The named-custom-provider runtime path returned a static api_mode, so a
providers: entry like opencode-go-bridge -> https://opencode.ai/zen/go/v1
sent responses-only models (grok-4.5, gpt-5.6-luna) to /chat/completions
and got HTTP 503 (#85589 repro). Now: when the provider name is in the
OpenCode family or the base_url is hosted on opencode.ai, derive api_mode
from the effective model and run the symmetric /v1 normalization — unless
the user declared an explicit transport, which stays authoritative.
5 new regression tests against a real temp HERMES_HOME config.
Teknium