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 Activate

• Building applications that call LLM APIs (Claude, 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_SONNET = "claude-sonnet-4-6"
MODEL_HAIKU = "claude-haiku-4-5-20251001"

_SONNET_TEXT_THRESHOLD = 10_000  # chars
_SONNET_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 >= _SONNET_TEXT_THRESHOLD or item_count >= _SONNET_ITEM_THRESHOLD:
        return MODEL_SONNET  # Complex task
    return MODEL_HAIKU  # 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 anthropic import (
    APIConnectionError,
    InternalServerError,
    RateLimitError,
)

_RETRYABLE_ERRORS = (APIConnectionError, RateLimitError, 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
    # AuthenticationError, BadRequestError 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 | |-------|---------------------|----------------------|---------------| | Haiku 4.5 | $0.80 | $4.00 | 1x | | Sonnet 4.6 | $3.00 | $15.00 | ~4x | | Opus 4.5 | $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

When to Use

• Any application calling Claude, 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

原文

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