diff --git a/langchain4j-vertex-ai-gemini/src/test/java/dev/langchain4j/model/vertexai/gemini/ContentsMapperTest.java b/langchain4j-vertex-ai-gemini/src/test/java/dev/langchain4j/model/vertexai/gemini/ContentsMapperTest.java
index 85c76259eb..62bd3f12df 100644
--- a/langchain4j-vertex-ai-gemini/src/test/java/dev/langchain4j/model/vertexai/gemini/ContentsMapperTest.java
+++ b/langchain4j-vertex-ai-gemini/src/test/java/dev/langchain4j/model/vertexai/gemini/ContentsMapperTest.java
@@ -115,4 +115,201 @@ class ContentsMapperTest {
         assertThat(contents.get(3).getPartsCount()).isEqualTo(1);
         assertThat(contents.get(3).getParts(0).getText()).isEqualTo("3+4 is smaller than 5+6");
     }
+
+    @Test
+    void should_handle_empty_message_list() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.systemInstruction).isNull();
+        assertThat(instructionAndContent.contents).isEmpty();
+    }
+
+    @Test
+    void should_handle_only_system_message() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(SystemMessage.from("Process data efficiently"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.systemInstruction).isNotNull();
+        assertThat(instructionAndContent.systemInstruction.getParts(0).getText())
+                .isEqualTo("Process data efficiently");
+        assertThat(instructionAndContent.contents).isEmpty();
+    }
+
+    @Test
+    void should_handle_only_user_message() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(UserMessage.from("Process this request"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.systemInstruction).isNull();
+        assertThat(instructionAndContent.contents).hasSize(1);
+        assertThat(instructionAndContent.contents.get(0).getParts(0).getText()).isEqualTo("Process this request");
+    }
+
+    @Test
+    void should_handle_multiple_system_messages() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(SystemMessage.from("First configuration"));
+        msgs.add(SystemMessage.from("Second configuration"));
+        msgs.add(UserMessage.from("Input data"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.systemInstruction).isNotNull();
+        assertThat(instructionAndContent.contents).hasSize(1);
+        assertThat(instructionAndContent.contents.get(0).getParts(0).getText()).isEqualTo("Input data");
+    }
+
+    @Test
+    void should_handle_consecutive_user_messages() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(UserMessage.from("First input"));
+        msgs.add(UserMessage.from("Second input"));
+        msgs.add(UserMessage.from("Third input"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.systemInstruction).isNull();
+        assertThat(instructionAndContent.contents).hasSize(3);
+        assertThat(instructionAndContent.contents.get(0).getParts(0).getText()).isEqualTo("First input");
+        assertThat(instructionAndContent.contents.get(1).getParts(0).getText()).isEqualTo("Second input");
+        assertThat(instructionAndContent.contents.get(2).getParts(0).getText()).isEqualTo("Third input");
+    }
+
+    @Test
+    void should_handle_consecutive_ai_messages() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(UserMessage.from("Request"));
+        msgs.add(AiMessage.from("Initial output"));
+        msgs.add(AiMessage.from("Additional output"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.contents).hasSize(3);
+        assertThat(instructionAndContent.contents.get(0).getParts(0).getText()).isEqualTo("Request");
+        assertThat(instructionAndContent.contents.get(1).getParts(0).getText()).isEqualTo("Initial output");
+        assertThat(instructionAndContent.contents.get(2).getParts(0).getText()).isEqualTo("Additional output");
+    }
+
+    @Test
+    void should_handle_tool_execution_without_ai_message() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(UserMessage.from("Execute task"));
+        msgs.add(ToolExecutionResultMessage.from(null, "process", "{\"status\": \"complete\"}"));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.contents).hasSize(2);
+        assertThat(instructionAndContent.contents.get(0).getParts(0).getText()).isEqualTo("Execute task");
+        assertThat(instructionAndContent
+                        .contents
+                        .get(1)
+                        .getParts(0)
+                        .getFunctionResponse()
+                        .getName())
+                .isEqualTo("process");
+    }
+
+    @Test
+    void should_handle_single_tool_execution_request() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        msgs.add(UserMessage.from("Request"));
+        ToolExecutionRequest request = ToolExecutionRequest.builder()
+                .name("Request")
+                .arguments("{\"type\": \"standard\"}")
+                .build();
+        msgs.add(AiMessage.from(request));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.contents).hasSize(2);
+        assertThat(instructionAndContent
+                        .contents
+                        .get(1)
+                        .getParts(0)
+                        .getFunctionCall()
+                        .getName())
+                .isEqualTo("Request");
+    }
+
+    @Test
+    void should_handle_tool_with_empty_arguments() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        ToolExecutionRequest request =
+                ToolExecutionRequest.builder().name("getStatus").arguments("{}").build();
+        msgs.add(AiMessage.from(request));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.contents).hasSize(1);
+        assertThat(instructionAndContent
+                        .contents
+                        .get(0)
+                        .getParts(0)
+                        .getFunctionCall()
+                        .getName())
+                .isEqualTo("getStatus");
+        assertThat(instructionAndContent
+                        .contents
+                        .get(0)
+                        .getParts(0)
+                        .getFunctionCall()
+                        .getArgs()
+                        .getFieldsCount())
+                .isEqualTo(0);
+    }
+
+    @Test
+    void should_handle_tool_with_null_arguments() {
+        // given
+        List<ChatMessage> msgs = new ArrayList<>();
+        ToolExecutionRequest request =
+                ToolExecutionRequest.builder().name("fetchData").arguments(null).build();
+        msgs.add(AiMessage.from(request));
+
+        // when
+        ContentsMapper.InstructionAndContent instructionAndContent = splitInstructionAndContent(msgs);
+
+        // then
+        assertThat(instructionAndContent.contents).hasSize(1);
+        assertThat(instructionAndContent
+                        .contents
+                        .get(0)
+                        .getParts(0)
+                        .getFunctionCall()
+                        .getName())
+                .isEqualTo("fetchData");
+    }
 }