Cost Engineering for LLMs

# script_id: day_033_cost_engineering_for_llms/token_counting
import tiktoken
from openai import OpenAI

def count_tokens(text: str, model: str = "gpt-4o") -> int:
    """Count tokens for a given text and model."""
    enc = tiktoken.encoding_for_model(model)
    return len(enc.encode(text))


def count_message_tokens(messages: list[dict], model: str = "gpt-4o") -> int:
    """Count tokens for a messages array (includes formatting overhead)."""
    enc = tiktoken.encoding_for_model(model)
    
    # OpenAI adds per-message overhead
    tokens_per_message = 3
    tokens_per_name = 1
    
    total = 0
    for message in messages:
        total += tokens_per_message
        for key, value in message.items():
            total += len(enc.encode(value))
            if key == "name":
                total += tokens_per_name
    
    total += 3  # Reply priming tokens
    return total


# Usage
messages = [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Summarize the French Revolution in 3 bullet points."},
]

input_tokens = count_message_tokens(messages)
print(f"Input tokens: {input_tokens}")
print(f"Estimated input cost (GPT-4o): ${input_tokens / 1_000_000 * 2.50:.6f}")

# script_id: day_033_cost_engineering_for_llms/cost_calculation
from dataclasses import dataclass


@dataclass
class ModelPricing:
    input_per_million: float   # USD per 1M input tokens
    output_per_million: float  # USD per 1M output tokens


PRICING = {
    "gpt-4o": ModelPricing(input_per_million=2.50, output_per_million=10.00),
    "gpt-4o-mini": ModelPricing(input_per_million=0.15, output_per_million=0.60),
    "claude-sonnet-4-5": ModelPricing(input_per_million=3.00, output_per_million=15.00),
    "claude-haiku-4-5": ModelPricing(input_per_million=0.80, output_per_million=4.00),
}


def calculate_cost(
    model: str,
    input_tokens: int,
    output_tokens: int
) -> float:
    """Calculate cost in USD for a single API call."""
    pricing = PRICING.get(model)
    if not pricing:
        raise ValueError(f"Unknown model: {model}")
    
    input_cost = (input_tokens / 1_000_000) * pricing.input_per_million
    output_cost = (output_tokens / 1_000_000) * pricing.output_per_million
    return input_cost + output_cost


# Example: A typical RAG query
rag_query_cost = calculate_cost(
    model="gpt-4o",
    input_tokens=1500,   # System prompt + context + question
    output_tokens=300,   # Answer
)
print(f"Cost per RAG query (GPT-4o): ${rag_query_cost:.6f}")
# → $0.006750

# Same query on GPT-4o-mini
rag_query_mini_cost = calculate_cost(
    model="gpt-4o-mini",
    input_tokens=1500,
    output_tokens=300,
)
print(f"Cost per RAG query (GPT-4o-mini): ${rag_query_mini_cost:.6f}")
# → $0.000405

# script_id: day_033_cost_engineering_for_llms/cost_calculation
def estimate_monthly_cost(
    daily_users: int,
    messages_per_user: float,
    avg_input_tokens: int,
    avg_output_tokens: int,
    llm_model: str,
    embedding_model: str = "text-embedding-3-small",
) -> dict:
    """Estimate monthly cost for a RAG chatbot."""
    
    # Embedding costs
    EMBEDDING_COST_PER_MILLION = {
        "text-embedding-3-small": 0.02,
        "text-embedding-3-large": 0.13,
    }
    
    monthly_queries = daily_users * messages_per_user * 30
    
    # Embedding: each query gets embedded (short, ~50 tokens)
    embedding_tokens = monthly_queries * 50
    embedding_cost = (embedding_tokens / 1_000_000) * EMBEDDING_COST_PER_MILLION[embedding_model]
    
    # LLM calls
    llm_cost = monthly_queries * calculate_cost(llm_model, avg_input_tokens, avg_output_tokens)
    
    total = embedding_cost + llm_cost
    cost_per_user_per_month = total / (daily_users * 30)
    
    return {
        "monthly_queries": monthly_queries,
        "embedding_cost_usd": round(embedding_cost, 2),
        "llm_cost_usd": round(llm_cost, 2),
        "total_usd": round(total, 2),
        "cost_per_user_per_month_usd": round(cost_per_user_per_month, 4),
    }


# Scenario 1: GPT-4o (premium quality)
premium = estimate_monthly_cost(
    daily_users=1000,
    messages_per_user=5,
    avg_input_tokens=1500,
    avg_output_tokens=300,
    llm_model="gpt-4o",
)
print("GPT-4o scenario:")
print(f"  Monthly cost: ${premium['total_usd']:,.2f}")
print(f"  Cost per user/month: ${premium['cost_per_user_per_month_usd']:.4f}")
# → Monthly cost: $1,012.50 (~$1K/month)

# Scenario 2: GPT-4o-mini (budget)
budget = estimate_monthly_cost(
    daily_users=1000,
    messages_per_user=5,
    avg_input_tokens=1500,
    avg_output_tokens=300,
    llm_model="gpt-4o-mini",
)
print("\nGPT-4o-mini scenario:")
print(f"  Monthly cost: ${budget['total_usd']:,.2f}")
print(f"  Cost per user/month: ${budget['cost_per_user_per_month_usd']:.4f}")
# → Monthly cost: ~$61/month (16x cheaper!)

# script_id: day_033_cost_engineering_for_llms/model_routing
from openai import OpenAI
from pydantic import BaseModel
from enum import Enum

client = OpenAI()


class QueryComplexity(str, Enum):
    SIMPLE = "simple"
    COMPLEX = "complex"


class ComplexityResult(BaseModel):
    complexity: QueryComplexity
    reason: str


def classify_query(query: str) -> QueryComplexity:
    """Classify query complexity using a cheap, fast model."""
    response = client.chat.completions.create(
        model="gpt-4o-mini",  # Always use cheap model for routing
        messages=[
            {
                "role": "system",
                "content": """Classify this query as 'simple' or 'complex'.
                
Simple: factual questions, lookups, definitions, yes/no, greetings
Complex: multi-step reasoning, code generation, analysis, comparisons, synthesis

Respond with JSON: {"complexity": "simple" or "complex", "reason": "brief reason"}"""
            },
            {"role": "user", "content": query},
        ],
        response_format={"type": "json_object"},
        max_tokens=50,  # Keep routing cheap
    )
    
    import json
    data = json.loads(response.choices[0].message.content)
    return QueryComplexity(data["complexity"])


def smart_query(query: str, context: str = "") -> tuple[str, str]:
    """Route query to appropriate model based on complexity.
    
    Returns: (answer, model_used)
    """
    complexity = classify_query(query)
    
    model = "gpt-4o" if complexity == QueryComplexity.COMPLEX else "gpt-4o-mini"
    
    messages = [
        {"role": "system", "content": "You are a helpful assistant."},
    ]
    if context:
        messages.append({"role": "system", "content": f"Context:\n{context}"})
    messages.append({"role": "user", "content": query})
    
    response = client.chat.completions.create(
        model=model,
        messages=messages,
    )
    
    return response.choices[0].message.content, model


# Test it
queries = [
    "What is the capital of France?",       # Simple
    "Compare and contrast React vs Vue for a large enterprise app with 50 developers",  # Complex
    "How do I install Python?",             # Simple
    "Design a caching strategy for a distributed system with 10M requests/day",  # Complex
]

for q in queries:
    answer, model_used = smart_query(q)
    print(f"Model: {model_used:15} | Query: {q[:50]}...")

# script_id: day_033_cost_engineering_for_llms/cost_calculation
import hashlib
import json
import time
from functools import wraps

# Simple in-memory cache (use Redis in production)
_cache: dict[str, dict] = {}


def cache_key(model: str, messages: list[dict]) -> str:
    """Generate a deterministic cache key."""
    content = json.dumps({"model": model, "messages": messages}, sort_keys=True)
    return hashlib.sha256(content.encode()).hexdigest()


def cached_completion(
    client,
    model: str,
    messages: list[dict],
    temperature: float = 0,
    ttl_seconds: int = 3600,
    **kwargs
):
    """LLM completion with exact-match caching.
    
    Only safe to cache when temperature=0 (deterministic).
    """
    if temperature != 0:
        # Non-deterministic: skip cache
        return client.chat.completions.create(
            model=model,
            messages=messages,
            temperature=temperature,
            **kwargs
        )
    
    key = cache_key(model, messages)
    now = time.time()
    
    if key in _cache:
        entry = _cache[key]
        if now - entry["timestamp"] < ttl_seconds:
            print(f"Cache HIT (saved ~${entry['estimated_cost']:.6f})")
            return entry["response"]
    
    # Cache MISS: make the real call
    response = client.chat.completions.create(
        model=model,
        messages=messages,
        temperature=temperature,
        **kwargs
    )
    
    cost = calculate_cost(
        model,
        response.usage.prompt_tokens,
        response.usage.completion_tokens
    )
    
    _cache[key] = {
        "response": response,
        "timestamp": now,
        "estimated_cost": cost,
    }
    
    return response

# script_id: day_033_cost_engineering_for_llms/semantic_cache
import numpy as np
from openai import OpenAI

client = OpenAI()

class SemanticCache:
    """Cache LLM responses based on semantic similarity of queries."""
    
    def __init__(self, similarity_threshold: float = 0.95):
        self.threshold = similarity_threshold
        self.entries: list[dict] = []  # In prod: use a vector DB
    
    def _embed(self, text: str) -> list[float]:
        response = client.embeddings.create(
            model="text-embedding-3-small",
            input=text,
        )
        return response.data[0].embedding
    
    def _cosine_similarity(self, a: list[float], b: list[float]) -> float:
        a, b = np.array(a), np.array(b)
        return float(np.dot(a, b) / (np.linalg.norm(a) * np.linalg.norm(b)))
    
    def get(self, query: str) -> str | None:
        """Find a cached response for a semantically similar query."""
        if not self.entries:
            return None
        
        query_embedding = self._embed(query)
        
        best_match = None
        best_score = 0
        
        for entry in self.entries:
            score = self._cosine_similarity(query_embedding, entry["embedding"])
            if score > best_score:
                best_score = score
                best_match = entry
        
        if best_score >= self.threshold:
            print(f"Semantic cache HIT (similarity: {best_score:.3f})")
            return best_match["response"]
        
        return None
    
    def set(self, query: str, response: str):
        """Cache a response with its query embedding."""
        embedding = self._embed(query)
        self.entries.append({
            "query": query,
            "embedding": embedding,
            "response": response,
        })


# Usage
cache = SemanticCache(similarity_threshold=0.92)

def cached_rag_query(query: str) -> str:
    # Check semantic cache first (cheap: just one embedding call)
    cached = cache.get(query)
    if cached:
        return cached
    
    # Full RAG pipeline (expensive)
    response = "... full LLM response ..."
    cache.set(query, response)
    return response

# These two queries should hit the same cache entry:
# "What is the return policy?" → cache miss, stores embedding
# "How do I return a product?" → cache hit! (semantically similar)

# script_id: day_033_cost_engineering_for_llms/prompt_caching
from anthropic import Anthropic

client = Anthropic()

# The system prompt and documents are cached after the first request
# Subsequent requests with the same prefix get 90% input discount
response = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    system=[
        {
            "type": "text",
            "text": "You are a legal document analyst...",  # Large system prompt
            "cache_control": {"type": "ephemeral"}
        }
    ],
    messages=[{"role": "user", "content": "Summarize section 3."}]
)

# Check cache usage in the response
print(f"Input tokens: {response.usage.input_tokens}")
print(f"Cache read tokens: {response.usage.cache_read_input_tokens}")
print(f"Cache creation tokens: {response.usage.cache_creation_input_tokens}")

# script_id: day_033_cost_engineering_for_llms/batch_api
from openai import OpenAI
import json

client = OpenAI()

# 1. Create a JSONL file with requests
requests = [
    {
        "custom_id": f"request-{i}",
        "method": "POST",
        "url": "/v1/chat/completions",
        "body": {
            "model": "gpt-4o-mini",
            "messages": [{"role": "user", "content": f"Summarize document {i}"}],
            "max_tokens": 500
        }
    }
    for i in range(100)
]

with open("batch_requests.jsonl", "w") as f:
    for req in requests:
        f.write(json.dumps(req) + "\n")

# 2. Upload and create batch
batch_file = client.files.create(file=open("batch_requests.jsonl", "rb"), purpose="batch")
batch = client.batches.create(input_file_id=batch_file.id, endpoint="/v1/chat/completions", completion_window="24h")

# 3. Check status (completes within 24h, usually much faster)
status = client.batches.retrieve(batch.id)
print(f"Status: {status.status}")  # validating, in_progress, completed

# script_id: day_033_cost_engineering_for_llms/cost_calculation
import time
import logging
from dataclasses import dataclass, field
from collections import defaultdict
from openai import OpenAI

logger = logging.getLogger(__name__)


@dataclass
class CostTracker:
    """Track LLM costs with per-user and global budgets."""
    
    daily_budget_usd: float = 100.0
    per_user_daily_budget_usd: float = 1.0
    
    _daily_spend: float = field(default=0.0, init=False)
    _user_spend: dict[str, float] = field(default_factory=lambda: defaultdict(float), init=False)
    _day: str = field(default_factory=lambda: time.strftime("%Y-%m-%d"), init=False)
    
    def _reset_if_new_day(self):
        today = time.strftime("%Y-%m-%d")
        if today != self._day:
            self._day = today
            self._daily_spend = 0.0
            self._user_spend.clear()
            logger.info("Daily cost counters reset")
    
    def check_budget(self, user_id: str) -> tuple[bool, str]:
        """Check if a user is within budget. Returns (allowed, reason)."""
        self._reset_if_new_day()
        
        if self._daily_spend >= self.daily_budget_usd:
            return False, f"Daily budget exhausted (${self._daily_spend:.2f}/${self.daily_budget_usd})"
        
        user_spend = self._user_spend[user_id]
        if user_spend >= self.per_user_daily_budget_usd:
            return False, f"User daily budget exhausted (${user_spend:.4f}/${self.per_user_daily_budget_usd})"
        
        return True, "OK"
    
    def record_cost(self, user_id: str, model: str, input_tokens: int, output_tokens: int):
        """Record cost after a successful API call."""
        cost = calculate_cost(model, input_tokens, output_tokens)
        self._daily_spend += cost
        self._user_spend[user_id] += cost
        
        logger.info(
            "llm_cost",
            extra={
                "user_id": user_id,
                "model": model,
                "input_tokens": input_tokens,
                "output_tokens": output_tokens,
                "cost_usd": cost,
                "daily_total_usd": self._daily_spend,
            }
        )
        
        # Alert at 80% of daily budget
        if self._daily_spend >= self.daily_budget_usd * 0.8:
            logger.warning(f"Daily budget at {self._daily_spend / self.daily_budget_usd:.0%}")
        
        return cost
    
    @property
    def daily_spend(self) -> float:
        self._reset_if_new_day()
        return self._daily_spend


# Global tracker instance
tracker = CostTracker(daily_budget_usd=50.0, per_user_daily_budget_usd=0.50)
client = OpenAI()


def guarded_completion(user_id: str, model: str, messages: list[dict], **kwargs):
    """LLM completion with budget enforcement and cost tracking."""
    
    # Check budget before calling
    allowed, reason = tracker.check_budget(user_id)
    if not allowed:
        raise PermissionError(f"Budget exceeded: {reason}")
    
    response = client.chat.completions.create(
        model=model,
        messages=messages,
        **kwargs
    )
    
    # Record actual cost after call
    cost = tracker.record_cost(
        user_id=user_id,
        model=model,
        input_tokens=response.usage.prompt_tokens,
        output_tokens=response.usage.completion_tokens,
    )
    
    return response, cost

# script_id: day_033_cost_engineering_for_llms/cost_conscious_chunking
# Cost-conscious chunking: fewer, better chunks
def optimize_context(chunks: list[str], query: str, max_tokens: int = 800) -> list[str]:
    """Select top chunks that fit within a token budget."""
    enc = tiktoken.encoding_for_model("gpt-4o")
    
    selected = []
    used_tokens = 0
    
    # Assume chunks are pre-ranked by relevance
    for chunk in chunks:
        chunk_tokens = len(enc.encode(chunk))
        if used_tokens + chunk_tokens <= max_tokens:
            selected.append(chunk)
            used_tokens += chunk_tokens
        else:
            break
    
    return selected