diff --git a/aana/core/models/base.py b/aana/core/models/base.py
index d129c53a..efb295e7 100644
--- a/aana/core/models/base.py
+++ b/aana/core/models/base.py
@@ -58,7 +58,7 @@ def merged_options(default_options: OptionType, options: OptionType) -> OptionTy
     if type(default_options) != type(options):
         raise ValueError("Option type mismatch.")  # noqa: TRY003
     default_options_dict = default_options.model_dump()
-    for k, v in options.model_dump().items():
+    for k, v in options.model_dump(exclude_unset=True).items():
         if v is not None:
             default_options_dict[k] = v
     return options.__class__.model_validate(default_options_dict)
diff --git a/aana/deployments/hf_text_generation_deployment.py b/aana/deployments/hf_text_generation_deployment.py
index 0d4be92b..a46c5d8e 100644
--- a/aana/deployments/hf_text_generation_deployment.py
+++ b/aana/deployments/hf_text_generation_deployment.py
@@ -68,8 +68,11 @@ async def generate_stream(
         prompt = str(prompt)
 
         if sampling_params is None:
-            sampling_params = SamplingParams()
-        sampling_params = merged_options(self.default_sampling_params, sampling_params)
+            sampling_params = self.default_sampling_params
+        else:
+            sampling_params = merged_options(
+                self.default_sampling_params, sampling_params
+            )
 
         prompt_input = self.tokenizer(
             prompt, return_tensors="pt", add_special_tokens=False
diff --git a/aana/deployments/idefics_2_deployment.py b/aana/deployments/idefics_2_deployment.py
index 6536bf88..09069e84 100644
--- a/aana/deployments/idefics_2_deployment.py
+++ b/aana/deployments/idefics_2_deployment.py
@@ -105,8 +105,11 @@ async def chat_stream(
         transformers.set_seed(42)
 
         if sampling_params is None:
-            sampling_params = SamplingParams()
-        sampling_params = merged_options(self.default_sampling_params, sampling_params)
+            sampling_params = self.default_sampling_params
+        else:
+            sampling_params = merged_options(
+                self.default_sampling_params, sampling_params
+            )
 
         messages, images = dialog.to_objects()
         text = self.processor.apply_chat_template(messages, add_generation_prompt=True)
@@ -191,8 +194,11 @@ async def chat_batch(
         transformers.set_seed(42)
 
         if sampling_params is None:
-            sampling_params = SamplingParams()
-        sampling_params = merged_options(self.default_sampling_params, sampling_params)
+            sampling_params = self.default_sampling_params
+        else:
+            sampling_params = merged_options(
+                self.default_sampling_params, sampling_params
+            )
 
         text_batch = []
         image_batch = []
diff --git a/aana/deployments/vllm_deployment.py b/aana/deployments/vllm_deployment.py
index 708849b5..50386cc6 100644
--- a/aana/deployments/vllm_deployment.py
+++ b/aana/deployments/vllm_deployment.py
@@ -227,8 +227,11 @@ async def generate_stream(  # noqa: C901
             prompt_token_ids = prompt
 
         if sampling_params is None:
-            sampling_params = SamplingParams()
-        sampling_params = merged_options(self.default_sampling_params, sampling_params)
+            sampling_params = self.default_sampling_params
+        else:
+            sampling_params = merged_options(
+                self.default_sampling_params, sampling_params
+            )
 
         json_schema = sampling_params.json_schema
         regex_string = sampling_params.regex_string
diff --git a/aana/tests/files/expected/image_text_generation/phi-3.5-vision-instruct_vllm_deployment_Starry_Night.jpeg_394ead9926577785413d0c748ebf9878.json b/aana/tests/files/expected/image_text_generation/phi-3.5-vision-instruct_vllm_deployment_Starry_Night.jpeg_394ead9926577785413d0c748ebf9878.json
index c44d4688..ba93432f 100644
--- a/aana/tests/files/expected/image_text_generation/phi-3.5-vision-instruct_vllm_deployment_Starry_Night.jpeg_394ead9926577785413d0c748ebf9878.json
+++ b/aana/tests/files/expected/image_text_generation/phi-3.5-vision-instruct_vllm_deployment_Starry_Night.jpeg_394ead9926577785413d0c748ebf9878.json
@@ -1 +1 @@
-" The painting is done by Vincent van Gogh."
\ No newline at end of file
+" Vincent van Gogh"
\ No newline at end of file
diff --git a/aana/tests/units/test_merge_options.py b/aana/tests/units/test_merge_options.py
index f95ddd09..6f813bfe 100644
--- a/aana/tests/units/test_merge_options.py
+++ b/aana/tests/units/test_merge_options.py
@@ -12,6 +12,7 @@ class MyOptions(BaseModel):
     field1: str
     field2: int | None = None
     field3: bool
+    field4: str = "default"
 
 
 def test_merged_options_same_type():
@@ -46,3 +47,17 @@ class AnotherOptions(BaseModel):
 
     with pytest.raises(ValueError):
         merged_options(default, to_merge)
+
+
+def test_merged_options_unset():
+    """Test merged_options with unset fields."""
+    default = MyOptions(field1="default1", field2=2, field3=True, field4="new_default")
+    to_merge = MyOptions(field1="merge1", field3=False)  # field4 is not set
+    merged = merged_options(default, to_merge)
+
+    assert merged.field1 == "merge1"
+    assert merged.field2 == 2
+    assert merged.field3 == False
+    assert (
+        merged.field4 == "new_default"
+    )  # Should retain value from default_options as it's not set in options