Prompt Caching: Cut LLM Costs Without Cutting Quality
Prompt caching: the fastest way to stop paying twice for the same tokens
If you call large language models all day, you are probably re-sending the same system instructions, tool schemas, policies, and long context blocks over and over. That repetition is expensive. Prompt caching is a simple optimization that lets the model reuse previously processed prompt content so you do not pay full price for identical input every time. In practice, this can cut token costs, lower latency, and make high-volume apps much easier to scale.
The idea is straightforward: if a prompt prefix stays the same across many requests, the provider can cache that prefix. On later requests, only the changing part needs to be processed from scratch. For teams shipping agents, support bots, code assistants, or document analyzers, the savings can be very real.
Where the savings come from
Most AI bills are driven by input tokens, not just outputs. A typical production request might include:
- A 1,500-token system prompt
- A 2,000-token tool schema and policy block
- A 6,000-token conversation or retrieved context
- A 200-token user message
If the first 3,500 tokens are repeated on every request, you are paying for the same text again and again. With prompt caching, those repeated tokens may be billed at a lower cached rate or avoid reprocessing overhead, depending on the model and provider. That means the more stable your prefix is, the better the economics.
Concrete example: assume 100,000 monthly requests, each with 3,500 repeated input tokens and 200 variable tokens. Without caching, that is 370 million input tokens. If caching reduces the effective cost of the repeated prefix by even 70%, the savings on the repeated portion alone can be substantial. On a high-usage app, that can mean hundreds or thousands of dollars saved per month.
What to cache first
Not every token is worth optimizing. Start with the parts of your prompt that are long, stable, and reused often:
- System instructions that define tone, rules, and behavior
- Tool definitions and function schemas
- Safety and compliance policies
- Static knowledge blocks such as product documentation or FAQs
- Long conversation headers that stay unchanged across turns
A good rule is this: if a prompt segment is identical across at least dozens of requests, it is a candidate for caching. If it changes constantly, caching will not help much.
How to structure prompts for caching
Prompt caching works best when your prompt layout is stable. Put the repeated content at the beginning and keep it byte-for-byte consistent. Even small changes can reduce cache hits. For example, changing whitespace, reordering tools, or inserting dynamic timestamps into the system prompt may break the reusable prefix.
Use this pattern:
- Fixed prefix: instructions, policies, schema, static context
- Dynamic suffix: user query, request-specific data, fresh context
For agentic workflows, keep the tool list in a single canonical order. For RAG applications, store retrieval snippets after the stable prefix, not before it. For chat apps, avoid adding variable metadata into the same block as your core instructions.
Real-world cost math
Let us say your app uses a model that charges more for uncached input than cached input. A request includes 5,000 repeated prefix tokens and 500 variable tokens. If you make 20,000 requests per day, that is 100 million repeated tokens daily. Even a modest reduction in effective cost for that repeated block can produce visible savings.
For smaller teams, the bigger win may be predictable spend. A support assistant that keeps the same policy and product guide in every call will usually see a much lower average token cost once caching is enabled. That makes it easier to estimate margins and avoid surprise bills when traffic spikes.
Why 59API is a smart low-cost choice
If you want to test prompt caching without overpaying for premium access, 59API is a strong option. It is an AI API relay with cheap, pay-as-you-go access to Claude models including Opus, Sonnet, Haiku, and Fable, plus GPT models. It is fully compatible with Claude Code, Codex, and any OpenAI SDK, so you can keep your current integrations and just point your client to https://api.59api.com.
That matters because prompt caching only helps if your underlying API costs are already efficient. 59API is among the cheapest relays, uses native official-quality models with no downgrade, and also offers a referral rebate. In other words, you get a low base rate, compatibility with existing tooling, and extra room to compound savings when you reduce token usage through caching.
Practical rollout checklist
- Measure your current prompt size and identify repeated tokens
- Separate fixed and dynamic content into clean prompt blocks
- Keep the cached prefix stable across requests
- Track cache hit rate and compare it to total input tokens
- Review monthly spend before and after rollout
- Test with your real workloads, not just synthetic prompts
If your application sends long, repeated prompts, prompt caching is one of the simplest cost-optimization moves you can make. Start by isolating stable instructions and schemas, then run a billing comparison after a few days of traffic. If you want a low-cost relay that works with the tools you already use, sign up for 59API and see how much token waste you can remove without changing model quality.