{"version":"2.1.98","anchor":"vertex-ai-model-upgrade-and-fallback-system","canonical_anchor":"vertex-ai-model-upgrade-and-fallback-system","heading":"Vertex AI Model Upgrade and Fallback System","tier":null,"area":null,"scope":null,"heads_up":null,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.98\/e\/vertex-ai-model-upgrade-and-fallback-system","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.98","markdown":"### Vertex AI Model Upgrade and Fallback System\n\n**What**\n\nAutomatic detection of stale Vertex model pins and probing for accessible fallback models when defaults are unavailable.\n\n**Details**\n\n- On startup (for first-party Vertex users), Claude Code checks whether pinned model versions are outdated compared to built-in defaults\n- For each stale tier, it probes Vertex AI to verify the upgrade candidate is accessible (one-token request, 20s timeout)\n- If upgrades are found, presents a dialog to update model pins\n- Separately, detects unpinned tiers where the default model may not be accessible and finds compatible fallbacks\n- Displays warnings when fallback models are used instead of defaults\n- Logs telemetry: `tengu_vertex_upgrade_check` and `tengu_vertex_fallback_check`\n- Auto-restarts Claude Code when model configuration changes\n\n**Evidence**\n\nVertex upgrade\/fallback system (search for `\"[vertex-upgrade] tiersWithPin=\"` and `\"[vertex-fallback] unpinnedTiers=\"`)"}