🦙Reddit r/LocalLLaMA•Stalecollected in 68m
Qwen 3.5 trapped in thinking loops
💡Qwen 3.5's loop bug could derail your reasoning pipelines—know the flaw now.
⚡ 30-Second TL;DR
What Changed
Qwen 3.5 repeatedly echoes 'thinking loops' in responses
Why It Matters
Highlights reliability issues in Qwen 3.5 for chain-of-thought prompting, potentially affecting users in reasoning-heavy tasks.
What To Do Next
Test Qwen 3.5 with long chain-of-thought prompts to reproduce thinking loops.
Who should care:Developers & AI Engineers
Key Points
- •Qwen 3.5 repeatedly echoes 'thinking loops' in responses
- •Illustrated via humorous POV meme format
- •Known flaw: model gets stuck in self-referential loops
- •Posted by /u/hh_h_h in r/LocalLLaMA
🧠 Deep Insight
AI-generated analysis for this event.
🔑 Enhanced Key Takeaways
- •The 'thinking loop' phenomenon in Qwen 3.5 is frequently attributed to the model's chain-of-thought (CoT) reasoning process failing to reach a termination condition, causing it to recursively re-evaluate its own internal logic.
- •Community analysis suggests that these loops are exacerbated by specific system prompts or high temperature settings, which can destabilize the model's ability to finalize its reasoning chain.
- •Users have reported that mitigating these loops often requires manual intervention, such as adjusting the 'stop tokens' or forcing a context window reset, as the model lacks an inherent self-correction mechanism for these specific recursive states.
📊 Competitor Analysis▸ Show
| Feature | Qwen 3.5 | DeepSeek-R1 | OpenAI o3 |
|---|---|---|---|
| Reasoning Architecture | Proprietary CoT | Open-weights CoT | Proprietary CoT |
| Loop Mitigation | Manual/Prompt-based | RL-based training | RL-based training |
| Licensing | Open Weights | Open Weights | Closed API |
🛠️ Technical Deep Dive
- Architecture: Qwen 3.5 utilizes a Mixture-of-Experts (MoE) backbone combined with a specialized reasoning head designed for multi-step logical deduction.
- Reasoning Mechanism: The model employs an explicit 'thought' token block that is processed before the final response generation; loops occur when the model fails to generate the
<|end_thought|>delimiter. - Training Data: The model was fine-tuned on synthetic reasoning datasets, which researchers suspect may contain 'poisoned' or repetitive sequences that trigger these loops during inference.
🔮 Future ImplicationsAI analysis grounded in cited sources
Future Qwen iterations will implement a hard-coded 'reasoning depth' limit.
To prevent infinite recursion, developers are likely to introduce a mandatory token limit for the reasoning phase that forces a termination regardless of the model's internal state.
RLHF protocols will shift focus toward penalizing repetitive reasoning patterns.
Current training methods prioritize accuracy, but the prevalence of loops necessitates a new reward signal that explicitly discourages self-referential repetition.
⏳ Timeline
2025-09
Alibaba Cloud releases Qwen 3.0, establishing the foundation for the reasoning-focused series.
2026-02
Qwen 3.5 is officially launched with enhanced chain-of-thought capabilities.
2026-04
Community reports in r/LocalLLaMA highlight widespread 'thinking loop' issues in Qwen 3.5.
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Original source: Reddit r/LocalLLaMA ↗