API Error 400 with custom model via ANTHROPIC_BASE_URL when plugins are loaded

Issue Description

When using Claude Code with a custom API endpoint (百炼 DashScope proxy) via environment variables ANTHROPIC_BASE_URL and ANTHROPIC_MODEL, the following error occurs on new sessions:

API Error: 400 Request body format invalid

The error also manifests as model resolution failures:

• API Error: 400 model 'claude-opus-4-8' is not supported.
• Model 'qwen3.6-plus' is not available. Your organization restricts model selection.

Root Cause Analysis

Through systematic debugging, I identified that:

1. --bare mode works perfectly - When all plugins are disabled, Claude Code connects to the custom API endpoint without issues.

1. Plugin loading triggers incompatible API requests - When plugins are loaded (any plugin: LSP, superpowers, claude-mem, etc.), Claude Code sends additional API requests that use:

• Model names like claude-opus-4-8 that aren't available on the custom endpoint
• Request body formats that the custom endpoint doesn't support

1. Configuration method matters - Using --settings <file> to specify configuration triggers the error, while relying on environment variables (ANTHROPIC_BASE_URL, ANTHROPIC_MODEL) works correctly in the current session.

Environment

• Claude Code version: 2.1.156
• Custom API endpoint: https://coding.dashscope.aliyuncs.com/apps/anthropic (百炼 DashScope proxy)
• Custom model: qwen3.6-plus
• OS: macOS (Darwin 25.4.0)
• Node.js: v18.18.0

Reproduction Steps

1. Set environment variables:

``bash
export ANTHROPIC_BASE_URL=https://coding.dashscope.aliyuncs.com/apps/anthropic
export ANTHROPIC_MODEL=qwen3.6-plus
export ANTHROPIC_AUTH_TOKEN=<your-token>
``

1. Create a settings file with plugins enabled:

``json
{
"model": "qwen3.6-plus",
"enabledPlugins": {
"typescript-lsp@claude-plugins-official": true,
"superpowers@claude-plugins-official": true
}
}
``

1. Start Claude Code:

``bash
claude --settings ~/.claude/settings-test.json
``

1. Observe error: API Error: 400 Request body format invalid

Workaround

Using --bare mode works:

claude --bare --settings ~/.claude/settings-test.json

Expected Behavior

Claude Code should either:

1. Respect the configured model for all API requests (including plugin-related ones)
2. Provide a way to disable plugin-specific API calls when using custom endpoints
3. Gracefully handle model unavailability without breaking the entire session

Additional Context

This is a significant issue for users who:

• Use Claude Code with custom API endpoints (common in enterprise/China deployments)
• Rely on plugins for development workflow (LSP, superpowers, memory systems)
• Need both custom models and plugin functionality simultaneously