diff --git a/hermes_cli/models.py b/hermes_cli/models.py
index d00b63c74f..8f9099987f 100644
--- a/hermes_cli/models.py
+++ b/hermes_cli/models.py
@@ -2051,7 +2051,12 @@ def normalize_copilot_model_id(
             return candidate
 
     if "/" in raw:
-        return raw.split("/", 1)[1].strip()
+        stripped = raw.split("/", 1)[1].strip()
+        # Enterprise BYOK custom models expose ``owner/sub/model`` ids (two
+        # slashes). A strip guess that still contains "/" cannot be a Copilot
+        # id, so pass the input through untouched instead of corrupting it.
+        if stripped and "/" not in stripped:
+            return stripped
     return raw
 
 
diff --git a/tests/hermes_cli/test_copilot_custom_model_ids.py b/tests/hermes_cli/test_copilot_custom_model_ids.py
new file mode 100644
index 0000000000..20e83ba399
--- /dev/null
+++ b/tests/hermes_cli/test_copilot_custom_model_ids.py
@@ -0,0 +1,37 @@
+"""Regression for #110597: Copilot custom/enterprise (BYOK) model ids must survive normalization.
+
+GitHub's enterprise custom-model (BYOK) catalog exposes Copilot ids shaped ``owner/sub/model``
+— that is, TWO slashes. With no candidate matching the (unreachable) catalog,
+``normalize_copilot_model_id`` stripped the FIRST path segment and returned the result:
+``acme-github-copilot/GLM/glm-5.2`` became ``GLM/glm-5.2``, the CLI's second normalization pass
+stripped again to ``glm-5.2``, and the Copilot API answered HTTP 400 ``model_not_supported``.
+Every such call failed, for both custom models a tenant had configured.
+"""
+
+import pytest
+
+from hermes_cli.models import normalize_copilot_model_id
+
+
+# ``owner/sub/model`` — the shape an enterprise BYOK custom model has (tenant name sanitized).
+CUSTOM_MODEL_IDS = [
+    "acme-github-copilot/QDeepseekV4/deepseek-flash",
+    "acme-github-copilot/GLM/glm-5.2",
+    "acme-github-copilot/HUAWEI/glm-5.2",
+]
+
+
+@pytest.mark.parametrize("model_id", CUSTOM_MODEL_IDS)
+def test_two_slash_custom_model_id_is_passed_through(model_id):
+    """A two-slash id IS a Copilot id — normalize nothing, catalog or no catalog."""
+    # ``hermes_cli/model_normalize.py`` calls this without an api_key, so the catalog is always
+    # empty on the runtime path that produced the 400.
+    assert normalize_copilot_model_id(model_id, catalog=[], api_key=None) == model_id
+    assert normalize_copilot_model_id(
+        model_id, catalog=[{"id": model_id}], api_key=None) == model_id
+
+
+def test_single_vendor_prefix_still_folds_to_the_bare_id():
+    """Contract preserved for the one-slash case this stripping exists for (#6879)."""
+    assert normalize_copilot_model_id(
+        "anthropic/claude-sonnet-4.6", catalog=[], api_key=None) == "claude-sonnet-4.6"