[BUG] WebFetch tool in Claude Code SDK calls US-region model when using Bedrock APAC region, causing 400 The provided model identifier is invalid

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Describe the bug

When running inference on AWS Bedrock with an APAC-region Claude model (e.g. apac.anthropic.claude-3-5-sonnet-*), if a tool invocation triggers WebFetch (web research) internally, the SDK unexpectedly attempts to call a US-region model (for example us.anthropic.claude-3-5-haiku-20241022-v1:0).
Because my Bedrock account is limited to the APAC region, this internal call is not permitted and the API responds:

API Error (us.anthropic.claude-3-5-haiku-20241022-v1:0): 
400 The provided model identifier is invalid.

Actual behavior

I think WebFetch internally calls a hard-coded US-region model (us.anthropic.claude-3-5-haiku-20241022-v1:0), leading to:

400 The provided model identifier is invalid.

Environment

• Claude Code SDK version: 0.0.20
• AWS Bedrock region: ap-northeast-1 (APAC)
• Python version: 3.12
• OS: Ubuntu 22.0.4 LTS

What Should Happen?

WebFetch (or any internal tool) should respect the region of the currently configured model and not attempt to call a US-region model when using an APAC-region Bedrock endpoint.

Error Messages/Logs

API Error (us.anthropic.claude-3-5-haiku-20241022-v1:0): 400 The provided model identifier is invalid.

Steps to Reproduce

• Use Claude Code SDK with AWS Bedrock, specifying an APAC region model (e.g. ANTHROPIC_MODEL='apac.anthropic.claude-3-7-sonnet-20250219-v1:0').
• Trigger a tool invocation that requires WebFetch (for example, navigating to a URL or performing a web search).
• Observe the logs: the SDK tries to call a US-region Haiku model and fails with a 400 error.

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

1.0.124

Platform

AWS Bedrock

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

This makes it impossible to use WebFetch in APAC regions even when the primary model is correctly configured for APAC.