Custom cloud endpoints (Together.ai, RunPod, Groq, etc.) lost their
API key after #4165 removed OPENAI_API_KEY .env saves. The key was
only saved to the custom_providers list which is unreachable at
runtime for plain 'custom' provider resolution.
Save model.api_key to config.yaml alongside model.provider and
model.base_url in all three custom endpoint code paths:
- _model_flow_custom (new endpoint with model name)
- _model_flow_custom (new endpoint without model name)
- _model_flow_named_custom (switching to a saved endpoint)
The runtime resolver already reads model.api_key (runtime_provider.py
line 224-228), so the key is picked up automatically. Each custom
endpoint carries its own key in config — no shared OPENAI_API_KEY
env var needed.
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