Feature Request: Allow custom model ID text input for Vertex AI (or add Gemini 3.1 models)

[distronode-com]

Hello! With the recent release of the new Gemini 3.1 models on Google Vertex AI (like gemini-3.1-flash-lite-preview and gemini-3.1-pro-preview-customtools), I am currently unable to select them because the Vertex AI provider UI uses a hardcoded dropdown list.

It would be fantastic if we could either get these new 3.1 models added to the dropdown, or even better, have the dropdown converted into (or accompanied by) a custom text input field so we can manually type in any new model IDs as they are released without having to wait for extension updates or hack our settings.json!

Thank you!

[roomote-v0]

I've analyzed this issue and here's my proposed implementation plan:

Current state:

• The UI already uses a ModelPicker component that supports typing custom model IDs (it shows a "Use custom model" option when text doesn't match a predefined model).
• The Gemini 3.1 models gemini-3.1-pro-preview and gemini-3.1-pro-preview-customtools are already in the vertex models list in packages/types/src/providers/vertex.ts.
• However, gemini-3.1-flash-lite-preview (mentioned in the issue) is missing from the list.
• The core problem: Both VertexHandler.getModel() (src/api/providers/vertex.ts) and AnthropicVertexHandler.getModel() (src/api/providers/anthropic-vertex.ts) silently ignore unknown model IDs and fall back to the default model. This means even if a user types a custom model ID in the UI, the backend discards it.

Proposed changes:

1. Add gemini-3.1-flash-lite-preview to packages/types/src/providers/vertex.ts with appropriate model info (context window, pricing, etc.)

2. Update VertexHandler.getModel() in src/api/providers/vertex.ts to pass through unknown model IDs with sensible default ModelInfo instead of falling back to the default model. When a model ID is not in the known list, use the user-provided ID with a generic ModelInfo (similar to how Bedrock handles custom ARNs).

3. Update AnthropicVertexHandler.getModel() in src/api/providers/anthropic-vertex.ts with the same custom model ID passthrough behavior for Claude models.

4. Add tests for the custom model ID passthrough in both handlers.

This approach solves both parts of the request: new models are added to the dropdown, and users can type any model ID for future models without waiting for extension updates.

Please mention @roomote in a comment saying that you approve this plan, and I'll proceed with the implementation.

[distronode-com]

@roomote I approve the plan as outlined.

[roomote-v0]

I've seen the approval and will proceed with the implementation plan. I'll open a PR to share progress shortly.

[roomote-v0]

Opened PR #12289 with my attempt at addressing this. It adds gemini-3.1-flash-lite-preview to the model list and enables custom model ID passthrough for both Vertex handlers so unknown model IDs are used as-is with sensible defaults instead of being silently discarded. Feedback welcome!