Claude Code on Google Cloud's Agent Platform changedgoogle-vertex-ai
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256256If you have not pinned a model and the current default is unavailable in your project, Claude Code falls back for the current session and shows a notice. It tries earlier versions of the default model first and, when the default is an Opus model and no Opus version is available, falls back to the default Sonnet model. The fallback is not persisted. Enable the newer model in [Model Garden](https://console.cloud.google.com/vertex-ai/model-garden) or [pin a version](#5-pin-model-versions) to make the choice permanent.
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258When you start the session on a specific Sonnet or Opus version, for example with `--model`, `ANTHROPIC_MODEL`, or the [`model` setting](/docs/en/settings), that version acts as the session's pinned default for the matching `sonnet` or `opus` alias. Claude Code skips the availability check for the built-in default your model replaces and starts on the model you configured, with no fallback notice.
258When you start the session on a specific Sonnet or Opus version, for example with `--model`, `ANTHROPIC_MODEL`, or the [`model` setting](/docs/en/settings-reference#model), that version acts as the session's pinned default for the matching `sonnet` or `opus` alias. Claude Code skips the availability check for the built-in default your model replaces and starts on the model you configured, with no fallback notice.
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260260Model aliases such as `opus` don't act as pins, and neither does a model ID Claude Code doesn't recognize.
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