DeepSeek V4 Flash API Cost: Thinking Mode Corrupts Strict JSON

DeepSeek V4 Flash costs $0.14 per million input tokens and $0.28 per million output, with cache hits at $0.0028, and the retrained 0731 build that now ships under that name has a defect you need to route around: with thinking on (the default) and a strict json_schema , integer fields came back corrupted in 8 of 13 default-thinking runs across two independent request paths. Turning thinking off fixed every run and cut the extraction to a seventh of the tokens. We measured deepseek-v4-flash-0731 on day one: the corruption, the sharper off-switch cliff the retrain introduced, the budget floor that rescues it, the 1,024-token cache pages, and what still separates the preview build and V4 Pro . TL;DR With default thinking plus strict json_schema , deepseek-v4-flash-0731 corrupted integer fields in 8 of 13 runs on two request paths; V4 Pro corrupted 2 of 4, and only the preview stayed clean. The 0731 retrain sharpened the off-switch cliff: 2-hop math fell 6/6 to 0/6. The cache serves 1,024-token pages from roughly a 1.1K-token floor, hits 0.3 seconds after priming, and entries outlive 45 minutes. enable_thinking: false fixed every structured run at a seventh of the tokens; for 2-hop math the safe thinking budget is 256. How do the three V4 builds compare on paper and on the meter? Same tokenizer, same cache, same thinking machinery; different prices, different failure modes. Everything measured below comes from identical probes run against all three (a dash means we did not probe that cell); the day-one analysis threads focus on benchmarks, so this is the operational half of the comparison: Flash 0731 Flash preview V4 Pro List price, in / out per 1M $0.14 / $0.28 $0.14 / $0.28 $0.435 / $0.87 Cache-hit input per 1M $0.0028 $0.0028 $0.003625 Thinking default on on on Strict JSON with thinking on 5/5 corrupted (our path) 4/4 clean 2/4 corrupted 2-hop math with thinking off 0/6 2/6 4/4 thinking_budget token-exact token-exact honored (4/4 at 16) Cache pages 1,024 tokens, hit a