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BES (Bidirectional Evolutionary Search)

Search framework from Harvard/MIT coupling forward evolutionary candidate search with backward goal decomposition.

Available in EUResearchLanguage ModelsResearch

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BES (Bidirectional Evolutionary Search) at a glance

What is BES (Bidirectional Evolutionary Search)?
BES (Bidirectional Evolutionary Search) is a research. Search framework from Harvard/MIT coupling forward evolutionary candidate search with backward goal decomposition.
Does BES (Bidirectional Evolutionary Search) work in the EU?
Available in the EU. Code and trained models are publicly released on GitHub by the authors, with no regional restrictions found; license not confirmed. Last verified 14 Nov 2026.
How much does BES (Bidirectional Evolutionary Search) cost?
Open source. Research code and trained models released by the authors; no hosted service or pricing found.
Where was BES (Bidirectional Evolutionary Search) shown?
In 1 creator video by AI Search. Most recently by AI Search on 31 May 2026, at 26:09 in “Self-improving AI, Opus 4.8, Nvidia bangers, game-ready 3D models, juggling robots: AI NEWS”.

Sources: the vendor's own pages (linked above) and creator videos at the linked timestamps. EU status is information, not legal advice; see how we check.

About

Bidirectional Evolutionary Search (BES) is a research search framework for self-improving language models and agentic systems, described in the paper "Self-Improving Language Models with Bidirectional Evolutionary Search" by Guowei Xu, Zhenting Qi, Huangyuan Su, Weirui Ye, Himabindu Lakkaraju, Sham M. Kakade and Yilun Du (Harvard University, MIT). It targets two limitations of best-of-N sampling and tree search: sparse verification signals, and candidates built mainly by autoregressive expansion, which keeps exploration within regions of high model probability.

Forward search adds evolution operators (combination, deletion, translocation, crossover) that edit and recombine partial trajectories to produce candidates that are hard to get from a single rollout. Backward search recursively decomposes the task into checkable sub-goals; each forward node is scored by how many sub-goals it satisfies, giving denser intermediate feedback. The authors also give theoretical motivation for both parts.

Reported results include gains on post-training tasks where mainstream algorithms such as GRPO and Tree-GRPO did not improve (e.g. multi-hop QA with Llama-3.2-3B and Llama-3.1-8B backbones), and competitive inference-time results on open problems (circle packing, Heilbronn problem) against OpenEvolve, GEPA and ShinkaEvolve. The authors state that code and trained models are released. A video creator noted the "self-improving" label is somewhat misleading. It is a research method, not a hosted product.

Key features
  • Forward search with evolution operators: combination, deletion, translocation, crossover
  • Backward search decomposing tasks into a tree of checkable sub-goals
  • Dense intermediate feedback instead of sparse verifier signals
  • Usable for post-training sample generation and inference-time search
  • Evaluated on multi-hop QA, logical reasoning, circle packing and Heilbronn problems
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