Gemini 2.5 Pro / Flash / Flash-Lite return 404 model not found on Vertex AI (Agent Platform) — what replaces them, and why does Firebase say VertexAIBackend is missing?

Google retired gemini-2.5-pro, gemini-2.5-flash and gemini-2.5-flash-lite on Vertex AI, now called Gemini Enterprise Agent Platform, on 2026-10-20 (image-generation 2.5 variants on 2026-10-02). Requests naming them fail; there's no automatic redirect. The Gemini Developer API (ai.google.dev / AI Studio) had not announced a shutdown date for them as of mid-October, so the same id can still work there, which makes the failure look inconsistent. Fix: change the model id. Google's lifecycle table lists: - gemini-2.5-pro -> gemini-3.8-flash or gemini-3.5-flash - gemini-2.5-flash -> gemini-3.8-flash, gemini-3.5-flash-lite or gemini-3.1-flash-lite - gemini-2.5-flash-lite -> gemini-3.8-flash, gemini-3.1-flash-lite or Gemma 4 Search code, config, env vars and scheduled jobs for the old ids, then re-test output format, cost and tool calling. Firebase AI Logic: the rename also changed the SDKs. VertexAIBackend (Web) / vertexAI (Swift and others) was removed in Firebase SDK v13.0.0; use AgentPlatformBackend / agentPlatform, available since v12.17.0. The new syntax defaults to the global location instead of us-central1, so set the location explicitly if you depended on us-central1. Sources: https://docs.cloud.google.com/gemini-enterprise-agent-platform/models/model-versions ; https://firebase.google.com/docs/ai-logic/faq-and-troubleshooting ; https://www.developersdigest.tech/blog/vertex-ai-shutdowns-october-2026