Gradient Descent Weekly — Issue #23

You’ve got a task.
Maybe it’s answering questions from documents.
Or generating domain-specific responses.
Or automating support.

You’re hearing two things:

“Just fine-tune the model!”
“No, use Retrieval-Augmented Generation (RAG)!”

So… which is it?

In this issue, we’re going to:

Break down what RAG and fine-tuning actually are
Compare strengths, weaknesses, and costs
Give you a decision matrix to pick the right path
Show real-world use cases

Let’s settle this once and for all.

🧠 First: What Are RAG and Fine-Tuning?

🧩 Retrieval-Augmented Generation (RAG)

Keep the base LLM frozen. Inject relevant context from your data into the prompt.

You don’t train the model
You retrieve relevant chunks (via search)
You feed them into the model prompt with the user query

🧠 Think of it like giving the LLM an open book exam.

🔧 Fine-Tuning

Actually retrain the model weights on your custom data.

The model learns from your examples
No retrieval — knowledge is embedded into weights
Needs GPUs, labeled data, MLOps muscle

🧠 Think of it like rewiring the LLM’s brain.

🥊 RAG vs Fine-Tuning: Head-to-Head

Feature	RAG	Fine-Tuning
🧠 Model Training	❌ None (zero-shot)	✅ Required
📦 Domain Knowledge	Retrieved on-the-fly	Baked into weights
🔄 Updating Data	Easy — just update your docs	Hard — retrain needed
💰 Cost	Lower infra cost, higher latency	High upfront GPU cost, faster inference
⚙️ Infra Complexity	Needs vector DB + search infra	Needs training + model hosting infra
📉 Drift Resistance	Resilient to changes in data	Prone to staleness
📚 Few-shot Learning	Limited, needs quality context windows	Good, if fine-tuned on similar tasks
🔒 Data Privacy	Easier to control (no training leakage)	Risk of memorization

🧪 When to Use RAG

✅ You have a lot of domain-specific text/data
✅ Your knowledge base changes often
✅ You can’t afford retraining pipelines
✅ You want faster iteration and low maintenance
✅ Your input size fits in the context window (or you chunk smartly)

Common RAG Use Cases:

Chatbots for internal documents
Enterprise knowledge Q&A
Legal, finance, healthcare document lookup
Personalized assistant apps
Custom search+summarize workflows

🧪 When to Use Fine-Tuning

✅ You need the model to follow custom behavior
✅ You have a very narrow domain (e.g., chemistry, contracts, insurance)
✅ You want better format adherence or output control
✅ You’ve hit RAG’s context limits
✅ You can invest in labeling, training, infra

Common Fine-Tuning Use Cases:

Agents/tools with specific response styles
Classification/regression tasks from text
Code generation for internal libraries
Custom tone-of-voice content generation
When latency at scale matters (RAG is slow)

🧩 What If You Combine Them?

Oh yes. The real pros do both.

Fine-tune a base model on your tone + format + data structure
→ Then augment it with RAG for up-to-date context.

💡 Example:

Fine-tune to generate structured support replies
RAG to inject the latest product release notes

This hybrid gives:

Better output quality
Lower hallucination risk
Dynamic + domain-smart performance

🧠 Decision Matrix: RAG vs Fine-Tuning

Scenario	Recommended
Need to keep model updated with changing docs	✅ RAG
Need control over output structure/style	✅ Fine-Tune
Low budget + fast MVP	✅ RAG
Model must recall info outside context window	✅ Fine-Tune
Real-time customer support from docs	✅ RAG
Legal document clause rewriting	✅ Fine-Tune
Internal Q&A over technical docs	✅ RAG
Email drafting with brand tone	✅ Fine-Tune
Long-term scalable infra	🤝 Hybrid

🧰 Tooling You Can Use

For RAG:

LangChain
LlamaIndex
Haystack
Pinecone, Weaviate, Qdrant, or FAISS
OpenAI Embeddings / HuggingFace

For Fine-Tuning:

OpenAI fine-tune API (GPT-3.5)
HuggingFace Trainer (PEFT, LoRA, DPO)
Axolotl / QLoRA
Amazon SageMaker JumpStart
Weights & Biases for experiment tracking

⚠️ Final Word of Warning

RAG hides latency.
Fine-tuning hides inflexibility.
Pick your poison — or better, balance them.

Don’t just do what’s hyped.
Ask:

What’s the user experience I want?
How often does my knowledge change?
What’s my budget and infra maturity?

Then build accordingly.

🔮 Up Next on Gradient Descent Weekly:

How to Build a Vector Database That Doesn’t Suck

🤖 RAG vs Fine-Tuning: Which One Is Right for You?