Originally published on rohitraj.tech
On July 24, 2026 at 15:59 UTC, DeepSeek fully retires the deepseek-chat and deepseek-reasoner model names — every API call still using them starts returning errors. The replacement names (deepseek-v4-flash, deepseek-v4-pro) take ten minutes to wire in, but two silent gotchas can wreck your bill or your latency: thinking mode moved from a model name to a request parameter, and the naive migration path can turn your cheapest endpoint into a reasoning-token furnace. Here is the exact before/after code, the Flash vs Pro decision table, the Anthropic-SDK routing trick, and how I would stage the cutover in production.
Read the full version with code samples, diagrams, and architecture details: DeepSeek V4 API Migration Guide: What Breaks on July 24, 2026 (and the 10-Minute Fix)
More engineering notes: rohitraj.tech/en/notes
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