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

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?

Summary

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.

The main loop resolves the same alias correctly through the env var. This is the bug reported in #73556, which was
closed as "not planned" after going stale, with no fix or maintainer comment. It still reproduces on 2.1.284.

Environment

• Claude Code 2.1.284 (CLI), Windows 11, Git Bash
• ~/.claude/settings.json env:

``json
"CLAUDE_CODE_USE_BEDROCK": "1",
"AWS_REGION": "us-east-1",
"ANTHROPIC_DEFAULT_SONNET_MODEL": "global.anthropic.claude-sonnet-5-5",
"ANTHROPIC_DEFAULT_OPUS_MODEL": "us.anthropic.claude-opus-5-5[1m]"
``

• No project-level or local settings override any model variable. env inside the session confirms

ANTHROPIC_DEFAULT_SONNET_MODEL=global.anthropic.claude-sonnet-5-5 is set in the process.

• No modelOverrides or availableModels are configured, so this is separate from #81995.

Reproduction

# 1. Main loop, alias -> works (env var honoured)
claude -p --model sonnet "Reply with only: OK"
# OK

# 2. Main loop, full ID -> works (ID is valid on this account)
claude -p --model global.anthropic.claude-sonnet-5-5 "Reply with only: OK"
# OK

# 3. Subagent, alias -> FAILS
claude -p --agents '{"alias-test":{"description":"test agent","prompt":"Reply with only: SUBAGENT-OK","model":"sonnet"}}' \
  "Call the Agent tool with subagent_type alias-test and prompt 'go'. Print its reply or the exact error."

# 4. Subagent, full ID -> works
claude -p --agents '{"fullid-test":{"description":"test agent","prompt":"Reply with only: SUBAGENT-OK","model":"global.anthropic.claude-sonnet-5-5"}}' \
  "Call the Agent tool with subagent_type fullid-test and prompt 'go'. Print its reply or the exact error."
# SUBAGENT-OK

A subagent file in .claude/agents/*.md with model: sonnet in its frontmatter fails the same way. So does
calling the Agent tool with model: "sonnet" on a built-in agent type.

Actual result (step 3)

Agent terminated early due to an API error: 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)

Expected result

Subagent alias resolution uses the same ANTHROPIC_DEFAULT_*_MODEL mapping as the main loop, so step 3 sends
global.anthropic.claude-sonnet-5-5 and succeeds.

Results matrix

| Path | Model requested | Model sent | Result |
|---|---|---|---|
| Main loop --model | sonnet | env override | OK |
| Main loop --model | global.anthropic.claude-sonnet-5-5 | same | OK |
| Subagent (--agents / frontmatter / Agent tool) | sonnet | us.anthropic.claude-sonnet-5-5 | 400 model_not_found |
| Subagent (--agents) | global.anthropic.claude-sonnet-5-5 | same | OK |

Likely cause (inference, not verified)

The subagent path seems to build the default Bedrock profile from AWS_REGION (us-east-1 -> us.) and never checks
ANTHROPIC_DEFAULT_SONNET_MODEL. In #73556 an EU region produced eu., which fits this. The opus and
haiku aliases don't fail for us only because their built-in us. IDs happen to be valid on this account. They
may be ignoring their overrides as well, but the error doesn't show.

Impact

• Custom agents pinned to sonnet for cost fail hard on Bedrock. Users then switch them to opus, which costs more,

or hard-code provider-specific IDs in agent files. Those files then can't be shared across providers.

• The error message points users at the wrong fix. They edit settings.json, which is already correct.

Workaround

Put the full provider ID in the agent frontmatter: model: global.anthropic.claude-sonnet-5-5.

---

*Investigated, reproduced and drafted by Claude Opus 5.5 (Claude Code 2.1.284) at the reporter's request; reproduction
steps were run and results recorded on 2026-09-30.*

🤖 Generated with Claude Code

What Should Happen?

Expected result

Subagent alias resolution uses the same ANTHROPIC_DEFAULT_*_MODEL mapping as the main loop, so step 3 sends
global.anthropic.claude-sonnet-5-5 and succeeds.

Results matrix

| Path | Model requested | Model sent | Result |
|---|---|---|---|
| Main loop --model | sonnet | env override | OK |
| Main loop --model | global.anthropic.claude-sonnet-5-5 | same | OK |
| Subagent (--agents / frontmatter / Agent tool) | sonnet | us.anthropic.claude-sonnet-5-5 | 400 model_not_found |
| Subagent (--agents) | global.anthropic.claude-sonnet-5-5 | same | OK |

Error Messages/Logs

Agent terminated early due to an API error: 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

Reproduction

# 1. Main loop, alias -> works (env var honoured)
claude -p --model sonnet "Reply with only: OK"
# OK

# 2. Main loop, full ID -> works (ID is valid on this account)
claude -p --model global.anthropic.claude-sonnet-5-5 "Reply with only: OK"
# OK

# 3. Subagent, alias -> FAILS
claude -p --agents '{"alias-test":{"description":"test agent","prompt":"Reply with only: SUBAGENT-OK","model":"sonnet"}}' \
  "Call the Agent tool with subagent_type alias-test and prompt 'go'. Print its reply or the exact error."

# 4. Subagent, full ID -> works
claude -p --agents '{"fullid-test":{"description":"test agent","prompt":"Reply with only: SUBAGENT-OK","model":"global.anthropic.claude-sonnet-5-5"}}' \
  "Call the Agent tool with subagent_type fullid-test and prompt 'go'. Print its reply or the exact error."
# SUBAGENT-OK

A subagent file in .claude/agents/*.md with model: sonnet in its frontmatter fails the same way. So does
calling the Agent tool with model: "sonnet" on a built-in agent type.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.284

Platform

AWS Bedrock

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_