GH-3657: Fix DeepSeek tool call content null issue

Closes #3657

* Add content fallback for DeepSeek when only tool calls are present
* Ensures AssistantMessage has proper output for downstream processing
* Add test case for AssistantMessage with tool calls only

Signed-off-by: Dongha Koo <[email protected]>

Author: Acacian

models/spring-ai-deepseek/src/main/java/org/springframework/ai/deepseek/DeepSeekChatModel.java

@@ -76,6 +76,7 @@
  * backed by {@link DeepSeekApi}.
  *
  * @author Geng Rong
+ * @author Dongha Koo
  */
 public class DeepSeekChatModel implements ChatModel {
 
@@ -338,7 +339,9 @@ private Generation buildGeneration(Choice choice, Map<String, Object> metadata)
 
 		String textContent = choice.message().content();
 		String reasoningContent = choice.message().reasoningContent();
-
+		if (textContent == null && !toolCalls.isEmpty()) {
+			textContent = "__tool_call__";
+		}
 		DeepSeekAssistantMessage assistantMessage = new DeepSeekAssistantMessage(textContent, reasoningContent,
 				metadata, toolCalls);
 		return new Generation(assistantMessage, generationMetadataBuilder.build());

spring-ai-client-chat/src/test/java/org/springframework/ai/chat/client/ChatClientResponseTests.java

@@ -17,9 +17,12 @@
 package org.springframework.ai.chat.client;
 
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.junit.jupiter.api.Test;
+import org.springframework.ai.chat.messages.AssistantMessage;
+import org.springframework.ai.chat.model.Generation;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
@@ -28,6 +31,7 @@
  * Unit tests for {@link ChatClientResponse}.
  *
  * @author Thomas Vitale
+ * @author Dongha Koo
  */
 class ChatClientResponseTests {
 
@@ -82,4 +86,15 @@ void whenMutateThenImmutableContext() {
 		assertThat(response.context()).containsEntry("key", "value");
 	}
 
+	@Test
+	void whenAssistantMessageHasOnlyToolCalls_thenContentIsToolCallMarker() {
+		var toolCall = new AssistantMessage.ToolCall("tool-1", "function", "doSomething", "{\"foo\":\"bar\"}");
+		var assistantMessage = new AssistantMessage(null, Map.of(), List.of(toolCall), List.of());
+
+		assertThat(assistantMessage.getDisplayText()).isEqualTo("__tool_call__");
+
+		var generation = new Generation(assistantMessage);
+		assertThat(generation.getOutput().getDisplayText()).isEqualTo("__tool_call__");
+	}
+
 }

spring-ai-model/src/main/java/org/springframework/ai/chat/messages/AssistantMessage.java

@@ -33,6 +33,7 @@
  *
  * @author Mark Pollack
  * @author Christian Tzolov
+ * @author Dongha Koo
  * @since 1.0.0
  */
 public class AssistantMessage extends AbstractMessage implements MediaContent {
@@ -104,4 +105,16 @@ public record ToolCall(String id, String type, String name, String arguments) {
 
 	}
 
+	/**
+	 * Returns a safe, non-null text representation. If text is null or empty and
+	 * toolCalls exist, returns "__tool_call__".
+	 */
+	public String getDisplayText() {
+		String text = super.getText();
+		if ((text == null || text.trim().isEmpty()) && this.hasToolCalls()) {
+			return "__tool_call__";
+		}
+		return (text != null) ? text : "";
+	}
+
 }