Update CHANGELOG.md with all implemented features

Comprehensive changelog update documenting:
- Agent interface and implementations (Phase 1)
- Responses API core with streaming (Phase 2)
- Conversation history and context management (Phase 3)
- Chat Completions wrapper (Phase 5)
- Advanced features and validation (Phase 6)
- Application infrastructure and setup
- Testing improvements (95 tests, 78.95% coverage)
- Documentation updates

Organized by implementation phases with detailed feature lists
and architectural changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -8,24 +8,96 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
-- Initial project structure with domain-based organization
-- OpenAI-compatible `/v1/chat/completions` endpoint with streaming support
-- OpenAI-compatible `/v1/models` endpoint
-- Health check and root endpoints
-- Comprehensive test suite with 62% coverage
-- Security-focused dependency management with CVE checking
-- FastAPI best practices implementation
-- SSE streaming with 20-second timeout protection
-- Custom Pydantic base models for consistent serialization
-- Application factory pattern for testability
-- Async Ollama client (ready for integration)
-- Complete documentation (README, AGENTS.md)
+
+#### Agent Interface (Phase 1)
+- Abstract `AgentInterface` base class for model abstraction
+- `LoremTesterAgent`: Full-featured mock agent with realistic behavior
+  - Configurable reasoning effort levels (none, minimal, low, medium, high, xhigh)
+  - Random tool/function call generation for testing
+  - Error triggers: rate_limit, context_overflow, invalid_tool
+  - Temperature-based response variation
+- `TatlockAgent`: Placeholder for future PydanticAI integration
+- `ModelRegistry`: Centralized model management and discovery
+- 18 agent tests with comprehensive coverage
+
+#### Responses API (Phases 2, 3, 6)
+- OpenAI Responses API format with structured output (`/v1/responses`)
+  - Reasoning items (thinking summaries with configurable effort)
+  - Function call items (tool execution simulation)
+  - Message items (assistant responses with output_text)
+  - Streaming and non-streaming modes
+- Real-time streaming with SSE-Starlette
+- Conversation history management (Phase 3):
+  - Hybrid client/server approach (client maintains state, server tracks)
+  - Auto-generated deterministic conversation IDs from message hash
+  - Configurable max turns with automatic trimming (default: 20)
+  - Context window management with approximate token counting
+  - Token usage statistics
+  - Placeholder for future vector memory (Qdrant)
+- Advanced features (Phase 6):
+  - Parameter validation with Pydantic field validators
+  - Temperature: 0.0-2.0 range enforcement
+  - Reasoning effort: 6 levels validation
+  - Max output tokens: positive integer enforcement
+  - Stop sequences: up to 4, non-empty strings
+  - Real-time stop sequence detection during streaming
+  - Real-time max tokens enforcement with token counting
+- 45 Responses API tests (router, error handling, history, advanced features)
+
+#### Chat Completions Wrapper (Phase 5)
+- OpenAI Chat Completions compatibility layer (`/v1/chat/completions`)
+- Single source of truth architecture (wraps Responses API)
+- Automatic reasoning generation
+- Converts reasoning items to `<think>` tags for Open WebUI
+- Pipeline prefix preservation
+- System message support
+- Enhanced error types (RateLimitError, ContextLengthError)
+- 12 Chat Completions tests (router + streaming wrapper)
+
+#### Application Infrastructure
+- FastAPI application factory pattern
+- CORS middleware with configurable origins
+- Global exception handlers:
+  - AppException handler for custom errors
+  - RequestValidationError handler for Pydantic validation
+  - General exception handler for unexpected errors
+- Lifespan management for startup/shutdown
+- OpenAPI schema with interactive documentation
+- 16 main application tests
+
+#### Testing Infrastructure
+- Comprehensive test suite: 95 tests, 78.95% coverage (up from 62%)
+- Async test support with pytest-asyncio
+- Test fixtures for sync and async clients
+- Integration tests for all API endpoints
+- Streaming functionality tests
+- Parameter validation tests
+- Error handling tests
+- Conversation history tests
+
+#### Documentation
+- Complete README.md rewrite with hybrid architecture
+- Architecture diagrams and decision documentation
+- AGENTS.md with technical implementation details
+- API usage examples for all endpoints
+- Conversation history guide
+- Open WebUI integration instructions
+- Troubleshooting section
+- Implementation planning documents
+
+### Changed
+- Hybrid architecture with Responses API as primary endpoint
+- Chat Completions now wraps Responses API (no duplicate logic)
+- Enhanced error handling with OpenAI-compatible format
+- Improved streaming with word-by-word delivery
+- Better test organization with domain-based structure
 
 ### Security
 - Minor version locking for all dependencies
 - All packages CVE-checked (as of 2025-12-06)
 - Environment variable protection via .gitignore
 - No known vulnerabilities in dependency tree
+- Input validation on all API endpoints
 
 ## [0.1.0] - 2025-12-06