fix(models): preserve OpenRouter variant tags (:free, :extended, :fast) during model switch (#6383)

Step c in switch_model() blindly converted the first colon to a slash for
aggregator providers, even when the model name already contained a slash
(vendor/model format). This mangled variant tags like :free into /free,
causing 400 Bad Request from the API.

Fix: skip the colon→slash conversion when the model already has a slash,
since the colon is a variant tag, not a vendor separator. The module
docstring already documented this intent (line 17-18) but the
implementation didn't enforce it.

Reported via Discord. Related to PR #6088 (which identified the same bug
but placed the fix in model_normalize.py instead of model_switch.py where
the actual mangling occurs).

hermes_cli/model_switch.py

@@ -537,8 +537,11 @@ def switch_model(
                     )
             else:
                 # --- Step c: On aggregator, convert vendor:model to vendor/model ---
+                # Only convert when there's no slash — a slash means the name
+                # is already in vendor/model format and the colon is a variant
+                # tag (:free, :extended, :fast) that must be preserved.
                 colon_pos = raw_input.find(":")
-                if colon_pos > 0 and is_aggregator(current_provider):
+                if colon_pos > 0 and "/" not in raw_input and is_aggregator(current_provider):
                     left = raw_input[:colon_pos].strip().lower()
                     right = raw_input[colon_pos + 1:].strip()
                     if left and right:

tests/hermes_cli/test_model_switch_variant_tags.py

@@ -0,0 +1,70 @@
+"""Tests for OpenRouter variant tag preservation in model switching.
+
+Regression test for GitHub PR #6088 / Discord report: OpenRouter model IDs
+with variant suffixes like ``:free``, ``:extended``, ``:fast`` were being
+mangled by the colon-to-slash conversion in model_switch.py Step c.
+
+The fix: Step c now skips colon→slash conversion when the model name already
+contains a forward slash (i.e. is already in ``vendor/model`` format), since
+the colon is a variant tag, not a vendor separator.
+"""
+import pytest
+from unittest.mock import patch
+
+from hermes_cli.model_switch import switch_model
+
+
+# Shared mock context — skip network calls, credential resolution, catalog lookups
+_MOCK_VALIDATION = {"accepted": True, "persist": True, "recognized": True, "message": None}
+
+
+def _run_switch(raw_input: str, current_provider: str = "openrouter") -> str:
+    """Run switch_model with mocked dependencies, return the resolved model name."""
+    with patch("hermes_cli.model_switch.resolve_alias", return_value=None), \
+         patch("hermes_cli.model_switch.list_provider_models", return_value=[]), \
+         patch("hermes_cli.runtime_provider.resolve_runtime_provider",
+               return_value={"api_key": "test", "base_url": "", "api_mode": "chat_completions"}), \
+         patch("hermes_cli.models.validate_requested_model", return_value=_MOCK_VALIDATION), \
+         patch("hermes_cli.model_switch.get_model_info", return_value=None), \
+         patch("hermes_cli.model_switch.get_model_capabilities", return_value=None), \
+         patch("hermes_cli.models.detect_provider_for_model", return_value=None):
+        result = switch_model(
+            raw_input=raw_input,
+            current_provider=current_provider,
+            current_model="anthropic/claude-sonnet-4.6",
+        )
+        assert result.success, f"switch_model failed: {result.error_message}"
+        return result.new_model
+
+
+class TestVariantTagPreservation:
+    """OpenRouter variant tags (:free, :extended, :fast) must survive model switching."""
+
+    @pytest.mark.parametrize("model,expected", [
+        ("nvidia/nemotron-3-super-120b-a12b:free", "nvidia/nemotron-3-super-120b-a12b:free"),
+        ("anthropic/claude-sonnet-4.6:extended", "anthropic/claude-sonnet-4.6:extended"),
+        ("meta-llama/llama-4-maverick:fast", "meta-llama/llama-4-maverick:fast"),
+    ])
+    def test_slash_format_preserves_variant_tag(self, model, expected):
+        """Models already in vendor/model:tag format must not have their tag mangled."""
+        assert _run_switch(model) == expected
+
+    def test_legacy_colon_format_converts_to_slash(self):
+        """Legacy vendor:model (no slash) should still be converted to vendor/model."""
+        result = _run_switch("nvidia:nemotron-3-super-120b-a12b")
+        assert result == "nvidia/nemotron-3-super-120b-a12b"
+
+    def test_legacy_colon_format_with_tag_converts_first_colon_only(self):
+        """vendor:model:free (no slash) → vendor/model:free — first colon becomes slash."""
+        result = _run_switch("nvidia:nemotron-3-super-120b-a12b:free")
+        assert result == "nvidia/nemotron-3-super-120b-a12b:free"
+
+    def test_bare_model_name_unaffected(self):
+        """Bare model names without colons or slashes should work normally."""
+        result = _run_switch("claude-sonnet-4.6")
+        assert result == "anthropic/claude-sonnet-4.6"
+
+    def test_already_correct_slug_no_tag(self):
+        """Standard vendor/model slugs without tags pass through unchanged."""
+        result = _run_switch("anthropic/claude-sonnet-4.6")
+        assert result == "anthropic/claude-sonnet-4.6"