diff --git a/models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/DeepSeekChatModel.java b/models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/DeepSeekChatModel.java
index fad9af7f8f..adf7502d76 100644
--- a/models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/DeepSeekChatModel.java
+++ b/models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/DeepSeekChatModel.java
@@ -430,14 +430,17 @@ else if (message.getMessageType() == MessageType.ASSISTANT) {
 					}).toList();
 				}
 				Boolean isPrefixAssistantMessage = null;
-				if (message instanceof DeepSeekAssistantMessage
-						&& Boolean.TRUE.equals(((DeepSeekAssistantMessage) message).getPrefix())) {
-					isPrefixAssistantMessage = true;
+				String reasoningContent = null;
+				if (message instanceof DeepSeekAssistantMessage deepSeekMsg) {
+					if (Boolean.TRUE.equals(deepSeekMsg.getPrefix())) {
+						isPrefixAssistantMessage = true;
+					}
+					reasoningContent = deepSeekMsg.getReasoningContent();
 				}
 				String text = assistantMessage.getText();
 				Assert.state(text != null, "text must not be null");
 				return List.of(new ChatCompletionMessage(text, ChatCompletionMessage.Role.ASSISTANT, null, null,
-						toolCalls, isPrefixAssistantMessage, null));
+						toolCalls, isPrefixAssistantMessage, reasoningContent));
 			}
 			else if (message.getMessageType() == MessageType.TOOL) {
 				ToolResponseMessage toolMessage = (ToolResponseMessage) message;
diff --git a/models/spring-ai-deepseek/src/test/java/org/springframework/ai/deepseek/DeepSeekChatCompletionRequestTests.java b/models/spring-ai-deepseek/src/test/java/org/springframework/ai/deepseek/DeepSeekChatCompletionRequestTests.java
index 56338f2657..b62d628a93 100644
--- a/models/spring-ai-deepseek/src/test/java/org/springframework/ai/deepseek/DeepSeekChatCompletionRequestTests.java
+++ b/models/spring-ai-deepseek/src/test/java/org/springframework/ai/deepseek/DeepSeekChatCompletionRequestTests.java
@@ -16,8 +16,11 @@
 
 package org.springframework.ai.deepseek;
 
+import java.util.List;
+
 import org.junit.jupiter.api.Test;
 
+import org.springframework.ai.chat.messages.UserMessage;
 import org.springframework.ai.chat.prompt.Prompt;
 import org.springframework.ai.deepseek.api.DeepSeekApi;
 
@@ -54,4 +57,39 @@ public void createRequestWithChatOptions() {
 		assertThat(request.temperature()).isEqualTo(99.9D);
 	}
 
+	@Test
+	public void reasoningContentIsPassedBackForDeepSeekAssistantMessage() {
+		var client = DeepSeekChatModel.builder().deepSeekApi(DeepSeekApi.builder().apiKey("TEST").build()).build();
+
+		DeepSeekAssistantMessage assistantMessage = new DeepSeekAssistantMessage.Builder().content("The answer is 42.")
+			.reasoningContent("I reasoned through the problem step by step.")
+			.build();
+
+		var prompt = new Prompt(
+				List.of(new UserMessage("What is the answer?"), assistantMessage, new UserMessage("Are you sure?")),
+				DeepSeekChatOptions.builder().model("deepseek-reasoner").build());
+
+		var request = client.createRequest(prompt, false);
+
+		assertThat(request.messages()).hasSize(3);
+		var assistantMsg = request.messages().get(1);
+		assertThat(assistantMsg.role()).isEqualTo(DeepSeekApi.ChatCompletionMessage.Role.ASSISTANT);
+		assertThat(assistantMsg.reasoningContent()).isEqualTo("I reasoned through the problem step by step.");
+	}
+
+	@Test
+	public void nullReasoningContentIsOmittedForPlainAssistantMessage() {
+		var client = DeepSeekChatModel.builder().deepSeekApi(DeepSeekApi.builder().apiKey("TEST").build()).build();
+
+		var prompt = new Prompt(List.of(new UserMessage("Hello"),
+				new org.springframework.ai.chat.messages.AssistantMessage("Hi!"), new UserMessage("How are you?")),
+				DeepSeekChatOptions.builder().model("deepseek-chat").build());
+
+		var request = client.createRequest(prompt, false);
+
+		assertThat(request.messages()).hasSize(3);
+		var assistantMsg = request.messages().get(1);
+		assertThat(assistantMsg.reasoningContent()).isNull();
+	}
+
 }