DeepSeek reroutes V4 Pro traffic to a Flash model

DeepSeek told users that once V4.1 Flash launches around September 10 Beijing time, every V4 Pro API request will route to V4.1 Flash instead and bill at Flash pricing, according to a r/LocalLLaMA thread. V4 Pro reached general availability August 13. Three weeks as the flagship, then a redirect to a cheaper model with no deprecation notice and no migration guide.

The numbers back up the swap. V4.1 Flash scores 82.7 on Terminal-Bench 2.1 at $0.14/$0.28 per million tokens, and on MathArena's AIME 2026 set it hits 95.83% against Pro's 96.67%, per The New Stack. Statistically indistinguishable math performance at about a ninth of the per-problem cost, running around 400 tokens per second. New pricing from September 10 drops the input cache-hit cost to $0.003 per million tokens.

DeepSeek had already opened an intermediate build to all API users on September 8 under the model ID deepseek-v4.1-flash-expires-on-0910, a self-destructing identifier good for exactly two days, no beta signup required, per another r/LocalLLaMA post.

Three more releases went up in the same 48 hours, chasing the same cost line down. Inception Labs released Mercury 2.5, a diffusion model running 1,107 tokens per second, discounted 80% to $0.04/$0.15 per million at Inception Labs. Desert Ant Labs gave away 18 on-device models free to 100,000 monthly devices with no per-token metering, according to its launch post. And a GitHub project called deltafin got the 2.8-trillion-parameter Kimi K3 running on a MacBook Pro by streaming 1.45TB of weights off four SSDs.

What to actually do

Pin model IDs explicitly in every client. DeepSeek just proved a vendor will redirect a paid tier to a different model while your evals still point at the old one. And if your pricing assumed a fixed cost per AI call, that assumption is now wrong every few months, not every few years.