diff --git a/src/tinker/lib/public_interfaces/training_client.py b/src/tinker/lib/public_interfaces/training_client.py
index d99a90a..47aeffb 100644
--- a/src/tinker/lib/public_interfaces/training_client.py
+++ b/src/tinker/lib/public_interfaces/training_client.py
@@ -263,7 +263,24 @@ async def forward_backward_custom_async(
         import torch
 
         # First do a forward pass and get logprobs
-        forward_future = await self.forward_async(data, "cross_entropy")
+        _loss_fn_input_keys = data[0].loss_fn_inputs.keys()
+        if "advantages" in _loss_fn_input_keys:
+            # Check for RL (`importance_sampling`) loss inputs
+            assert "advantages" in _loss_fn_input_keys, "advantages must be in loss_fn_inputs"
+            assert "target_tokens" in _loss_fn_input_keys, "target_tokens must be in loss_fn_inputs"
+            assert "logprobs" in _loss_fn_input_keys, "logprobs must be in loss_fn_inputs"
+            _loss_fn = "importance_sampling"
+
+        elif "weights" in _loss_fn_input_keys:
+            # Check for supervised learning loss inputs
+            assert "weights" in _loss_fn_input_keys, "weights must be in loss_fn_inputs"
+            assert "target_tokens" in _loss_fn_input_keys, "target_tokens must be in loss_fn_inputs"
+            _loss_fn = "cross_entropy"
+        else:
+            assert False, "Invalid loss function inputs"
+
+        # Compute on-policy logprobs
+        forward_future = await self.forward_async(data, _loss_fn)
         forward_result = await forward_future.result_async()
         logprobs_list: List[torch.Tensor] = []
         for out in forward_result.loss_fn_outputs:
@@ -280,7 +297,7 @@ async def forward_backward_custom_async(
             grads.append(logprob.grad)
 
         linear_loss_data = []
-        for datum, grad in zip(data, grads):
+        for datum, grad in zip(data, grads, strict=True):
             loss_fn_inputs: Any = {
                 "target_tokens": datum.loss_fn_inputs["target_tokens"],
                 "weights": -grad,  # Pass PyTorch tensor directly (will be converted to TensorData)