Describe the bug

When the primary provider hits a rate limit, Hermes falls back to a model whose provider is defined in custom_providers. However, the fallback request is sent to the primary provider's base_url instead of the fallback provider's own base_url, causing errors.

To Reproduce

1. Configure a tentative primary provider that you know will hit rate limiting (e.g., Nous Portal with rate_limit: True or in a busy account):

   model:
     provider: nous
     model: deepseek/deepseek-v4-pro
     base_url: https://inference-api.nousresearch.com/v1

2. Define a fallback model whose provider lives in custom_providers:

   fallback_model:
     provider: aliyun-singapore
     model: qwen3.6-plus
   
   custom_providers:
     - name: aliyun-singapore
       base_url: https://dashscope.aliyuncs.com/compatible-mode/v1
       api_key: ${ALIYUN_SINGAPORE_API_KEY}
       models:
         qwen3.6-plus:
           context_length: 1000000

3. Trigger a rate limit on the primary provider.

4. Observe the fallback request.

Expected behavior

The fallback request should be sent to https://dashscope.aliyuncs.com/compatible-mode/v1 (the aliyun-singapore provider's own base_url), using Authorization: Bearer <aliyun-singapore-api-key>.

Actual behavior

The fallback request is sent to https://inference-api.nousresearch.com/v1 (the primary nous provider's base_url), which naturally doesn't have the model qwen3.6-plus, resulting in:

⚠️  API call failed (attempt 1/3): NotFoundError [HTTP 404]
   🔌 Provider: aliyun-singapore  Model: qwen3.6-plus
   🌐 Endpoint: https://inference-api.nousresearch.com/v1
   📝 Error: HTTP 404: Model 'qwen3.6-plus' not found. The requested model does not exist in our configuration or OpenRouter catalog.

Root cause hypothesis

The fallback resolution logic correctly identifies the provider name (aliyun-singapore), but fails to look up its base_url from the custom_providers list. Instead it inherits the primary provider's base_url. This likely affects all fallback providers defined in custom_providers — they all end up hitting the primary provider's endpoint with an incompatible model name.

Workaround

Use custom_openrouter as the fallback provider (OpenRouter resolves model routing itself, so the wrong base_url to Nous is harmless for OpenRouter models):

fallback_model:
  provider: custom_openrouter
  model: deepseek/deepseek-v4-flash

But this is16 only a workaround. The root issue remains.

Environment

• Hermes Agent: git HEAD
• Config version: _config_version: 22
  -alan macOS + CLI/TUI mode