ModelBest and Tsinghua Release PostTrainBench: Agents Execute Well but Are Strategy-Locked
The ModelBest and Tsinghua team analyzed 1,338 complete training trajectories based on PostTrainBench, covering 7 benchmarks, 4 base models, and Claude Code, Codex CLI, and OpenCode, running for 10 hours on a single H100. Results show that the agents' average score rose from 33.34 to 48.55 with experience, but among 3,557 pairs of adjacent experiments, only 2.1% actually switched strategies; all execution-layer suggestions were adopted, while none of the strategy-layer suggestions were adopted. At the same checkpoint, a single sentence of human guidance to switch strategies was enough to outperform autonomous continuation, with GSM8K improving by 17.44pp, HumanEval by 10.80pp, and AIME 2025 by 6.67pp.
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AI score 64
Based on 1,338 complete training trajectories, it breaks agent capability into two layers—execution and strategy—and quantifies the gains from human mid-course intervention.
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An agent that can run a full post-training pipeline almost never reconsiders its initial strategy. We analyze 1,338 complete training trajectories from PostTrainBench — 7 benchmarks, 4 base models, Claude Code / Codex CLI / OpenCode, 10h on one H100 — and split capability into EXECUTION vs STRATEGY.
1️⃣ Qualified executor, locked strategy. Agents improve over base models on every benchmark; with experience, avg score goes 33.34 → 48.55. ALL execution-level suggestions were adopted, but NONE of the strategy-level suggestions (e.g., switching to RL, adding SFT warmup) were adopted. Only 2.1% of 3,557 adjacent experiment pairs actually switch strategy. Defaults follow the agent, not the task: Claude Code starts full-parameter SFT 71.9% of the time; Codex CLI starts PEFT/LoRA 89.5%.
2️⃣ Experience and compute don't unlock it. Extra experience and memory raise execution but never leave the locked strategy; more reasoning tokens buy finer execution rather than better direction — on AIME 2025, ~14M extra tokens solve just 1 more problem.
3️⃣ A mid-run human guidance wins everywhere. At the same checkpoint, same remaining budget, one sentence switching to the alternative strategy beats autonomous continuation: GSM8K +17.44pp, HumanEval +10.80pp, AIME 2025 +6.67pp.
The missing ability isn't executing a new strategy — it's the trigger to spontaneously restart strategy decisions mid-trajectory.
📄 Paper: https://t.co/yuI2gosUXC #AIAgent #PostTraining #LLM #RLHF #GRPO #PPO #RSI #AgentBenchmark #PostTrainBench #THUNLP