diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index d720111509e..4c0b105da04 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -1518,10 +1518,15 @@ def _normalize_main_model_assignment(provider: str, model: str) -> tuple[str, st
     # mismatch, entry missing from custom_providers/providers) must still
     # not be treated as a stray vendor prefix -- it isn't a known Hermes
     # provider/alias, but it also isn't the analytics-vendor case this
-    # fallback exists for.
+    # fallback exists for. Match only the durable named-custom syntax
+    # (bare "custom" bucket, or "custom:<name>" per
+    # ``providers.custom_provider_slug``) -- a bare ``startswith("custom")``
+    # would also swallow unrelated unconfigured vendor names that merely
+    # happen to start with "custom" (e.g. "customproxy").
+    is_custom_provider_slug = canonical == "custom" or canonical.startswith("custom:")
     if (
         canonical not in _KNOWN_PROVIDER_NAMES
-        and not canonical.startswith("custom")
+        and not is_custom_provider_slug
         and "/" in model_in
     ):
         # Vendor prefix posing as a provider (analytics fallback). Resolve
diff --git a/tests/hermes_cli/test_normalize_main_model_assignment.py b/tests/hermes_cli/test_normalize_main_model_assignment.py
index 830875ca986..2d1e6e99b70 100644
--- a/tests/hermes_cli/test_normalize_main_model_assignment.py
+++ b/tests/hermes_cli/test_normalize_main_model_assignment.py
@@ -9,23 +9,82 @@ a slash-bearing model id (``ollama/glm-5.2``) was indistinguishable from the
 "vendor prefix posing as a provider" analytics-fallback case, and got silently
 rewritten to ``provider: openrouter`` in ``config.yaml`` -- reassigning the
 provider entirely, not just mangling the model id.
+
+A properly *configured* ``custom:<name>`` provider is now resolved earlier,
+via ``resolve_custom_provider`` against ``custom_providers``/``providers`` in
+config -- that's the primary, intended path and isn't this module's concern
+to re-test. What's tested here is the fallback safety net for when that
+resolution comes up empty (typo, config drift, entry removed) -- a
+``custom:<name>`` slug must still not be misread as a stray analytics vendor
+prefix and reassigned to openrouter, even though it isn't in
+``_KNOWN_PROVIDER_NAMES`` either.
 """
 
+from unittest.mock import patch
+
 from hermes_cli.web_server import _normalize_main_model_assignment
 
 
-class TestNamedCustomProviderIsNotTreatedAsStrayVendorPrefix:
-    def test_named_custom_provider_slug_is_preserved(self):
-        assert _normalize_main_model_assignment("custom:litellm", "ollama/glm-5.2") == (
-            "custom:litellm",
-            "ollama/glm-5.2",
-        )
+def _no_custom_providers_configured():
+    """Patch load_config so resolve_user_provider/resolve_custom_provider
+    both come up empty, forcing execution into the fallback path under
+    test -- independent of whatever config.yaml happens to be on disk."""
+    return patch("hermes_cli.web_server.load_config", return_value={})
+
+
+class TestUnresolvedNamedCustomProviderIsNotTreatedAsStrayVendorPrefix:
+    """Covers the case where ``resolve_custom_provider`` finds no match --
+    e.g. ``custom:litellm`` was configured once, then the entry was renamed
+    or dropped from ``custom_providers``, but old sessions/config still
+    reference the old slug.
+    """
+
+    def test_unresolved_named_custom_provider_slug_is_preserved(self):
+        with _no_custom_providers_configured():
+            assert _normalize_main_model_assignment("custom:litellm", "ollama/glm-5.2") == (
+                "custom:litellm",
+                "ollama/glm-5.2",
+            )
 
     def test_bare_custom_bucket_is_preserved(self):
-        assert _normalize_main_model_assignment("custom", "ollama/glm-5.2") == (
-            "custom",
-            "ollama/glm-5.2",
-        )
+        with _no_custom_providers_configured():
+            assert _normalize_main_model_assignment("custom", "ollama/glm-5.2") == (
+                "custom",
+                "ollama/glm-5.2",
+            )
+
+    def test_unconfigured_non_custom_vendor_name_still_falls_back(self):
+        """A name that merely starts with the substring "custom" but isn't
+        the durable ``custom:<name>`` syntax (no colon) is NOT exempted --
+        it's just another unknown vendor label and should still hit the
+        openrouter fallback like any other unrecognized provider string.
+        """
+        with _no_custom_providers_configured():
+            assert _normalize_main_model_assignment(
+                "customproxy", "anthropic/claude-opus-4.6"
+            ) == ("openrouter", "anthropic/claude-opus-4.6")
+
+
+class TestConfiguredNamedCustomProviderResolvesViaPrimaryPath:
+    """The primary, intended path: a ``custom:<name>`` slug that IS present
+    in ``custom_providers`` resolves through ``resolve_custom_provider``
+    before the fallback under test above is ever reached.
+    """
+
+    def test_configured_named_custom_provider_resolves(self):
+        cfg = {
+            "custom_providers": [
+                {
+                    "name": "litellm",
+                    "base_url": "http://localhost:4000/v1",
+                    "key_env": "LITELLM_API_KEY",
+                }
+            ]
+        }
+        with patch("hermes_cli.web_server.load_config", return_value=cfg):
+            assert _normalize_main_model_assignment(
+                "custom:litellm", "ollama/glm-5.2"
+            ) == ("custom:litellm", "ollama/glm-5.2")
 
 
 class TestStrayVendorPrefixFallbackStillWorks: