GH-3118: Fix NPE in OpenAIChatModel compatible streaming api with vertex ai gemini

Fixes: #3118

Signed-off-by: jonghoon park <dev@jonghoonpark.com>

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/OpenAiChatModel.java

@@ -104,6 +104,7 @@ import org.springframework.util.StringUtils;
  * @author Ilayaperumal Gopinathan
  * @author Alexandros Pappas
  * @author Soby Chacko
+ * @author Jonghoon Park
  * @see ChatModel
  * @see StreamingChatModel
  * @see OpenAiApi
@@ -304,15 +305,15 @@ public class OpenAiChatModel implements ChatModel {
 			Flux<ChatResponse> chatResponse = completionChunks.map(this::chunkToChatCompletion)
 				.switchMap(chatCompletion -> Mono.just(chatCompletion).map(chatCompletion2 -> {
 					try {
-						@SuppressWarnings("null")
-						String id = chatCompletion2.id();
+						// If an id is not provided, set to "NO_ID" (for compatible APIs).
+						String id = chatCompletion2.id() == null ? "NO_ID" : chatCompletion2.id();
 
 						List<Generation> generations = chatCompletion2.choices().stream().map(choice -> { // @formatter:off
 							if (choice.message().role() != null) {
 								roleMap.putIfAbsent(id, choice.message().role().name());
 							}
 							Map<String, Object> metadata = Map.of(
-									"id", chatCompletion2.id(),
+									"id", id,
 									"role", roleMap.getOrDefault(id, ""),
 									"index", choice.index(),
 									"finishReason", choice.finishReason() != null ? choice.finishReason().name() : "",

models/spring-ai-openai/src/test/java/org/springframework/ai/openai/chat/proxy/VertexAIGeminiWithOpenAiChatModelIT.java

@@ -0,0 +1,121 @@
+/*
+ * Copyright 2025-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.ai.openai.chat.proxy;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledIfEnvironmentVariable;
+import org.springframework.ai.chat.messages.AssistantMessage;
+import org.springframework.ai.chat.messages.Message;
+import org.springframework.ai.chat.messages.UserMessage;
+import org.springframework.ai.chat.model.ChatResponse;
+import org.springframework.ai.chat.model.Generation;
+import org.springframework.ai.chat.prompt.Prompt;
+import org.springframework.ai.chat.prompt.SystemPromptTemplate;
+import org.springframework.ai.model.SimpleApiKey;
+import org.springframework.ai.model.tool.ToolCallingManager;
+import org.springframework.ai.openai.OpenAiChatModel;
+import org.springframework.ai.openai.OpenAiChatOptions;
+import org.springframework.ai.openai.api.OpenAiApi;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Bean;
+import org.springframework.core.io.Resource;
+import reactor.core.publisher.Flux;
+
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Jonghoon Park
+ */
+@SpringBootTest(classes = VertexAIGeminiWithOpenAiChatModelIT.Config.class)
+@EnabledIfEnvironmentVariable(named = "GEMINI_API_KEY", matches = ".+")
+class VertexAIGeminiWithOpenAiChatModelIT {
+
+	private static final String VERTEX_AI_GEMINI_BASE_URL = "https://generativelanguage.googleapis.com";
+
+	private static final String VERTEX_AI_GEMINI_DEFAULT_MODEL = "gemini-2.0-flash";
+
+	@Value("classpath:/prompts/system-message.st")
+	private Resource systemResource;
+
+	@Autowired
+	private OpenAiChatModel chatModel;
+
+	@Test
+	void roleTest() {
+		UserMessage userMessage = new UserMessage(
+				"Tell me about 3 famous pirates from the Golden Age of Piracy and what they did.");
+		SystemPromptTemplate systemPromptTemplate = new SystemPromptTemplate(this.systemResource);
+		Message systemMessage = systemPromptTemplate.createMessage(Map.of("name", "Bob", "voice", "pirate"));
+		Prompt prompt = new Prompt(List.of(userMessage, systemMessage));
+		ChatResponse response = this.chatModel.call(prompt);
+		assertThat(response.getResults()).hasSize(1);
+		assertThat(response.getResults().get(0).getOutput().getText()).contains("Blackbeard");
+	}
+
+	@Test
+	void streamRoleTest() {
+		UserMessage userMessage = new UserMessage(
+				"Tell me about 3 famous pirates from the Golden Age of Piracy and what they did.");
+		SystemPromptTemplate systemPromptTemplate = new SystemPromptTemplate(this.systemResource);
+		Message systemMessage = systemPromptTemplate.createMessage(Map.of("name", "Bob", "voice", "pirate"));
+		Prompt prompt = new Prompt(List.of(userMessage, systemMessage));
+		Flux<ChatResponse> flux = this.chatModel.stream(prompt);
+
+		List<ChatResponse> responses = flux.collectList().block();
+		assertThat(responses.size()).isGreaterThan(1);
+
+		String stitchedResponseContent = responses.stream()
+			.map(ChatResponse::getResults)
+			.flatMap(List::stream)
+			.map(Generation::getOutput)
+			.map(AssistantMessage::getText)
+			.collect(Collectors.joining());
+
+		assertThat(stitchedResponseContent).contains("Blackbeard");
+	}
+
+	@SpringBootConfiguration
+	static class Config {
+
+		@Bean
+		public OpenAiApi chatCompletionApi() {
+			return OpenAiApi.builder()
+				.baseUrl(VERTEX_AI_GEMINI_BASE_URL)
+				.completionsPath("/v1beta/openai/chat/completions")
+				.apiKey(new SimpleApiKey(System.getenv("GEMINI_API_KEY")))
+				.build();
+		}
+
+		@Bean
+		public OpenAiChatModel openAiClient(OpenAiApi openAiApi) {
+			return OpenAiChatModel.builder()
+				.openAiApi(openAiApi)
+				.toolCallingManager(ToolCallingManager.builder().build())
+				.defaultOptions(OpenAiChatOptions.builder().model(VERTEX_AI_GEMINI_DEFAULT_MODEL).build())
+				.build();
+		}
+
+	}
+
+}