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+---
+name: fine-tuning-with-trl
+description: Fine-tune LLMs using reinforcement learning with TRL - SFT for instruction tuning, DPO for preference alignment, PPO/GRPO for reward optimization, and reward model training. Use when need RLHF, align model with preferences, or train from human feedback. Works with HuggingFace Transformers.
+version: 1.0.0
+author: Orchestra Research
+license: MIT
+dependencies: [trl, transformers, datasets, peft, accelerate, torch]
+metadata:
+  hermes:
+    tags: [Post-Training, TRL, Reinforcement Learning, Fine-Tuning, SFT, DPO, PPO, GRPO, RLHF, Preference Alignment, HuggingFace]
+
+---
+
+# TRL - Transformer Reinforcement Learning
+
+## Quick start
+
+TRL provides post-training methods for aligning language models with human preferences.
+
+**Installation**:
+```bash
+pip install trl transformers datasets peft accelerate
+```
+
+**Supervised Fine-Tuning** (instruction tuning):
+```python
+from trl import SFTTrainer
+
+trainer = SFTTrainer(
+    model="Qwen/Qwen2.5-0.5B",
+    train_dataset=dataset,  # Prompt-completion pairs
+)
+trainer.train()
+```
+
+**DPO** (align with preferences):
+```python
+from trl import DPOTrainer, DPOConfig
+
+config = DPOConfig(output_dir="model-dpo", beta=0.1)
+trainer = DPOTrainer(
+    model=model,
+    args=config,
+    train_dataset=preference_dataset,  # chosen/rejected pairs
+    processing_class=tokenizer
+)
+trainer.train()
+```
+
+## Common workflows
+
+### Workflow 1: Full RLHF pipeline (SFT → Reward Model → PPO)
+
+Complete pipeline from base model to human-aligned model.
+
+Copy this checklist:
+
+```
+RLHF Training:
+- [ ] Step 1: Supervised fine-tuning (SFT)
+- [ ] Step 2: Train reward model
+- [ ] Step 3: PPO reinforcement learning
+- [ ] Step 4: Evaluate aligned model
+```
+
+**Step 1: Supervised fine-tuning**
+
+Train base model on instruction-following data:
+
+```python
+from transformers import AutoModelForCausalLM, AutoTokenizer
+from trl import SFTTrainer, SFTConfig
+from datasets import load_dataset
+
+# Load model
+model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B")
+tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-0.5B")
+
+# Load instruction dataset
+dataset = load_dataset("trl-lib/Capybara", split="train")
+
+# Configure training
+training_args = SFTConfig(
+    output_dir="Qwen2.5-0.5B-SFT",
+    per_device_train_batch_size=4,
+    num_train_epochs=1,
+    learning_rate=2e-5,
+    logging_steps=10,
+    save_strategy="epoch"
+)
+
+# Train
+trainer = SFTTrainer(
+    model=model,
+    args=training_args,
+    train_dataset=dataset,
+    tokenizer=tokenizer
+)
+trainer.train()
+trainer.save_model()
+```
+
+**Step 2: Train reward model**
+
+Train model to predict human preferences:
+
+```python
+from transformers import AutoModelForSequenceClassification
+from trl import RewardTrainer, RewardConfig
+
+# Load SFT model as base
+model = AutoModelForSequenceClassification.from_pretrained(
+    "Qwen2.5-0.5B-SFT",
+    num_labels=1  # Single reward score
+)
+tokenizer = AutoTokenizer.from_pretrained("Qwen2.5-0.5B-SFT")
+
+# Load preference data (chosen/rejected pairs)
+dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")
+
+# Configure training
+training_args = RewardConfig(
+    output_dir="Qwen2.5-0.5B-Reward",
+    per_device_train_batch_size=2,
+    num_train_epochs=1,
+    learning_rate=1e-5
+)
+
+# Train reward model
+trainer = RewardTrainer(
+    model=model,
+    args=training_args,
+    processing_class=tokenizer,
+    train_dataset=dataset
+)
+trainer.train()
+trainer.save_model()
+```
+
+**Step 3: PPO reinforcement learning**
+
+Optimize policy using reward model:
+
+```bash
+python -m trl.scripts.ppo \
+    --model_name_or_path Qwen2.5-0.5B-SFT \
+    --reward_model_path Qwen2.5-0.5B-Reward \
+    --dataset_name trl-internal-testing/descriptiveness-sentiment-trl-style \
+    --output_dir Qwen2.5-0.5B-PPO \
+    --learning_rate 3e-6 \
+    --per_device_train_batch_size 64 \
+    --total_episodes 10000
+```
+
+**Step 4: Evaluate**
+
+```python
+from transformers import pipeline
+
+# Load aligned model
+generator = pipeline("text-generation", model="Qwen2.5-0.5B-PPO")
+
+# Test
+prompt = "Explain quantum computing to a 10-year-old"
+output = generator(prompt, max_length=200)[0]["generated_text"]
+print(output)
+```
+
+### Workflow 2: Simple preference alignment with DPO
+
+Align model with preferences without reward model.
+
+Copy this checklist:
+
+```
+DPO Training:
+- [ ] Step 1: Prepare preference dataset
+- [ ] Step 2: Configure DPO
+- [ ] Step 3: Train with DPOTrainer
+- [ ] Step 4: Evaluate alignment
+```
+
+**Step 1: Prepare preference dataset**
+
+Dataset format:
+```json
+{
+  "prompt": "What is the capital of France?",
+  "chosen": "The capital of France is Paris.",
+  "rejected": "I don't know."
+}
+```
+
+Load dataset:
+```python
+from datasets import load_dataset
+
+dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")
+# Or load your own
+# dataset = load_dataset("json", data_files="preferences.json")
+```
+
+**Step 2: Configure DPO**
+
+```python
+from trl import DPOConfig
+
+config = DPOConfig(
+    output_dir="Qwen2.5-0.5B-DPO",
+    per_device_train_batch_size=4,
+    num_train_epochs=1,
+    learning_rate=5e-7,
+    beta=0.1,  # KL penalty strength
+    max_prompt_length=512,
+    max_length=1024,
+    logging_steps=10
+)
+```
+
+**Step 3: Train with DPOTrainer**
+
+```python
+from transformers import AutoModelForCausalLM, AutoTokenizer
+from trl import DPOTrainer
+
+model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct")
+tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct")
+
+trainer = DPOTrainer(
+    model=model,
+    args=config,
+    train_dataset=dataset,
+    processing_class=tokenizer
+)
+
+trainer.train()
+trainer.save_model()
+```
+
+**CLI alternative**:
+```bash
+trl dpo \
+    --model_name_or_path Qwen/Qwen2.5-0.5B-Instruct \
+    --dataset_name argilla/Capybara-Preferences \
+    --output_dir Qwen2.5-0.5B-DPO \
+    --per_device_train_batch_size 4 \
+    --learning_rate 5e-7 \
+    --beta 0.1
+```
+
+### Workflow 3: Memory-efficient online RL with GRPO
+
+Train with reinforcement learning using minimal memory.
+
+For in-depth GRPO guidance — reward function design, critical training insights (loss behavior, mode collapse, tuning), and advanced multi-stage patterns — see **[references/grpo-training.md](references/grpo-training.md)**. A production-ready training script is in **[templates/basic_grpo_training.py](templates/basic_grpo_training.py)**.
+
+Copy this checklist:
+
+```
+GRPO Training:
+- [ ] Step 1: Define reward function
+- [ ] Step 2: Configure GRPO
+- [ ] Step 3: Train with GRPOTrainer
+```
+
+**Step 1: Define reward function**
+
+```python
+def reward_function(completions, **kwargs):
+    """
+    Compute rewards for completions.
+
+    Args:
+        completions: List of generated texts
+
+    Returns:
+        List of reward scores (floats)
+    """
+    rewards = []
+    for completion in completions:
+        # Example: reward based on length and unique words
+        score = len(completion.split())  # Favor longer responses
+        score += len(set(completion.lower().split()))  # Reward unique words
+        rewards.append(score)
+    return rewards
+```
+
+Or use a reward model:
+```python
+from transformers import pipeline
+
+reward_model = pipeline("text-classification", model="reward-model-path")
+
+def reward_from_model(completions, prompts, **kwargs):
+    # Combine prompt + completion
+    full_texts = [p + c for p, c in zip(prompts, completions)]
+    # Get reward scores
+    results = reward_model(full_texts)
+    return [r["score"] for r in results]
+```
+
+**Step 2: Configure GRPO**
+
+```python
+from trl import GRPOConfig
+
+config = GRPOConfig(
+    output_dir="Qwen2-GRPO",
+    per_device_train_batch_size=4,
+    num_train_epochs=1,
+    learning_rate=1e-5,
+    num_generations=4,  # Generate 4 completions per prompt
+    max_new_tokens=128
+)
+```
+
+**Step 3: Train with GRPOTrainer**
+
+```python
+from datasets import load_dataset
+from trl import GRPOTrainer
+
+# Load prompt-only dataset
+dataset = load_dataset("trl-lib/tldr", split="train")
+
+trainer = GRPOTrainer(
+    model="Qwen/Qwen2-0.5B-Instruct",
+    reward_funcs=reward_function,  # Your reward function
+    args=config,
+    train_dataset=dataset
+)
+
+trainer.train()
+```
+
+**CLI**:
+```bash
+trl grpo \
+    --model_name_or_path Qwen/Qwen2-0.5B-Instruct \
+    --dataset_name trl-lib/tldr \
+    --output_dir Qwen2-GRPO \
+    --num_generations 4
+```
+
+## When to use vs alternatives
+
+**Use TRL when:**
+- Need to align model with human preferences
+- Have preference data (chosen/rejected pairs)
+- Want to use reinforcement learning (PPO, GRPO)
+- Need reward model training
+- Doing RLHF (full pipeline)
+
+**Method selection**:
+- **SFT**: Have prompt-completion pairs, want basic instruction following
+- **DPO**: Have preferences, want simple alignment (no reward model needed)
+- **PPO**: Have reward model, need maximum control over RL
+- **GRPO**: Memory-constrained, want online RL
+- **Reward Model**: Building RLHF pipeline, need to score generations
+
+**Use alternatives instead:**
+- **HuggingFace Trainer**: Basic fine-tuning without RL
+- **Axolotl**: YAML-based training configuration
+- **LitGPT**: Educational, minimal fine-tuning
+- **Unsloth**: Fast LoRA training
+
+## Common issues
+
+**Issue: OOM during DPO training**
+
+Reduce batch size and sequence length:
+```python
+config = DPOConfig(
+    per_device_train_batch_size=1,  # Reduce from 4
+    max_length=512,  # Reduce from 1024
+    gradient_accumulation_steps=8  # Maintain effective batch
+)
+```
+
+Or use gradient checkpointing:
+```python
+model.gradient_checkpointing_enable()
+```
+
+**Issue: Poor alignment quality**
+
+Tune beta parameter:
+```python
+# Higher beta = more conservative (stays closer to reference)
+config = DPOConfig(beta=0.5)  # Default 0.1
+
+# Lower beta = more aggressive alignment
+config = DPOConfig(beta=0.01)
+```
+
+**Issue: Reward model not learning**
+
+Check loss type and learning rate:
+```python
+config = RewardConfig(
+    learning_rate=1e-5,  # Try different LR
+    num_train_epochs=3  # Train longer
+)
+```
+
+Ensure preference dataset has clear winners:
+```python
+# Verify dataset
+print(dataset[0])
+# Should have clear chosen > rejected
+```
+
+**Issue: PPO training unstable**
+
+Adjust KL coefficient:
+```python
+config = PPOConfig(
+    kl_coef=0.1,  # Increase from 0.05
+    cliprange=0.1  # Reduce from 0.2
+)
+```
+
+## Advanced topics
+
+**SFT training guide**: See [references/sft-training.md](references/sft-training.md) for dataset formats, chat templates, packing strategies, and multi-GPU training.
+
+**DPO variants**: See [references/dpo-variants.md](references/dpo-variants.md) for IPO, cDPO, RPO, and other DPO loss functions with recommended hyperparameters.
+
+**Reward modeling**: See [references/reward-modeling.md](references/reward-modeling.md) for outcome vs process rewards, Bradley-Terry loss, and reward model evaluation.
+
+**Online RL methods**: See [references/online-rl.md](references/online-rl.md) for PPO, GRPO, RLOO, and OnlineDPO with detailed configurations.
+
+**GRPO deep dive**: See [references/grpo-training.md](references/grpo-training.md) for expert-level GRPO patterns — reward function design philosophy, training insights (why loss increases, mode collapse detection), hyperparameter tuning, multi-stage training, and troubleshooting. Production-ready template in [templates/basic_grpo_training.py](templates/basic_grpo_training.py).
+
+## Hardware requirements
+
+- **GPU**: NVIDIA (CUDA required)
+- **VRAM**: Depends on model and method
+  - SFT 7B: 16GB (with LoRA)
+  - DPO 7B: 24GB (stores reference model)
+  - PPO 7B: 40GB (policy + reward model)
+  - GRPO 7B: 24GB (more memory efficient)
+- **Multi-GPU**: Supported via `accelerate`
+- **Mixed precision**: BF16 recommended (A100/H100)
+
+**Memory optimization**:
+- Use LoRA/QLoRA for all methods
+- Enable gradient checkpointing
+- Use smaller batch sizes with gradient accumulation
+
+## Resources
+
+- Docs: https://huggingface.co/docs/trl/
+- GitHub: https://github.com/huggingface/trl
+- Papers:
+  - "Training language models to follow instructions with human feedback" (InstructGPT, 2022)
+  - "Direct Preference Optimization: Your Language Model is Secretly a Reward Model" (DPO, 2023)
+  - "Group Relative Policy Optimization" (GRPO, 2024)
+- Examples: https://github.com/huggingface/trl/tree/main/examples/scripts
+
+
+