diff --git a/agent/reasoning_promotion.py b/agent/reasoning_promotion.py
index 52a4def074..af6fe68a54 100644
--- a/agent/reasoning_promotion.py
+++ b/agent/reasoning_promotion.py
@@ -57,10 +57,17 @@ def answer_in_reasoning_capability(agent: Any) -> bool:
         return configured
     # The provider-level ``capabilities:`` block, re-read on the live route like the per-model key
     # so it holds on CLI/TUI (no constructor ``capabilities=``) and survives /model switches.
-    for entry in _entries_for_route(base_url, custom_providers, None):
-        configured = (entry.get("capabilities") or {}).get(_ANSWER_IN_REASONING_CAPABILITY)
-        if isinstance(configured, bool):
-            return configured
+    # Several entries may share one endpoint (LiteLLM/vLLM router): only the live ``custom:<slug>``
+    # entry counts; without one, every flagged sibling on the endpoint must opt in.
+    from hermes_cli.providers import custom_provider_slug
+
+    entries = list(_entries_for_route(base_url, custom_providers, None))
+    live = [e for e in entries
+            if custom_provider_slug(str(e.get("name") or ""), str(e.get("provider_key") or "")) == provider]
+    flags = {(e.get("capabilities") or {}).get(_ANSWER_IN_REASONING_CAPABILITY) for e in live or entries}
+    flags &= {True, False}
+    if flags:
+        return flags == {True}
 
     if _NEMOTRON_PARSER_MODEL_MARKER not in model.lower():
         return False
diff --git a/tests/agent/test_empty_terminal_reasoning_surface.py b/tests/agent/test_empty_terminal_reasoning_surface.py
index e39d9909c0..52532bdf67 100644
--- a/tests/agent/test_empty_terminal_reasoning_surface.py
+++ b/tests/agent/test_empty_terminal_reasoning_surface.py
@@ -27,11 +27,13 @@ sys.modules.setdefault("firecrawl", types.SimpleNamespace(Firecrawl=object))
 sys.modules.setdefault("fal_client", types.SimpleNamespace())
 
 
-# Provider-level ``capabilities:`` opt-ins: the fixture's own route and a second custom provider.
+# Provider-level ``capabilities:`` opt-ins: the fixture's own route and a second custom provider
+# (plus an un-opted sibling on that provider's endpoint).
 _OPTED_IN_PROVIDERS = [
     {"name": "local", "base_url": "https://example.invalid/v1", "capabilities": {"answer_in_reasoning": True}},
     {"name": "acme", "base_url": "https://llm.example.com/v1", "model": "acme/reasoner",
      "capabilities": {"answer_in_reasoning": True}},
+    {"name": "acme-plain", "base_url": "https://llm.example.com/v1"},  # same endpoint, no opt-in
 ]
 
 
@@ -188,6 +190,7 @@ def test_length_cut_reasoning_is_not_promoted(tmp_path, monkeypatch):
     ("custom", "https://llm.example.com/v1", "acme/reasoner", "private thoughts that must not be shown", 1),
     ("custom", "https://llm.example.com/v1", "acme/after-model-switch", "private thoughts that must not be shown", 1),
     ("custom", "https://fallback.example.com/v1", "acme/reasoner", "the visible answer", 2),
+    ("custom:acme-plain", "https://llm.example.com/v1", "acme/reasoner", "the visible answer", 2),
 ])
 def test_reasoning_promotion_requires_a_trusted_route(tmp_path, monkeypatch, provider, base_url, model, final, calls):
     """Private reasoning on an untrusted route retries to the visible answer and never