Bug: Custom data parts break subsequent messages with Gemini

[daneatmastra]

This issue was created from Discord post 1465700804320170072:

Issue:

After using writer.custom() in a tool, I cannot send new messages. Getting this error:
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

Reproduction:

execute: async (inputData, { writer }) => {
  await writer?.custom({
    type: "data-chart",
    data: inputData,
  });
  
  return { success: true };
}

After the tool executes and returns custom data parts, the next user message fails with the Gemini error above.

[daneatmastra]

Thank you for reporting this issue! We have labeled it for trio-tb and we will look into it as soon as possible.

If you're experiencing an error, please provide a minimal reproducible example whenever possible to help us resolve it quickly.