GenerateContent method return nothing

[jeanphi-baconnais]

Hi 👋
I created a tool to create images from a prompt to change elements on a template. I use generateContent method (in synchrone) with some content part.

In the majority of case, Gemini gives me a new image from the template. But depending of the template file, the generateContent method return a object not null but with nothing in parts method.

On templates, I use the same weight (900 ko), I don't understand why Gemini doesn't return anything in some use cases. And I am asking how debug this behavior? Can I check log anywhere? I look in Log explorer but nothing ...

The code in case I make something wrong. I use the API key to execute this. I tried with Vertex but I have others error about configuration on my project.

var response = client.models.generateContent(
            "gemini-2.5-flash-image-preview",
            Content.fromParts(
                    Part.fromBytes(Files.readAllBytes(templateFile), "image/png")),
                    Part.fromBytes(Files.readAllBytes(file1), "image/png")),
                    Part.fromBytes(Files.readAllBytes(file2), "image/png")),
                    Part.fromText(prompt)
            ),
           null);

    for (Part part : Objects.requireNonNull(response.parts())) {
        if (part.inlineData().isPresent()) {

[hemasekhar-p]

Thank you for bringing this to our attention. The behavior you are observing is actually part of the API's safety design. When the model prevents generation due to Safety Filters or Recitation checks, the API returns a response with parts list as empty(means no content generated) and finishReasons explicitly set to SAFETY, RECITATION, or OTHER.

In your current implementation, the code iterates over response.parts() immediately. Because this list is empty during a safety block, the loop is skipped entirely, making it appear as though the method returned nothing without warning.

So, we recommend adding a check for the finishReason on the candidate before processing the parts. If the reason is anything other than STOP, you should handle the interruption rather than attempting to read the image or content directly. Please refer the bellow code and Safety feedback section In the below documentation.

Reference Doc

Solution Code :

package com.example.repros.dec2025;
import com.example.repros.constants.GenAIConstants;
import com.google.genai.Client;
import com.google.genai.types.Content;
import com.google.genai.types.GenerateContentResponse;
import com.google.genai.types.Part;
import com.google.genai.types.Candidate;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Base64;
import java.util.Objects;
public class GenerateContentMethodReturnNothing {
	
   private static final String TEMPLATE_PATH = "img1.jpeg";
   private static final String FILE_1_PATH = "img2.jpeg";
   private static final String FILE_2_PATH = "img3.jpeg";
  
   public static void verifyTesting() {
   	
   	try(Client client = Client.builder()
       	    .apiKey(GenAIConstants.GOOGLE_API_KEY)
       	    .vertexAI(false)
       	    .build()){
   	
   	byte[] templateBytes = loadFile(TEMPLATE_PATH);
       byte[] file1Bytes = loadFile(FILE_1_PATH);
       byte[] file2Bytes = loadFile(FILE_2_PATH);
      
       String prompt = "Generate a new image based on this template.";
   	GenerateContentResponse response = client.models.generateContent(
   		    "gemini-2.5-flash-image-preview",
   		    Content.fromParts(
   		        Part.fromBytes(templateBytes, "image/png"),
   		        Part.fromBytes(file1Bytes, "image/png"),
   		        Part.fromBytes(file2Bytes, "image/png"),
   		        Part.fromText(prompt)
   		    ),
   		    null
   		);
   		// 1. Check if we have candidates
   		if (response.candidates() != null && !response.candidates().isEmpty()) {
   		    Candidate candidate = response.candidates().get().getFirst();
   		    // 2. Check the Finish Reason
   		    // If it is NOT "STOP", the model blocked the generation
   		    if (!"STOP".equals(candidate.finishReason().get().toString())) {
   		        System.out.println("Generation stopped. Reason: " + candidate.finishReason().toString());
   		       
   		        // Print safety ratings to see what triggered the block
   		        if (candidate.safetyRatings() != null) {
   		            candidate.safetyRatings().get().forEach(rating ->
   		                System.out.println(rating.category() + ": " + rating.probability())
   		            );
   		        }
   		        return;
   		    }
   		}
   		// 3. Safe to process parts
   		for (Part part : Objects.requireNonNull(response.parts())) {
   		    if (part.inlineData().isPresent()) {
   		    	saveImage(part.inlineData().get().data().get().toString(), "output_image.png");
   		    }
   		}
   		
   	} catch (Exception e) {
           e.printStackTrace();
       }
   }
  
   private static byte[] loadFile(String filePath) throws IOException {
       Path path = Paths.get(filePath);
       if (!Files.exists(path)) {
           throw new IOException("File not found: " + filePath);
       }
       return Files.readAllBytes(path);
   }
   private static void saveImage(String data, String outputPath) {
       try (FileOutputStream fos = new FileOutputStream(outputPath)) {
           byte[] imageBytes = Base64.getDecoder().decode(data);
           fos.write(imageBytes);
           System.out.println("Saved image to: " + new File(outputPath).getAbsolutePath());
       } catch (IOException e) {
           System.err.println("Failed to save image: " + e.getMessage());
       }
   }
  
    
}

[jeanphi-baconnais]

Thank you for this response @hemasekhar-p ! I will try to implement this at the beginning of January.

[hemasekhar-p]

Once you tried and implemented, please let us know your feedback wether you are able to resolve your issue or still you are facing same issue. we are happy to help you further to investigation and resolve your issue.

[jeanphi-baconnais]

Hello @hemasekhar-p , since my upgrade with Gemini 3 I never have this problem. But that is clear and I add this control in my application.

[hemasekhar-p]

@jeanphi-baconnais , Thank you for your confirmation. if the issue is now resolved, please proceed with closing this ticket as completed?