[claude-code] [BUG] [bedrock] Subagent `sonnet` alias ignores ANTHROPIC_DEFAULT_SONNET_MODEL and sends an invalid `us.` profile ID (regression of / same as #73556)

## Observed symptom [BUG] [bedrock] Subagent `sonnet` alias ignores ANTHROPIC_DEFAULT_SONNET_MODEL and sends an invalid `us.` profile ID (regression of / same as #73556) - [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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 On Amazon Bedrock, when a subagent requests the `sonnet` alias, Claude Code ignores `ANTHROPIC_DEFAULT_SONNET_MODEL` and sends a built-in, region-prefixed profile ID (`us.anthropic.claude-sonnet-5-5`). Bedrock rejects that ID with a 400 error, and the subagent dies before its first tool call. The alias can come from agent frontmatter (`model: sonnet`) or from the Agent tool's `model` parameter. ## Literal error text ``` ). Bedrock rejects that ID with a 400 error, and the subagent dies before its first tool call. The alias can come from agent frontmatter ( ``` ## Environment GitHub anthropics/claude-code #98298 ## Trigger / context Public GitHub report in anthropics/claude-code (FRESH_7D). ## Source evidence - https://github.com/anthropics/claude-code/issues/98298 (2026-10-01T05:11:48Z) — evidence_class: PRIMARY_ISSUE_REPORT — #98298 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:44:03.802689Z - scout_id: C7913c3e5ba ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.