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Context Window Optimization: Managing Token Limits

1 mars 2025

11 min read

Ailog Research Team

Strategies for fitting more information in limited context windows: compression, summarization, smart selection, and window management techniques.

The Context Window Challenge

LLMs have fixed context windows:

Model	Context Window
GPT-3.5 Turbo	16K tokens
GPT-4	8K / 32K tokens
GPT-4 Turbo	128K tokens
Claude 2	100K tokens
Claude 3	200K tokens
Llama 2	4K tokens
Gemini 1.5 Pro	1M tokens

The problem:

System prompt: 500 tokens
User query: 100 tokens
Retrieved contexts: 5 chunks × 512 tokens = 2,560 tokens
Conversation history: 1,000 tokens
─────────────────────────────────────────
Total input: 4,160 tokens
Max output: 1,000 tokens
─────────────────────────────────────────
Total: 5,160 tokens (fits in 8K window)

As your RAG system scales:

More retrieved chunks
Longer conversations
More complex prompts
Larger documents

You'll hit context limits. Optimization is essential.

Token Counting

Accurate Token Counting

DEVELOPERpython
import tiktoken

def count_tokens(text: str, model="gpt-4") -> int:
    encoding = tiktoken.encoding_for_model(model)
    return len(encoding.encode(text))

# Example
text = "Hello, how are you?"
tokens = count_tokens(text)  # ~5 tokens

Context Budget Calculation

DEVELOPERpython
class ContextBudget:
    def __init__(self, model="gpt-4", max_output_tokens=1000):
        self.model = model
        self.max_output = max_output_tokens

        # Context windows
        self.windows = {
            "gpt-3.5-turbo": 16385,
            "gpt-4": 8192,
            "gpt-4-32k": 32768,
            "gpt-4-turbo": 128000,
            "claude-2": 100000,
            "claude-3": 200000,
        }

        self.total_window = self.windows.get(model, 8192)
        self.available_input = self.total_window - max_output_tokens

    def allocate(self, system=500, query=200, history=1000):
        """
        Allocate tokens for different components
        """
        fixed_tokens = system + query + history
        available_for_context = self.available_input - fixed_tokens

        return {
            'system_prompt': system,
            'query': query,
            'history': history,
            'context': available_for_context,
            'output': self.max_output,
            'total': fixed_tokens + available_for_context + self.max_output
        }

# Usage
budget = ContextBudget(model="gpt-4")
allocation = budget.allocate()

print(f"Available for retrieved context: {allocation['context']} tokens")
# Available for retrieved context: 5492 tokens

Chunk Selection Strategies

Top-K with Token Limit

DEVELOPERpython
def select_chunks_within_budget(chunks: List[dict], budget: int, model="gpt-4") -> List[dict]:
    """
    Select as many top chunks as possible within token budget
    """
    selected = []
    total_tokens = 0

    for chunk in chunks:
        chunk_tokens = count_tokens(chunk['content'], model)

        if total_tokens + chunk_tokens <= budget:
            selected.append(chunk)
            total_tokens += chunk_tokens
        else:
            break

    return selected

# Usage
chunks = retriever.retrieve(query, k=20)  # Get more than needed
selected = select_chunks_within_budget(chunks, budget=5000)
# Might return 8-12 chunks depending on length

Priority-Based Selection

DEVELOPERpython
def priority_select_chunks(chunks: List[dict], budget: int, weights: dict) -> List[dict]:
    """
    Select chunks based on multiple criteria
    """
    # Score chunks
    for chunk in chunks:
        score = (
            weights['relevance'] * chunk['similarity_score'] +
            weights['recency'] * chunk['recency_score'] +
            weights['authority'] * chunk['authority_score']
        )
        chunk['priority_score'] = score

    # Sort by priority
    sorted_chunks = sorted(chunks, key=lambda x: x['priority_score'], reverse=True)

    # Select within budget
    return select_chunks_within_budget(sorted_chunks, budget)

Compression Techniques

Extractive Summarization

Extract only relevant sentences.

DEVELOPERpython
from transformers import pipeline

summarizer = pipeline("summarization", model="facebook/bart-large-cnn")

def compress_chunk(chunk: str, max_length: int = 130) -> str:
    """
    Compress chunk while preserving key information
    """
    summary = summarizer(
        chunk,
        max_length=max_length,
        min_length=30,
        do_sample=False
    )

    return summary[0]['summary_text']

# Usage
original = "Very long chunk of text..." # 500 tokens
compressed = compress_chunk(original, max_length=130)  # ~100 tokens

LLM-Based Compression

DEVELOPERpython
async def llm_compress_context(query: str, chunk: str, llm) -> str:
    """
    Use LLM to extract only relevant information
    """
    prompt = f"""Extract only the information relevant to the query from this text.

Query: {query}

Text: {chunk}

Relevant excerpt:"""

    return await llm.generate(prompt, max_tokens=200)

# Example
query = "How do I reset my password?"
chunk = "Our system offers many features including user management, password reset, data export..."

compressed = await llm_compress_context(query, chunk, llm)
# "Password reset: Click 'Forgot Password' on the login page..."

Semantic Compression

Remove redundant information across chunks.

DEVELOPERpython
def remove_redundant_chunks(chunks: List[str], threshold=0.85) -> List[str]:
    """
    Remove chunks with high semantic overlap
    """
    embeddings = embed_batch(chunks)
    selected = [0]  # Always keep first (highest relevance)

    for i in range(1, len(chunks)):
        # Check similarity to already selected
        max_similarity = max(
            cosine_similarity(embeddings[i], embeddings[j])
            for j in selected
        )

        # Only add if sufficiently different
        if max_similarity < threshold:
            selected.append(i)

    return [chunks[i] for i in selected]

Sliding Window Approaches

Chunked Processing

Process long documents in windows.

DEVELOPERpython
def sliding_window_qa(query: str, long_document: str, window_size=2000, stride=1000):
    """
    Process long document with sliding window
    """
    answers = []

    # Create windows
    tokens = tokenize(long_document)

    for i in range(0, len(tokens), stride):
        window = tokens[i:i + window_size]
        window_text = detokenize(window)

        # Generate answer for this window
        answer = llm.generate(
            query=query,
            context=window_text
        )

        if answer and answer != "Information not found":
            answers.append({
                'answer': answer,
                'position': i,
                'confidence': estimate_confidence(answer)
            })

    # Combine answers (take highest confidence or synthesize)
    return best_answer(answers)

Hierarchical Processing

Process at multiple granularities.

DEVELOPERpython
async def hierarchical_processing(query: str, document: str, llm):
    """
    1. Summarize entire document
    2. Find relevant sections in summary
    3. Process full sections for answer
    """

    # Level 1: Document summary
    doc_summary = await llm.generate(
        f"Summarize this document:\n\n{document}",
        max_tokens=500
    )

    # Level 2: Identify relevant sections
    relevance_check = await llm.generate(
        f"Which parts of this summary are relevant to: {query}\n\nSummary: {doc_summary}",
        max_tokens=100
    )

    # Level 3: Process full relevant sections
    relevant_sections = extract_sections(document, relevance_check)

    # Generate final answer from relevant sections
    answer = await llm.generate(
        query=query,
        context=relevant_sections
    )

    return answer

Conversation History Management

Fixed Window

Keep only recent history.

DEVELOPERpython
class FixedWindowHistory:
    def __init__(self, max_turns=5):
        self.max_turns = max_turns
        self.history = []

    def add_turn(self, query: str, answer: str):
        self.history.append({'query': query, 'answer': answer})

        # Keep only recent turns
        if len(self.history) > self.max_turns:
            self.history = self.history[-self.max_turns:]

    def get_context(self) -> str:
        return "\n".join([
            f"User: {turn['query']}\nAssistant: {turn['answer']}"
            for turn in self.history
        ])

Token-Based Window

Keep history within token budget.

DEVELOPERpython
class TokenBudgetHistory:
    def __init__(self, max_tokens=2000, model="gpt-4"):
        self.max_tokens = max_tokens
        self.model = model
        self.history = []

    def add_turn(self, query: str, answer: str):
        self.history.append({'query': query, 'answer': answer})
        self._trim_to_budget()

    def _trim_to_budget(self):
        while self.history:
            context = self.get_context()
            tokens = count_tokens(context, self.model)

            if tokens <= self.max_tokens:
                break

            # Remove oldest turn
            self.history.pop(0)

    def get_context(self) -> str:
        return "\n".join([
            f"User: {turn['query']}\nAssistant: {turn['answer']}"
            for turn in self.history
        ])

Summarized History

Summarize old history to save tokens.

DEVELOPERpython
class SummarizedHistory:
    def __init__(self, llm, summary_threshold=10):
        self.llm = llm
        self.summary_threshold = summary_threshold
        self.summary = ""
        self.recent_history = []

    async def add_turn(self, query: str, answer: str):
        self.recent_history.append({'query': query, 'answer': answer})

        # When recent history gets long, summarize
        if len(self.recent_history) >= self.summary_threshold:
            await self._summarize_old_turns()

    async def _summarize_old_turns(self):
        # Summarize all but last 3 turns
        to_summarize = self.recent_history[:-3]

        if to_summarize:
            history_text = format_history(to_summarize)

            new_summary = await self.llm.generate(
                f"Summarize this conversation:\n\n{self.summary}\n\n{history_text}",
                max_tokens=200
            )

            self.summary = new_summary
            self.recent_history = self.recent_history[-3:]

    async def get_context(self) -> str:
        recent = format_history(self.recent_history)

        if self.summary:
            return f"Earlier: {self.summary}\n\nRecent:\n{recent}"
        else:
            return recent

Prompt Optimization

Template Compression

DEVELOPERpython
# Verbose prompt (wasteful)
verbose_prompt = """
You are a helpful AI assistant. Your job is to answer questions based on the provided context.
Please read the context carefully and provide accurate answers. If you don't know the answer,
say so. Always be polite and professional.

Context: {context}

Question: {query}

Answer:
"""

# Compressed prompt (efficient)
compressed_prompt = """Answer based on context. Say "I don't know" if uncertain.

Context: {context}

Q: {query}
A:"""

# Token savings: ~50 tokens per query

Dynamic Prompts

Adjust prompt based on query complexity.

DEVELOPERpython
def get_optimal_prompt(query: str, context: str, complexity: str) -> str:
    if complexity == "simple":
        # Minimal prompt for simple queries
        return f"Context: {context}\n\nQ: {query}\nA:"

    elif complexity == "medium":
        # Standard prompt
        return f"Answer based on context:\n\n{context}\n\nQ: {query}\nA:"

    else:
        # Detailed prompt for complex queries
        return f"""Analyze the context carefully and provide a detailed answer.

Context: {context}

Question: {query}

Detailed answer:"""

Adaptive Context Loading

Lazy Loading

Load context incrementally.

DEVELOPERpython
async def adaptive_context_loading(query: str, vector_db, llm, max_chunks=10):
    """
    Start with few chunks, add more if needed
    """
    chunk_counts = [3, 5, 8, max_chunks]

    for num_chunks in chunk_counts:
        # Retrieve chunks
        chunks = await vector_db.search(query, k=num_chunks)
        context = format_chunks(chunks)

        # Generate answer
        answer = await llm.generate(query=query, context=context)

        # Check confidence
        confidence = await estimate_confidence(answer, llm)

        if confidence > 0.8:
            return answer  # Good enough

    # Used all chunks, return best effort
    return answer

Confidence-Based Retrieval

DEVELOPERpython
async def confidence_based_retrieval(query: str, vector_db, llm):
    """
    Retrieve more context if initial answer has low confidence
    """
    # Start with top 3
    chunks = await vector_db.search(query, k=3)
    context = format_chunks(chunks)

    answer = await llm.generate(query=query, context=context)
    confidence = await estimate_confidence(answer, llm)

    # If low confidence, retrieve more
    if confidence < 0.6:
        additional_chunks = await vector_db.search(query, k=7)
        chunks.extend(additional_chunks[3:])  # Skip first 3 (duplicates)
        context = format_chunks(chunks)

        answer = await llm.generate(query=query, context=context)

    return answer

Multi-Turn Optimization

Context Carryover

Avoid re-sending unchanged context.

DEVELOPERpython
class EfficientConversation:
    def __init__(self, llm):
        self.llm = llm
        self.static_context = None
        self.conversation_history = []

    async def query(self, user_query: str, retrieve_new_context=True):
        # Retrieve context only if query changed topic
        if retrieve_new_context:
            self.static_context = await retrieve_context(user_query)

        # Build minimal prompt
        prompt = f"""Context (same as before): [Ref: {hash(self.static_context)}]

Previous conversation:
{format_recent_history(self.conversation_history[-2:])}

New question: {user_query}

Answer:"""

        answer = await self.llm.generate(prompt)

        self.conversation_history.append({
            'query': user_query,
            'answer': answer
        })

        return answer

Monitoring Token Usage

DEVELOPERpython
class TokenUsageTracker:
    def __init__(self):
        self.usage = []

    def track(self, prompt_tokens: int, completion_tokens: int, model: str):
        self.usage.append({
            'timestamp': time.time(),
            'prompt_tokens': prompt_tokens,
            'completion_tokens': completion_tokens,
            'total_tokens': prompt_tokens + completion_tokens,
            'model': model
        })

    def get_stats(self):
        if not self.usage:
            return {}

        total_tokens = sum(u['total_tokens'] for u in self.usage)
        avg_prompt = np.mean([u['prompt_tokens'] for u in self.usage])
        avg_completion = np.mean([u['completion_tokens'] for u in self.usage])

        return {
            'total_tokens': total_tokens,
            'avg_prompt_tokens': avg_prompt,
            'avg_completion_tokens': avg_completion,
            'num_requests': len(self.usage)
        }

# Usage
tracker = TokenUsageTracker()

response = await llm.generate(prompt)
tracker.track(
    prompt_tokens=count_tokens(prompt),
    completion_tokens=count_tokens(response),
    model="gpt-4"
)

stats = tracker.get_stats()
print(f"Average prompt tokens: {stats['avg_prompt_tokens']}")

Best Practices

Measure first: Count tokens before optimizing
Allocate budgets: Reserve tokens for each component
Compress intelligently: Only compress what won't hurt quality
Trim history: Don't send full conversation every time
Start small: Retrieve fewer chunks, expand if needed
Monitor usage: Track token consumption over time
Test impact: Ensure compression doesn't hurt quality

Trade-offs

Strategy	Token Savings	Quality Impact	Complexity
Chunk selection	20-40%	Low	Low
Extractive summary	50-70%	Medium	Medium
LLM compression	60-80%	Low-Medium	Medium
Prompt optimization	10-30%	Low	Low
History summarization	40-60%	Low	Medium
Adaptive loading	Variable	Low	High

Next Steps

You now have a complete understanding of RAG fundamentals, from embeddings and chunking to production deployment and optimization. Apply these guides incrementally, measure results, and iterate based on your specific use case and constraints.