Custom agents with model: opus fail on Bedrock provider - x-anthropic-billing-header reserved keyword error

Bug Description

Custom agents defined in ~/.claude/agents/*.md that specify model: opus (or inherit Opus from the parent session) fail to spawn when using the Bedrock provider. The error is:

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 with Bedrock provider in settings.json:

{
  "provider": "bedrock",
  "model": "us.anthropic.claude-opus-4-6-v1"
}

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

---
name: my-agent
model: opus
---

You are a helpful agent.

1. Spawn the agent via Task tool:

Task(subagent_type="my-agent", prompt="Hello")

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

What Works vs What Fails

| Scenario | Result |
|----------|--------|
| Custom agent + Bedrock + model: opus | FAILS |
| Custom agent + Bedrock + model: sonnet | Works |
| Custom agent + Bedrock + no model (inherits opus) | FAILS |
| Built-in agent (general-purpose, Explore) + Bedrock + opus | Works |

Root Cause Hypothesis

When spawning custom agents through the Bedrock provider with the Opus model, Claude Code appears to incorrectly inject the x-anthropic-billing-header HTTP header into the system prompt content sent to the Bedrock Converse API. This does not happen with built-in agent types, suggesting the custom agent code path handles Bedrock headers differently.

Workaround

Pass model: sonnet explicitly when spawning custom agents via the Task tool. The custom agent's full system prompt is preserved, just running on Sonnet instead of Opus.

Environment

• Platform: Windows 11
• Provider: Bedrock (us.anthropic.claude-opus-4-6-v1)
• Claude Code version: Latest as of 2026-02-10
• Agent teams feature enabled: CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
• All 5 custom agents affected (qa-test-engineer, requirements-guardian, elite-security-auditor, elite-documentation-architect, devops-guardian)