fix: filter empty-text parts in GoogleGenAiChatModel to prevent API errors

Fixes #4556

When the Google GenAI API returns a candidate whose parts contain no text
(e.g. thought-signature-only or server-side tool-invocation parts), the
response converter was producing AssistantMessages with empty content.
When these empty messages were stored in chat memory and sent in subsequent
requests, the Google API rejected them with 'missing parts field' errors,
breaking multi-turn conversations with tool calling.

Two fixes applied:

1. responseCandidateToGeneration: add filter on part.text() presence
   so only parts with actual text content produce generations.
   Uses part.text().get() after the filter guard (no orElse needed).

2. toGeminiContent: filter out Content items whose parts list is empty
   before building the API request, so a stale empty-content
   AssistantMessage in chat memory cannot cause a 400 error.

Also adds two unit tests in GoogleGenAiChatModelExtendedUsageTests
covering both the empty-part candidate response and the clean text-part
round-trip, so regressions are caught without a live API key.

Signed-off-by: anuragg-saxenaa <anuragg.saxenaa@gmail.com>

Author: anuragg-saxenaa

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");
+	}
+
 }