Unable to submit request because it must include at least one parts field

[ankargren]

Describe the bug A root agent is unable to transfer to subagents.

To Reproduce Steps to reproduce the behavior:

1. Install @google/adk v0.2.0
2. Add:

import { LlmAgent } from '@google/adk';

export const jokeAgent = new LlmAgent({
  name: 'tell_a_joke_agent',
  model: 'gemini-2.0-flash',
  description: 'Tell a joke',
  instruction: `You are a helpful assistant that tells a joke.`,
});

export const philosophicalAgent = new LlmAgent({
  name: 'philosophical_agent',
  model: 'gemini-2.0-flash',
  description: 'Say a philosophical statement',
  instruction: `You are a helpful assistant that says a philosophical statement.`,
});

export const rootAgent = new LlmAgent({
  name: 'root_agent',
  model: 'gemini-2.0-flash',
  description: 'Root agent',
  instruction: `You are a helpful assistant that is the root agent.`,
  subAgents: [jokeAgent, philosophicalAgent],
});

3. Start the local web UI and trigger one of the subagents, like tell me a joke
4. See the error appear: Unable to submit request because it must include at least one parts field, which describes the prompt input. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

Expected behavior It should be able to transfer to a subagent

Screenshots

Desktop (please complete the following information):

• OS: macOS (Darwin 25.1.0, arm64)
• TS version/environment: 5.9.3
• ADK version(see maven dependency): 0.2.0