diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index 5b136bd820e6..d720111509e6 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -1472,6 +1472,14 @@ def _normalize_main_model_assignment(provider: str, model: str) -> tuple[str, st
        known but ``poolside`` isn't) but the model is a vendor-prefixed
        aggregator slug, keep the user's CURRENT aggregator if they're on
        one, else fall back to openrouter.
+
+       Named custom providers (``custom:litellm``, etc.) are excluded from
+       this fallback: ``_KNOWN_PROVIDER_NAMES`` only lists the bare
+       ``"custom"`` bucket, never a specific ``custom:<name>`` slug, so
+       without this exclusion every named custom provider paired with a
+       slash-bearing model (e.g. ``ollama/glm-5.2`` behind a LiteLLM proxy)
+       looked exactly like the stray-vendor-prefix case above and got
+       silently reassigned to ``openrouter``.
     2. Model-format normalization for the resolved provider via
        ``normalize_model_for_provider`` (e.g. ``anthropic/claude-opus-4.6``
        on native anthropic → ``claude-opus-4-6``).
@@ -1506,7 +1514,16 @@ def _normalize_main_model_assignment(provider: str, model: str) -> tuple[str, st
     if custom_provider is not None:
         return custom_provider.id, model_in
 
-    if canonical not in _KNOWN_PROVIDER_NAMES and "/" in model_in:
+    # A named custom provider that didn't resolve above (typo, config
+    # 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.
+    if (
+        canonical not in _KNOWN_PROVIDER_NAMES
+        and not canonical.startswith("custom")
+        and "/" in model_in
+    ):
         # Vendor prefix posing as a provider (analytics fallback). Resolve
         # against the user's current provider when it's an aggregator that
         # serves vendor-prefixed slugs; otherwise default to openrouter.
diff --git a/tests/hermes_cli/test_normalize_main_model_assignment.py b/tests/hermes_cli/test_normalize_main_model_assignment.py
new file mode 100644
index 000000000000..830875ca9863
--- /dev/null
+++ b/tests/hermes_cli/test_normalize_main_model_assignment.py
@@ -0,0 +1,43 @@
+"""Regression tests for ``_normalize_main_model_assignment`` (POST /api/model/set).
+
+Named custom providers are represented as ``custom:<name>`` slugs everywhere
+else in the codebase (``runtime_provider.py``, ``model_switch.py``), but
+``_KNOWN_PROVIDER_NAMES`` only lists the bare ``"custom"`` bucket. Before this
+fix, persisting a main-slot assignment for a named custom provider (e.g. a
+LiteLLM proxy fronting Ollama, registered as ``custom:litellm``) together with
+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.
+"""
+
+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 test_bare_custom_bucket_is_preserved(self):
+        assert _normalize_main_model_assignment("custom", "ollama/glm-5.2") == (
+            "custom",
+            "ollama/glm-5.2",
+        )
+
+
+class TestStrayVendorPrefixFallbackStillWorks:
+    """The original bug this function fixes: an analytics row with no
+    ``billing_provider`` falls back to the model's vendor prefix as the
+    "provider" (e.g. ``provider="anthropic"`` from
+    ``modelVendor("anthropic/claude-opus-4.6")``). That must still resolve
+    to the native provider with its model normalized -- unaffected by the
+    ``custom:`` exclusion above.
+    """
+
+    def test_known_native_provider_still_normalizes_model(self):
+        assert _normalize_main_model_assignment(
+            "anthropic", "anthropic/claude-opus-4.6"
+        ) == ("anthropic", "claude-opus-4-6")