DeepSeek V4 Pro: 90% of Claude performance at 5-7x lower cost

DeepSeek V4 Pro costs $0.435/M input tokens vs Claude Sonnet 4's $3, achieving 90% of Claude's coding performance in production use. The user built custom harness features like hash-anchored editing and sticky prefix caching to close the gap, noting long-horizon planning and UI work remain weaker areas.