docs: add FAQ section for common issues

Cover three frequently reported problems:
- usage.input_tokens error from misconfigured ANTHROPIC_BASE_URL
- Cannot find package 'bundle' from outdated Bun version
- How to use non-Anthropic models (OpenAI/DeepSeek/Ollama)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.en.md

@@ -252,6 +252,42 @@ src/
 
 ---
 
+## FAQ
+
+### Q: `undefined is not an object (evaluating 'usage.input_tokens')`
+
+**Cause**: `ANTHROPIC_BASE_URL` is misconfigured. The API endpoint is returning HTML or another non-JSON format instead of a valid Anthropic protocol response.
+
+This project uses the **Anthropic Messages API protocol**. `ANTHROPIC_BASE_URL` must point to an endpoint compatible with Anthropic's `/v1/messages` interface. The Anthropic SDK automatically appends `/v1/messages` to the base URL, so:
+
+- MiniMax: `ANTHROPIC_BASE_URL=https://api.minimaxi.com/anthropic` ✅
+- OpenRouter: `ANTHROPIC_BASE_URL=https://openrouter.ai/api` ✅
+- OpenRouter (wrong): `ANTHROPIC_BASE_URL=https://openrouter.ai/anthropic` ❌ (returns HTML)
+
+If your model provider only supports the OpenAI protocol, you need a proxy like LiteLLM for protocol translation. See the [Third-Party Models Guide](docs/third-party-models.en.md).
+
+### Q: `Cannot find package 'bundle'`
+
+```
+error: Cannot find package 'bundle' from '.../claude-code-haha/src/entrypoints/cli.tsx'
+```
+
+**Cause**: Your Bun version is too old and doesn't support the required `bun:bundle` built-in module.
+
+**Fix**: Upgrade Bun to the latest version:
+
+```bash
+bun upgrade
+```
+
+### Q: How to use OpenAI / DeepSeek / Ollama or other non-Anthropic models?
+
+This project only supports the Anthropic protocol. If your model provider doesn't natively support the Anthropic protocol, you need a proxy like [LiteLLM](https://github.com/BerriAI/litellm) for protocol translation (OpenAI → Anthropic).
+
+See the [Third-Party Models Guide](docs/third-party-models.en.md) for detailed setup instructions.
+
+---
+
 ## Disclaimer
 
 This repository is based on the Claude Code source leaked from the Anthropic npm registry on 2026-03-31. All original source code copyrights belong to [Anthropic](https://www.anthropic.com). It is provided for learning and research purposes only.