cost-aware-llm-pipeline

# Cost-Aware LLM Pipeline

Patterns for controlling LLM API costs while maintaining quality. Combines model routing, budget tracking, retry logic, and prompt caching into a composable pipeline.

## When to Use

- Building applications that call LLM APIs (Gemini, GPT, etc.)
- Processing batches of items with varying complexity
- Need to stay within a budget for API spend
- Optimizing cost without sacrificing quality on complex tasks

## Core Concepts

### 1. Model Routing by Task Complexity

Automatically select cheaper models for simple tasks, reserving expensive models for complex ones.

```python
MODEL_FLASH = "gemini-2.5-flash"
MODEL_FLASH_LITE = "gemini-2.5-flash-lite"

_FLASH_TEXT_THRESHOLD = 10_000  # chars
_FLASH_ITEM_THRESHOLD = 30     # items

def select_model(
    text_length: int,
    item_count: int,
    force_model: str | None = None,
) -> str:
    """Select model based on task complexity."""
    if force_model is not None:
        return force_model
    if text_length >= _FLASH_TEXT_THRESHOLD or item_count >= _FLASH_ITEM_THRESHOLD:
        return MODEL_FLASH  # Complex task
    return MODEL_FLASH_LITE  # Simple task (3-4x cheaper)
```

### 2. Immutable Cost Tracking

Track cumulative spend with frozen dataclasses. Each API call returns a new tracker — never mutates state.

```python
from dataclasses import dataclass

@dataclass(frozen=True, slots=True)
class CostRecord:
    model: str
    input_tokens: int
    output_tokens: int
    cost_usd: float

@dataclass(frozen=True, slots=True)
class CostTracker:
    budget_limit: float = 1.00
    records: tuple[CostRecord, ...] = ()

    def add(self, record: CostRecord) -> "CostTracker":
        """Return new tracker with added record (never mutates self)."""
        return CostTracker(
            budget_limit=self.budget_limit,
            records=(*self.records, record),
        )

    @property
    def total_cost(self) -> float:
        return sum(r.cost_usd for r in self.records)

    @property
    def over_budget(self) -> bool:
        return self.total_cost > self.budget_limit
```

### 3. Narrow Retry Logic

Retry only on transient errors. Fail fast on authentication or bad request errors.

```python
from google.api_core.exceptions import (
    ServiceUnavailable,
    ResourceExhausted,
    InternalServerError,
)

_RETRYABLE_ERRORS = (ServiceUnavailable, ResourceExhausted, InternalServerError)
_MAX_RETRIES = 3

def call_with_retry(func, *, max_retries: int = _MAX_RETRIES):
    """Retry only on transient errors, fail fast on others."""
    for attempt in range(max_retries):
        try:
            return func()
        except _RETRYABLE_ERRORS:
            if attempt == max_retries - 1:
                raise
            time.sleep(2 ** attempt)  # Exponential backoff
    # PermissionDenied, InvalidArgument etc. → raise immediately
```

### 4. Prompt Caching

Cache long system prompts to avoid resending them on every request.

```python
messages = [
    {
        "role": "user",
        "content": [
            {
                "type": "text",
                "text": system_prompt,
                "cache_control": {"type": "ephemeral"},  # Cache this
            },
            {
                "type": "text",
                "text": user_input,  # Variable part
            },
        ],
    }
]
```

## Composition

Combine all four techniques in a single pipeline function:

```python
def process(text: str, config: Config, tracker: CostTracker) -> tuple[Result, CostTracker]:
    # 1. Route model
    model = select_model(len(text), estimated_items, config.force_model)

    # 2. Check budget
    if tracker.over_budget:
        raise BudgetExceededError(tracker.total_cost, tracker.budget_limit)

    # 3. Call with retry + caching
    response = call_with_retry(lambda: client.messages.create(
        model=model,
        messages=build_cached_messages(system_prompt, text),
    ))

    # 4. Track cost (immutable)
    record = CostRecord(model=model, input_tokens=..., output_tokens=..., cost_usd=...)
    tracker = tracker.add(record)

    return parse_result(response), tracker
```

## Pricing Reference (2025-2026)

| Model | Input ($/1M tokens) | Output ($/1M tokens) | Relative Cost |
|-------|---------------------|----------------------|---------------|
| Flash Lite 2.5 | $0.80 | $4.00 | 1x |
| Flash 2.5 | $3.00 | $15.00 | ~4x |
| Pro 2.5 | $15.00 | $75.00 | ~19x |

## Best Practices

- **Start with the cheapest model** and only route to expensive models when complexity thresholds are met
- **Set explicit budget limits** before processing batches — fail early rather than overspend
- **Log model selection decisions** so you can tune thresholds based on real data
- **Use prompt caching** for system prompts over 1024 tokens — saves both cost and latency
- **Never retry on authentication or validation errors** — only transient failures (network, rate limit, server error)

## Anti-Patterns to Avoid

- Using the most expensive model for all requests regardless of complexity
- Retrying on all errors (wastes budget on permanent failures)
- Mutating cost tracking state (makes debugging and auditing difficult)
- Hardcoding model names throughout the codebase (use constants or config)
- Ignoring prompt caching for repetitive system prompts

## Ideal For

- Any application calling Gemini, OpenAI, or similar LLM APIs
- Batch processing pipelines where cost adds up quickly
- Multi-model architectures that need intelligent routing
- Production systems that need budget guardrails