[BUG] Agent system and WebFetch fail with Bedrock EU regions - model region not respected

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?

Description:
When using Claude Code with AWS Bedrock in eu-central-1 region, the agent system
and WebFetch tool fail with "400 The provided model identifier is invalid" errors.

Environment:

• AWS_REGION=eu-central-1
• ANTHROPIC_MODEL=eu.anthropic.claude-sonnet-4-5-20250929-v1:0
• CLAUDE_CODE_USE_BEDROCK=1

The tools attempt to use US region model IDs (us.anthropic.claude-*) instead of
EU region IDs (eu.anthropic.claude-*) when spawning subprocesses or agents.

What Should Happen?

Expected: Tools should respect AWS_REGION and use corresponding regional model IDs
Actual: Tools hardcode US region model IDs, causing Bedrock to reject them

Error Messages/Logs

Steps to Reproduce

Test Case 1: Agent system (Task tool)

1. Create an agent configuration file: .claude/agents/test-agent.md

---
name: test-agent
description: A test agent
model: sonnet
---

You are a test agent.

1. In Claude Code, invoke the agent:

> as test-agent say hello

1. Or programmatically via Task tool:

Use Task tool with:
- subagent_type: "test-agent"
- prompt: "Say hello"

Expected Result: Agent should use eu.anthropic.claude-sonnet-4-5-20250929-v1:0 or equivalent EU model

Actual Result:

API Error (us.anthropic.claude-sonnet-4-5-20250929-v1:0): 400 The provided model identifier is invalid.

Test Case 2: WebFetch tool

1. Configure Claude Code with Bedrock EU region (as above)

1. Use WebFetch tool:

WebFetch tool with:
- url: "https://example.com"
- prompt: "Get the page content"

Expected Result: WebFetch should use EU region model IDs for processing

Actual Result:

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

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.31 (Claude Code)

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_