Why the Session title Fails to Auto-Update for Vertex Models on OpenCode Desktop #31879

Description

Why the Title Fails to Auto-Update for Vertex Models
The issue lies in a specific quirk of how OpenCode constructs the prompt for title generation compared to how Google's Vertex / Gemini API expects to receive messages.

Here is exactly what happens:

The Hardcoded Title Prompt: Inside OpenCode's source code, the function responsible for auto-generating titles (SessionPrompt.ensureTitle) constructs the background API request like this:
messages: [
{ role: "user", content: "Generate a title for this conversation:\n" },
...msgs // <-- This is your actual chat history
]
Consecutive User Messages: Because your chat history (...msgs) also starts with your initial user prompt, OpenCode ends up sending an array that starts with two consecutive user messages in a row.
The API Difference:
Providers like OpenAI and Anthropic are completely fine with receiving multiple user messages back-to-back. They just process them sequentially, which is why title generation works flawlessly for them.
However, Google's Vertex AI (Gemini) API strictly enforces alternating roles. It requires the sequence to be exactly user → model → user → model.
The Silent Failure: When the Vertex API receives the two consecutive user messages from OpenCode, it instantly rejects the request with a 400 Bad Request / Invalid Argument error. Because this happens in an asynchronous background task, OpenCode simply swallows the error, aborts the title generation, and leaves your session stuck on the default "New session - [Timestamp]" title.

Plugins

NA

OpenCode version

OpenCode Desktop Version 1.16.2 (1.16.2)

Steps to reproduce

1. Launch OpenCode Desktop.
2. Click on New Session.
3. Select Provider 'Vertex' (API key) - not Vertex (Anthropic).
4. Select and model (example Gemini 3.5 Flash).
5. Send the agent a prompt/chat message.
6. Look above on the Session Summary or in the workspace session history, it will remain 'New session' and not use the title generation.
7. Works perfectly fine in new sessions when using any other provider.

Screenshot and/or share link

Operating System

Mac OS 26.5.1 (25F80)

Terminal

OpenCode Desktop Version 1.16.2 (1.16.2)