Description

resolve_provider_client() in agent/auxiliary_client.py handles named custom providers (from custom_providers list in config.yaml) by unconditionally creating an OpenAI(...) client, even when the provider's api_mode is anthropic_messages.

# Line ~1460 in v0.9.0
client = OpenAI(api_key=custom_key, base_url=custom_base)

This means custom providers pointing to Anthropic-compatible endpoints (corporate proxies, self-hosted deployments) send requests in OpenAI format, causing ChatCompletion.choices = None crashes.

Context

This is the third layer of the bug reported in #8857. PR #8871 (merged in v0.9.0) correctly fixed:

1. _normalize_aux_provider() stripping named provider identity for vision tasks
2. resolve_vision_provider_client() not propagating api_mode downstream

But the underlying issue remains: even with api_mode correctly propagated, the named custom provider code path never checks it.

Expected Behavior

When a named custom provider has api_mode: anthropic_messages (either in config or passed as parameter), resolve_provider_client() should create an AnthropicAuxiliaryClient wrapping the native Anthropic SDK — mirroring how _try_anthropic() already handles the built-in anthropic provider.

Suggested Fix

In the named custom provider block of resolve_provider_client():

effective_api_mode = api_mode or custom_entry.get("api_mode", "")
if effective_api_mode == "anthropic_messages":
    try:
        from agent.anthropic_adapter import build_anthropic_client
        real_client = build_anthropic_client(custom_key, custom_base)
        client = AnthropicAuxiliaryClient(
            real_client, final_model, custom_key, custom_base)
    except ImportError:
        logger.warning(
            "resolve_provider_client: anthropic SDK not available "
            "for custom provider %r, falling back to OpenAI client",
            provider)
        client = OpenAI(api_key=custom_key, base_url=custom_base)
else:
    client = OpenAI(api_key=custom_key, base_url=custom_base)

Environment

• Hermes v0.9.0 (v2026.4.13), macOS
• Custom provider: corporate Anthropic proxy with api_mode: anthropic_messages
• Patched locally and working in production

Related

• Vision auxiliary client drops api_mode for custom providers, causing crash #8857 (original vision crash report)
• PR fix(agent): propagate api_mode to vision provider resolution #8871 (partial fix, merged in v0.9.0)