claude_code_oauth_token (sk-ant-oat01-*) fails with Header 14 invalid value

[7da11]

Bug Description

When using claude_code_oauth_token with a token generated by claude setup-token (prefix sk-ant-oat01-*), the action fails with:

API Error: Header '14' has invalid value: '***'

The same token works correctly when used locally with claude -p.

Steps to Reproduce

1. Generate OAuth token: claude setup-token (Max 5x plan, sk-ant-oat01-* format)
2. Store as GitHub secret CLAUDE_CODE_OAUTH_TOKEN
3. Configure workflow with claude_code_oauth_token parameter
4. Trigger via @claude comment on a PR

Observed Behavior

The OIDC and app token exchange succeed, but Claude Code SDK fails:

OIDC token successfully obtained
App token successfully obtained
...
SDK execution error: Error: Claude Code returned an error result: API Error: Header '14' has invalid value

Also Tried

• Passing the same OAuth token via anthropic_api_key parameter: Invalid API key error (expected, different code path)
• Token works locally: ANTHROPIC_API_KEY=sk-ant-oat01-... claude -p "echo test" succeeds

Environment

• Plan: Max 5x
• Auth method: claude.ai (OAuth)
• Action version: v1 (SHA: 51ea8ea)
• Runner: ubuntu-latest (Ubuntu 24.04)
• Workflow permissions include id-token: write
• claude auth status locally shows: loggedIn: true, authMethod: claude.ai, subscriptionType: max