[BUG] using default region and model ID in bedrock.py causes Strands hello world example to fail in non-US regions. model selection happen before region resolution

[cerashdan]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.5.0

Python Version

3.12.3

Operating System

Linux - Ubuntu

Installation Method

pip

Steps to Reproduce

To reproduce the issue:

1. Pick a region that is not US-based (e.g. eu-central-1)
2. Follow the steps to deploy the agent using AgentCore startkit - Get started with the Amazon Bedrock AgentCore Runtime starter toolkit
3. As the AI agent is deployed regionally, it will override eu-central-1 for the AWS region in models\bedrock.py and use US model ID: us.anthropic.claude-sonnet-4-20250514-v1:0
4. the us.anthropic.claude-sonnet-4-20250514-v1:0 is an invalid model ID in EU regions.

Expected Behavior

The hello world example in Get started with the Amazon Bedrock AgentCore Runtime starter toolkit should work out of the box - allowing developers to get started quickly to use Strands SDK with AgentCore.

Actual Behavior

The hello world example in Get started with the Amazon Bedrock AgentCore Runtime starter toolkit fails in non-US regions due to invalid model identifier and the developer has to go through the logs to realize the issue.

Additional Context

Possible Solution

I suggest that we replace those lines

DEFAULT_BEDROCK_MODEL_ID = "us.anthropic.claude-sonnet-4-20250514-v1:0" DEFAULT_BEDROCK_REGION = "us-west-2"

with an internal function that will resolve CRIS model ID for the appropriate AWS partition:

def _get_default_model_for_region(region: str) -> str:
    region_models = {
        "us-east-1": "us.anthropic.claude-sonnet-4-20250514-v1:0",
        "us-west-2": "us.anthropic.claude-sonnet-4-20250514-v1:0",
        "eu-central-1": "eu.anthropic.claude-sonnet-4-20250514-v1:0",
        "eu-west-1": "eu.anthropic.claude-sonnet-4-20250514-v1:0",
    }
    return region_models.get(region, "us.anthropic.claude-sonnet-4-20250514-v1:0")

DEFAULT_BEDROCK_REGION = "us-west-2"

and the config update:

self.config = BedrockModel.BedrockConfig(model_id=DEFAULT_BEDROCK_MODEL_ID)

with:

resolved_region = region_name or (boto_session or boto3.Session()).region_name or os.environ.get("AWS_REGION") or DEFAULT_BEDROCK_REGION
self.config = BedrockModel.BedrockConfig(model_id=_get_default_model_for_region(resolved_region))

Related Issues

No response

[zastrowm]

Team is on-board for making this a better default.

Specifically we should have a default per geography (eu regions use eu-, us reasons use us-)

[cerashdan]

This PR #770 dynamically resolve the default model ID for the right AWS partition allowing the code to run in future regions where Bedrock might be available.