Production breaking: Vercel AI SDK @ai-sdk/google suddenly fails for all Gemini 2.5 Flash users. Function calling streams throw AI_TypeValidationError because Gemini introduced 'thought' parts (reasoning tokens) in API responses that the SDK Zod schema did not recognize — it only accepted text/functionCall/inlineData. 32 GitHub reactions, 22 comments, fix confirmed by Vercel team and shipped in @ai-sdk/google@1.2.19. Three resolution paths: upgrade, disable thinking, or downgrade model. Affects all Next.js / Vercel AI SDK apps using Gemini 2.5 Flash with tools.

An environment variable is undefined at runtime in a Next.js app.