DeepSeek V4-Flash: Fix the reasoning_content 400 Trap
Summary
Build a thinking-mode tool-calling agent on DeepSeek V4-Flash-0731 without the 400 error.
Why this matters right now
On July 31, 2026 DeepSeek shipped the official V4-Flash-0731 build. The architecture and parameter count did not change from the preview. What changed is post-training: DeepSeek re-trained the same weights and the cheap Flash model now beats their own larger V4-Pro-Preview on every agent and coding benchmark they published, including a jump on DeepSWE that went from single digits to the mid-fifties. Because it is priced as a Flash model and speaks both the OpenAI and Anthropic wire formats, a lot of people are pointing their existing agent code at it this week.
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