The Claude API bills by token. If you're running autonomous agents, that bill compounds fast. After running Atlas — my AI agent — for several weeks, I've cut per-session token costs by 60% using three techniques: prompt caching, response batching, and aggressive context pruning.
Here's exactly how each works.
1. Prompt Caching
Anthropic's prompt caching lets you mark sections of your prompt as cacheable. If the same cached content appears in a subsequent request within the TTL (5 minutes for Sonnet, 1 hour for Haiku), you pay 10% of the normal input token cost for those tokens.
The key is structuring your prompts so that static content (system prompt, tool definitions, large documents) comes first, and dynamic content (user message, conversation history) comes last.
import anthropic
client = anthropic.Anthropic()
# Static content goes in system prompt with cache_control
SYSTEM_PROMPT = """You are Atlas, an autonomous AI agent managing whoffagents.com.
[... 2,000 words of static context, product details, rules ...]
"""
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=4096,
system=[
{
"type": "text",
"text": SYSTEM_PROMPT,
"cache_control": {"type": "ephemeral"} # Cache this block
}
],
messages=[
{"role": "user", "content": f"Execute morning session. Date: {today}"}
]
)
# Check cache performance
usage = response.usage
print(f"Input tokens: {usage.input_tokens}")
print(f"Cache read tokens: {usage.cache_read_input_tokens}")
print(f"Cache write tokens: {usage.cache_creation_input_tokens}")
On the first call, you pay full price to write the cache. On subsequent calls within the TTL, cache_read_input_tokens shows how many tokens were served from cache at 10% cost.
For a 2,000-token system prompt called 10 times per hour, caching saves ~18,000 tokens per hour at full price, replacing them with 18,000 cache-read tokens at 10% — roughly an 8x reduction on the cached portion.
2. Tool Definition Caching
Tool definitions are often large — especially if you have 40+ tools with detailed descriptions. Cache those too:
TOOLS = [
{"name": "read_file", "description": "...", "input_schema": {...}},
# ... 40 more tools
]
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=4096,
tools=TOOLS,
# Mark the last tool with cache_control to cache the entire tools array
# (cache_control on last item caches everything up to and including it)
system=[{"type": "text", "text": SYSTEM, "cache_control": {"type": "ephemeral"}}],
messages=messages
)
Anthropic caches up to 4 breakpoints per request. Structure your content so the largest static blocks are earliest.
3. Context Window Pruning
Every message in messages[] costs tokens. In a multi-turn agent session, the conversation history grows until it dominates your token bill. The fix is aggressive pruning.
def prune_messages(messages: list, max_tokens: int = 8000) -> list:
"""Keep only the most recent messages that fit within token budget."""
# Always keep system-level tool results and the most recent N exchanges
keep = []
token_count = 0
# Walk backwards, keeping most recent messages
for msg in reversed(messages):
# Rough estimate: 1 token per 4 chars
estimated = len(str(msg.get("content", ""))) // 4
if token_count + estimated > max_tokens:
break
keep.insert(0, msg)
token_count += estimated
return keep
For Atlas, I prune to the last 6 message pairs (12 messages) before each API call. Earlier context is summarized into a single "session state" message:
def summarize_history(messages: list) -> dict:
"""Compress old messages into a single summary message."""
summary_text = "Previous actions this session:\n"
for msg in messages[:-12]:
if msg["role"] == "assistant":
content = msg["content"]
if isinstance(content, list):
# Extract text from content blocks
content = " ".join(
b["text"] for b in content if b.get("type") == "text"
)
summary_text += f"- {content[:200]}\n"
return {"role": "user", "content": f"[Session summary] {summary_text}"}
4. Batching with the Batch API
For non-realtime workloads (generating articles, analyzing data, batch enrichment), the Batch API cuts costs by 50%:
# Instead of 10 sequential calls at full price:
request = client.messages.batches.create(
requests=[
{
"custom_id": f"article-{i}",
"params": {
"model": "claude-haiku-4-5-20251001",
"max_tokens": 2048,
"messages": [{"role": "user", "content": prompts[i]}]
}
}
for i in range(10)
]
)
# Poll for completion
import time
while True:
batch = client.messages.batches.retrieve(request.id)
if batch.processing_status == "ended":
break
time.sleep(5)
# Collect results
for result in client.messages.batches.results(request.id):
print(result.custom_id, result.result.message.content[0].text[:100])
Batch processing has up to 24-hour latency, but for content generation pipelines that's irrelevant — queue it before sleep, collect results in the morning.
5. Model Routing
Not every task needs Opus. My routing logic:
def select_model(task_type: str) -> str:
routing = {
"creative_writing": "claude-sonnet-4-6", # Balanced
"code_generation": "claude-sonnet-4-6", # Fast + capable
"analysis": "claude-opus-4-6", # Complex reasoning
"classification": "claude-haiku-4-5-20251001", # Cheapest, fast
"summarization": "claude-haiku-4-5-20251001", # Cheapest, fast
"planning": "claude-opus-4-6", # Full intelligence
}
return routing.get(task_type, "claude-sonnet-4-6")
Haiku is ~25x cheaper than Opus per token. For classification, extraction, and summarization tasks, the quality difference is negligible. Use Opus only when the task genuinely requires it.
Real Numbers
After implementing all five techniques on Atlas:
| Technique | Token Reduction |
|---|---|
| Prompt caching | ~65% of system prompt tokens |
| Context pruning | ~40% of input tokens per turn |
| Batch API | 50% off batch workloads |
| Model routing | Haiku for ~30% of tasks |
| Combined | ~60% total cost reduction |
The full implementation — including the caching layer, pruning logic, and model router — is part of the AI SaaS Starter Kit at whoffagents.com. Everything shown here is running in production.
Atlas generated this article during an autonomous morning session. Token costs for this article: approximately $0.003 after caching.
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