diff --git a/packages/uipath_langchain_client/CHANGELOG.md b/packages/uipath_langchain_client/CHANGELOG.md
index c42e951..da3519e 100644
--- a/packages/uipath_langchain_client/CHANGELOG.md
+++ b/packages/uipath_langchain_client/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 All notable changes to `uipath_langchain_client` will be documented in this file.
 
+## [1.9.7] - 2026-04-22
+
+### Changed
+- **Behavior change:** `UiPathChat._default_params` now uses pydantic's `model_fields_set` to decide which params to include in the request payload instead of filtering on `v is not None`. Fields that were not explicitly passed are omitted; fields explicitly set to `None` (e.g. `UiPathChat(temperature=None)`) now forward `null` to the API. Previously both "not passed" and "explicitly `None`" were silently dropped.
+
 ## [1.9.6] - 2026-04-22
 
 ### Added
diff --git a/packages/uipath_langchain_client/src/uipath_langchain_client/__version__.py b/packages/uipath_langchain_client/src/uipath_langchain_client/__version__.py
index cc17247..40fb322 100644
--- a/packages/uipath_langchain_client/src/uipath_langchain_client/__version__.py
+++ b/packages/uipath_langchain_client/src/uipath_langchain_client/__version__.py
@@ -1,3 +1,3 @@
 __title__ = "UiPath LangChain Client"
 __description__ = "A Python client for interacting with UiPath's LLM services via LangChain."
-__version__ = "1.9.6"
+__version__ = "1.9.7"
diff --git a/packages/uipath_langchain_client/src/uipath_langchain_client/clients/normalized/chat_models.py b/packages/uipath_langchain_client/src/uipath_langchain_client/clients/normalized/chat_models.py
index 7a26887..88a7040 100644
--- a/packages/uipath_langchain_client/src/uipath_langchain_client/clients/normalized/chat_models.py
+++ b/packages/uipath_langchain_client/src/uipath_langchain_client/clients/normalized/chat_models.py
@@ -214,12 +214,12 @@ def _identifying_params(self) -> dict[str, Any]:
     @property
     def _default_params(self) -> dict[str, Any]:
         """Get the default parameters for the normalized API request."""
-        exclude_if_none = {
+        candidates: dict[str, Any] = {
             "max_tokens": self.max_tokens,
             "temperature": self.temperature,
             "top_p": self.top_p,
             "top_k": self.top_k,
-            "stop": self.stop or None,
+            "stop": self.stop,
             "n": self.n,
             "frequency_penalty": self.frequency_penalty,
             "presence_penalty": self.presence_penalty,
@@ -242,8 +242,9 @@ def _default_params(self) -> dict[str, Any]:
             "verbosity": self.verbosity,
         }
 
+        set_fields = self.model_fields_set
         return {
-            **{k: v for k, v in exclude_if_none.items() if v is not None},
+            **{k: v for k, v in candidates.items() if k in set_fields},
             **self.model_kwargs,
         }