Prompt Caching

Prompt Caching

Prompt caching is the easiest 5× cost win in production. Most teams botch the key design and never see the hit rate they were promised.

The model providers do the heavy lifting — your job is to put the right content above the cache boundary and to measure whether the cache is actually hitting.

When to use this skill

• The user has a stable system prompt or tool list ≥1k tokens on a hot path.
• The user reports cost or latency problems on repeated calls.
• The user is comparing Anthropic, OpenAI, Bedrock, or Gemini for an agent.
• The user mentions "cache hit rate" but cannot quote theirs.

Decision flow

1. Is there a stable prefix ≥1k tokens that repeats across calls? → cache it.
2. Does the cached content change less often than every 5 minutes? → cache it.
3. Is the hot-path call volume ≥10/min from the same prefix? → cache it.
4. Is the content user-specific but stable for that user's session? → per-user cache with a session-scoped key.
5. Is the content fully dynamic per call? → no cache. Compress the prompt instead.

The five rules

1. Static content above, dynamic below. Tools, system prompt, large docs at the top. User turn at the bottom. Any dynamic injection above the cache boundary destroys the cache.
2. Cache boundaries are explicit. Anthropic uses cache_control: ephemeral breakpoints; OpenAI auto-caches with hashing. Know which knobs your provider gives you.
3. TTL is short. 5 minutes typical (Anthropic ephemeral). Plan for cold starts after idle periods, not just initial requests.
4. Measure hit rate. Every provider returns cache-read and cache-write token counts. Log them. Dashboard them.
5. Cache-aware prompt layout costs nothing. Refactor the prompt order before enabling caching.

What to cache, in priority order

• Tool/function definitions. Long, stable, expensive on every call.
• System prompt. Especially when it includes few-shot examples or rubrics.
• Long documents (RAG context, schemas, transcripts) that repeat across turns of one conversation.
• Conversation history above the latest turn — for multi-turn agents.

Anti-patterns to flag immediately

• Dynamic timestamp or request-id in the system prompt. Single character above the boundary = 0% hit rate.
• Reordering tools per call. Tool list order matters for the hash; keep it deterministic.
• Caching everything. Cache writes cost more than reads; cache the stable part only.
• No cache-hit dashboard. "I think it's caching" is not a metric.
• Caching across users where prompts include other users' data. Privacy bug and low hit rate.
• Assuming the cache survives idle periods. Anthropic ephemeral cache is 5 minutes; OpenAI's is longer but not infinite.

Provider notes (one-liners)

• Anthropic (Claude). Explicit cache_control breakpoints on system/messages/tools. Up to 4 breakpoints. 5-minute TTL. Read tokens billed at 10%; write tokens at 125%.
• OpenAI. Automatic prefix caching for ≥1024-token prompts. No explicit breakpoints. Hit reported in usage.prompt_tokens_details.cached_tokens.
• Google (Gemini). Both implicit prefix caching and explicit cachedContent for long-lived caches (hours-to-days). Use explicit cache for large stable docs.
• AWS Bedrock. Inherits per-provider semantics; verify the model + region supports caching before designing around it.

Questions to ask the user

1. What is the prefix length and call rate for the hot path?
2. What is the current hit rate? If unknown, that's task one.
3. Which dynamic value is highest in the prompt today, and can it move below the boundary?
4. Is the workload interactive (cache survives between user turns) or batch (caches go cold between requests)?
5. What is the cost target per task, and what fraction of that is prompt tokens?

The hard line

If you can't tell me your cache hit rate, you don't have caching — you have wishful thinking. Instrument first; tune second.

Why this exists

Caching is the single biggest lever on production cost for any agent with a non-trivial system prompt or tool list, yet most teams either skip it or implement it in a way that quietly fails. A 10-minute prompt-layout refactor often beats weeks of model-swapping. See [link to article on prompt caching].

References

• references/anthropic-caching.md — breakpoint placement, billing math, gotchas.
• references/openai-caching.md — what auto-caches, what doesn't, how to read cached_tokens.
• references/gemini-caching.md — implicit vs explicit cache, when to use which.
• references/cost-math.md — when caching breaks even.

Related skills

• Caching is the first cost lever — [[agent-cost-modeling]].
• Cache hits cut TTFT, not just cost — [[latency-budgeting]].
• Prefix layout depends on architecture — [[agent-architecture-patterns]].
• Tool definitions cache (or quietly don't) — [[tool-use-schema-design]].