MCP tool can no longer return image attachments

[psemeniuk]

Description

Regression where MCP tool results containing an image are no longer preserved/rendered correctly.

I reproduced this with a minimal standalone local MCP server whose tool returns a single tiny PNG image and no text.

1.17.4 works.

1.17.5+ is broken.

In the broken versions, the image is dropped before the model sees it. The model only receives/mentions a JSON success indicator like {"ok":true} instead of an actual image.

Plugins

none

OpenCode version

1.17.5

Steps to reproduce

1. Create a local MCP server with one tool, for example image_only_result.
2. Make that tool return a tool result with a single inline PNG image and no text:

{
  "content": [
    {
      "type": "image",
      "data": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAADElEQVR4nGP4z8AAAAMBAQDJ/pLvAAAAAElFTkSuQmCC",
      "mimeType": "image/png"
    }
  ],
  "structuredContent": { "ok": true },
  "isError": false
}

3. Create a minimal agent/command that calls the tool exactly once and then reports whether it can see an image.
4. Run it with opencode 1.17.4 and then with opencode 1.17.5.

Expected:

• the tool result keeps its image attachment
• the model can see the image

Actual on 1.17.5+:

• the tool result is collapsed to text/structured output
• no image attachment is preserved
• the model reports that it cannot see an actual image

Concrete result from my minimal repro:

• 1.17.4: model response said it could see a tiny PNG image
• 1.17.5+: model response said it only saw text/metadata / {"ok":true}

Screenshot and/or share link

No public share link. I can provide the minimal repro files if needed.

Operating System

macOS 26.5.1 arm64

Terminal

Ghostty