New Model, Old Habits: Behavior Transfer under Model Swapping in Multi-Session LLM Agents
Organizations: Graduate School of Artificial Intelligence, POSTECH · Meta · Department of Computer Science and Engineering, POSTECH
Abstract
Multi-session agents can switch between LLMs to balance performance and inference cost. Such switches require transferring a history for continuity, yet their effects on the recipient remain underexplored. Through controlled experiments with multi-session tasks, we uncover behavior transfer: recipients tend to adopt reasoning and action patterns from the models that generated their histories. This effect is most evident with raw-history transfer but is substantially reduced by summarization. Based on these findings, we propose Direction- and Authorship-aware Summarization of History (DASH) that preserves stronger-model behavior while minimizing weak-to-strong transfer via summarization. Experiments with repeated swaps show that DASH outperforms raw-history transfer and naive summarization baselines. These results suggest that effective multi-model orchestration requires considering not only which model to use, but also which behavioral patterns to carry forward.
Figures & tables
| History transfer | Qwen3.5-4B Qwen3.5-27B | Qwen3.5-4B DeepSeek | Gemma-4-E4B Gemma-4-31B | Gemma-4-E4B DeepSeek | |
|---|---|---|---|---|---|
| Weak-first ( ) | |||||
| Web | 23.5 | 32.5 | 24.8 | 27.5 | |
| 27.2 | 39.5 | 30.5 | 35.7 | ||
| 32.3 | 39.8 | 31.7 | 35.5 | ||
| Travel | 29.3 | 32.2 | 18.6 | 27.5 | |
| 31.3 | 47.8 | 24.8 | 40.9 | ||
Appendix figures & tables18 assets
Supplementary material from the paper’s appendix.
Appendix
| Environment | Tasks | Sessions | Swap points | |
|---|---|---|---|---|
| Web Shopping | 150 | 900 | 6 | 450 |
| Travel Planning | 270 | 1,869 | 5–8 | 773 |
| Web Search | 221 | 1,641 | 4–16 | 633 |
| Math | 40 | 354 | 2–16 | 115 |
| Physics | 20 | 86 | 2–12 | 47 |
| Environment | Max tokens | Reasoning budget | Max steps |
|---|---|---|---|
| Web Shopping | 4,096 | 3,584 | 20 |
| Travel Planning | 32,768 | 22,938 | 30 |
| Web Search | 15,000 † | 1,024 | 35 |
| Math, Physics | 8,192 | – | 3 |
| Feature | Session-level unit | Definition |
|---|---|---|
| Web Shopping | ||
| Words per query | Mean words | Mean number of words per search[...] query. |
| Query copies option text | Fraction of queries | Queries containing at least 80% of the content words of one listed option. |
| Repeats identical query | 0/1 | 1 if an identical search[...] action is issued more than once in the session. |
| Plan statement in reasoning | Fraction of calls | first , next, , then , I will , I’ll , let me , plan , step , now I . |
| Options covered by queries | Fraction of options | Listed options with at least 50% of their content words contained in at least one query. |
| Feature | Session-level unit | Definition |
| Travel Planning | ||
| Budget terms in reasoning | Matches per 1k chars | budget , cost(s) , price(s) , dollar amounts, per person/night , total , cheaper , expensive . |
| Meal slots filled in plan | Fraction of slots | Breakfast, lunch, and dinner slots with plan entries other than - . |
| “Format/exactly” in reasoning | Matches per 1k chars | format* , exact(ly) . |
| No tool call | 0/1 | 1 if the plan is produced without any tool call. |
| Max reasoning length | Thousand characters | Maximum reasoning length across the session’s model calls, divided by 1,000. |
| Physics | Math | |||
| History used | Self | Inherited | Self | Inherited |
| All paired swap points | ||||
| History cut ratio (%) | 23.0 | 18.9 | 13.4 | 17.8 |
| Continuation cut ratio (%) | 15.2 | 6.3 | 13.9 | 14.5 |
| Continuation PS (%) | 44.9 | 52.7 | 25.3 | 24.1 |
| Swap effect | ||||
| Self-history baseline | Normalized shift | |||||||||
| Feature | 4B | 27B | DS | G | ||||||
| Web Shopping | ||||||||||
| Words per query | 3.82 | 8.18 | 9.14 | 10.87 | 0.85 | 0.94 | 0.73 | 0.97 | 0.88 | 0.87 |
| Query copies option text | 0.07 | 0.39 | 0.58 | 0.58 | 0.92 | 0.87 | 0.84 | 0.90 | 1.10 | 1.07 |
| Repeats identical query | 0.08 | 0.48 | 0.32 | 0.30 | 0.74 | 0.76 | 0.78 | 0.96 | 0.44 | 0.84 |
| Plan statement in reasoning | 0.90 | 0.46 | 0.55 | 0.85 | 1.00 | 1.06 | 1.00 | 1.59 | 0.74 | 0.55 |
| Self-history baseline | Normalized shift | |||||||||
| Feature | 4B | 27B | DS | G | ||||||
| Web Shopping | ||||||||||
| Words per query | 3.82 | 8.18 | 9.14 | 10.87 | 0.70 | 0.44 | 0.68 | 0.61 | 0.32 | 0.76 |
| Query copies option text | 0.07 | 0.39 | 0.58 | 0.58 | 0.83 | 0.33 | 0.81 | 0.54 | 0.50 | 0.98 |
| Repeats identical query | 0.08 | 0.48 | 0.32 | 0.30 | 0.17 | 0.08 | 0.13 | 0.22 | 0.13 | 0.16 |
| Plan statement in reasoning | 0.90 | 0.46 | 0.55 | 0.85 | 0.00 | 0.27 | 0.51 | 0.05 | 0.40 | |
| Self-history baseline | Normalized shift | |||||||||
| Feature | 4B | 27B | DS | G | ||||||
| Web Shopping | ||||||||||
| Words per query | 3.82 | 8.18 | 9.14 | 10.87 | 0.46 | 0.40 | 0.04 | 0.00 | ||
| Query copies option text | 0.07 | 0.39 | 0.58 | 0.58 | 0.53 | 0.52 | 0.00 | |||
| Repeats identical query | 0.08 | 0.48 | 0.32 | 0.30 | 0.30 | 0.01 | 0.33 | 0.02 | 0.56 | |
| Plan statement in reasoning | 0.90 | 0.46 | 0.55 | 0.85 | 0.01 | 0.07 | 0.78 | 0.09 | 0.03 | 0.09 |
| Direction | Model / Swap | Subset | Math | Physics |
| No swap (Self) | Qwen3.5-4B | 14.8 | 25.6 | |
| Qwen3.5-27B | 25.2 | 55.4 | ||
| DeepSeek | 25.3 | 44.9 | ||
| Gemma | 28.4 | 57.4 | ||
| Weak strong | Qwen3.5-4B Qwen3.5-27B | All paired | 26.4 | 47.6 |
| Correctness | 26.1 | 59.5 |
| Swap | Model / Swap | Context | Web Shopping | Travel Planning | Web Search |
|---|---|---|---|---|---|
| No swap (Self) | Qwen3.5-4B | 12.9 | 14.4 | 9.4 | |
| Qwen3.5-27B | 62.1 | 51.8 | 15.9 | ||
| Gemma | 65.8 | 24.8 | 14.1 | ||
| DeepSeek | 62.4 | 57.8 | 29.3 | ||
| Weak strong | Qwen3.5-4B Qwen3.5-27B | 28.7 | 28.5 | 13.8 | |
| 35.9 | 26.2 | 13.7 |
| Swap | Model / Swap | Subset | Web Shopping | Travel Planning | Web Search |
|---|---|---|---|---|---|
| Weak strong | Qwen3.5-4B Qwen3.5-27B | All paired | 28.7 | 28.5 | 13.8 |
| Correctness-controlled | 39.7 | 29.8 | 12.4 | ||
| # tasks (controlled / all) | 45 / 150 | 111 / 270 | 204 / 221 | ||
| Qwen3.5-4B DeepSeek | All paired | 47.6 | 44.4 | 23.8 | |
| Correctness-controlled | 49.3 | 37.6 | 21.3 | ||
| # tasks (controlled / all) | 36 / 150 | 88 / 270 | 189 / 221 |
LLM Agents
Large language model agents use language models to plan, use tools, and act across multi-step tasks and environments.
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