Explore subagents ignore ANTHROPIC_SMALL_FAST_MODEL and use hardcoded Haiku model with wrong region/format

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?

Explore subagents are ignoring the ANTHROPIC_SMALL_FAST_MODEL environment variable configured in ~/.claude/settings.json. Instead, they use a hardcoded model ID (anthropic.claude-haiku-4-5-20251001-v1:0) in the wrong region (us-east-1) and wrong format (direct model ID instead of inference profile).

This causes 403 authorization errors when AWS IAM permissions are scoped to specific regions or inference profiles.

What Should Happen?

When ANTHROPIC_SMALL_FAST_MODEL is configured in ~/.claude/settings.json, Explore subagents should:

Respect the configured model/inference profile
Use the configured AWS region
Use the inference profile format if specified (e.g., us.anthropic.*)

The main conversation correctly respects these settings - only Explore subagents ignore them.

Error Messages/Logs

403 {"Message":"User is not authorized to perform: bedrock:InvokeModelWithResponseStream on resource: arn:aws:bedrock:us-east-1::foundation-model/anthropic.claude-haiku-4-5-20251001-v1:0 because no identity-based policy allows the bedrock:InvokeModelWithResponseStream action"}

Steps to Reproduce

Configure AWS Bedrock with IAM permissions scoped to us-east-2 and inference profiles only

Create ~/.claude/settings.json:

json{
  "model": "opus",
  "env": {
    "CLAUDE_CODE_USE_BEDROCK": "1",
    "AWS_REGION": "us-east-2",
    "AWS_PROFILE": "ai-ws-dev-user",
    "ANTHROPIC_MODEL": "us.anthropic.claude-opus-4-5-20251101-v1:0",
    "ANTHROPIC_SMALL_FAST_MODEL": "us.anthropic.claude-sonnet-4-20250514-v1:0"
  }
}

Launch Claude Code in any project

Trigger an Explore subagent (e.g., ask Claude to search the codebase)

Observe 403 error attempting to use anthropic.claude-haiku-4-5-20251001-v1:0 in us-east-1

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

N/A

Claude Code Version

2.1.12

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

This appears related to #18950 (subagents not inheriting settings.json configuration). While version 2.1.0 included fixes for subagent model inheritance, this specific case remains broken.
Workaround: Add IAM permissions for anthropic.claude-haiku-4-5-20251001-v1:0 in us-east-1, but this defeats the purpose of configurable model/region settings.
Impact: Explore subagents are completely non-functional for users with region-scoped or inference-profile-scoped IAM permissions, forcing manual file exploration workflows and significantly degrading the development experience.