper relies on this without support.

    Momentum, technicals, analyst sentiment, and composite scoring may all point in the same direction during a strong trend, but they diverge during regime shifts.

    “momentum, technicals, analyst sentiment, and composite scoring may all point in the same direction during a strong trend, but they diverge during regime shifts”
  • The paper relies on this without support.

    Frequent large weight shifts are penalized by the market.

    “frequent large weight shifts are penalized by the market.”
9 of 9

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

5 of 5

Citations

Cites

Show all 13 references

Research works printed but not linked: 8

  1. Olivier Chapelle and Lihong Li. An empirical evaluation of thompson sampling. Advances in Neural Information Processing Systems, 24, 2011.
  2. Amirata Ghorbani and James Zou. Data shapley: Equitable valuation of data for machine learning. Proceedings of the 36th International Conference on Machine Learning, pp. 2242-2251, 2019.
  3. Carlos E. Jimenez, John Yang, Alexander Wettig, Shunyu Yao, Kexin Pei, Ofir Press, and Karthik Narasimhan. SWE-bench: Can language models resolve real-world GitHub issues? Proceedings of the 12th International Conference on Learning Representations, 2024.
  4. Scott M. Lundberg and Su-In Lee. A unified approach to interpreting model predictions. Advances in Neural Information Processing Systems, 30, 2017.
  5. Lloyd S. Shapley. A Value for n-Person Games, volume 2. Princeton University Press, 1953.
  6. Shunyu Yao, Jeffrey Zhao, Dian Yu, Nan Du, Izhak Shafran, Karthik Narasimhan, and Yuan Cao. ReAct: Synergizing reasoning and acting in language models. Proceedings of the 11th International Conference on Learning Representations, 2023.
  7. Jenny Zhang, Bingchen Zhao, Wannan Yang, Jakob Foerster, Jeff Clune, Minqi Jiang, Sam Devlin, and Tatiana Shavrina. Hyperagents. arXiv, 2026.
  8. Shuyan Zhou, Frank F. Xu, Hao Zhu, Xuhui Zhou, Robert Lo, Abishek Sridhar, Xianyi Cheng, Tianyue Ou, Yonatan Bisk, Daniel Fried, Uri Alon, and Graham Neubig. Webarena: A realistic web environment for building autonomous agents. Proceedings of the 12th International Conference on Learning Representations, 2024.

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Metadata

Authors
Xu, W., Han, J., Guo, M., Mei, K., Zhu, X., Zhang, H., & Metaxas, D. N.
Affiliations
Rutgers, The State University of New Jersey (US)
Venue
Findings of the Association for Computational Linguistics: EMNLP 2026
Published
2026
Pages
27857-27885
OpenAlex
W7155488304
Cited by
10
References
13
Citations by year
2026: 9
FWCI
53.61
Type
conference paper
Language
en
Open access
Locations
  • Findings of the Association for Computational Linguistics: EMNLP (Findings of EMNLP) (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • arXiv (Cornell University)
Topics
  • Social Sciences › Decision Sciences › Management Science and Operations Research › Advanced Bandit Algorithms Research
  • Physical Sciences › Computer Science › Artificial Intelligence › AI-based Problem Solving and Planning
  • Physical Sciences › Computer Science › Artificial Intelligence › Data Stream Mining Techniques
Keywords
LLM agents, self-improving agents, portfolio optimization, open-ended environments

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