fix(provider/gemini): Prevent empty parts in assistant messages

[monotykamary]

This PR addresses an issue where cancelling a chat request and promptly starting a new one could lead to an Error 400, Message: * GenerateContentRequest.contents[38].parts: contents.parts must not be empty. when using the Gemini provider.

The root cause was that assistant messages (model responses) were being included in the conversation history sent to the Gemini API even if they were empty (i.e., contained no text content or tool calls). This typically occurred during rapid interactions like cancelling and restarting a chat.

The fix modifies the convertMessages function in internal/llm/provider/gemini.go to ensure that an assistant message is only added to the history if msg.Content().String() != "" or len(msg.ToolCalls()) > 0. This prevents empty parts from being sent to the Gemini API.

Changes:

• Modified internal/llm/provider/gemini.go:
  • Added a conditional check before appending assistant messages to the history. An assistant message is now only appended if it has actual content (text or tool calls).

To Test:

1. Checkout this branch.
2. Rebuild the opencode application (go build -o opencode).
3. Configure Gemini as the AI provider.
4. Attempt to reproduce the error by starting a chat, cancelling it (Ctrl+X), and immediately starting a new chat or sending a new message.
5. The "contents.parts must not be empty" error should no longer occur.

┃ Hello. Who are you?

┃ I am OpenCode, an interactive CLI tool that helps users with software engineering tasks. Gemini 2.5 Pro (4ms)

┃ Write an essay on the Great Gatsby.

<cancel>

┃ ?

┃ I am a software engineering assistant and cannot write an essay. My purpose is to help with software development tasks. Gemini 2.5 Pro (9ms)

┃ What have I said so far to you?

┃ Here is a summary of our conversation so far:
┃
┃ 1. You said: "Hello. Who are you?"
┃ 2. I said: "I am OpenCode, an interactive CLI tool that helps users with software engineering tasks."
┃ 3. You said: "Write an essay on the Great Gatsby.?"
┃ 4. I said: "I am a software engineering assistant and cannot write an essay. My purpose is to help with software development tasks."
┃ 5. You said: "What have I said so far to you?" Gemini 2.5 Pro (5ms)

[kujtimiihoxha]

Thanks for the PR