Slashing LLM Bills by 58%: Building an OpenTelemetry Semantic Cache Proxy in FastAPI

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Slashing LLM Bills by 58%: Building an OpenTelemetry Semantic Cache Proxy in FastAPI

Production LLM deployments come with a dirty secret: between 30% and 60% of LLM queries in enterprise SaaS applications are duplicates or slight paraphrases. Support chat queries, recurring agent tasks, automated enrichment jobs, and document Q&A pipelines repeatedly hit upstream inference providers (OpenAI, Anthropic, Mistral) with functionally identical prompts.

The result? Insane monthly inference bills, unpredictable latency spikes, and engineering teams flying completely blind without granular, per-tenant observability.

In this technical breakdown, we'll build a production-grade, drop-in OpenAI-compatible reverse proxy using asynchronous Python (FastAPI), Redis Vector Search for semantic caching, and OpenTelemetry (OTel) for unified span and token metrics.

1. The Bottleneck: The High Cost of Fragile Commercial Gateways

When engineering leaders realize LLM spending is spiraling out of control, they usually evaluate two paths:

Proprietary Observability & Gateway Platforms: Commercial APMs and specialized LLM SaaS gateways charge recurring per-seat fees or per-million-token markups. Furthermore, sending unredacted tenant prompts to a third-party gateway creates severe data privacy and compliance liabilities.
Naive Exact-Match Caching: Hashing prompts (SHA256(prompt)) in Redis yields miserable cache hit rates (under 4%) because even an extra whitespace or punctuation mark invalidates the cache.

Traditional Flow:
Client ──> Commercial Gateway ($$$ per token) ──> OpenAI API ($$$ per run)

Optimized Self-Hosted Flow:
Client ──> FastAPI Proxy ──> Redis Vector Search (Cosine >= 0.92) ──> Cache HIT (0ms upstream cost)
                    │ (Cache MISS)
                    └───> Upstream Provider (OTel Metric Exported)

To solve this sustainably, we need an in-house proxy that acts as an exact drop-in replacement (base_url="http://proxy-host/v1"), verifies semantic similarity via vector embeddings, and tracks granular OTel spans directly into Grafana, Jaeger, or Prometheus.

2. The Architecture

The proxy pipeline executes in five sequential phases:

Intercept & Tenant Auth: Extract headers (Authorization, X-Tenant-ID) and validate against token budget quotas.
Embedding & Similarity Probe: Vectorize the incoming user prompt using a lightweight embedding model (e.g., text-embedding-3-small or an on-premise SentenceTransformer).
Redis Vector Similarity Query: Perform a K-Nearest Neighbors (KNN) search over indexed prompt embeddings using cosine similarity. If similarity >= threshold (e.g., 0.92), return cached completions instantly.
Circuit Breaking & Token Guardrails: If cache misses, verify the tenant's real-time token spend bucket. If the sliding-window budget is exceeded, terminate with HTTP 429 without hitting upstream.
OTel Span Finalization: Emit OTLP metrics (prompt tokens, completion tokens, latency, cache status) and pipe them directly into your APM backend.

3. The Code & Logic

Let's construct the core engine. Below is the semantic cache router and vector manager implemented in FastAPI and Redis.

Step 1: Redis Semantic Cache Setup

import numpy as np
from redis.asyncio import Redis
from redis.commands.search.field import VectorField, TextField
from redis.commands.search.indexDefinition import IndexDefinition, IndexType
from redis.commands.search.query import Query

INDEX_NAME = "idx:semantic_cache"
VECTOR_DIM = 1536  # text-embedding-3-small dimension

async def init_redis_indices(redis_client: Redis):
    try:
        await redis_client.ft(INDEX_NAME).info()
    except Exception:
        schema = (
            TextField("prompt_text"),
            TextField("completion_text"),
            VectorField(
                "prompt_vector",
                "HNSW",
                {
                    "TYPE": "FLOAT32",
                    "DIM": VECTOR_DIM,
                    "DISTANCE_METRIC": "COSINE",
                }
            )
        )
        definition = IndexDefinition(prefix=["cache:prompt:"], index_type=IndexType.HASH)
        await redis_client.ft(INDEX_NAME).create_index(schema, definition=definition)

Step 2: Semantic Matching Engine

async def check_semantic_cache(redis_client: Redis, query_vector: list[float], threshold: float = 0.92):
    query_bytes = np.array(query_vector, dtype=np.float32).tobytes()

    # KNN search retrieving nearest prompt vector
    query = (
        Query("*=>[KNN 1 @prompt_vector $vec AS score]")
        .sort_by("score")
        .return_fields("completion_text", "score")
        .dialect(2)
    )

    results = await redis_client.ft(INDEX_NAME).search(query, query_params={"vec": query_bytes})

    if results.docs:
        doc = results.docs[0]
        # Cosine distance: 0 = identical, 2 = opposite. Similarity = 1 - distance
        distance = float(doc.score)
        similarity = 1.0 - distance

        if similarity >= threshold:
            return doc.completion_text, similarity

    return None, 0.0

Step 3: OpenTelemetry Instrumented Proxy Route

from fastapi import FastAPI, Request, HTTPException
from opentelemetry import trace, metrics
import httpx

app = FastAPI()
tracer = trace.get_tracer("llm-proxy")
meter = metrics.get_meter("llm-proxy")

token_counter = meter.create_counter(
    name="llm_tokens_consumed_total",
    description="Total tokens consumed split by tenant and cache hit status"
)

@app.post("/v1/chat/completions")
async def chat_completions_proxy(request: Request):
    tenant_id = request.headers.get("X-Tenant-ID", "default_tenant")
    payload = await request.json()
    messages = payload.get("messages", [])
    last_prompt = messages[-1]["content"] if messages else ""

    with tracer.start_as_current_span("llm_completion_router") as span:
        span.set_attribute("llm.tenant_id", tenant_id)

        # 1. Compute embedding (simplified dummy hook)
        prompt_vec = await compute_embedding(last_prompt)

        # 2. Check Semantic Cache
        cached_response, similarity = await check_semantic_cache(app.state.redis, prompt_vec, threshold=0.92)

        if cached_response:
            span.set_attribute("llm.cache_hit", True)
            span.set_attribute("llm.cosine_similarity", similarity)
            token_counter.add(0, {"tenant": tenant_id, "cache_hit": "true"})

            return {
                "id": "cached-completion",
                "object": "chat.completion",
                "choices": [{
                    "index": 0,
                    "message": {"role": "assistant", "content": cached_response},
                    "finish_reason": "stop"
                }],
                "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0}
            }

        # 3. Cache Miss - Forward to OpenAI
        span.set_attribute("llm.cache_hit", False)
        async with httpx.AsyncClient() as client:
            upstream_resp = await client.post(
                "https://api.openai.com/v1/chat/completions",
                headers={"Authorization": request.headers.get("Authorization")},
                json=payload,
                timeout=30.0
            )

        data = upstream_resp.json()

        # Extract token metrics for OTel
        usage = data.get("usage", {})
        total_tokens = usage.get("total_tokens", 0)
        token_counter.add(total_tokens, {"tenant": tenant_id, "cache_hit": "false"})

        # Store in Redis vector cache asynchronously
        reply_text = data["choices"][0]["message"]["content"]
        await store_cache(app.state.redis, last_prompt, prompt_vec, reply_text)

        return data

4. Deployment & Real-World Performance

When pushing a semantic proxy to production, three critical real-world edge cases must be handled:

Sliding-Window Circuit Breakers: If an automated workflow goes rogue (e.g., an n8n webhook looping uncontrollably), simple rate limits won't save you from a massive bill. Implement atomic Redis INCRBY operations keyed by tenant and current minute (tenant:{id}:budget:{YYYYMMDDHHmm}). If the tenant exceeds their token ceiling, trip the circuit and return an HTTP 429 Too Many Requests instantly.
Dynamic Similarity Thresholds: Not all workloads tolerate loose semantic matches. A support FAQ chatbot operates well at a 0.88 similarity threshold, whereas legal or financial extraction tasks require 0.97 or strictly deterministic cache hits. Make the threshold configurable via HTTP headers (X-Semantic-Threshold: 0.95).
Connection Pooling & Downstream Timeouts: Set strict connect timeouts (typically 2.0s) and read timeouts (30.0s) on your HTTPX client pool. Ensure vector embeddings run asynchronously so cache lookup overhead adds less than 12ms of total latency.

5. Conclusion & Ready-to-Use Workflow

You now have the architectural blueprint to eliminate redundant API calls, enforce strict per-tenant token guardrails, and export enterprise-grade OpenTelemetry metrics without paying software seat taxes.

You can implement this architecture from scratch using the code snippets above. However, if you want a complete, battle-tested, production-ready solution with Docker Compose stacks, automated migrations, Jaeger/Grafana dashboards, and test fixtures ready for zero-downtime deployment, grab the turnkey package: