Prompt Caching

Caching strategies for LLM prompts including Anthropic prompt caching, response caching, and CAG (Cache Augmented Generation)

Capabilities

• prompt-cache
• response-cache
• kv-cache
• cag-patterns
• cache-invalidation

Prerequisites

• Knowledge: Caching fundamentals, LLM API usage, Hash functions
• Skills_recommended: context-window-management

Scope

• Does_not_cover: CDN caching, Database query caching, Static asset caching
• Boundaries: Focus is LLM-specific caching, Covers prompt and response caching

Ecosystem

Primary_tools

• Anthropic Prompt Caching - Native prompt caching in Claude API
• Redis - In-memory cache for responses (ioredis on servers; @upstash/redis over HTTP on serverless, see upstash-redis)
• OpenAI Caching - Automatic caching in OpenAI API

Patterns

Anthropic Prompt Caching

Use Claude's native prompt caching for repeated prefixes

When to use: Using Claude API with stable system prompts or context

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic();

// Cache the stable parts of your prompt async function queryWithCaching(userQuery: string) { const response = await client.messages.create({ model: "claude-sonnet-4-20250514", max_tokens: 1024, system: [ { type: "text", text: LONG_SYSTEM_PROMPT, // Your detailed instructions cache_control: { type: "ephemeral" } // Cache this! }, { type: "text", text: KNOWLEDGE_BASE, // Large static context cache_control: { type: "ephemeral" } } ], messages: [ { role: "user", content: userQuery } // Dynamic part ] });

// Check cache usage
console.log(`Cache read: ${response.usage.cache_read_input_tokens}`);
console.log(`Cache write: ${response.usage.cache_creation_input_tokens}`);

return response;

}

// Measure actual cache-read/write costs and latency for the selected model/provider.

Response Caching

Cache full LLM responses for identical or similar queries

When to use: Same queries asked repeatedly

import { createHash } from 'crypto'; import Redis from 'ioredis';

const redis = new Redis(process.env.REDIS_URL); // Serverless/edge alternative without a persistent connection: // import { Redis } from '@upstash/redis'; const redis = Redis.fromEnv(); // then use redis.set(key, value, { ex: ttl })

class ResponseCache { private ttl = 3600; // 1 hour default

// Exact match caching
async getCached(prompt: string): Promise<string | null> {
    const key = this.hashPrompt(prompt);
    return await redis.get(`response:${key}`);
}

async setCached(prompt: string, response: string): Promise<void> {
    const key = this.hashPrompt(prompt);
    await redis.set(`response:${key}`, response, 'EX', this.ttl);
}

private hashPrompt(prompt: string): string {
    return createHash('sha256').update(prompt).digest('hex');
}

// Semantic similarity caching
async getSemanticallySimilar(
    prompt: string,
    threshold: number = 0.95
): Promise<string | null> {
    const embedding = await embed(prompt);
    const similar = await this.vectorCache.search(embedding, 1);

    if (similar.length && similar[0].similarity > threshold) {
        return await redis.get(`response:${similar[0].id}`);
    }
    return null;
}

// Temperature-aware caching
async getCachedWithParams(
    prompt: string,
    params: { temperature: number; model: string }
): Promise<string | null> {
    // Only cache low-temperature responses
    if (params.temperature > 0.5) return null;

    const key = this.hashPrompt(
        `${prompt}|${params.model}|${params.temperature}`
    );
    return await redis.get(`response:${key}`);
}

}

Cache Augmented Generation (CAG)

Pre-cache documents in prompt instead of RAG retrieval

When to use: Document corpus is stable and fits in context

// CAG: Pre-compute document context, cache in prompt // Better than RAG when: // - Documents are stable // - Total fits in context window // - Latency is critical

class CAGSystem { private cachedContext: string | null = null; private lastUpdate: number = 0;

async buildCachedContext(documents: Document[]): Promise<void> {
    // Pre-process and format documents
    const formatted = documents.map(d =>
        `## ${d.title}\n${d.content}`
    ).join('\n\n');

    // Store with timestamp
    this.cachedContext = formatted;
    this.lastUpdate = Date.now();
}

async query(userQuery: string): Promise<string> {
    // Use cached context directly in prompt
    const response = await client.messages.create({
        model: "claude-sonnet-4-20250514",
        max_tokens: 1024,
        system: [
            {
                type: "text",
                text: "You are a helpful assistant with access to the following documentation.",
                cache_control: { type: "ephemeral" }
            },
            {
                type: "text",
                text: this.cachedContext ?? "",  // verify initialized context before querying
                cache_control: { type: "ephemeral" }
            }
        ],
        messages: [{ role: "user", content: userQuery }]
    });

    return response.content.filter(block => block.type === "text").map(block => block.text).join("\n");
}

// Periodic refresh
async refreshIfNeeded(documents: Document[]): Promise<void> {
    const stale = Date.now() - this.lastUpdate > 3600000;  // 1 hour
    if (stale) {
        await this.buildCachedContext(documents);
    }
}

}

// CAG vs RAG decision matrix: // | Factor | CAG Better | RAG Better | // |------------------|------------|------------| // | Corpus size | < 100K tokens | > 100K tokens | // | Update frequency | Low | High | // | Latency needs | Critical | Flexible | // | Query specificity| General | Specific |

Sharp Edges

Cache miss causes latency spike with additional overhead

Severity: HIGH

Situation: Slow response when cache miss, slower than no caching

Symptoms:

• Slow responses on cache miss
• Cache hit rate below 50%
• Higher latency than uncached

Why this breaks: Cache check adds latency. Cache write adds more latency. Miss + overhead > no caching.

Recommended fix:

// Optimize for cache misses, not just hits

class OptimizedCache { async queryWithCache(prompt: string): Promise<string> { const cacheKey = this.hash(prompt);

    // Non-blocking cache check
    const cachedPromise = this.cache.get(cacheKey);
    // Defer paid provider call until bounded cache lookup misses.

    // Race: use cache if available before LLM returns
    const cached = await Promise.race([
        cachedPromise,
        sleep(50).then(() => null) // also catch cache errors per availability policy  // 50ms cache timeout
    ]);

    if (cached) {
        // Cancel LLM request if possible
        return cached;
    }

    // Cache miss: continue with LLM
    const response = await this.queryLLM(prompt);

    // Async cache write (don't block response)
    this.cache.set(cacheKey, response).catch(console.error);

    return response;
}

}

// Alternative: Probabilistic caching // Only cache if query matches known high-frequency patterns class SelectiveCache { private patterns: Map<string, number> = new Map();

shouldCache(prompt: string): boolean {
    const pattern = this.extractPattern(prompt);
    const frequency = this.patterns.get(pattern) || 0;

    // Only cache high-frequency patterns
    return frequency > 10;
}

recordQuery(prompt: string): void {
    const pattern = this.extractPattern(prompt);
    this.patterns.set(pattern, (this.patterns.get(pattern) || 0) + 1);
}

}

Cached responses become incorrect over time

Severity: HIGH

Situation: Users get outdated or wrong information from cache

Symptoms:

• Users report wrong information
• Answers don't match current data
• Complaints about outdated responses

Why this breaks: Source data changed. No cache invalidation. Long TTLs for dynamic data.

Recommended fix:

// Implement proper cache invalidation

class InvalidatingCache { // Version-based invalidation private cacheVersion = 1;

getCacheKey(prompt: string): string {
    return `v${this.cacheVersion}:${this.hash(prompt)}`;
}

invalidateAll(): void {
    this.cacheVersion++;
    // Old keys automatically become orphaned
}

// Content-hash invalidation
async setWithContentHash(
    key: string,
    response: string,
    sourceContent: string
): Promise<void> {
    const contentHash = this.hash(sourceContent);
    await this.cache.set(key, {
        response,
        contentHash,
        timestamp: Date.now()
    });
}

async getIfValid(
    key: string,
    currentSourceContent: string
): Promise<string | null> {
    const cached = await this.cache.get(key);
    if (!cached) return null;

    // Check if source content changed
    const currentHash = this.hash(currentSourceContent);
    if (cached.contentHash !== currentHash) {
        await this.cache.delete(key);
        return null;
    }

    return cached.response;
}

// Event-based invalidation
onSourceUpdate(sourceId: string): void {
    // Invalidate all caches that used this source
    this.invalidateByTag(`source:${sourceId}`);
}

}

Prompt caching doesn't work due to prefix changes

Severity: MEDIUM

Situation: Cache misses despite similar prompts

Symptoms:

• Cache hit rate lower than expected
• Cache creation tokens high, read low
• Similar prompts not hitting cache

Why this breaks: Anthropic caching requires exact prefix match. Timestamps or dynamic content in prefix. Different message order.

Recommended fix:

// Structure prompts for optimal caching

class CacheOptimizedPrompts { // WRONG: Dynamic content in cached prefix buildPromptBad(query: string): SystemMessage[] { return [ { type: "text", text: You are helpful. Current time: ${new Date()}, // BREAKS CACHE! cache_control: { type: "ephemeral" } } ]; }

// RIGHT: Static prefix, dynamic at end
buildPromptGood(query: string): SystemMessage[] {
    return [
        {
            type: "text",
            text: STATIC_SYSTEM_PROMPT,  // Never changes
            cache_control: { type: "ephemeral" }
        },
        {
            type: "text",
            text: STATIC_KNOWLEDGE_BASE,  // Rarely changes
            cache_control: { type: "ephemeral" }
        }
        // Dynamic content goes in messages, NOT system
    ];
}

// Prefix ordering matters
buildWithConsistentOrder(components: string[]): SystemMessage[] {
    // Sort components for consistent ordering
    const sorted = [...components]; // preserve semantic order; make construction deterministic
    return sorted.map((c, i) => ({
        type: "text",
        text: c,
        cache_control: i === sorted.length - 1
            ? { type: "ephemeral" }
            : undefined  // Only cache the full prefix
    }));
}

}

Validation Checks

Caching High Temperature Responses

Severity: WARNING

Message: Caching with high temperature. Responses are non-deterministic.

Fix action: Define task-specific freshness/reuse policy; temperature is not a determinism or safety guarantee

Cache Without TTL

Severity: WARNING

Message: Cache without TTL. May serve stale data indefinitely.

Fix action: Set appropriate TTL based on data freshness requirements

Dynamic Content in Cached Prefix

Severity: WARNING

Message: Dynamic content in cached prefix. Will cause cache misses.

Fix action: Move dynamic content outside of cache_control blocks

No Cache Metrics

Severity: INFO

Message: Cache without hit/miss tracking. Can't measure effectiveness.

Fix action: Add cache hit/miss metrics and logging

Collaboration

Delegation Triggers

• context window|token -> context-window-management (Need context optimization)
• rag|retrieval -> rag-implementation (Need retrieval system)
• memory -> conversation-memory (Need memory persistence)

High-Performance LLM System

Skills: prompt-caching, context-window-management, rag-implementation

Workflow:

1. Analyze query patterns
2. Implement prompt caching for stable prefixes
3. Add response caching for frequent queries
4. Consider CAG for stable document sets
5. Monitor and optimize hit rates

Related Skills

Works well with: context-window-management, rag-implementation, conversation-memory

When to Use

• User mentions or implies: prompt caching
• User mentions or implies: cache prompt
• User mentions or implies: response cache
• User mentions or implies: cag
• User mentions or implies: cache augmented

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

All cache classes are sketches with adapters/helpers/types omitted. Cache keys must include tenant/user authorization scope, model/version, full instructions, tool/schema versions, generation parameters and source versions; hashing a prompt does not anonymize cached sensitive content. Separate exact response caching from provider prefix/KV caching, which still generates a fresh answer and has provider-specific minimum tokens, TTL and pricing. Semantic similarity is not answer equivalence; do not reuse across permissions or safety-sensitive contexts without validation. Treat cached documents as untrusted source data, not authoritative system instructions. Persist version/invalidation state across workers/restarts, serialize Redis objects explicitly and apply retention/deletion policies. The100K-token and0.5-temperature thresholds are illustrative, not universal.