diff --git a/sota-implementations/decision_transformer/utils.py b/sota-implementations/decision_transformer/utils.py
index d4a67e7d3a9..415e19a1f7c 100644
--- a/sota-implementations/decision_transformer/utils.py
+++ b/sota-implementations/decision_transformer/utils.py
@@ -109,7 +109,7 @@ def make_transformed_env(base_env, env_cfg, obs_loc, obs_std, train=False):
         )
 
     # copy action from the input tensordict to the output
-    transformed_env.append_transform(TensorDictPrimer(action=base_env.action_spec))
+    transformed_env.append_transform(TensorDictPrimer(base_env.full_action_spec))
 
     transformed_env.append_transform(DoubleToFloat())
     obsnorm = ObservationNorm(
diff --git a/test/test_transforms.py b/test/test_transforms.py
index cc3ca40b059..44ebce72c5c 100644
--- a/test/test_transforms.py
+++ b/test/test_transforms.py
@@ -7408,7 +7408,7 @@ def make_env():
     def test_trans_parallel_env_check(self, maybe_fork_ParallelEnv):
         env = TransformedEnv(
             maybe_fork_ParallelEnv(2, ContinuousActionVecMockEnv),
-            TensorDictPrimer(mykey=Unbounded([2, 4])),
+            TensorDictPrimer(mykey=Unbounded([4])),
         )
         try:
             check_env_specs(env)
@@ -7423,11 +7423,39 @@ def test_trans_parallel_env_check(self, maybe_fork_ParallelEnv):
                 pass
 
     @pytest.mark.parametrize("spec_shape", [[4], [2, 4]])
-    def test_trans_serial_env_check(self, spec_shape):
-        env = TransformedEnv(
-            SerialEnv(2, ContinuousActionVecMockEnv),
-            TensorDictPrimer(mykey=Unbounded(spec_shape)),
-        )
+    @pytest.mark.parametrize("expand_specs", [True, False, None])
+    def test_trans_serial_env_check(self, spec_shape, expand_specs):
+        if expand_specs is None:
+            with pytest.warns(FutureWarning, match=""):
+                env = TransformedEnv(
+                    SerialEnv(2, ContinuousActionVecMockEnv),
+                    TensorDictPrimer(
+                        mykey=Unbounded(spec_shape), expand_specs=expand_specs
+                    ),
+                )
+                env.observation_spec
+        elif expand_specs is True:
+            shape = spec_shape[:-1]
+            env = TransformedEnv(
+                SerialEnv(2, ContinuousActionVecMockEnv),
+                TensorDictPrimer(
+                    Composite(mykey=Unbounded(spec_shape), shape=shape),
+                    expand_specs=expand_specs,
+                ),
+            )
+        else:
+            # If we don't expand, we can't use [4]
+            env = TransformedEnv(
+                SerialEnv(2, ContinuousActionVecMockEnv),
+                TensorDictPrimer(
+                    mykey=Unbounded(spec_shape), expand_specs=expand_specs
+                ),
+            )
+            if spec_shape == [4]:
+                with pytest.raises(ValueError):
+                    env.observation_spec
+                return
+
         check_env_specs(env)
         assert "mykey" in env.reset().keys()
         r = env.rollout(3)
@@ -10310,9 +10338,8 @@ def _make_transform_env(self, out_key, base_env):
         transform = KLRewardTransform(actor, out_keys=out_key)
         return Compose(
             TensorDictPrimer(
-                primers={
-                    "sample_log_prob": Unbounded(shape=base_env.action_spec.shape[:-1])
-                }
+                sample_log_prob=Unbounded(shape=base_env.action_spec.shape[:-1]),
+                shape=base_env.shape,
             ),
             transform,
         )
diff --git a/torchrl/envs/transforms/transforms.py b/torchrl/envs/transforms/transforms.py
index 14d4133412c..5b2dafc9b04 100644
--- a/torchrl/envs/transforms/transforms.py
+++ b/torchrl/envs/transforms/transforms.py
@@ -4984,6 +4984,7 @@ def __init__(
         | Dict[NestedKey, float]
         | Dict[NestedKey, Callable] = None,
         reset_key: NestedKey | None = None,
+        expand_specs: bool = None,
         **kwargs,
     ):
         self.device = kwargs.pop("device", None)
@@ -4995,8 +4996,10 @@ def __init__(
                 )
             kwargs = primers
         if not isinstance(kwargs, Composite):
-            kwargs = Composite(kwargs)
+            kwargs = Composite(**kwargs)
         self.primers = kwargs
+        self.expand_specs = expand_specs
+
         if random and default_value:
             raise ValueError(
                 "Setting random to True and providing a default_value are incompatible."
@@ -5089,15 +5092,25 @@ def transform_observation_spec(self, observation_spec: Composite) -> Composite:
             )
 
         if self.primers.shape != observation_spec.shape:
-            if self.primers.shape == () and self.parent.batch_size != ():
+            if self.expand_specs:
                 self.primers = self._expand_shape(self.primers)
-            else:
+            elif self.expand_specs is None:
+                warnings.warn(
+                    f"expand_specs wasn't specified in the {type(self).__name__} constructor. "
+                    f"The current behaviour is that the transform will attempt to set the shape of the composite "
+                    f"spec, and if this can't be done it will be expanded. "
+                    f"From v0.8, a mismatched shape between the spec of the transform and the env's batch_size "
+                    f"will raise an exception.",
+                    category=FutureWarning,
+                )
                 try:
                     # We try to set the primer shape to the observation spec shape
                     self.primers.shape = observation_spec.shape
                 except ValueError:
                     # If we fail, we expand them to that shape
                     self.primers = self._expand_shape(self.primers)
+            else:
+                self.primers.shape = observation_spec.shape
 
         device = observation_spec.device
         observation_spec.update(self.primers.clone().to(device))
diff --git a/torchrl/modules/tensordict_module/rnn.py b/torchrl/modules/tensordict_module/rnn.py
index 57bcac94cf4..68309c346cd 100644
--- a/torchrl/modules/tensordict_module/rnn.py
+++ b/torchrl/modules/tensordict_module/rnn.py
@@ -653,7 +653,8 @@ def make_tuple(key):
             {
                 in_key1: Unbounded(shape=(self.lstm.num_layers, self.lstm.hidden_size)),
                 in_key2: Unbounded(shape=(self.lstm.num_layers, self.lstm.hidden_size)),
-            }
+            },
+            expand_specs=True,
         )
 
     @property
@@ -1467,7 +1468,8 @@ def make_tuple(key):
         return TensorDictPrimer(
             {
                 in_key1: Unbounded(shape=(self.gru.num_layers, self.gru.hidden_size)),
-            }
+            },
+            expand_specs=True,
         )
 
     @property