Configuring Custom Models: Vision & Parallel Tool Calls

Overview

This page documents configuration patterns for using custom models (e.g. Qwen, custom fine-tunes) with LiteLLM as a proxy and OpenCode as the agentic frontend.

When using non-standard models, both LiteLLM and OpenCode require explicit configuration to advertise and support advanced capabilities like vision (image input) and parallel tool calls. Without proper setup, these features silently fail.

Vision / Multimodal Support for Custom Models

Configuring image support for custom models in a llama.cpp + LiteLLM + opencode setup.

The Problem

LiteLLM does not infer vision support for arbitrary custom_openai models. OpenCode also performs its own preflight modality check and does not read LiteLLM's supports_vision metadata for custom providers. Both layers must be configured independently.

LiteLLM Side

Add supports_vision: true under the LiteLLM model's model_info so LiteLLM and its model metadata advertise image capability
LiteLLM's OpenAI-compatible/custom OpenAI chat path does not normally strip OpenAI-style image blocks — it preserves image_url content and normalizes string image URLs into { "url": ... }

OpenCode Side

For opencode custom providers, image support must be declared per model with modalities.input including image
If modalities is set, include both input and output
Without modalities.input: ["text", "image"], attachments get replaced with an error: ERROR: Cannot read image (this model does not support image input). Inform the user.

Working OpenCode Model Shape

"Qwen3.6-35B-A3B": {
  "name": "Qwen3.6-35B-A3B",
  "limit": {
    "context": 262144,
    "output": 65536
  },
  "modalities": {
    "input": ["text", "image"],
    "output": ["text"]
  }
}

Parallel Tool Calls

Enabling parallel tool calls for a custom model served through LiteLLM, used by OpenCode.

The Problem

The parallel_tool_calls: true in model_info is metadata only — it does not auto-forward to the API. Without explicit pass-through, the downstream server never receives the signal.

OpenCode Side

No config change needed in opencode.jsonc
OpenCode uses the AI SDK which auto-detects parallel tool call capability from the provider
The tool_call boolean in model config is for declaring tool support, not parallel behavior
Your setup (jamesravey_litellm provider with @ai-sdk/openai-compatible) handles this at the SDK level

LiteLLM Side

Fix: Add parallel_tool_calls inside litellm_params:

"litellm_params": {
  ...
  "parallel_tool_calls": true
}

Prerequisites

LiteLLM must be v1.61.0+ (parallel tool calls pass-through added then)
The downstream inference server (vLLM, TGI, etc.) must support parallel tool calls

Files Reference

~/.config/opencode/opencode.jsonc — OpenCode model config (no changes needed for parallel tool calls)
LiteLLM model config — add parallel_tool_calls to litellm_params and supports_vision: true to model_info