[BUG] /model persists us. prefix for Opus 4.6 on EU Bedrock (eu-west-1), causing 400 in new sessions

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?

When using AWS Bedrock in eu-west-1, running /model and selecting Opus 4.6 writes us.anthropic.claude-opus-4-6-v1 into ~/.claude/settings.json. This model ID is invalid in eu-west-1 — only eu.anthropic.claude-opus-4-6-v1 and global.anthropic.claude-opus-4-6-v1 exist there.

The session where /model was used continues to work (presumably resolves the model differently at runtime), but every new session reads the persisted us. model ID from settings.json and fails with:

API Error (us.anthropic.claude-opus-4-6-v1): 400 The provided model identifier is invalid.

Sonnet 4.6 is unaffected — /model correctly persists eu.anthropic.claude-sonnet-4-6 for Sonnet.

Expected Behavior

/model should persist either:

• The short alias (opus) and resolve the regional prefix at runtime, OR
• The correct regional prefix (eu. when AWS_REGION=eu-west-1)

Steps to Reproduce

1. Configure Bedrock with AWS_REGION=eu-west-1:

``json
{
"env": {
"CLAUDE_CODE_USE_BEDROCK": "1",
"AWS_REGION": "eu-west-1",
"AWS_PROFILE": "your-profile"
}
}
``

1. Start Claude Code, run /model, select Opus 4.6
2. Send any message — works in this session
3. Check ~/.claude/settings.json — shows "model": "us.anthropic.claude-opus-4-6-v1"
4. Open a new Claude Code session — fails with 400

Verification

# Only eu. and global. profiles exist in eu-west-1:
aws bedrock list-inference-profiles --region eu-west-1 \
  --query 'inferenceProfileSummaries[?contains(inferenceProfileId,`opus-4-6`)].inferenceProfileId'
# ["eu.anthropic.claude-opus-4-6-v1", "global.anthropic.claude-opus-4-6-v1"]

Environment

• Claude Code version: 2.1.100 (bug NOT present in 2.1.97)
• API provider: Amazon Bedrock
• AWS region: eu-west-1
• OS: macOS Darwin 25.3.0
• Shell: zsh

Related Issues

• #20594 — ANTHROPIC_MODEL ignored, cross-region inference profile used instead
• #27241 — Haiku model ID missing cross-region prefix
• #25193 — Teammates receive normalized model ID instead of Bedrock model ID