What
When AWS_REGION or AWS_DEFAULT_REGION is set to something that does not validate, Bedrock region detection now reports the source as env-invalid rather than pretending it used the default, and falls back to us-east-1.
Evidence
env-invalid
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Names in the bundleAWS_REGIONAWS_DEFAULT_REGION
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up AWS_REGION, on Claude Code on Amazon Bedrock.
* You only need to set `AWS_REGION` to override your AWS profile's region or when your profile has no region. Claude Code resolves the region in this order:amazon-bedrock see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up AWS_REGION, on Claude Code on Amazon Bedrock.