[BUG] Subagent spawning fails with model: opus on AWS Bedrock — "x-anthropic-billing-header is a reserved keyword

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 Claude Code with AWS Bedrock (CLAUDE_CODE_USE_BEDROCK=1), spawning subagents (via the Task tool) with model: opus fails with an API 400 error. The same agents work fine with model: sonnet or model: haiku. The main conversation itself runs on opus via Bedrock without issue.

What Should Happen?

Subagents with model: opus should work on AWS Bedrock the same way the main conversation does. The billing header should be sent as an HTTP header, not embedded in the system prompt content.

Error Messages/Logs

API Error: 400 x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt.

Steps to Reproduce

1. Configure Claude Code to use AWS Bedrock:

``json
// .claude/settings.local.json
{
"env": {
"CLAUDE_CODE_USE_BEDROCK": "1",
"AWS_REGION": "us-east-1"
}
}
``

1. Create a custom agent at .claude/agents/my-agent.md:

``yaml
---
name: my-agent
description: "A test agent"
model: opus
---
You are a helpful assistant.
``

1. Use the Task tool to spawn the agent:

``
Task(subagent_type="my-agent", prompt="Read any file and return its first line.")
``

1. Result: API Error: 400 x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt.

1. Change the agent to model: sonnet or pass model: sonnet as a Task tool parameter — it works.

Test Matrix

| Agent Type | Model | Backend | Result |
|-----------|-------|---------|--------|
| Custom agent | opus (frontmatter) | Bedrock | FAIL |
| Built-in general-purpose | opus (Task param) | Bedrock | FAIL |
| Custom agent | haiku (Task param override) | Bedrock | OK |
| Custom agent | sonnet (Task param override) | Bedrock | OK |
| Built-in mcp-explorer | haiku (frontmatter) | Bedrock | OK |
| Built-in general-purpose | sonnet (Task param) | Bedrock | OK |
| Main conversation | opus | Bedrock | OK |

Root Cause Analysis

Claude Code sends an x-anthropic-billing-header with attribution data on every API call (visible in debug logs):

[DEBUG] attribution header x-anthropic-billing-header: cc_version=2.1.37.3a3; cc_entrypoint=cli; cch=00000;

When spawning opus-model subagents through Bedrock, this header appears to be injected into the system prompt content rather than sent as an HTTP header. Bedrock's API validation rejects requests containing x-anthropic-billing-header as a reserved keyword in the system prompt body.

This does not affect:

• The main conversation (opus on Bedrock works fine)
• Subagents using model: haiku or model: sonnet
• Direct Anthropic API usage (not Bedrock)

Environment

• Claude Code version: 2.1.37
• API backend: AWS Bedrock (CLAUDE_CODE_USE_BEDROCK=1)
• AWS Region: us-east-1
• Model ID (from config): us.anthropic.claude-opus-4-6-v1
• OS: Linux (devcontainer, linux/amd64)
• Node.js: 22.x

Workaround

Change model: opus to model: sonnet in custom agent frontmatter (.claude/agents/*.md). Sonnet subagents work correctly on Bedrock.

Expected Behavior

Subagents with model: opus should work on AWS Bedrock the same way the main conversation does. The billing header should be sent as an HTTP header, not embedded in the system prompt content.

Impact

Any custom agent or built-in agent configured to use opus is completely broken on Bedrock deployments. Users must downgrade all subagents to sonnet/haiku as a workaround.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.37

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_