diff --git a/doc/source/changelog.rst b/doc/source/changelog.rst
index 3b0f2ec..625d7da 100644
--- a/doc/source/changelog.rst
+++ b/doc/source/changelog.rst
@@ -2,9 +2,15 @@
 Changelog
 #########
 
+Version 4.0.0rc2 (2025-02-23)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- feat(optimize): add option to pass keyword arguments to pipeline during optimization
+
 Version 4.0.0rc1 (2025-02-11)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+- feat(optimize): add option to pass keyword arguments to pipeline during optimization
 - BREAKING: drop support for `Python` < 3.10
 - BREAKING: switch to native namespace package 
 - BREAKING: remove `pyannote.pipeline.blocks` submodule
diff --git a/src/pyannote/pipeline/optimizer.py b/src/pyannote/pipeline/optimizer.py
index 4b4de05..1057093 100644
--- a/src/pyannote/pipeline/optimizer.py
+++ b/src/pyannote/pipeline/optimizer.py
@@ -30,7 +30,7 @@
 import time
 import warnings
 from pathlib import Path
-from typing import Iterable, Optional, Callable, Generator, Union, Dict
+from typing import Iterable, Optional, Callable, Generator, Mapping, Union, Dict
 
 import numpy as np
 import optuna.logging
@@ -231,7 +231,16 @@ def objective(trial: Trial) -> float:
                 # process input with pipeline
                 # (and keep track of processing time)
                 before_processing = time.time()
-                output = pipeline(input)
+
+                # get optional kwargs to be passed to the pipeline
+                # (e.g. num_speakers for speaker diarization). they
+                # must be stored in a 'pipeline_kwargs' key in the
+                # `input` dictionary.
+                if isinstance(input, Mapping):
+                    pipeline_kwargs = input.get("pipeline_kwargs", {})
+                else:
+                    pipeline_kwargs = {}
+                output = pipeline(input, **pipeline_kwargs)
                 after_processing = time.time()
                 processing_time.append(after_processing - before_processing)