diff --git a/src/jaxsim/api/contact.py b/src/jaxsim/api/contact.py
index 15ea7df18..5de7e43ce 100644
--- a/src/jaxsim/api/contact.py
+++ b/src/jaxsim/api/contact.py
@@ -365,10 +365,12 @@ def jacobian(
 
     # Adjust the output representation.
     match output_vel_repr:
+
         case VelRepr.Inertial:
             O_J_WC = W_J_WC
 
         case VelRepr.Body:
+
             W_H_C = transforms(model=model, data=data)
 
             def body_jacobian(W_H_C: jtp.Matrix, W_J_WC: jtp.Matrix) -> jtp.Matrix:
@@ -381,9 +383,11 @@ def body_jacobian(W_H_C: jtp.Matrix, W_J_WC: jtp.Matrix) -> jtp.Matrix:
             O_J_WC = jax.vmap(body_jacobian)(W_H_C, W_J_WC)
 
         case VelRepr.Mixed:
+
             W_H_C = transforms(model=model, data=data)
 
             def mixed_jacobian(W_H_C: jtp.Matrix, W_J_WC: jtp.Matrix) -> jtp.Matrix:
+
                 W_H_CW = W_H_C.at[0:3, 0:3].set(jnp.eye(3))
 
                 CW_X_W = jaxsim.math.Adjoint.from_transform(
diff --git a/src/jaxsim/api/kin_dyn_parameters.py b/src/jaxsim/api/kin_dyn_parameters.py
index 579bee865..f9ffbc0a7 100644
--- a/src/jaxsim/api/kin_dyn_parameters.py
+++ b/src/jaxsim/api/kin_dyn_parameters.py
@@ -184,6 +184,7 @@ def κb(link_index: jtp.IntLike) -> jtp.Vector:
             carry0 = κb, link_index
 
             def scan_body(carry: tuple, i: jtp.Int) -> tuple[tuple, None]:
+
                 κb, active_link_index = carry
 
                 κb, active_link_index = jax.lax.cond(
@@ -225,12 +226,14 @@ def scan_body(carry: tuple, i: jtp.Int) -> tuple[tuple, None]:
         )
 
     def __eq__(self, other: KynDynParameters) -> bool:
+
         if not isinstance(other, KynDynParameters):
             return False
 
         return hash(self) == hash(other)
 
     def __hash__(self) -> int:
+
         return hash(
             (
                 hash(self.number_of_links()),
@@ -640,6 +643,7 @@ def build_from_inertial_parameters(
     def build_from_flat_parameters(
         index: jtp.IntLike, parameters: jtp.VectorLike
     ) -> LinkParameters:
+
         index = jnp.array(index).squeeze().astype(int)
 
         m = jnp.array(parameters[0]).squeeze().astype(float)
@@ -664,11 +668,7 @@ def flat_parameters(params: LinkParameters) -> jtp.Vector:
 
         return (
             jnp.hstack(
-                [
-                    params.mass,
-                    params.center_of_mass.squeeze(),
-                    params.inertia_elements,
-                ]
+                [params.mass, params.center_of_mass.squeeze(), params.inertia_elements]
             )
             .squeeze()
             .astype(float)
diff --git a/src/jaxsim/rbda/contacts/common.py b/src/jaxsim/rbda/contacts/common.py
index 9ff071071..072317f50 100644
--- a/src/jaxsim/rbda/contacts/common.py
+++ b/src/jaxsim/rbda/contacts/common.py
@@ -17,31 +17,27 @@ class ContactsState(abc.ABC):
     def build(cls, **kwargs) -> ContactsState:
         """
         Build the contact state object.
+
         Returns:
             The contact state object.
         """
 
-        return cls(**kwargs)
-
     @classmethod
     @abc.abstractmethod
     def zero(cls, **kwargs) -> ContactsState:
         """
         Build a zero contact state.
+
         Returns:
             The zero contact state.
         """
 
-        return cls.build(**kwargs)
-
     @abc.abstractmethod
     def valid(self, **kwargs) -> bool:
         """
         Check if the contacts state is valid.
         """
 
-        return True
-
 
 class ContactsParams(abc.ABC):
     """
@@ -57,8 +53,7 @@ def build(cls) -> ContactsParams:
             The `ContactsParams` instance.
         """
 
-        raise NotImplementedError
-
+    @abc.abstractmethod
     def valid(self, *args, **kwargs) -> bool:
         """
         Check if the parameters are valid.
@@ -66,12 +61,11 @@ def valid(self, *args, **kwargs) -> bool:
             True if the parameters are valid, False otherwise.
         """
 
-        return True
-
 
 class ContactModel(abc.ABC):
     """
     Abstract class representing a contact model.
+
     Attributes:
         parameters: The parameters of the contact model.
         terrain: The terrain model.
@@ -86,12 +80,16 @@ def compute_contact_forces(
         position: jtp.Vector,
         velocity: jtp.Vector,
         **kwargs,
-    ) -> tuple[Any, ...]:
+    ) -> tuple[jtp.Vector, tuple[Any, ...]]:
         """
         Compute the contact forces.
+
         Args:
-            position: The position of the collidable point.
-            velocity: The velocity of the collidable point.
+            position: The position of the collidable point w.r.t. the world frame.
+            velocity:
+                The linear velocity of the collidable point (linear component of the mixed 6D velocity).
+
         Returns:
-            A tuple containing the contact force and additional information.
+            A tuple containing as first element the computed 6D contact force applied to the contact point and expressed in the world frame,
+            and as second element a tuple of optional additional information.
         """
diff --git a/src/jaxsim/rbda/contacts/soft.py b/src/jaxsim/rbda/contacts/soft.py
index 1362d68f9..f199efdb6 100644
--- a/src/jaxsim/rbda/contacts/soft.py
+++ b/src/jaxsim/rbda/contacts/soft.py
@@ -31,6 +31,7 @@ class SoftContactsParams(ContactsParams):
     )
 
     def __hash__(self) -> int:
+
         from jaxsim.utils.wrappers import HashedNumpyArray
 
         return hash(
@@ -42,6 +43,7 @@ def __hash__(self) -> int:
         )
 
     def __eq__(self, other: SoftContactsParams) -> bool:
+
         if not isinstance(other, SoftContactsParams):
             return NotImplemented
 
@@ -126,6 +128,20 @@ def build_default_from_jaxsim_model(
 
         return SoftContactsParams.build(K=K, D=D, mu=μc)
 
+    def valid(self) -> bool:
+        """
+        Check if the parameters are valid.
+
+        Returns:
+            `True` if the parameters are valid, `False` otherwise.
+        """
+
+        return (
+            jnp.all(self.K >= 0.0)
+            and jnp.all(self.D >= 0.0)
+            and jnp.all(self.mu >= 0.0)
+        )
+
 
 @jax_dataclasses.pytree_dataclass
 class SoftContacts(ContactModel):