LLM Cost Optimization

Skill — LLM Cost Optimization

When this skill activates

Any task involving LLM API cost reduction, prompt compression, semantic caching, model cascading/routing, batch API usage, or token budget management.

Mandatory actions when this skill is active

Before writing any code

1. Establish baseline costs (cost per query, daily/monthly spend, cost by feature).
2. Identify the biggest cost drivers (which prompts, which models, which features).
3. Set cost reduction targets with quality guardrails.

During implementation

• Implement semantic caching for repeated/similar queries.
• Use model cascading (start cheap, escalate only when needed).
• Add token estimation before API calls (pre-flight cost check).

After implementation

• Monitor cost per query, cache hit rate, and cascade escalation rate.
• Set up cost anomaly alerts (spike detection).
• Document cost optimization decisions in ARCHITECTURE.md.

Prompt Compression

System Prompt Optimization

• Remove redundant instructions (LLMs don't need repetition like humans).
• Use abbreviations and compact formatting in system prompts.
• Reference items by ID rather than including full content.
• Cache static system prompts (most providers support this).

Context Window Efficiency

• Include only relevant context (not entire documents).
• Summarize long documents before including in prompt.
• Use structured formats (JSON/YAML) over verbose prose for data.
• Remove examples from system prompt once model demonstrates understanding.

Token Reduction Techniques

Semantic Caching

Concept

• Hash similar queries → return cached response if semantically equivalent.
• Not exact-match caching — uses embedding similarity.
• Threshold: if query embedding distance < 0.05, serve cached response.

Implementation

1. Embed incoming query
2. Search cache for similar queries (cosine similarity > 0.95)
3. If hit: return cached response (cost = ~$0)
4. If miss: call LLM, store response in cache with query embedding

Cache Invalidation

• TTL-based: expire after N hours (for time-sensitive data).
• Event-based: invalidate when underlying data changes.
• Version-based: invalidate when prompt/model version changes.

Expected Performance

• Cache hit rate: 20-60% for typical applications.
• Cost reduction: proportional to hit rate.
• Latency improvement: 10-100x faster on cache hits.

Model Cascading

Pattern

Query → Haiku/Small Model → Quality Check → Pass? → Return
                                                → Fail? → Sonnet/Large Model → Return

Implementation Rules

• Start with cheapest model capable of the task.
• Define quality gate (confidence score, format validation, length check).
• Escalate to more expensive model only when quality gate fails.
• Track escalation rate (target: < 20% of queries escalate).

Model Tier Pricing (Approximate)

Routing Heuristics

• Classification/extraction → always use cheap model.
• Code generation → medium model (escalate if syntax errors).
• Complex reasoning → start medium, escalate if confidence low.
• Safety-critical → always use expensive model (no cascading).

Batch API Usage

When to Use

• Non-real-time workloads (background processing, ETL, reports).
• Large volume of similar requests.
• Typical discount: 50% cheaper than synchronous API.

Batch-Eligible Workloads

• Document summarization pipelines.
• Nightly content generation.
• Bulk classification/tagging.
• Training data generation.
• Automated evaluations.

Implementation

• Queue requests during the day.
• Submit batch job during off-peak (overnight).
• Process results next morning.
• Set up retry for failed items in batch.

Token Estimation

Pre-Flight Cost Check

estimated_tokens = count_tokens(system_prompt + context + query)
estimated_cost = estimated_tokens * price_per_token
if estimated_cost > budget_threshold:
    compress_context()  # or reject query

Token Counting

• Use tiktoken (OpenAI) or provider-specific tokenizer.
• Count BEFORE sending to API (not after).
• Include expected output tokens in estimate.
• Set max_tokens to limit output cost.

Budget Controls

• Per-query budget: reject or compress if estimated cost too high.
• Per-user budget: track cumulative cost, throttle when approaching limit.
• Per-feature budget: allocate cost budgets to product features.

Output Token Reduction

Techniques

• Set max_tokens to reasonable limit for the task.
• Instruct model to be concise: "Answer in 2-3 sentences."
• Use structured output (JSON) to prevent verbose prose.
• Ask for key information only, not explanations (when appropriate).

Output Cost Impact

Cost Monitoring

Key Metrics

Dashboard Requirements

• Cost breakdown by: feature, model, endpoint, user tier.
• Trend lines: daily, weekly, monthly.
• Forecast: projected monthly cost at current rate.
• Anomaly alerts: immediate notification on cost spikes.

Optimization Feedback Loop

Monitor costs → Identify top cost drivers → Apply optimization →
Measure improvement → Adjust thresholds → Repeat monthly

Self-check before task completion

Before marking a task done when this skill was active:

• Did I read the full SKILL.md before starting? (Not just the triggers)
• Is semantic caching implemented for repeated queries?
• Is model cascading configured (cheap first, escalate on failure)?
• Are token budgets estimated before API calls?
• Is cost monitoring in place with anomaly alerts?
• Are batch APIs used for non-real-time workloads?
• Is prompt compression applied to system prompts?