diff --git a/hermes_cli/runtime_provider.py b/hermes_cli/runtime_provider.py
index a0ef239493..9588dc08a3 100644
--- a/hermes_cli/runtime_provider.py
+++ b/hermes_cli/runtime_provider.py
@@ -1098,6 +1098,7 @@ def _resolve_named_custom_runtime(
     requested_provider: str,
     explicit_api_key: Optional[str] = None,
     explicit_base_url: Optional[str] = None,
+    target_model: Optional[str] = None,
 ) -> Optional[Dict[str, Any]]:
     # Bare `provider="custom"` with an explicit base_url (e.g. propagated
     # from a `model_aliases:` direct-alias resolution) — build a runtime
@@ -1238,6 +1239,43 @@ def _resolve_named_custom_runtime(
     request_overrides = _custom_provider_request_overrides(custom_provider)
     if request_overrides:
         result["request_overrides"] = request_overrides
+
+    # Custom providers in the OpenCode family (name extends opencode-go/zen,
+    # or base_url hosted on opencode.ai) serve models behind different API
+    # surfaces per model — a static api_mode 503s for /v1/responses-only
+    # models like grok-4.5 (#85589). Re-derive api_mode from the effective
+    # model and normalize the /v1 suffix, exactly like the built-in
+    # opencode-zen/go paths do.
+    from hermes_cli.models import opencode_provider_family
+
+    _oc_family = opencode_provider_family(requested_provider)
+    if _oc_family is None:
+        try:
+            from utils import base_url_hostname
+
+            if base_url_hostname(base_url).lower() == "opencode.ai":
+                _oc_family = (
+                    "opencode-go" if "/zen/go" in base_url.lower() else "opencode-zen"
+                )
+        except Exception:
+            _oc_family = None
+    if _oc_family is not None and not custom_provider.get("api_mode"):
+        from hermes_cli.models import (
+            normalize_opencode_base_url,
+            opencode_model_api_mode,
+        )
+
+        _effective_model = str(
+            target_model
+            or custom_provider.get("model")
+            or _get_model_config().get("default")
+            or ""
+        ).strip()
+        if _effective_model:
+            result["api_mode"] = opencode_model_api_mode(_oc_family, _effective_model)
+        result["base_url"] = normalize_opencode_base_url(
+            _oc_family, result["api_mode"], result["base_url"]
+        )
     return result
 
 
@@ -1847,6 +1885,7 @@ def resolve_runtime_provider(
         requested_provider=requested_provider,
         explicit_api_key=explicit_api_key,
         explicit_base_url=explicit_base_url,
+        target_model=target_model,
     )
     if custom_runtime:
         custom_runtime["requested_provider"] = requested_provider
diff --git a/tests/hermes_cli/test_custom_opencode_family_runtime.py b/tests/hermes_cli/test_custom_opencode_family_runtime.py
new file mode 100644
index 0000000000..74a90ba4d3
--- /dev/null
+++ b/tests/hermes_cli/test_custom_opencode_family_runtime.py
@@ -0,0 +1,79 @@
+"""Custom OpenCode-family provider runtime resolution (#85589).
+
+A user-defined provider whose name extends an OpenCode family slug
+(``opencode-go-bridge``) — or whose base_url is hosted on opencode.ai —
+must get the same per-model api_mode derivation and /v1 base-url
+normalization as the built-in ``opencode-go`` / ``opencode-zen``
+providers. Before the fix, such providers resolved with a static
+api_mode (chat_completions), so /v1/responses-only models like
+``grok-4.5`` failed with HTTP 503 "Endpoint unavailable".
+"""
+
+import os
+import tempfile
+
+import pytest
+
+
+@pytest.fixture()
+def _bridge_env(monkeypatch, tmp_path):
+    hermes_home = tmp_path / ".hermes"
+    hermes_home.mkdir()
+    monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+    monkeypatch.setenv("OPENCODE_GO_BRIDGE_API_KEY", "sk-test-bridge")
+
+    def write_config(provider_name: str, extra: str = "", default: str = "grok-4.5"):
+        (hermes_home / "config.yaml").write_text(
+            f"""
+model:
+  provider: {provider_name}
+  default: {default}
+providers:
+  {provider_name}:
+    base_url: https://opencode.ai/zen/go/v1
+    key_env: OPENCODE_GO_BRIDGE_API_KEY
+{extra}"""
+        )
+
+    return write_config
+
+
+def _resolve(provider, model):
+    from hermes_cli.runtime_provider import resolve_runtime_provider
+
+    return resolve_runtime_provider(requested=provider, target_model=model)
+
+
+class TestCustomOpencodeFamilyRuntime:
+    def test_family_provider_routes_grok_to_responses(self, _bridge_env):
+        """The #85589 repro: grok-4.5 on opencode-go-bridge → codex_responses."""
+        _bridge_env("opencode-go-bridge")
+        r = _resolve("opencode-go-bridge", "grok-4.5")
+        assert r["api_mode"] == "codex_responses"
+        assert r["base_url"].rstrip("/").endswith("/zen/go/v1")
+
+    def test_family_provider_routes_minimax_to_messages_and_strips_v1(self, _bridge_env):
+        _bridge_env("opencode-go-bridge")
+        r = _resolve("opencode-go-bridge", "minimax-m2.7")
+        assert r["api_mode"] == "anthropic_messages"
+        # /v1 stripped so the Anthropic SDK's own /v1/messages doesn't double up
+        assert r["base_url"].rstrip("/").endswith("/zen/go")
+
+    def test_family_provider_keeps_chat_models_on_v1(self, _bridge_env):
+        _bridge_env("opencode-go-bridge")
+        r = _resolve("opencode-go-bridge", "deepseek-v4-flash")
+        assert r["api_mode"] == "chat_completions"
+        assert r["base_url"].rstrip("/").endswith("/zen/go/v1")
+
+    def test_explicit_transport_in_config_wins(self, _bridge_env):
+        """A user-declared transport is authoritative — no family override."""
+        _bridge_env("opencode-go-bridge", extra="    transport: chat_completions\n")
+        r = _resolve("opencode-go-bridge", "grok-4.5")
+        assert r["api_mode"] == "chat_completions"
+
+    def test_arbitrary_name_on_opencode_host_derives_family(self, _bridge_env):
+        """A provider with a non-family name pointing at opencode.ai still
+        gets per-model routing (host match)."""
+        _bridge_env("my-oc-proxy", default="gpt-5.6-luna")
+        r = _resolve("my-oc-proxy", "gpt-5.6-luna")
+        assert r["api_mode"] == "codex_responses"