fix(downloads): route unknown checkpoint baseModels to diffusion models by default

CivitAI labels new DiT architectures (MiniMax H3, future Flux/Wan/Qwen
variants) as model.type "Checkpoint" with plain "Model" file entries,
so the DIFFUSION_MODEL_BASE_MODELS allowlist could never keep up and
such downloads were mis-routed to the checkpoint roots (e.g. model
2877206 / version 3374439). The set of true full-checkpoint families is
closed, so the baseModel fallback is inverted:

1. file type UNet/Diffusion Model -> unet (unchanged)
2. baseModel in DIFFUSION_MODEL_BASE_MODELS (now incl. MiniMax H3) -> unet
3. baseModel in new CHECKPOINT_BASE_MODELS (SD 1.x/2.x/3.x, SDXL, Pony,
   Illustrious, NoobAI) -> checkpoint
4. unknown/empty baseModel -> new unknown_base_model_routing setting,
   defaulting to diffusion models

The setting is exposed under Settings > Downloads, validated in
SettingsManager, and threaded into both the download manager and the
download routing endpoint so they keep agreeing.

tests/routes/test_download_routing_handlers.py

@@ -73,6 +73,42 @@ async def test_regular_checkpoint_stays_on_checkpoint_root():
     assert payload["root_kind"] == "checkpoint"
 
 
+@pytest.mark.asyncio
+async def test_unknown_base_model_routes_to_unet_by_default():
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest(
+            {
+                "model_type": "checkpoint",
+                "base_model": "Brand New Arch",
+                "file_types": ["Model"],
+            }
+        )
+    )
+    payload = json.loads(response.text)
+    assert payload["is_diffusion_model"] is True
+    assert payload["root_kind"] == "unet"
+
+
+@pytest.mark.asyncio
+async def test_unknown_base_model_honors_checkpoint_routing_setting():
+    get_settings_manager().settings["unknown_base_model_routing"] = "checkpoint"
+
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest(
+            {
+                "model_type": "checkpoint",
+                "base_model": "Brand New Arch",
+                "file_types": ["Model"],
+            }
+        )
+    )
+    payload = json.loads(response.text)
+    assert payload["is_diffusion_model"] is False
+    assert payload["root_kind"] == "checkpoint"
+
+
 @pytest.mark.asyncio
 async def test_lora_is_never_diffusion():
     handler = DownloadRoutingHandler()

tests/services/test_download_manager_basic.py

@@ -2209,7 +2209,7 @@ async def test_download_falls_back_to_rest_name_when_mini_fails(
     metadata_provider.payload = {
         "id": 42,
         "model": {"type": "Checkpoint", "tags": ["fantasy"]},
-        "baseModel": "BaseModel",
+        "baseModel": "SDXL 1.0",
         "creator": {"username": "Author"},
         "files": [
             {

tests/services/test_download_routing.py

@@ -20,9 +20,47 @@ def test_base_model_fallback_routes_to_unet():
     )
 
 
-def test_regular_checkpoint_stays_on_checkpoint_roots():
+def test_minimax_h3_routes_to_unet():
+    """CivitAI model 2877206: type "Checkpoint", baseModel "MiniMax H3",
+    all file entries typed "Model"."""
+    assert is_diffusion_model_download(
+        "checkpoint", file_types=["Model"], base_model="MiniMax H3"
+    )
+
+
+@pytest.mark.parametrize("base_model", ["SDXL 1.0", "Illustrious", "SD 1.5", "SD 3.5 Large"])
+def test_known_checkpoint_base_models_stay_on_checkpoint_roots(base_model):
+    """CHECKPOINT_BASE_MODELS members never route to unet, even when the
+    unknown-base-model default is diffusion."""
     assert not is_diffusion_model_download(
-        "checkpoint", file_types=["Model"], base_model="SDXL 1.0"
+        "checkpoint", file_types=["Model"], base_model=base_model
+    )
+
+
+def test_unknown_base_model_defaults_to_unet():
+    """New DiT families appear faster than the allowlist can track them, so
+    unknown baseModels route to unet by default."""
+    assert is_diffusion_model_download(
+        "checkpoint", file_types=["Model"], base_model="Brand New Arch"
+    )
+
+
+def test_unknown_base_model_honors_checkpoint_setting():
+    assert not is_diffusion_model_download(
+        "checkpoint",
+        file_types=["Model"],
+        base_model="Brand New Arch",
+        unknown_base_model_default="checkpoint",
+    )
+
+
+def test_unknown_base_model_checkpoint_setting_still_loses_to_file_type():
+    """The file-type signal stays first regardless of the setting."""
+    assert is_diffusion_model_download(
+        "checkpoint",
+        file_types=["Diffusion Model"],
+        base_model="SDXL 1.0",
+        unknown_base_model_default="checkpoint",
     )
 
 
@@ -35,9 +73,15 @@ def test_non_checkpoint_types_never_route_to_unet():
     )
 
 
-def test_empty_inputs_stay_on_checkpoint_roots():
-    assert not is_diffusion_model_download("checkpoint")
-    assert not is_diffusion_model_download("checkpoint", file_types=[], base_model="")
+def test_empty_base_model_follows_unknown_default():
+    assert is_diffusion_model_download("checkpoint")
+    assert is_diffusion_model_download("checkpoint", file_types=[], base_model="")
+    assert not is_diffusion_model_download(
+        "checkpoint",
+        file_types=[],
+        base_model="",
+        unknown_base_model_default="checkpoint",
+    )
 
 
 from py.services.download_routing import resolve_other_download_sub_type

tests/services/test_settings_manager.py

@@ -1300,6 +1300,21 @@ def test_setting_download_skip_base_models_normalizes_string_input(manager):
     assert manager.get("download_skip_base_models") == ["SDXL 1.0", "Pony"]
 
 
+def test_unknown_base_model_routing_defaults_to_diffusion_model(manager):
+    assert manager.get("unknown_base_model_routing") == "diffusion_model"
+
+
+def test_unknown_base_model_routing_is_normalized(manager):
+    manager.set("unknown_base_model_routing", "CHECKPOINT")
+    assert manager.get("unknown_base_model_routing") == "checkpoint"
+
+    manager.set("unknown_base_model_routing", "bogus")
+    assert manager.get("unknown_base_model_routing") == "diffusion_model"
+
+    manager.set("unknown_base_model_routing", None)
+    assert manager.get("unknown_base_model_routing") == "diffusion_model"
+
+
 def test_skip_previously_downloaded_model_versions_defaults_false(manager):
     assert manager.get_skip_previously_downloaded_model_versions() is False