DeepSeek V4 Flash 0731 Corrupts JSON Integer Fields When Thinking Mode Is On

DeepSeek's retrained V4 Flash 0731 model has a documented defect where enabling its default thinking mode causes integer field corruption in strict JSON schema outputs, with 8 of 13 test runs returning wrong values despite schema-valid responses. The failure was observed across two independent request paths, with corrupted outputs including incorrect line-item counts and slightly wrong totals — errors silent enough to pass validation and reach production pipelines. Disabling thinking via the enable_thinking: false parameter resolved every corrupted run and reduced token usage to roughly one-seventh of the default. The corruption appears tied to the presence of thinking rather than its budget size, as even a capped 64-token budget produced the same errors. The preview build of V4 Flash remained clean across all four probed runs, while V4 Pro also showed corruption in 2 of 4 runs, suggesting the issue extends across the broader V4 thinking model family.