Both functionTool & VertexRagRetrieval works partially

[harry-yoo-eb]

** Please make sure you read the contribution guide and file the issues in the right place. **
Contribution guide.

Describe the bug
400 returned by gemini 2.5 flash
only when

• functionResponse exists in contents when tools contain both function tools AND VertexRagRetrieval

To Reproduce
Steps to reproduce the behavior:

1. Prepare MultiToolAgent.java to execute
2. Update MultiToolAgent.java code with

        // Define the location for your service
        String location = "us-central1"; // Your target region

        // Construct the regional endpoint
        String endpoint = String.format("%s-aiplatform.googleapis.com:443", location);

        // Create service settings with the explicit endpoint
        VertexRagServiceSettings vertexRagServiceSettings =
                VertexRagServiceSettings.newBuilder().setEndpoint(endpoint).build();

        // Create the client with the new settings
        VertexRagServiceClient vertexRagServiceClient =
                VertexRagServiceClient.create(vertexRagServiceSettings);
...
...
                .tools(
                        FunctionTool.create(MultiToolAgent.class, "getWeather"),
                        new VertexAiRagRetrieval(
                                "my docs",
                                "This provides my docs",
                                vertexRagServiceClient,
                                "projects/{prj-id}/locations/us-central1",
                                List.of(RetrieveContextsRequest.VertexRagStore.RagResource.newBuilder().setRagCorpus("projects/{prj-id}/locations/us-central1/ragCorpora/{corpus_id}").build()),
                                null)
                )

3. Start MultiToolAgent.java
4. Ask questions related to function call
   -> 400 returned
   java.util.concurrent.ExecutionException: com.google.genai.errors.ClientException: 400 Bad Request. Request contains an invalid argument.

Expected behavior
LLM answers correctly, not 400

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Java version:
• ADK version(see maven dependency): 0.2.0

Additional context

• This happens only when input prompt triggers function when both functions tool & vertexRagRetrieval exist.
• That means, this does not happen with either tool
• This does not happen when input prompt triggers only vertexRagRetrieval in LLM. (answer is correct based on my RAG)

[onestardao]

This matches No.13 — Multi-Agent Chaos from our Problem Map, where multiple agents or tools overwrite or misalign logic.
We’ve compiled a full list of 16 common AI failure patterns like this one and helped 80+ developers fix them using a Semantic Firewall layer — no infra changes needed.

The project is MIT-licensed and reached 600+ GitHub stars in 60 days.
If you’re curious, leave a comment and I can share the link.