fix: filter empty-text parts in GoogleGenAiChatModel to prevent API errors (#4556)

models/spring-ai-google-genai/src/main/java/org/springframework/ai/google/genai/GoogleGenAiChatModel.java

@@ -728,19 +728,23 @@ protected List<Generation> responseCandidateToGeneration(Candidate candidate) {
 				.orElse(List.of())
 				.stream()
 				.filter(part -> part.toolCall().isEmpty() && part.toolResponse().isEmpty())
+				.filter(part -> StringUtils.hasText(part.text().orElse("")))
 				.map(part -> {
 					var partMessageMetadata = new HashMap<>(messageMetadata);
 					partMessageMetadata.put("isThought", part.thought().orElse(false));
 					return AssistantMessage.builder()
-						.content(part.text().orElse(""))
+						.content(part.text().get())
 						.properties(partMessageMetadata)
 						.build();
 				})
 				.map(assistantMessage -> new Generation(assistantMessage, chatGenerationMetadata))
 				.toList();
 
-			// If all parts were server-side tool invocations, return a single generation
-			// with empty text but with the server-side tool invocation metadata
+			// If all parts were server-side tool invocations or had no text content,
+			// return a single generation with the metadata (no empty text parts).
+			// Empty text AssistantMessages must not be added to chat history because
+			// the Google API rejects subsequent requests containing content with no parts.
+			// See: https://github.com/spring-projects/spring-ai/issues/4556
 			if (generations.isEmpty()) {
 				AssistantMessage assistantMessage = AssistantMessage.builder()
 					.content("")
@@ -1005,6 +1009,7 @@ private List<Content> toGeminiContent(List<Message> instructions) {
 				.role(toGeminiMessageType(message.getMessageType()).getValue())
 				.parts(messageToGeminiParts(message))
 				.build())
+			.filter(content -> content.parts().isPresent() && !content.parts().get().isEmpty())
 			.toList();
 
 		return contents;

models/spring-ai-google-genai/src/test/java/org/springframework/ai/google/genai/GoogleGenAiChatModelExtendedUsageTests.java

@@ -441,4 +441,77 @@ void testUsageWithNullMetadata() {
 		assertThat(genAiUsage.getCachedContentTokenCount()).isNull();
 	}
 
+	@Test
+	void testResponseCandidateWithEmptyTextPartsProducesNoEmptyGenerations() {
+		// Regression test for https://github.com/spring-projects/spring-ai/issues/4556
+		// A candidate whose parts have no text (e.g. thought-signature-only parts) must not
+		// produce an AssistantMessage with empty content, because the Google API rejects
+		// subsequent requests that include Content with an empty parts list.
+
+		// Part with no text — simulates a thought-signature-only part returned by Gemini
+		Part emptyTextPart = Part.builder().build(); // no .text(...)
+
+		Content responseContent = Content.builder().parts(emptyTextPart).build();
+
+		GenerateContentResponseUsageMetadata usageMetadata = GenerateContentResponseUsageMetadata.builder()
+			.promptTokenCount(10)
+			.candidatesTokenCount(0)
+			.totalTokenCount(10)
+			.build();
+
+		Candidate candidate = Candidate.builder().content(responseContent).index(0).build();
+
+		GenerateContentResponse mockResponse = GenerateContentResponse.builder()
+			.candidates(List.of(candidate))
+			.usageMetadata(usageMetadata)
+			.modelVersion("gemini-2.0-flash")
+			.build();
+
+		this.chatModel.setMockGenerateContentResponse(mockResponse);
+
+		UserMessage userMessage = new UserMessage("Hello");
+		Prompt prompt = new Prompt(List.of(userMessage));
+		ChatResponse response = this.chatModel.call(prompt);
+
+		// The response must have exactly one generation (the empty-content fallback),
+		// and that generation must not contain null content.
+		assertThat(response.getResults()).isNotNull();
+		assertThat(response.getResults()).hasSize(1);
+		assertThat(response.getResults().get(0).getOutput().getText()).isNotNull();
+	}
+
+	@Test
+	void testToGeminiContentFiltersOutEmptyPartContent() {
+		// Regression test for https://github.com/spring-projects/spring-ai/issues/4556
+		// toGeminiContent must not include Content items whose parts list is empty,
+		// because the Google API rejects requests with empty-parts Content entries.
+
+		// Candidate with a real text part
+		Part textPart = Part.builder().text("Hello from Gemini").build();
+		Content responseContent = Content.builder().parts(textPart).build();
+
+		GenerateContentResponseUsageMetadata usageMetadata = GenerateContentResponseUsageMetadata.builder()
+			.promptTokenCount(5)
+			.candidatesTokenCount(4)
+			.totalTokenCount(9)
+			.build();
+
+		Candidate candidate = Candidate.builder().content(responseContent).index(0).build();
+
+		GenerateContentResponse mockResponse = GenerateContentResponse.builder()
+			.candidates(List.of(candidate))
+			.usageMetadata(usageMetadata)
+			.modelVersion("gemini-2.0-flash")
+			.build();
+
+		this.chatModel.setMockGenerateContentResponse(mockResponse);
+
+		UserMessage userMessage = new UserMessage("Hi");
+		Prompt prompt = new Prompt(List.of(userMessage));
+		ChatResponse response = this.chatModel.call(prompt);
+
+		assertThat(response.getResults()).hasSize(1);
+		assertThat(response.getResults().get(0).getOutput().getText()).isEqualTo("Hello from Gemini");
+	}
+
 }