{"version":"2.1.285","anchor":"third-party-model-env-vars-haiku-handled-like-opussonnet","canonical_anchor":"third-party-model-env-vars-haiku-handled-like-opussonnet","heading":"Bedrock and Vertex: Haiku default tracking and automatic fallback when a model is unavailable","tier":"notice","area":"Models","url":"https:\/\/changelogs.core-directive.com\/v\/2.1.285\/e\/third-party-model-env-vars-haiku-handled-like-opussonnet","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.285","markdown":"### Bedrock and Vertex: Haiku default tracking and automatic fallback when a model is unavailable\n\nOn Bedrock and Vertex, a probed Haiku default no longer counts as your own setting, and models your account cannot use now fall back to the next tier\n\n**Unclear.** What switches the fallback ladder on and which providers it covers are not known.\n\n**What**\n\nWhen Claude Code runs through a third-party cloud provider such as Amazon Bedrock or Google Vertex, it checks which models your account can use. It can also fill in the default model environment variables for you. An environment variable is a named setting passed to the program when it starts. This release extends that handling and adds an automatic fallback:\n\n- `ANTHROPIC_DEFAULT_HAIKU_MODEL` now gets a `CLAUDE_CODE_3P_PROBE_WROTE_HAIKU_DEFAULT` marker. When Claude Code's own availability check wrote the value, it is no longer treated as something you set yourself. This already worked this way for Opus and Sonnet. Before, the Haiku variable always counted as user-set.\n\n- New `CLAUDE_CODE_3P_SEEDED_SONNET_DEFAULT` and `CLAUDE_CODE_3P_SEEDED_OPUS_DEFAULT` variables record when a Sonnet or Opus default was filled in automatically. These markers are passed on to child processes along with the other model variables.\n\n- A new fallback ladder moves a model tier down to the next model when your account cannot access the one it would use. The ladder follows the model environment variables. The error shown reads \"is not available to this account. Ask your cloud admin to enable it, or choose another model.\"\n\n- When the Opus tier falls back, `ANTHROPIC_DEFAULT_OPUS_MODEL_NAME` and `ANTHROPIC_DEFAULT_OPUS_MODEL_DESCRIPTION` are rewritten to say Opus is unavailable and to name the model used instead.\n\n- When the default Sonnet model is unavailable on Bedrock or Vertex, the replacement model is now also registered with Claude Code, not only written to the environment variable. This path sits behind a server-controlled switch, `tengu_bedrock_default_fallback`.\n\n- `CLAUDE_CODE_DISABLE_MODEL_ACCESS_FALLBACK` turns the fallback ladder off, alongside the existing `CLAUDE_CODE_NO_MODEL_FALLBACK`.\n\n**Why**\n\nIf your cloud account has not enabled a model, Claude Code may now switch to a different one instead of failing. The model picker will say so. If you would rather get an error than a silent downgrade, set `CLAUDE_CODE_DISABLE_MODEL_ACCESS_FALLBACK`.\n\n- Area: Models\n- Tier: You'll notice\n- Useful: 4\/5\n- Signal: 2\/5"}