Practical playbook for maximizing prompt cache hit rate and minimizing token spend on LLM/agentic systems (Anthropic, OpenAI, Google Gemini, self-hosted vLLM). Covers prompt structure rules, multi-turn cache breakpoint placement, tool definition stability, dynamic content handling, TTL choice, multi-tenant isolation, and known anti-patterns.
Installation
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Practical playbook for maximizing prompt cache hit rate and minimizing token spend on LLM/agentic systems (Anthropic, OpenAI, Google Gemini, self-hosted vLLM). Covers prompt structure rules, multi-turn cache breakpoint placement, tool definition stability, dynamic content handling, TTL choice, multi-tenant isolation, and known anti-patterns.
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Prompt Caching Playbook for Production LLM/Agentic Systems
A practical lens + ordered playbook for structuring prompts and message arrays so the provider can reuse the KV-cache across requests. Get token cost down 5–50× on input and TTFT down 13–85% without touching the model.
This skill covers Anthropic Claude, OpenAI GPT/o-series, Google Gemini, and self-hosted vLLM. The structural rules are the same across all four; only syntax differs.
How to apply
Diagnose first. Look at the user's existing prompt structure or system architecture before suggesting changes. Static-vs-dynamic content placement and tool definition stability are the two highest-leverage variables — check those before discussing TTL or breakpoint count.
Apply the universal rule. Static prefix → dynamic suffix. Order across all providers: tools → system → conversation history → new user message. Anything that changes per request must live after the last cache breakpoint.
Name what NOT to do. The anti-patterns list in §6 below catches ~80% of real production cache misses. Surface them proactively whenever the user describes their prompt structure.
Quantify the win. Every recommendation should come with an estimated cost/latency saving anchored in real numbers from §8 (worked examples) or §7 (provider mechanics), so the user can prioritize.
Recommend a monitoring loop. Cache hit rate must be logged per-request and dashboarded per-session, per-tenant. Without observability, regressions are invisible until the next billing cycle.
TL;DR — the rules that matter
Static prefix → dynamic suffix. Order: tools → system static → system dynamic → long context / RAG → conversation history → new user message. Cache breakpoint goes after the last stable block.
Multi-turn: previous user and assistant messages stay in place (append-only). New user message appended at the end. Cache breakpoint slides forward each turn.
Tool definitions are part of the cached prefix. Reorder tools = full cache invalidation. Add/remove an MCP tool mid-session = full invalidation (system + all history).
Cache hit pricing: ~10% of base input (Anthropic, Google) or ~50% (OpenAI). Cache write costs: 1.25× base on Anthropic for 5min TTL, 2× for 1h TTL; OpenAI and Google charge nothing extra for writes.
TTL choice: 5 min default refreshes on every hit — fine for active sessions. 1 hour (Anthropic) or 24h (OpenAI) for spaced-out workloads (review agents, batch jobs).
Anti-patterns that kill the cache: timestamps/UUIDs at start of system prompt, live git status in every prompt, middleware injecting trace IDs, reordering tools, switching models mid-session, aggressive history reformatting, putting tool results inside the cached prefix.
Decision tree — what to do for the user's stack
Is the user designing a new system?
├── YES → §1 (canonical structure) + §3 (provider-specific syntax) + §4 (monitoring)
└── NO, debugging existing system:
├── "Cache hit rate is low / unstable"
│ → §6 (anti-patterns checklist) — walk through one by one
├── "Bill is too high"
│ → §5 (cost math) → §1 → §6
├── "Hit rate was good, now it's not"
│ → check for recent changes: tools added? middleware deployed? model switched?
│ → §6 #4, #5, #6, #7
├── "Migrating providers"
│ → §3 (per-provider mechanics) + §2 (multi-turn nuances per provider)
├── Multi-tenant SaaS concerns
│ → §7 (cache isolation)
└── "What's the right TTL"
→ §3 TTL subsection — depends on inter-request gap
For deep mechanics on any specific provider, read references/anthropic.md, references/openai.md, references/gemini.md, or references/self-hosted.md. For full benchmark numbers and the academic findings, see references/benchmarks.md. For analysis of OSS agents (OpenCode, Aider, Cline, OpenHands, Codex CLI, Claude Code), see references/oss-agents.md.
1. The canonical structure
All major providers use prefix matching by exact byte equality. The KV-cache stores attention Key/Value tensors for the prefix; on hit, the model skips re-processing those tokens and resumes forward pass only from the first un-cached byte. Any byte change in the prefix invalidates everything after it.
The mandatory ordering across Anthropic, OpenAI, Google, and vLLM:
[1] Tool definitions ← most stable, cached first (part of prefix)
[2] System prompt — static ← role, style, conventions, AGENTS.md/rules
[3] System prompt — dynamic ← timestamp, cwd, user_location (END of system!)
[4] Long context / RAG ← documents, codebase chunks (if reused)
[5] Conversation history ← user/assistant turns, append-only
[6] New user message ← always last
Cache breakpoint (where the provider should stop hashing the prefix and start treating content as variable) goes after the last stable block. In Anthropic terms — that's where cache_control is placed. In OpenAI/Google — the system decides automatically based on the prefix matcher.
Canonical system prompt template
# === BLOCK A: ROLE (stable forever) ===
You are an expert software engineering agent. Your role is...
[2000–4000 tokens of stable instructions]
# === BLOCK B: TOOL USAGE GUIDELINES (stable per agent version) ===
When calling tools, you must...
[1000–3000 tokens]
# === BLOCK C: PROJECT CONVENTIONS (AGENTS.md / .cursorrules / etc.) ===
[variable per project but stable within a project]
# === BLOCK D: STYLE / OUTPUT FORMAT ===
[stable]
# === [CACHE BREAKPOINT HERE] ===
# === BLOCK E: DYNAMIC ENVIRONMENT (after breakpoint, NOT cached) ===
<env>
date: 2026-05-14T10:30:00Z
cwd: /home/user/projects/foo
git_branch: feature/auth-v2
</env>
2. Multi-turn behavior — how the breakpoint moves
The most common confusion: "do I keep the previous user message in the array?" Yes — the previous user and assistant messages stay in place. Append-only.
Turn 2: prefix [tools][system][user_1][assistant_1] is cached (3 of 4 blocks reused from turn 1).
Turn 3: prefix [tools][system][user_1][assistant_1][user_2][assistant_2] is cached.
Provider mechanics:
Anthropic — top-level cache_control: {"type":"ephemeral"} automatically applies the breakpoint to the last cacheable block and advances it. Or place explicit cache_control on the last user message manually (up to 4 breakpoints).
OpenAI — fully automatic, no markers. The provider hashes the prefix on every request and routes to a machine that has it cached.
Google Gemini — implicit caching is automatic. Explicit Context Caching API is incompatible with tool use in the same request (limitation as of early 2026).
Compaction / history truncation
When context grows past the limit, agents must compact. Compaction is the #1 cache-killer in long sessions because the summarized prefix breaks byte equality.
Two strategies:
Make the summary itself stable. Compact once, then the summarized prefix becomes the new stable prefix. Subsequent turns build on top of it. Hit rate recovers after one cold turn.
Pruning, not summarizing. Drop oldest tool outputs while keeping the message structure intact. Some agents (OpenHands, OpenCode) protect the most recent N tokens and mark older tool outputs as "compacted" without rewriting them.
3. Provider-specific mechanics
Compressed cheat sheet. For full mechanics, read the relevant reference file.
Batch eval / nightly jobs: Anthropic 1h, OpenAI 24h. The 2× write multiplier on Anthropic is recouped after ~3 reads.
RAG over stable docs: Anthropic 1h for the document context, 5min for conversation history (mix is allowed — 1h must come before 5min in the request).
For deep provider-specific details (4-breakpoint placement on Anthropic, prompt_cache_key granularity on OpenAI, Context Caching API on Google, vLLM prefix caching tuning), see the reference files.
Hit rate over time per session. Should rise to 80%+ after turn 2–3. If it falls within a session, something is mutating the prefix mid-conversation.
Hit rate per tenant/segment. Outliers reveal poorly-structured per-tenant prefixes.
Unique system_prompt_hash count per day. If you logically have "v1" but see 50 distinct hashes, an invisible mutator (analytics injector, trace ID middleware, plugin) is corrupting the prefix.
Cost split: new input / cache read / cache write / output. Ratio of write to read should drop fast in active sessions.
Alert thresholds:
Hit rate < 70% sustained for >5min on an agentic workload → investigate.
cached_tokens field is 0 for >10 consecutive requests on a long-prefix workload → check for prefix mutation or known provider bugs (Gemini-with-tools edge cases, OpenRouter sticky-routing issues).
5. Cost math — worked examples
Example A: 10K-token system prompt, 1M requests/month, Claude Sonnet-class model
Base input $3/M, cache write 5min $3.75/M, cache read $0.30/M.
Scenario
Hit rate
Monthly input cost
No caching
0%
$30,000
Naive caching (bad structure)
30%
~$8,775
Good structure
80%
~$990
Best-practice multi-breakpoint
92%
~$576
52× difference between "no cache" and "92% hit rate." Output cost unchanged.
Example B: 50-turn agent session, 20K system prompt
No caching: ~$3.50–4.50/session
Caching system only: ~$0.50–0.70/session
Multi-breakpoint sliding: ~$0.19/session
10K sessions/day → $35K/mo vs $5K/mo vs $1.9K/mo input.
Example C: Cheap-tier model (e.g., Flash-class), 10K system, 1M req/mo
Base $0.50/M, cache hit $0.05/M (90% off).
Hit rate
Monthly cost
0%
$5,000
80%
$1,400
For high-volume workloads, the cheap-tier + good caching combo lands input cost at ~$1.5K/month per million requests. Output billed separately.
6. Anti-patterns checklist (the cache-killers)
Walk through this list when debugging low hit rate.
#
Anti-pattern
Effect
1
Timestamp / current date in start of system prompt
0% hit rate; every request is full reprocess
2
Session ID / request UUID anywhere in system
Same
3
Live git status / file listing in every prompt
Cache invalidates on every file edit
4
Middleware injecting trace ID / analytics token into system
Invisible mutator — hardest to find
5
Reorder of tool definitions between requests
Full invalidation of tools+system+messages
6
Adding/removing a tool mid-session (MCP, plugin)
Same
7
Switching model mid-session
Cache is per-model — cold start on new
8
Aggressive summarization/compaction every N turns
Each rewrite breaks byte equality
9
Reformatting history (case, whitespace, JSON re-serialization)
Two-letter change = thousands of tokens missed (empirically)
10
Streaming response with no usage telemetry
You lose visibility into cache stats
11
Anthropic routed through OpenRouter without sticky routing
Hit rate stays flat regardless of session length
12
Tool results placed inside the cached prefix
Dynamic content invalidates the prefix
13
Mixing explicit Context Cache with tool use on Google models
Documented API incompatibility
14
OpenAI: too-narrow prompt_cache_key (RPM per key < ~15)
Overflow routes to cold machines
15
cache_control breakpoint placed on a block that changes
Pay for write every time, never read
The most expensive failure mode in practice is #4 (middleware). Observability layers (Sentry, LangSmith, Helicone, custom analytics) often inject trace IDs into the system prompt. They work "correctly" and silently cost 10× more on input — discovered only when looking at the next bill.
7. Multi-tenant cache isolation
Cross-tenant data leakage via cache is not a real risk on any of the three major providers. Cache hashes exact content; a different tenant's prefix won't match. You do not need to add per-tenant salt to the prefix — that would kill the global cross-tenant cache and lose all the savings.
What you might need depending on compliance:
Anthropic — cache entries isolated between organizations and between workspaces (on Bedrock and Microsoft Foundry). Direct Anthropic API: org-level only.
OpenAI — granulate prompt_cache_key per tenant segment, not per user. Aim for >10 RPM per key. Per-user keys with low traffic suffer from overflow.
Google — implicit caching is content-hashed and per-project. No per-tenant config needed.
Self-hosted vLLM — use cache_salt parameter to isolate per-tenant on shared GPU.
ZDR (zero data retention) compatibility: all three major providers state cache lives in RAM/VRAM only, not written to disk, with short TTL. Anthropic's docs explicitly note "raw text of prompts is not stored — only KV representations and cryptographic hashes in memory."
8. Production code templates
Anthropic-style (explicit cache_control)
import anthropic
client = anthropic.Anthropic()
STATIC_SYSTEM = open("prompts/system.md").read()
TOOLS = load_tool_definitions() # stable order, frozen per releasedefcall_agent(history, new_user_msg, dynamic_env):
# Mark last tool to cache the whole tools array
tools = TOOLS.copy()
tools[-1] = {**tools[-1], "cache_control": {"type": "ephemeral"}}
# System: static cached, dynamic appended without cache_control
system = [
{"type": "text", "text": STATIC_SYSTEM,
"cache_control": {"type": "ephemeral", "ttl": "1h"}},
{"type": "text",
"text": f"<env>\ndate: {dynamic_env['date']}\ncwd: {dynamic_env['cwd']}\n</env>"},
]
# Messages: history + new user with sliding breakpoint
messages = history + [{
"role": "user",
"content": [{
"type": "text", "text": new_user_msg,
"cache_control": {"type": "ephemeral"}, # sliding BP
}],
}]
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=4096,
tools=tools, system=system, messages=messages,
)
u = response.usage
hit_rate = u.cache_read_input_tokens / max(
1, u.cache_read_input_tokens + u.cache_creation_input_tokens + u.input_tokens
)
return response, hit_rate
OpenAI-style (automatic + cache key)
from openai import OpenAI
client = OpenAI()
defcall_agent(segment: str, messages: list):
response = client.chat.completions.create(
model="gpt-4o",
messages=messages,
# Granulate per segment, not per user — keep RPM/key above ~15
prompt_cache_key=f"agent-prod-v1-{segment}",
)
cached = response.usage.prompt_tokens_details.cached_tokens or0
total = response.usage.prompt_tokens
return response, cached / total
Google-style (implicit, with tools — watch for known caveats)
# Pseudocode — actual SDK varies; the structural rule is what matters.defcall_with_implicit(system: str, history: list, new_msg: str, tools=None):
"""For agents with tools: implicit caching only. Some provider+model
combinations have known issues activating implicit cache when tools
are present — monitor cached_content_token_count in production."""
response = generate_content(
model=MODEL,
system_instruction=system, # stable, at start
contents=history + [new_msg],
tools=tools or [],
# Ensure system + history >= model's min threshold (1024–4096 tokens)
)
cached = response.usage_metadata.cached_content_token_count or0return response, cached
9. Audit checklist for existing systems
Walk through this when reviewing a user's existing agent/LLM stack:
Every LLM call logs cache_read, cache_write, new_input token counts.
System prompt is hashed on send — metric "unique system_prompt_hash per day" exists.
Tool definitions: one shared list, fixed order, versioned with the release.
Timestamp / date / cwd / random_id are out of the start of system prompt.
Conversation history is append-only — never reformatted between turns.
Compaction triggers only on overflow, and the summarized prefix is itself stable across subsequent turns.
Model is not switched mid-session.
Anthropic: cache_control present on at least one block (or top-level auto).
OpenAI: prompt_cache_key set, granular enough but >10 RPM/key.
Google: cached_content_token_count > 0 verified in production traces (especially with tools).
Dashboard shows hit rate per session, p50/p95 TTFT, cost split.
Alert configured for hit_rate < threshold (e.g., 70% for agents).
Middleware/plugins audited for injection into system prompt.
If using LiteLLM/OpenRouter/proxy — sticky routing verified, otherwise prefer direct provider for cache-heavy workloads.
Reference files
For deeper mechanics on specific topics, load the corresponding file from references/:
references/anthropic.md — full Anthropic mechanics: 4-breakpoint placement, automatic vs explicit modes, 1h vs 5min TTL break-even math, hierarchy of invalidation, response usage parsing, Bedrock/Vertex caveats.
references/oss-agents.md — reverse-engineered cache strategies of OpenCode (the reference implementation), Aider, Cline/Roo Code, Continue, OpenHands, OpenAI Codex CLI, and what's known about Claude Code and Cursor.
references/benchmarks.md — production hit-rate numbers from various agents, the Don't Break the Cache arxiv paper findings (41–80% cost savings, 13–31% TTFT improvement), and the methodology behind them.
Lumer et al., Don't Break the Cache: An Evaluation of Prompt Caching for Long-Horizon Agentic Tasks, arxiv 2601.06007 (v2, Jan 2026) — 41–80% cost, 13–31% TTFT.
Production analysis:
Anthropic engineering threads on Claude Code (cache hit rate ~92% in production, SEV-level alerting).
LMCache blog (Dec 2025) — trace analysis of Claude Code.
