Semantic Caching

Cache LLM responses by semantic similarity.

When to Use

• Repeated similar queries
• High-volume LLM applications
• Cost reduction (70-95% savings)
• Latency optimization

Cache Hierarchy

Request → L1 (Exact) → L2 (Semantic) → L3 (Prompt) → L4 (LLM)
           ~1ms         ~10ms           ~2s          ~3s
         100% save    100% save       90% save    Full cost

Redis Semantic Cache

from redisvl.index import SearchIndex
from redisvl.query import VectorQuery

class SemanticCacheService:
    def __init__(self, redis_url: str, threshold: float = 0.92):
        self.client = Redis.from_url(redis_url)
        self.threshold = threshold

    async def get(self, content: str, agent_type: str) -> dict | None:
        embedding = await embed_text(content[:2000])

        query = VectorQuery(
            vector=embedding,
            vector_field_name="embedding",
            filter_expression=f"@agent_type:{{{agent_type}}}",
            num_results=1
        )

        results = self.index.query(query)

        if results:
            distance = float(results[0].get("vector_distance", 1.0))
            if distance <= (1 - self.threshold):
                return json.loads(results[0]["response"])

        return None

    async def set(self, content: str, response: dict, agent_type: str):
        embedding = await embed_text(content[:2000])
        key = f"cache:{agent_type}:{hash_content(content)}"

        self.client.hset(key, mapping={
            "agent_type": agent_type,
            "embedding": embedding,
            "response": json.dumps(response),
            "created_at": time.time(),
        })
        self.client.expire(key, 86400)  # 24h TTL

Similarity Thresholds

Multi-Level Lookup

async def get_llm_response(query: str, agent_type: str) -> dict:
    # L1: Exact match (in-memory LRU)
    cache_key = hash_content(query)
    if cache_key in lru_cache:
        return lru_cache[cache_key]

    # L2: Semantic similarity (Redis)
    similar = await semantic_cache.get(query, agent_type)
    if similar:
        lru_cache[cache_key] = similar  # Promote to L1
        return similar

    # L3/L4: LLM call with prompt caching
    response = await llm.generate(query)

    # Store in caches
    await semantic_cache.set(query, response, agent_type)
    lru_cache[cache_key] = response

    return response

Key Decisions

Common Mistakes

• Threshold too low (false positives)
• No cache warming (cold start)
• Missing metadata filters
• Not promoting L2 hits to L1

Related Skills

• prompt-caching - Provider-native caching
• embeddings - Vector generation
• cache-cost-tracking - Langfuse integration