Studio: surface the real reason a model fails validation (#6398)

* Studio: surface the real reason a model fails validation

validate_model caught every error and returned a blank "Invalid model" while
logging the actual cause. Selecting a GGUF model without a built llama-server,
for example, raises a deliberately actionable RuntimeError ("llama-server binary
not found - cannot load GGUF models. Run setup.sh ...") that the user never saw,
leaving them with an unexplained "Invalid model".

Surface RuntimeError and ValueError messages (path-redacted, and wrapped with the
existing "not supported yet" hint where it applies) in the 400 detail, matching
what the native-path branch already does. Any other exception type stays generic
so an unexpected internal error never leaks its details to the client.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Test: accept positional args in the from_identifier mock

Make the mock robust to a future from_identifier signature that passes
positional arguments, per review feedback.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

studio/backend/routes/inference.py

@@ -2578,6 +2578,20 @@ async def validate_model(
             f"Error validating model identifier '{request.model_path}': {e}",
             exc_info = True,
         )
+        # RuntimeError / ValueError carry intentional, actionable messages here
+        # (e.g. "llama-server binary not found - cannot load GGUF models. Run
+        # setup.sh ..."), so surface them instead of a blank "Invalid model".
+        # Path-redact for safety and keep any other exception type generic so an
+        # unexpected internal error never leaks its details to the client.
+        if isinstance(e, (RuntimeError, ValueError)):
+            msg = redact_native_paths(str(e)).strip()
+            if msg:
+                if any(h.lower() in msg.lower() for h in not_supported_hints):
+                    msg = f"This model is not supported yet. Try a different model. (Original error: {msg})"
+                raise HTTPException(
+                    status_code = 400,
+                    detail = msg,
+                )
         raise HTTPException(
             status_code = 400,
             detail = "Invalid model",

studio/backend/tests/test_validate_model_error.py

@@ -0,0 +1,86 @@
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0
+
+"""routes/inference.py::validate_model surfaces actionable RuntimeError/ValueError
+messages (e.g. "llama-server binary not found - run setup.sh") instead of a blank
+"Invalid model", while keeping unexpected exceptions generic so internals never
+leak to the client.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import sys
+from pathlib import Path
+
+import pytest
+
+_BACKEND = Path(__file__).resolve().parents[1]
+if str(_BACKEND) not in sys.path:
+    sys.path.insert(0, str(_BACKEND))
+
+pytest.importorskip("fastapi")
+
+from fastapi import HTTPException  # noqa: E402
+
+import routes.inference as inf  # noqa: E402
+from models.inference import ValidateModelRequest  # noqa: E402
+
+
+def _provoke(
+    monkeypatch,
+    exc: BaseException,
+    *,
+    native: bool = False,
+) -> HTTPException:
+    """Drive validate_model so from_identifier raises ``exc``; return the
+    HTTPException it converts that into."""
+    monkeypatch.setattr(
+        inf,
+        "_resolve_model_identifier_for_request",
+        lambda request, operation: ("org/repo", "org/repo", native),
+    )
+
+    def _raise(*_args, **_kwargs):
+        raise exc
+
+    monkeypatch.setattr(inf.ModelConfig, "from_identifier", staticmethod(_raise))
+
+    req = ValidateModelRequest(model_path = "org/repo")
+    with pytest.raises(HTTPException) as excinfo:
+        asyncio.run(inf.validate_model(req, current_subject = "tester"))
+    return excinfo.value
+
+
+def test_runtime_error_surfaces_actionable_message(monkeypatch):
+    err = RuntimeError(
+        "llama-server binary not found - cannot load GGUF models. "
+        "Run setup.sh to build it, or set LLAMA_SERVER_PATH."
+    )
+    http = _provoke(monkeypatch, err)
+    assert http.status_code == 400
+    assert "llama-server binary not found" in http.detail
+    assert http.detail != "Invalid model"
+
+
+def test_value_error_not_supported_is_wrapped(monkeypatch):
+    http = _provoke(monkeypatch, ValueError("architecture FooBar is not supported"))
+    assert http.status_code == 400
+    assert "not supported yet" in http.detail.lower()
+    # Original cause is preserved for context.
+    assert "FooBar" in http.detail
+
+
+def test_unexpected_exception_stays_generic(monkeypatch):
+    # A non-user-facing exception type must NOT have its message surfaced.
+    http = _provoke(monkeypatch, KeyError("secret-internal-detail"))
+    assert http.status_code == 400
+    assert http.detail == "Invalid model"
+    assert "secret-internal-detail" not in http.detail
+
+
+def test_empty_runtime_error_falls_back_to_generic(monkeypatch):
+    # A RuntimeError with no message should not produce an empty 400 detail.
+    http = _provoke(monkeypatch, RuntimeError(""))
+    assert http.status_code == 400
+    assert http.detail == "Invalid model"