diff --git a/tests/hermes_cli/test_runtime_provider_resolution.py b/tests/hermes_cli/test_runtime_provider_resolution.py
index bf2ea27cd..f251a3f24 100644
--- a/tests/hermes_cli/test_runtime_provider_resolution.py
+++ b/tests/hermes_cli/test_runtime_provider_resolution.py
@@ -1170,7 +1170,18 @@ def test_opencode_go_glm_defaults_to_chat_completions(monkeypatch):
     assert resolved["base_url"] == "https://opencode.ai/zen/go/v1"
 
 
-def test_opencode_go_configured_api_mode_still_overrides_default(monkeypatch):
+def test_opencode_go_model_derivation_beats_stale_persisted_api_mode(monkeypatch):
+    """opencode-zen/go re-derive api_mode from the effective model on every
+    resolve, ignoring any persisted ``api_mode`` in config. Refs #16878 /
+    PR #16888: the persisted mode from the previous default model must not
+    leak across /model switches (a stale ``anthropic_messages`` on a
+    chat_completions target would strip /v1 from base_url and 404).
+
+    minimax-m2.5 is an Anthropic-routed model on opencode-go, so even when
+    the config claims ``api_mode: chat_completions`` the runtime must pick
+    ``anthropic_messages`` — the model dictates the mode, not the stale
+    persisted setting.
+    """
     monkeypatch.setattr(rp, "resolve_provider", lambda *a, **k: "opencode-go")
     monkeypatch.setattr(
         rp,
@@ -1187,7 +1198,7 @@ def test_opencode_go_configured_api_mode_still_overrides_default(monkeypatch):
     resolved = rp.resolve_runtime_provider(requested="opencode-go")
 
     assert resolved["provider"] == "opencode-go"
-    assert resolved["api_mode"] == "chat_completions"
+    assert resolved["api_mode"] == "anthropic_messages"
 
 
 def test_named_custom_provider_anthropic_api_mode(monkeypatch):