The author reports that DeepSeek 4.1 Flash performs indistinguishably from frontier models such as Opus 5.5 in daily coding tasks while costing dramatically less. Over a month of heavy use across twelve projects, the model’s per‑session expense rarely exceeded one dollar, made possible by a subscription to OpenCode Go at $10 per month that effectively provides unlimited usage. The key technical advancement enabling this efficiency is a reduction in the model’s key‑value (KV) cache size by roughly 437× relative to its earlier V1 version. Because the KV cache dominates GPU memory consumption during long inference runs, this shrinkage allows all‑day coding sessions to stay within a low‑cost envelope and reduces associated energy and water usage. The same cache‑optimization technique has also contributed to Opus 5.5’s own efficiency gains. For occasional critical checks, the author supplements DeepSeek 4.1 Flash with Opus 5.5 for final code reviews, then delegates fixes back to the cheaper model. Overall, the combination of comparable capability, a 437× KV‑cache compression, and sub‑dollar operational costs makes DeepSeek 4.1 Flash a practical, sustainable alternative to more expensive frontier LLMs for everyday development workflows.