diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..d6c7bff
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,207 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+DeepLX is a serverless translation service optimized for Cloudflare Workers. It provides a free alternative to translation APIs by proxying requests to DeepL and Google Translate services through intelligent load balancing, rate limiting, and caching mechanisms.
+
+## Development Commands
+
+### Running & Deployment
+- `npm run dev` - Start local development server with Wrangler
+- `npm run deploy` - Deploy to Cloudflare Workers production
+- `npm run cf-typegen` - Generate TypeScript types for Cloudflare Workers bindings
+
+### Testing
+- `npm test` - Run all tests
+- `npm run test:unit` - Run unit tests only (tests in tests/lib/)
+- `npm run test:integration` - Run integration tests (tests/integration/)
+- `npm run test:performance` - Run performance tests (tests/performance/)
+- `npm run test:coverage` - Generate coverage report
+- `npm run test:watch` - Run tests in watch mode
+- `npm run test:debug` - Run tests with Node debugging enabled
+
+### Code Quality
+- `npm run lint` - Type-check with TypeScript (no emit, runs `tsc --noEmit`)
+
+## Architecture
+
+### Request Flow
+1. Request arrives at Hono router (src/index.ts)
+2. CORS preflight handling (if OPTIONS)
+3. Security middleware validates input and extracts client IP
+4. Rate limiting checks (two-level: client IP + proxy backend)
+5. Cache lookup (two-level: in-memory + KV)
+6. If cache miss:
+   - Proxy manager selects available proxy endpoint
+   - Circuit breaker checks if proxy is healthy
+   - Retry logic with exponential backoff
+   - Translation query to provider (DeepL or Google)
+7. Cache successful translation
+8. Return standardized response
+
+### Core Components
+
+#### Translation Providers
+- **DeepL Query Engine** ([src/lib/query.ts](src/lib/query.ts)): Main translation logic for DeepL using JSONRPC API
+- **Google Translate Service** ([src/lib/services/googleTranslate.ts](src/lib/services/googleTranslate.ts)): Google Translate integration
+
+#### Support Systems
+- **Two-Level Cache** ([src/lib/cache.ts](src/lib/cache.ts)): In-memory Map + Cloudflare KV with 1-hour TTL
+- **Token Bucket Rate Limiter** ([src/lib/rateLimit.ts](src/lib/rateLimit.ts)): Dual rate limiting (per-client IP + per-proxy) with dynamic limits based on proxy count
+- **Circuit Breaker** ([src/lib/circuitBreaker.ts](src/lib/circuitBreaker.ts)): Prevents cascade failures by temporarily blocking failing proxy endpoints (states: CLOSED/OPEN/HALF_OPEN)
+- **Proxy Manager** ([src/lib/proxyManager.ts](src/lib/proxyManager.ts)): Random proxy selection with browser fingerprinting (User-Agent, Accept-Language rotation)
+- **Retry Logic** ([src/lib/retryLogic.ts](src/lib/retryLogic.ts)): Exponential backoff retry mechanism (max 3 retries, 1s initial delay, 2x backoff factor)
+
+#### Configuration
+All configurable constants are centralized in [src/lib/config.ts](src/lib/config.ts):
+- Rate limits: `RATE_LIMIT_CONFIG` (dynamic based on proxy count)
+- Cache TTL: `CACHE_CONFIG`
+- Retry settings: `DEFAULT_RETRY_CONFIG`
+- Payload limits: `PAYLOAD_LIMITS`
+- Request timeout: `REQUEST_TIMEOUT`
+
+### API Endpoints
+
+The application exposes three POST endpoints (defined in [src/index.ts](src/index.ts)):
+- `/deepl` - DeepL translation (recommended)
+- `/google` - Google Translate
+- `/translate` - Legacy endpoint (uses DeepL for backward compatibility)
+- `/debug` - Debug endpoint (only available when `DEBUG_MODE=true`)
+
+All endpoints use the same `handleTranslation()` function with a provider parameter.
+
+### Environment Configuration
+
+Required environment variables in [wrangler.jsonc](wrangler.jsonc):
+- `DEBUG_MODE` - Enable debug endpoint (default: "false")
+- `PROXY_URLS` - Comma-separated list of XDPL proxy endpoints for DeepL
+
+Required KV namespaces:
+- `CACHE_KV` - Translation result cache
+- `RATE_LIMIT_KV` - Rate limit token buckets
+
+Required bindings:
+- `ANALYTICS` - Cloudflare Analytics Engine dataset
+
+### Scheduled Tasks
+
+The worker includes a scheduled event handler that runs every 5 minutes (configured in `wrangler.jsonc` triggers):
+- Clears in-memory cache to prevent memory leaks
+- Called via `handleScheduled()` in [src/index.ts](src/index.ts)
+
+## Key Implementation Details
+
+### Rate Limiting Strategy
+- **Client-level**: Token bucket with dynamic limit = `(proxy_count × 8 requests/sec) × 60 = tokens/minute`
+- **Proxy-level**: 8 tokens/sec per proxy with 16 token burst capacity
+- Uses two-level caching (in-memory 5s TTL + KV 1h TTL) for performance
+- See `checkCombinedRateLimit()` in [src/lib/rateLimit.ts](src/lib/rateLimit.ts)
+
+### Caching Strategy
+- **In-memory cache**: JavaScript Map for fast lookups (cleared every 5 minutes by scheduled task)
+- **KV cache**: Cloudflare KV for persistence (1-hour TTL)
+- Cache key generation: hash of `text:source_lang:target_lang:provider`
+- See [src/lib/cache.ts](src/lib/cache.ts)
+
+### Circuit Breaker Pattern
+- Opens after 5 consecutive failures
+- 30-second recovery timeout
+- Requires 3 consecutive successes to close
+- One circuit breaker instance per proxy URL
+- See [src/lib/circuitBreaker.ts](src/lib/circuitBreaker.ts)
+
+### Security Features
+- Input validation and sanitization in [src/lib/security.ts](src/lib/security.ts) and [src/lib/validation.ts](src/lib/validation.ts)
+- Maximum text length: 5000 characters (configurable in `PAYLOAD_LIMITS`)
+- Language code validation with whitelist
+- Client IP extraction from CF-Connecting-IP or X-Forwarded-For headers
+- CORS handling via `handleCORSPreflight()`
+
+## Testing Strategy
+
+Tests are organized in the [tests/](tests/) directory:
+- `tests/lib/` - Unit tests for individual library modules
+- `tests/integration/` - Integration tests for API endpoints
+- `tests/performance/` - Performance and load tests
+- `tests/setup.ts` - Jest setup file
+- `tests/utils/testHelpers.ts` - Shared test utilities
+
+Test configuration in [jest.config.js](jest.config.js):
+- Uses `ts-jest` preset for TypeScript
+- 30-second test timeout
+- Coverage collected from `src/**/*.ts`
+
+When writing tests:
+- Mock Cloudflare Workers bindings (KV, Analytics)
+- Use `testHelpers.ts` utilities for creating mock environments
+- Integration tests should test full request/response cycle
+- Performance tests should validate response time constraints
+
+## TypeScript Configuration
+
+- **Target**: ES2020 with WebWorker lib
+- **Module**: ESNext with bundler resolution (Cloudflare Workers uses bundling)
+- **Strict mode**: Enabled
+- Types: `@cloudflare/workers-types`, `jest`
+- Entry point: [src/index.ts](src/index.ts)
+
+## Contributing Guidelines
+
+From [CONTRIBUTING.md](CONTRIBUTING.md):
+
+### Branch naming:
+- `feature/description` - New features
+- `fix/description` - Bug fixes
+- `docs/description` - Documentation
+- `refactor/description` - Code refactoring
+- `test/description` - Test improvements
+
+### Commit format:
+Use conventional commits: `type(scope): description`
+- Types: feat, fix, docs, style, refactor, test, chore
+
+### Code style:
+- 2 spaces indentation
+- Single quotes for strings
+- Trailing commas in multiline structures
+- Lines under 100 characters
+- Explicit types for function parameters/returns
+- Avoid `any` type
+
+### Before submitting PR:
+1. Run `npm test` (all tests must pass)
+2. Run `npm run lint` (type check)
+3. Add tests for new functionality
+4. Update documentation if needed
+5. Ensure code coverage doesn't decrease
+
+## Performance Optimization
+
+Optimized for Cloudflare Workers serverless environment:
+- Minimize memory allocations (cache cleanup every 5 minutes)
+- Efficient token bucket algorithm (O(1) rate limit checks)
+- Two-level caching reduces KV read latency
+- Browser fingerprinting uses pre-defined arrays (no runtime generation)
+- Async KV writes don't block responses (fire-and-forget pattern in rate limiter)
+
+## Common Debugging Scenarios
+
+### Enable Debug Mode
+Set `DEBUG_MODE=true` in wrangler.jsonc to enable the `/debug` endpoint for request inspection.
+
+### Rate Limit Issues
+- Check dynamic rate limits with `getDynamicRateLimits(env)` in [src/lib/rateLimit.ts](src/lib/rateLimit.ts)
+- Rate limits scale with proxy count: more proxies = higher client limits
+- Clear rate limit cache: delete keys matching `rate_limit:*` in `RATE_LIMIT_KV`
+
+### Circuit Breaker Tripped
+- Circuit opens after 5 consecutive proxy failures
+- Wait 30 seconds for automatic HALF_OPEN state
+- Check proxy health in proxy manager logs
+
+### Cache Issues
+- In-memory cache is cleared every 5 minutes by scheduled task
+- KV cache has 1-hour TTL
+- Cache key collision: verify `generateCacheKey()` produces unique keys for different inputs
diff --git a/package-lock.json b/package-lock.json
index df2c40c..e9cd939 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -79,6 +79,7 @@
       "integrity": "sha512-UlLAnTPrFdNGoFtbSXwcGFQBtQZJCNjaN6hQNP3UPvuNXT1i82N26KL3dZeIpNalWywr9IuQuncaAfUaS1g6sQ==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@ampproject/remapping": "^2.2.0",
         "@babel/code-frame": "^7.27.1",
@@ -626,7 +627,8 @@
       "resolved": "https://registry.npmjs.org/@cloudflare/workers-types/-/workers-types-4.20250722.0.tgz",
       "integrity": "sha512-pTY+A07DTSacgUBYcVEEb78/KG7THdcRpPqXLeH/A/LHHobAddgN4zyXBldsoZuzy7bD9tZYJW+wkcyR4k7fDA==",
       "dev": true,
-      "license": "MIT OR Apache-2.0"
+      "license": "MIT OR Apache-2.0",
+      "peer": true
     },
     "node_modules/@cspotcode/source-map-support": {
       "version": "0.8.1",
@@ -1546,6 +1548,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "caniuse-lite": "^1.0.30001726",
         "electron-to-chromium": "^1.5.173",
@@ -2554,6 +2557,7 @@
       "integrity": "sha512-NIy3oAFp9shda19hy4HK0HRTWKtPJmGdnvywu01nOqNC2vZg+Z+fvJDxpMQA88eb2I9EcafcdjYgsDthnYTvGw==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@jest/core": "^29.7.0",
         "@jest/types": "^29.6.3",
@@ -4154,6 +4158,7 @@
       "integrity": "sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==",
       "dev": true,
       "license": "Apache-2.0",
+      "peer": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
@@ -4192,6 +4197,7 @@
       "integrity": "sha512-B06u0wXkEd+o5gOCMl/ZHl5cfpYbDZKAT+HWTL+Hws6jWu7dCiqBBXXXzMFcFVJb8D4ytAnYmxJA83uwOQRSsg==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "defu": "^6.1.4",
         "exsolve": "^1.0.4",
@@ -4290,6 +4296,7 @@
       "dev": true,
       "hasInstallScript": true,
       "license": "Apache-2.0",
+      "peer": true,
       "bin": {
         "workerd": "bin/workerd"
       },
diff --git a/src/lib/proxyManager.ts b/src/lib/proxyManager.ts
index 443c3dc..4c5ab4e 100644
--- a/src/lib/proxyManager.ts
+++ b/src/lib/proxyManager.ts
@@ -68,18 +68,23 @@ export async function selectProxy(env: Env): Promise<ProxyEndpoint | null> {
 
 /**
  * Generate realistic browser fingerprint headers
- * Creates randomized headers to mimic real browser requests
+ * Creates randomized headers to mimic real browser requests and prevent 503 errors
+ * Enhanced with comprehensive headers to bypass DeepL's bot detection
  * @returns Record<string, string> - Object containing HTTP headers
  */
 export function generateBrowserFingerprint(): Record<string, string> {
   return {
     "User-Agent": USER_AGENTS[Math.floor(Math.random() * USER_AGENTS.length)],
+    Accept: "*/*",
     "Accept-Language":
       ACCEPT_LANGUAGES[Math.floor(Math.random() * ACCEPT_LANGUAGES.length)],
-    Accept: "application/json, text/plain, */*",
-    "Accept-Encoding": "gzip, deflate, br",
+    "Accept-Encoding": "gzip, deflate, br, zstd",
+    Origin: "https://www.deepl.com",
+    Referer: "https://www.deepl.com/",
+    "Sec-Fetch-Dest": "empty",
+    "Sec-Fetch-Mode": "cors",
+    "Sec-Fetch-Site": "same-site",
     DNT: "1",
     Connection: "keep-alive",
-    "Upgrade-Insecure-Requests": "1",
   };
 }
diff --git a/tests/utils/testHelpers.ts b/tests/utils/testHelpers.ts
index 881dfbc..0ef5126 100644
--- a/tests/utils/testHelpers.ts
+++ b/tests/utils/testHelpers.ts
@@ -186,12 +186,6 @@ export function createTestEnvironment() {
   const env: Env = {
     CACHE_KV: cacheKV as any,
     RATE_LIMIT_KV: rateLimitKV as any,
-
-            { status: 200 }
-          )
-        ),
-      }),
-    } as any,
     ANALYTICS: {
       writeDataPoint: jest.fn(),
     } as any,