diff --git a/pyproject.toml b/pyproject.toml
index bee146c..b45848d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -32,7 +32,7 @@ classifiers = [
     "Topic :: Software Development :: User Interfaces",
     "Framework :: FastAPI",
 ]
-dependencies = ["qh", "ju", "i2", "dol", "meshed"]
+dependencies = ["qh", "ju>=0.1.33", "i2", "dol", "meshed"]
 
 [project.urls]
 Homepage = "https://github.com/i2mint/uf"
diff --git a/tests/test_specs.py b/tests/test_specs.py
index 110638d..adc4bb2 100644
--- a/tests/test_specs.py
+++ b/tests/test_specs.py
@@ -104,3 +104,112 @@ def test_get_func():
     func = store.get_func('sample_add')
     assert func == sample_add
     assert func(10, 20) == 30
+
+
+# --- ju integration (i2mint/uf#4) -------------------------------------------
+# These tests take the ju path with no fallback guard. If ju is not installed
+# they skip with a visible reason; if ju is installed but fails to import (e.g.
+# an undeclared dependency of ju), they fail with the real traceback.
+
+import importlib
+import importlib.abc
+import sys
+import warnings
+
+from uf.specs import JuNotInstalledWarning, _import_func_to_form_spec
+
+
+def _require_ju():
+    """Import ``ju.rjsf``; skip only if ``ju`` itself is absent."""
+    try:
+        return importlib.import_module('ju.rjsf')
+    except ModuleNotFoundError as e:
+        if e.name == 'ju':
+            pytest.skip(f'ju not installed ({e.name!r} missing)')
+        raise
+
+
+def test_ju_path_is_taken_without_fallback():
+    """The spec must come from ju, not from the basic fallback."""
+    _require_ju()
+    with warnings.catch_warnings():
+        warnings.simplefilter('error', JuNotInstalledWarning)
+        spec = FunctionSpecStore([sample_add])['sample_add']
+
+    schema = spec['schema']
+    assert schema['properties'] == {
+        'x': {'type': 'integer'},
+        'y': {'type': 'integer'},
+    }
+    # The basic fallback never emits a uiSchema; ju always does.
+    assert spec['uiSchema'], 'empty uiSchema: the basic fallback ran, not ju'
+
+
+def test_ju_required_fields():
+    """Params without defaults are required; defaulted ones are not (ju>=0.1.33)."""
+    _require_ju()
+    schema = FunctionSpecStore([sample_greet])['sample_greet']['schema']
+    assert schema['required'] == ['name']
+    assert schema['properties']['greeting'].get('default') == 'Hello'
+
+
+class _JuAbsentFinder(importlib.abc.MetaPathFinder):
+    """Make ``ju`` look uninstalled, exactly as a real absence does."""
+
+    def find_spec(self, fullname, path=None, target=None):
+        if fullname == 'ju':
+            raise ModuleNotFoundError("No module named 'ju'", name='ju')
+        return None
+
+
+def _block_ju(monkeypatch):
+    for name in [m for m in sys.modules if m == 'ju' or m.startswith('ju.')]:
+        monkeypatch.delitem(sys.modules, name)
+    monkeypatch.setattr(sys, 'meta_path', [_JuAbsentFinder(), *sys.meta_path])
+
+
+def test_missing_ju_falls_back_with_visible_warning(monkeypatch):
+    _block_ju(monkeypatch)
+    with pytest.warns(JuNotInstalledWarning, match='not installed'):
+        spec = FunctionSpecStore([sample_greet])['sample_greet']
+    assert spec['schema']['required'] == ['name']
+    assert spec['uiSchema'] == {}
+
+
+class _BrokenJuRjsfFinder(importlib.abc.MetaPathFinder):
+    """Simulate ju being installed but one of its own dependencies missing."""
+
+    def find_spec(self, fullname, path=None, target=None):
+        if fullname == 'ju.rjsf':
+            raise ModuleNotFoundError("No module named 'dill'", name='dill')
+        return None
+
+
+def test_broken_ju_dependency_raises_instead_of_falling_back(monkeypatch):
+    _require_ju()
+    monkeypatch.delitem(sys.modules, 'ju.rjsf', raising=False)
+    monkeypatch.setattr(sys, 'meta_path', [_BrokenJuRjsfFinder(), *sys.meta_path])
+    monkeypatch.delattr(sys.modules['ju'], 'rjsf', raising=False)
+    with pytest.raises(ModuleNotFoundError) as excinfo:
+        _import_func_to_form_spec()
+    assert excinfo.value.name == 'dill'
+
+
+def test_missing_ju_rjsf_submodule_raises(monkeypatch):
+    """ju present but ju.rjsf absent (too-old ju) is a broken install, not 'absent'."""
+    _require_ju()
+    monkeypatch.setitem(sys.modules, 'ju.rjsf', None)
+    monkeypatch.delattr(sys.modules['ju'], 'rjsf', raising=False)
+    with pytest.raises(ModuleNotFoundError) as excinfo:
+        _import_func_to_form_spec()
+    assert excinfo.value.name == 'ju.rjsf'
+
+
+def test_other_import_errors_propagate(monkeypatch):
+    """An ImportError that is not 'ju is absent' (e.g. a renamed symbol) raises."""
+    _require_ju()
+    fake = type(sys)('ju.rjsf')  # module without func_to_form_spec
+    monkeypatch.setitem(sys.modules, 'ju.rjsf', fake)
+    monkeypatch.setattr(sys.modules['ju'], 'rjsf', fake, raising=False)
+    with pytest.raises(ImportError, match='func_to_form_spec'):
+        _import_func_to_form_spec()
diff --git a/uf/specs.py b/uf/specs.py
index cc9ba97..1d645c5 100644
--- a/uf/specs.py
+++ b/uf/specs.py
@@ -4,10 +4,46 @@
 form specs and OpenAPI schemas.
 """
 
+import warnings
 from typing import Callable, Iterable, Optional, Any
 from collections.abc import Mapping
 from functools import cached_property
 
+# Only the absence of top-level ``ju`` means "not installed" (fall back). A missing
+# ``ju.rjsf`` (too-old ju, stray namespace dir) or a missing dependency *of* ju is
+# a broken install: let it raise.
+_JU_MODULE_NAMES = ("ju",)
+
+
+class JuNotInstalledWarning(UserWarning):
+    """Emitted when ``ju`` is absent and uf falls back to basic form specs."""
+
+
+def _import_func_to_form_spec() -> Optional[Callable]:
+    """Return ``ju.rjsf.func_to_form_spec``, or ``None`` if ``ju`` is not installed.
+
+    Only the absence of ``ju`` itself is tolerated (with a visible warning).
+    Any other import failure -- a missing dependency of ``ju``, a renamed
+    symbol -- is a broken environment and propagates, so it fails loudly
+    instead of silently degrading every generated form.
+
+    >>> _import_func_to_form_spec() is None or callable(_import_func_to_form_spec())
+    True
+    """
+    try:
+        from ju.rjsf import func_to_form_spec
+    except ModuleNotFoundError as e:
+        if e.name not in _JU_MODULE_NAMES:
+            raise
+        warnings.warn(
+            f"{e.name!r} is not installed; uf is falling back to basic form "
+            "specs (no ju/RJSF features). Install it with `pip install ju`.",
+            JuNotInstalledWarning,
+            stacklevel=4,  # caller -> __getitem__ -> _generate_spec -> here
+        )
+        return None
+    return func_to_form_spec
+
 
 class FunctionSpecStore(Mapping):
     """A mapping from function names to their RJSF specifications.
@@ -88,39 +124,34 @@ def _generate_spec(self, func_name: str) -> dict:
         """
         func = self._funcs[func_name]
 
-        try:
-            # Import ju.rjsf for form spec generation
-            from ju.rjsf import func_to_form_spec
-
-            # Generate form spec
-            # ju nests the spec under a "rjsf" field by default; uf wants it flat
-            rjsf_config = {"nest_under_field": None, **self._rjsf_config}
-            form_spec = func_to_form_spec(func, **rjsf_config)
-
-            # Apply custom UI schema factory if provided
-            if self._ui_schema_factory:
-                ui_schema = self._ui_schema_factory(func)
-                if "uiSchema" in form_spec:
-                    form_spec["uiSchema"].update(ui_schema)
-                else:
-                    form_spec["uiSchema"] = ui_schema
-
-            return {
-                "schema": form_spec.get("schema", {}),
-                "uiSchema": form_spec.get("uiSchema", {}),
-                "func": func,
-                "name": func_name,
-                "description": func.__doc__ or f"Execute {func_name}",
-            }
-
-        except ImportError:
-            # Fallback to basic spec if ju.rjsf not available
+        func_to_form_spec = _import_func_to_form_spec()
+        if func_to_form_spec is None:
             return self._generate_basic_spec(func_name, func)
 
+        # ju nests the spec under a "rjsf" field by default; uf wants it flat
+        rjsf_config = {"nest_under_field": None, **self._rjsf_config}
+        form_spec = func_to_form_spec(func, **rjsf_config)
+
+        # Apply custom UI schema factory if provided
+        if self._ui_schema_factory:
+            ui_schema = self._ui_schema_factory(func)
+            if "uiSchema" in form_spec:
+                form_spec["uiSchema"].update(ui_schema)
+            else:
+                form_spec["uiSchema"] = ui_schema
+
+        return {
+            "schema": form_spec.get("schema", {}),
+            "uiSchema": form_spec.get("uiSchema", {}),
+            "func": func,
+            "name": func_name,
+            "description": func.__doc__ or f"Execute {func_name}",
+        }
+
     def _generate_basic_spec(self, func_name: str, func: Callable) -> dict:
         """Generate a basic specification without ju.rjsf.
 
-        This is a fallback for when ju.rjsf is not available. It creates
+        This is the fallback used when ``ju`` is not installed. It creates
         a minimal JSON Schema from function signature.
 
         Args: