Studio Images: clearer error for an unsupported diffusion model

When a repo id resolves to no diffusion family the load raised 'Could not infer a
diffusion family... Pass family_override (z-image)', which points at an unrelated
family and doesn't say what is supported. Replace it with a message that lists the
supported families (from a new supported_family_names helper) and notes that video
models and image models whose diffusers transformer has no single-file loader are
not supported. Applies to both the diffusers and native sd.cpp load paths. Also
refreshes two stale family-registry comments that still called FLUX.2-dev omitted.

studio/backend/core/inference/diffusion.py

@@ -33,6 +33,7 @@ from .diffusion_families import (
     detect_family,
     resolve_base_repo,
     resolve_local_gguf_child,
+    supported_family_names,
 )
 from .diffusion_device import (
     DiffusionDeviceTarget,
@@ -399,7 +400,10 @@ class DiffusionBackend:
         fam = detect_family(repo_id, family_override)
         if fam is None:
             raise ValueError(
-                f"Could not infer a diffusion family for '{repo_id}'. Pass family_override (z-image)."
+                f"'{repo_id}' is not a supported diffusion image model. Supported families: "
+                f"{', '.join(supported_family_names())}. If this is a variant of one of them, "
+                f"pass family_override with that family name. (Video models and image models "
+                f"whose diffusers transformer has no single-file loader are not supported.)"
             )
         # Non-GGUF loads (a single-file safetensors transformer, or a full pipeline)
         # are gated to the unsloth org or a local path -- they fetch + deserialise

studio/backend/core/inference/diffusion_families.py

@@ -95,9 +95,9 @@ class DiffusionFamily:
 # Keyed by architecture, not per model variant: a checkpoint's specific base repo
 # is read from its HF base_model tag at load time, so one entry covers Turbo/full,
 # schnell/dev, etc. base_repo here is only a fallback. Only archs whose diffusers
-# transformer supports from_single_file load here (ERNIE-Image does not, yet).
-# FLUX.2-dev and FLUX.2-klein-9B are left out only because their base diffusers
-# repos are gated; the open klein-4B base stands in for the klein family below.
+# transformer supports from_single_file load here (ERNIE-Image does not, yet; LTX
+# video models are out of scope). FLUX.2-klein-9B shares the klein family (its base
+# repo is resolved per-variant), and FLUX.2-dev has its own family below.
 _FAMILIES: tuple[DiffusionFamily, ...] = (
     DiffusionFamily(
         name = "flux.1",
@@ -114,8 +114,8 @@ _FAMILIES: tuple[DiffusionFamily, ...] = (
         ),
     ),
     # FLUX.2-klein is a distinct pipeline (Flux2KleinPipeline) with a Qwen3 text
-    # encoder, not the Mistral-based Flux2Pipeline; it must precede a generic
-    # flux match. The base Flux2Pipeline (FLUX.2-dev) is gated, so it's omitted.
+    # encoder, not the Mistral-based Flux2Pipeline; it must precede a generic flux
+    # match. The Mistral-based Flux2Pipeline is the separate flux.2-dev family below.
     DiffusionFamily(
         name = "flux.2-klein",
         pipeline_class = "Flux2KleinPipeline",
@@ -284,6 +284,12 @@ def detect_family(repo_id: str, override: Optional[str] = None) -> Optional[Diff
     return None
 
 
+def supported_family_names() -> tuple[str, ...]:
+    """Family names accepted as ``family_override`` and shown in the unknown-model
+    error. Kept in registry order so the message lists what the backend can load."""
+    return tuple(fam.name for fam in _FAMILIES)
+
+
 def resolve_base_repo(fam: DiffusionFamily, base_repo: Optional[str]) -> str:
     """The companion diffusers repo: caller-supplied if given, else the family fallback."""
     base = (base_repo or "").strip()

studio/backend/core/inference/sd_cpp_backend.py

@@ -40,6 +40,7 @@ from core.inference.diffusion_families import (
     family_sd_cpp_supported,
     resolve_base_repo,
     resolve_local_gguf_child,
+    supported_family_names,
 )
 from core.inference.diffusion_memory import (
     OFFLOAD_GROUP,
@@ -247,7 +248,11 @@ class SdCppDiffusionBackend:
             )
         fam = detect_family(repo_id, family_override)
         if fam is None:
-            raise ValueError(f"Could not infer a diffusion family for '{repo_id}'.")
+            raise ValueError(
+                f"'{repo_id}' is not a supported diffusion image model. Supported families: "
+                f"{', '.join(supported_family_names())}. If this is a variant of one of them, "
+                f"pass family_override with that family name."
+            )
         if not family_sd_cpp_supported(fam):
             raise ValueError(f"Family '{fam.name}' has no native sd.cpp asset mapping.")
 

studio/backend/tests/test_diffusion_backend.py

@@ -26,6 +26,7 @@ from core.inference.diffusion_families import (
     detect_family,
     resolve_base_repo,
     resolve_local_gguf_child,
+    supported_family_names,
 )
 
 
@@ -82,6 +83,16 @@ def test_detect_family_override():
     assert detect_family("local/path", override = "not-a-family") is None
 
 
+def test_supported_family_names():
+    names = supported_family_names()
+    # The unknown-model error lists these, so the key families must be present.
+    for expected in ("flux.1", "flux.2-klein", "flux.2-dev", "qwen-image", "z-image"):
+        assert expected in names
+    # Every listed name is a valid family_override (round-trips through detect_family).
+    for name in names:
+        assert detect_family("some/unknown-repo", override = name) is not None
+
+
 def test_resolve_base_repo():
     fam = detect_family("x", override = "z-image")
     assert resolve_base_repo(fam, None) == fam.base_repo