diff --git a/effectful/internals/unification.py b/effectful/internals/unification.py
index ba770ad3..ad022a63 100644
--- a/effectful/internals/unification.py
+++ b/effectful/internals/unification.py
@@ -539,6 +539,17 @@ def _unify_generic(typ, subtyp, subs: Substitutions) -> Substitutions:
             typing.get_origin(typ), collections.abc.Generator
         ):
             return unify(typing.get_args(typ)[0], typing.get_args(subtyp)[0], subs)
+        elif typing.get_origin(subtyp) is effectful.ops.types.Operation and not (
+            isinstance(typing.get_origin(typ), type)
+            and issubclass(typing.get_origin(typ), effectful.ops.types.Operation)
+        ):
+            # An Operation[P, R] is a Callable[P, R] (gh #669): unify the pattern
+            # against the operation's parameter/return signature. ``Operation``'s
+            # args are (params, return) just like ``Callable``'s, except params is
+            # a tuple (or ``...``) rather than a list.
+            op_params, op_ret = typing.get_args(subtyp)
+            callable_params = op_params if op_params is ... else list(op_params)
+            return unify(typ, collections.abc.Callable[callable_params, op_ret], subs)  # type: ignore
         elif typing.get_origin(typ) == typing.get_origin(subtyp):
             return unify(typing.get_args(typ), typing.get_args(subtyp), subs)
         elif types.get_original_bases(typing.get_origin(subtyp)):
@@ -556,6 +567,17 @@ def _unify_generic(typ, subtyp, subs: Substitutions) -> Substitutions:
         and issubclass(subtyp, typing.get_origin(typ))
     ):
         return subs  # implicit expansion to subtyp[Any]
+    elif isinstance(typ, GenericAlias):
+        # Special case for treating arrays as iterables of arrays
+        try:
+            import jax
+
+            if typing.get_origin(typ) is collections.abc.Iterable and issubclass(
+                subtyp, jax.Array
+            ):
+                return unify(typing.get_args(typ)[0], jax.Array, subs)
+        except ImportError:
+            pass
     raise TypeError(f"Cannot unify generic type {typ} with {subtyp} given {subs}.")
 
 
diff --git a/tests/test_internals_unification.py b/tests/test_internals_unification.py
index fc1a4ba1..fe3a9ed0 100644
--- a/tests/test_internals_unification.py
+++ b/tests/test_internals_unification.py
@@ -1967,3 +1967,69 @@ class Info(typing.TypedDict):
 
     subs = unify(collections.abc.Mapping, Info)
     assert subs == {}
+
+
+def test_unify_jax_array_iterable():
+    import jax
+
+    subs = unify(collections.abc.Iterable[T], jax.Array)
+    assert subs == {T: jax.Array}
+
+
+def test_unify_operation_callable():
+    """An ``Operation[P, R]`` unifies as a ``Callable[P, R]`` (gh #669)."""
+    from effectful.ops.types import Operation
+
+    # TypeVar params bind to the operation's parameter/return types
+    assert unify(collections.abc.Callable[[T], V], Operation[[int], int]) == {
+        T: int,
+        V: int,
+    }
+    # a repeated TypeVar binds consistently
+    assert unify(collections.abc.Callable[[T], T], Operation[[int], int]) == {T: int}
+    # multiple parameters
+    assert unify(collections.abc.Callable[[T, U], V], Operation[[int, str], bool]) == {
+        T: int,
+        U: str,
+        V: bool,
+    }
+    # ``...`` parameters in the pattern ignore the operation's parameter types
+    assert unify(collections.abc.Callable[..., V], Operation[[int], int]) == {V: int}
+    # fully concrete: nothing to bind
+    assert unify(collections.abc.Callable[[int], int], Operation[[int], int]) == {}
+    # nested: an operation-valued argument
+    assert unify(
+        collections.abc.Callable[[T], list[V]], Operation[[int], list[str]]
+    ) == {T: int, V: str}
+
+
+def test_unify_operation_callable_failure():
+    """An arity mismatch between the Callable pattern and the Operation fails."""
+    from effectful.ops.types import Operation
+
+    with pytest.raises(TypeError):
+        unify(collections.abc.Callable[[T, U], V], Operation[[int], int])
+    with pytest.raises(TypeError):
+        unify(collections.abc.Callable[[T], V], Operation[[int, str], bool])
+
+
+def test_operation_unifies_with_callable_param_gh669():
+    """An Operation passed where a ``Callable`` is expected infers correctly.
+
+    Regression test for gh #669: calling an operation whose parameter is typed
+    ``Callable[[S], T]`` with another operation should unify and infer the return
+    type, rather than raising ``Cannot unify generic type ...``.
+    """
+    from effectful.ops.semantics import typeof
+    from effectful.ops.types import NotHandled, Operation
+
+    @Operation.define
+    def f(x: int) -> int:
+        raise NotHandled
+
+    @Operation.define
+    def g[S, R](x: collections.abc.Callable[[S], R]) -> R:
+        raise NotHandled
+
+    term = g(f)
+    assert typeof(term) is int