Problem

When fallback_providers references a provider defined in custom_providers but does not supply an inline base_url, _try_activate_fallback() passed explicit_base_url=None to resolve_provider_client(). The resolution then fell through to the primary provider's endpoint, silently misdirecting the fallback request.

Reported in #15743.

Root cause

_try_activate_fallback() only sourced fb_base_url_hint from the fallback entry's own base_url field and from KNOWN_PROVIDERS. Named custom_providers entries were never consulted, so any provider whose connectivity lives entirely in custom_providers (no built-in entry) produced a None hint.

Fix

Before calling resolve_provider_client(), when neither fb_base_url_hint nor fb_api_key_hint is set, attempt a lookup via _get_named_custom_provider(fb_provider) and propagate base_url, api_key, and key_env as explicit hints:

if not fb_base_url_hint and not fb_api_key_hint:
    try:
        from hermes_cli.runtime_provider import _get_named_custom_provider
        _cp = _get_named_custom_provider(fb_provider)
        if _cp:
            fb_base_url_hint = (_cp.get("base_url") or "").strip() or None
            _cp_key = (_cp.get("api_key") or "").strip()
            _cp_key_env = (_cp.get("key_env") or "").strip()
            if not fb_api_key_hint:
                if _cp_key:
                    fb_api_key_hint = _cp_key or None
                elif _cp_key_env:
                    fb_api_key_hint = os.getenv(_cp_key_env, "").strip() or None
    except Exception:
        pass

The entire block is wrapped in except Exception: pass so a config-load failure degrades gracefully — resolve_provider_client() still runs with whatever it can derive from the provider name alone, exactly as before.

Precedence is preserved: the lookup only runs when both hints are empty. An inline base_url in the fallback entry always takes priority.

Tests

Three new tests in TestCustomProviderFallbackBaseUrl (appended to tests/run_agent/test_provider_fallback.py):

All 22 tests in the file pass (22 passed).

Checklist

• Fixes the reported symptom end-to-end (wrong endpoint → correct endpoint)
• Inline base_url continues to take precedence (no behaviour change for existing configs)
• Lookup failure is silently swallowed — no regression for providers that aren't in custom_providers
• Three targeted tests cover the happy path, the no-op path, and the error path
• No new dependencies introduced

Closes #15743