diff --git a/sbi/inference/posteriors/vector_field_posterior.py b/sbi/inference/posteriors/vector_field_posterior.py
index 8d81fc2b8..cac80df73 100644
--- a/sbi/inference/posteriors/vector_field_posterior.py
+++ b/sbi/inference/posteriors/vector_field_posterior.py
@@ -291,6 +291,7 @@ def _sample_via_diffusion(
         max_sampling_batch_size: int = 10_000,
         show_progress_bars: bool = True,
         save_intermediate: bool = False,
+        **kwargs,
     ) -> Tensor:
         r"""Return samples from posterior distribution $p(\theta|x)$.
 
diff --git a/sbi/samplers/rejection/rejection.py b/sbi/samplers/rejection/rejection.py
index 892334050..13ebcd929 100644
--- a/sbi/samplers/rejection/rejection.py
+++ b/sbi/samplers/rejection/rejection.py
@@ -11,6 +11,7 @@
 from torch import Tensor, as_tensor
 from tqdm.auto import tqdm
 
+from sbi.sbi_types import AcceptRejectFn, SampleProposal
 from sbi.utils.sbiutils import gradient_ascent
 
 
@@ -214,8 +215,8 @@ def log_prob(self, theta: Tensor, **kwargs) -> Tensor:
 
 @torch.no_grad()
 def accept_reject_sample(
-    proposal: Callable,
-    accept_reject_fn: Callable,
+    proposal: SampleProposal,
+    accept_reject_fn: AcceptRejectFn,
     num_samples: int,
     num_xos: int = 1,
     show_progress_bars: bool = False,
@@ -242,12 +243,12 @@ def accept_reject_sample(
            density during evaluation of the posterior.
 
     Args:
-        proposal: A callable that takes `sample_shape` as arguments (and kwargs as
-            needed). Returns samples from the proposal distribution with shape
-            (*sample_shape, event_dim).
-        accept_reject_fn: Function that evaluates which samples are accepted or
-            rejected. Must take a batch of parameters and return a boolean tensor which
-            indicates which parameters get accepted.
+        proposal: A callable following the `SampleProposal` protocol, i.e., takes
+            `sample_shape` as first argument (and kwargs as needed). Returns samples
+            from the proposal distribution with shape (*sample_shape, event_dim).
+        accept_reject_fn: A callable following the `AcceptRejectFn` protocol that
+            evaluates which samples are accepted or rejected. Takes a batch of
+            parameters and returns a boolean tensor indicating which are accepted.
         num_samples: Desired number of samples.
         num_xos: Number of conditions for batched_sampling (currently only accepting
             one batch dimension for the condition).
@@ -329,7 +330,7 @@ def accept_reject_sample(
 
         # Sample and reject.
         candidates = proposal(
-            (sampling_batch_size,),  # type: ignore
+            torch.Size((sampling_batch_size,)),
             **proposal_sampling_kwargs,
         )
         # SNPE-style rejection-sampling when the proposal is the neural net.
diff --git a/sbi/sbi_types.py b/sbi/sbi_types.py
index 425a900ad..1d62e8eb4 100644
--- a/sbi/sbi_types.py
+++ b/sbi/sbi_types.py
@@ -1,7 +1,7 @@
 # This file is part of sbi, a toolkit for simulation-based inference. sbi is licensed
 # under the Apache License Version 2.0, see <https://www.apache.org/licenses/>
 
-from typing import Optional, Sequence, Tuple, TypeVar, Union
+from typing import Optional, Protocol, Sequence, Tuple, TypeVar, Union
 
 import numpy as np
 import torch
@@ -35,10 +35,33 @@
 PyroTransformedDistribution: TypeAlias = TransformedDistribution
 TorchTensor: TypeAlias = Tensor
 
+
+class SampleProposal(Protocol):
+    """Protocol for sample proposal callables used in rejection sampling.
+
+    Any callable that takes a sample shape and optional keyword arguments
+    and returns a Tensor of samples satisfies this protocol.
+    """
+
+    def __call__(self, sample_shape: torch.Size, **kwargs) -> Tensor: ...
+
+
+class AcceptRejectFn(Protocol):
+    """Protocol for accept/reject functions used in rejection sampling.
+
+    Any callable that takes a batch of parameters (theta) and returns a boolean
+    Tensor indicating which samples are accepted satisfies this protocol.
+    """
+
+    def __call__(self, theta: Tensor) -> Tensor: ...
+
+
 __all__ = [
+    "AcceptRejectFn",
     "Array",
     "Shape",
     "OneOrMore",
+    "SampleProposal",
     "ScalarFloat",
     "TensorBoardSummaryWriter",
     "TorchTransform",
diff --git a/sbi/utils/restriction_estimator.py b/sbi/utils/restriction_estimator.py
index ffbaf9665..1a0de4182 100644
--- a/sbi/utils/restriction_estimator.py
+++ b/sbi/utils/restriction_estimator.py
@@ -688,7 +688,9 @@ def sample(
 
         if sample_with == "rejection":
             samples, acceptance_rate = rejection.accept_reject_sample(
-                proposal=self._prior.sample,
+                proposal=lambda sample_shape, **kwargs: self._prior.sample(
+                    sample_shape
+                ),
                 accept_reject_fn=self._accept_reject_fn,
                 num_samples=num_samples,
                 show_progress_bars=show_progress_bars,