[BUG] Bedrock: subagent model alias resolved with the parent session's inference-profile prefix, ignoring ANTHROPIC_DEFAULT_<TIER>_MODEL

What's Wrong?

On Bedrock, a subagent's model alias is resolved with the parent session's inference-profile prefix instead of the configured ANTHROPIC_DEFAULT_<TIER>_MODEL value. When the parent runs on a us. profile and the configured Sonnet model is a global. profile (Sonnet 5.5 has no us. profile in us-east-1), every Sonnet subagent fails at its first call.

What Should Happen?

A subagent with model: sonnet (frontmatter or per-call) should use ANTHROPIC_DEFAULT_SONNET_MODEL exactly as configured, global.anthropic.claude-sonnet-5-5, the same way a top-level claude -p --model sonnet session does.

Error Messages/Logs

API Error (us.anthropic.claude-sonnet-5-5): 400 The provided model identifier is invalid.. Try /model to switch to us.anthropic.claude-sonnet-5. (error type model_not_found, HTTP 400, model sent to the API: us.anthropic.claude-sonnet-5-5)

Steps to Reproduce

1. Settings env: CLAUDE_CODE_USE_BEDROCK=1, AWS_REGION=us-east-1, ANTHROPIC_DEFAULT_OPUS_MODEL=us.anthropic.claude-opus-5-5, ANTHROPIC_DEFAULT_SONNET_MODEL=global.anthropic.claude-sonnet-5-5.
2. Define a subagent with model: sonnet in its frontmatter.
3. echo 'Reply with exactly: OK' | claude -p --model sonnet --output-format json: works, and modelUsage shows global.anthropic.claude-sonnet-5-5.
4. From a session on us.anthropic.claude-opus-5-5, dispatch the subagent: it fails with the error above, and the model sent is us.anthropic.claude-sonnet-5-5.
5. From a session on global.anthropic.claude-opus-5-5 (claude -p --model global.anthropic.claude-opus-5-5 --allowedTools Agent), dispatch the same subagent: it works, and modelUsage shows global.anthropic.claude-sonnet-5-5.

Workaround: put the session model and all ANTHROPIC_DEFAULT_*_MODEL values on the same prefix.

Claude Model

Opus (parent), Sonnet (subagent)

Is this a regression?

I don't know

Claude Code Version

2.1.286 (Claude Code)

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

zsh

Additional Information

This is the bug from #73556 (same symptom with an eu. prefix), which was closed as stale with a request to reopen if it's still relevant. It still reproduces on 2.1.286. The new finding is where the prefix comes from: it is taken from the parent session's model. Step 5 shows that moving the parent to a global. profile makes the same subagent resolve correctly.