LLM Response Caching Layer

LLM Response Caching Layer

Implement semantic and exact-match caching for LLM API responses to reduce costs 40-60% and cut P50 latency from seconds to milliseconds.

Activation Triggers

Activate on: "cache LLM responses", "semantic cache", "reduce OpenAI costs", "LLM API caching", "cache embeddings", "deduplicate LLM calls", "response memoization for AI"

NOT for: General HTTP/CDN caching (caching-strategies), browser cache headers (caching-strategies), or database query caching (ORM-specific)

Quick Start

1. Classify request types — Deterministic (structured extraction, classification) vs. creative (open-ended generation). Deterministic requests are highly cacheable.
2. Choose cache strategy — Exact-match (hash of prompt + params) for deterministic requests, semantic similarity for natural language queries.
3. Set up cache store — Redis for exact-match (fast, TTL-native), vector DB for semantic cache (similarity search).
4. Integrate as middleware — Wrap your LLM client with a cache-check layer that intercepts before API calls.
5. Monitor hit rates — Target 30-50% hit rate for mixed workloads, 70%+ for classification/extraction.

Core Capabilities

Architecture Patterns

Pattern 1: Two-Tier LLM Cache

LLM Request
    │
    ▼
[Exact Match Cache (Redis)] ──hit──→ Return cached response (< 5ms)
    │ miss
    ▼
[Semantic Cache (Vector DB)] ──hit (similarity > 0.95)──→ Return cached response (< 50ms)
    │ miss
    ▼
[LLM API Call] ──→ response ──→ Store in both caches ──→ Return response

# Two-tier caching middleware
import hashlib, json, numpy as np

class LLMCacheMiddleware:
    def __init__(self, redis_client, vector_db, embedder, threshold=0.95):
        self.redis = redis_client
        self.vdb = vector_db
        self.embedder = embedder
        self.threshold = threshold

    def cache_key(self, prompt: str, model: str, **params) -> str:
        blob = json.dumps({"prompt": prompt, "model": model, **params}, sort_keys=True)
        return f"llm:{hashlib.sha256(blob.encode()).hexdigest()}"

    async def query(self, prompt: str, model: str, **params) -> str:
        # Tier 1: Exact match
        key = self.cache_key(prompt, model, **params)
        cached = await self.redis.get(key)
        if cached:
            return json.loads(cached)["response"]  # < 5ms

        # Tier 2: Semantic match
        query_emb = self.embedder.embed(prompt)
        results = self.vdb.search(query_emb, top_k=1)
        if results and results[0].score >= self.threshold:
            return results[0].payload["response"]  # < 50ms

        # Cache miss: call LLM
        response = await llm_call(prompt, model, **params)

        # Store in both tiers
        await self.redis.setex(key, 86400, json.dumps({"response": response}))
        self.vdb.upsert(query_emb, {"prompt": prompt, "response": response})
        return response

Pattern 2: Cache-Aware Request Classification

Incoming Request
    │
    ▼
[Classify Cacheability]
    ├── temperature == 0 AND structured output → HIGHLY CACHEABLE (TTL: 7 days)
    ├── temperature == 0 AND free-form         → CACHEABLE (TTL: 24 hours)
    ├── temperature > 0 AND repeated pattern   → SEMANTIC CACHE ONLY (TTL: 1 hour)
    └── temperature > 0 AND unique/creative    → DO NOT CACHE

Pattern 3: Versioned Cache with Model Upgrades

Cache Key = hash(prompt + model_version + system_prompt_version + params)

Model upgrade (gpt-4o-2026-01 → gpt-4o-2026-03):
  → All cache keys change automatically (model_version in hash)
  → Old cache entries expire via TTL
  → No manual invalidation needed

Anti-Patterns

1. Caching creative/high-temperature responses — Temperature > 0.7 means the user expects variety. Caching returns identical responses and feels broken.
2. Global similarity threshold — A 0.95 threshold works for factual Q&A but is too loose for code generation and too tight for casual chat. Tune per endpoint.
3. No cache versioning on model upgrades — Switching from GPT-4o to Claude returns stale GPT-4o responses. Include model version in cache keys.
4. Infinite TTL — Even deterministic responses can become stale when underlying data changes. Set TTL based on data freshness requirements (hours to days, not forever).
5. Caching without cost tracking — You cannot prove ROI without measuring: cache hit rate, tokens saved, dollars saved, latency improvement.

Quality Checklist

• Request types classified by cacheability (deterministic vs. creative)
• Exact-match cache uses hash of full request (prompt + model + params)
• Semantic cache threshold tuned per use case (tested on sample queries)
• Cache key includes model version for automatic invalidation on upgrades
• TTL set based on data freshness requirements (not infinite)
• High-temperature (> 0.7) requests excluded from caching
• Cache hit/miss rates monitored with alerting on rate drops
• Cost savings calculated and reported: tokens saved, dollars saved per day
• Cache store sized for expected working set (Redis memory, vector DB storage)
• Graceful degradation: cache failure falls through to LLM API (never blocks)