Vision Fallback

Auto-describe images via a vision model when the active model lacks vision support.

When you paste/drop an image into an OpenCode session whose active model can't read images (e.g. deepseek-v4-flash), this plugin transparently:

1. Detects the image part in the message
2. Calls a vision-capable model (e.g. mimo-v2.5)
3. Replaces the image with a text description
4. Main model receives the description as plain text

No manual model switching. No "this model does not support image input" errors.

Pipeline

┌─────────────┐
│  User        │
│  paste/drop  │
│  image       │
└──────┬──────┘
       ▼
┌─────────────┐     ┌──────────────────────────┐
│  OpenCode   │     │  Plugin                   │
│             │     │                           │
│  message    │────▶│  hook: messages.transform │
│  with image │     │                           │
│  FilePart   │     │  image part found?        │
└──────┬──────┘     └────────────┬─────────────┘
       │                         │ yes
       │                         ▼
       │              ┌──────────────────────────┐
       │              │  call vision model       │
       │              │  (mimo-v2.5)             │
       │              └────────────┬─────────────┘
       │                           ▼
       │              ┌──────────────────────────┐
       │              │  get text description    │
       │              └────────────┬─────────────┘
       │                           ▼
       │              ┌──────────────────────────┐
       │              │  replace image part      │
       │              │  with description text   │
       │              └────────────┬─────────────┘
       │                           │
       ▼                           ▼
┌──────────────────────────────────────────────┐
│  messages converted → no images left to strip │
└──────────────────────┬───────────────────────┘
                       ▼
┌──────────────────────────────────────────────┐
│  main model (deepseek-v4-flash)               │
│  receives text description                    │
└──────────────────────────────────────────────┘

Step-by-step

┌─────────────────────────────────────────────────────────────┐
│ 1. User pastes image → OpenCode adds FilePart (image/png)   │
│ 2. Hook fires (experimental.chat.messages.transform)        │
│ 3. Plugin detects image part                                 │
│ 4. Plugin calls vision model (mimo-v2.5) via API             │
│ 5. Vision model returns text description                     │
│ 6. Plugin replaces image part with [Image: ...] text part    │
│ 7. Messages converted → no images left to strip              │
│ 8. Main model (deepseek-v4-flash) receives text description  │
└─────────────────────────────────────────────────────────────┘

With vs without the plugin

sequenceDiagram
    participant U as User
    participant O as OpenCode
    participant P as Plugin
    participant V as Vision Model
    participant M as Main Model

    Note over U,M: WITHOUT plugin
    U->>O: paste image
    O->>M: ERROR: model does not support image input
    M-->>U: "I can't see the image"

    Note over U,M: WITH plugin
    U->>O: paste image
    O->>P: messages.transform hook
    P->>V: describe this image (mimo-v2.5)
    V-->>P: text description
    P->>O: replace image with description text
    O->>M: text description only
    M-->>U: reasoning based on image content

Why

Many strong coding models (deepseek-v4-flash, GLM, Haiku) don't support image input. Without a fallback, OpenCode replaces pasted images with an error string (ERROR: Cannot read image...), and the model has no idea what you showed it.

This plugin solves that by giving the model a text description of the image instead — so the main model can reason about the image without needing vision capability.

Install

Add the plugin to your opencode.json:

{
  "plugin": ["./opencode-vision-fallback/src/index.ts"]
}

Or via npm (once published):

{
  "plugin": ["opencode-vision-fallback"]
}

Configuration

All options are optional. Configure via the plugin options array:

{
  "plugin": [["opencode-vision-fallback", {
    "vision_model": "mimo-v2.5",
    "base_url": "https://opencode.ai/zen/go/v1",
    "api_key_env": "OPENCODE_API_KEY",
    "auth_provider": "opencode-go",
    "max_tokens": 1000,
    "timeout_ms": 30000,
    "mime_prefix": "image/",
    "prompt": "Describe this image in detail. Focus on: text, UI elements, code, diagrams."
  }]]
}

Option	Default	Description
vision_model	mimo-v2.5	Vision-capable model id
base_url	https://opencode.ai/zen/go/v1	OpenAI-compatible base URL
api_key_env	OPENCODE_API_KEY	Env var holding the API key
auth_provider	opencode-go	Key in auth.json to fall back to
max_tokens	1000	Description length cap
timeout_ms	30000	Per-image request timeout
mime_prefix	image/	Which file mimes to process
prompt	(default)	Description prompt

API key resolution

The plugin looks for the key in this order:

1. Env var (api_key_env, default OPENCODE_API_KEY)
2. auth.json under auth_provider key (default opencode-go)

The auth.json path is ~/.local/share/opencode/auth.json, overridable via OPENCODE_AUTH_FILE.

How it works

The problem in OpenCode

OpenCode's provider transform (packages/opencode/src/provider/transform.ts) has an unsupportedParts() function. When the active model's capabilities.input doesn't include image, it replaces image parts with error text before the request reaches the LLM:

ERROR: Cannot read "clipboard" (this model does not support image input). Inform the user.

This happens at the provider layer — so by the time the model sees the message, the image is already gone.

The fix: intercept before the transform

This plugin hooks into OpenCode's experimental.chat.messages.transform plugin hook. In the session pipeline (packages/opencode/src/session/prompt.ts), this hook runs:

messages prepared
  → plugin.trigger("experimental.chat.messages.transform", ...)   ← plugin runs here
  → MessageV2.toModelMessagesEffect(msgs, model)                  ← images get stripped here

The plugin mutates output.messages in place — replacing each image FilePart with a text Part that carries the vision model's description. When unsupportedParts() later runs, there are no image parts left to strip.

Message format

OpenCode v2 message parts use the FilePart schema for attachments:

type FilePart = {
  type: "file"
  mime: string        // e.g. "image/png"
  filename?: string
  url: string         // data: URL or file path
  source?: FilePartSource
}

The plugin matches parts where type === "file" and mime starts with the configured mime_prefix (default image/).

Vision call

For each matched image, the plugin makes an OpenAI-compatible chat completion request:

POST {base_url}/chat/completions
{
  "model": "mimo-v2.5",
  "messages": [{
    "role": "user",
    "content": [
      { "type": "text", "text": "<prompt>" },
      { "type": "image_url", "image_url": { "url": "<image url>" } }
    ]
  }],
  "max_tokens": 1000
}

The returned description replaces the image part:

msg.parts[idx] = {
  type: "text",
  text: `[Image: ${description}]`
}

The main model then receives the description as plain text.

OpenCode features used

Feature	Where
Plugin hook experimental.chat.messages.transform	packages/opencode/src/session/prompt.ts — runs after messages are prepared, before LLM dispatch
Plugin options (PluginOptions second arg)	packages/opencode/src/plugin/index.ts — server(input, load.options)
Message V2 schema (FilePart)	packages/schema/src/v1/session.ts — type: "file", mime, url fields
Structured logging (client.app.log)	client.app.log({ body: { service, level, message } }) — goes to opencode.log, not the TUI
Auth storage	~/.local/share/opencode/auth.json — provider key lookup

Logging

Logs go to OpenCode's structured log (~/.local/share/opencode/log/opencode.log) tagged service: "vision-fallback" — nothing prints into the TUI chat.

level=INFO message="detected 1 image(s), routing to vision model"
level=INFO message="image replaced with text description"
level=INFO message="processed 1 image(s) via vision model"

Failure behavior

• No API key → image left for main model (degrades to current OpenCode behavior, logs a warning)
• Vision API error → image left untouched, error logged
• Timeout → aborts after timeout_ms, image left untouched
• Unknown mime → not processed

The plugin never breaks the main pipeline — all failures degrade gracefully.

License

MIT