The Lab Notes

Anthropic

Claude Code gives custom Bedrock and Vertex names the short prompt

Claude Code 2.1.312 sends a shorter system prompt to unrecognized model names on Bedrock, Vertex, Foundry and Mantle, including custom deployment names.

The Lab Notes ·

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Claude Code now sends a shorter system prompt to any model name it does not recognize on Amazon Bedrock, Google Vertex AI, Foundry and Mantle. That includes custom deployment names that front Sonnet or Haiku. Admins who name their own deployments get the change without asking for it.

The change is in Claude Code 2.1.312, published on GitHub Oct. 27, 2026. The system prompt is the standing instructions Claude Code sends the model before every request. Setting CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT=0 keeps the full prompt.

Custom Names and ARNs Trigger It

The changelog names custom deployment names and inference profile ARNs as the examples. A custom deployment name is a name an admin picks for a deployment. An inference profile ARN is the identifier Amazon Bedrock gives a model endpoint.

Earlier changelog entries already cover unrecognized model IDs. Claude Code shows a startup warning and, in print mode, writes a [claude-code:unrecognized_model] line to stderr. Mapping the model with modelOverrides silences the warning.

Flagged Messages Switch Models Too

The same release widens the automatic model switches that follow a flagged message. They now cover model names Claude Code does not recognize, such as custom deployment names. Setting CLAUDE_CODE_DISABLE_REFUSAL_FALLBACK=1 turns them off.

Our View

We think the trigger matters more than the list of providers. The shorter prompt and the automatic switches depend on whether Claude Code recognizes the name. A team that points Claude Code at a Bedrock inference profile ARN or a Vertex deployment it named itself gets the new behavior on update, even when Sonnet sits behind the name.

Admins and developers in that position have to decide whether they want it. Set CLAUDE_CODE_SIMPLE_SYSTEM_PROMPT=0 to get the full prompt back. Set CLAUDE_CODE_DISABLE_REFUSAL_FALLBACK=1 to turn the automatic switches off. Set them as environment variables wherever Claude Code runs.

Either choice holds for a team only if those variables reach every developer machine and build job that uses the custom name.