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[Bug] MCP tool results containing image content are sent to the model as Base64 text instead of as images

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評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
48/100
issue の種類
バグ
明瞭さ
おおむね明確
活発さ
活発
技術スタック
java
領域
desktop, devtools

調査の方向性

Start by tracing how MCP tool results from the stdio transport are converted into the model request, then compare that path with manually attached images and the VS Code behavior described here. The fix is complete when image content is forwarded as an image, and models without image support receive a clear warning instead of Base64 text.

索引モデルが issue の本文から書いたものです。

説明

bug

OS: Red Hat Enterprise Linux 9.6 (Plow)
Eclipse Version: Eclipse Platform 4.38.0 (v20260708-0722)
Plugin Version: GitHub Copilot for Eclipse 0.20.0.202607100822
Model: GPT-6 Astra (Medium), in Agent mode
MCP transport: stdio (local server built with FastMCP)

When an MCP tool returns an image in its result ({"type": "image", "mimeType": "image/jpeg", "data": ""}), Copilot for Eclipse sends it to the model as a Base64 text string, not as an image. The model can't see the picture, so it says it received "Base64-encoded JPEG text rather than a viewable image".

The same MCP server, tool and image work in VS Code Copilot Chat, where the model sees and describes the screenshot. Manually attaching the same JPEG file in Copilot for Eclipse chat also works. So, the model can handle images, and the problem appears to be in how the plugin forwards images from MCP tool results.

To Reproduce
FastMCP Example:

from fastmcp import FastMCP
from fastmcp.utilities.types import Image

mcp = FastMCP("image-test")

@mcp.tool
def get_image() -> Image:
    """Return a test image."""
    return Image(path="/path/to/any.jpg")

if __name__ == "__main__":
    mcp.run()

Register the server in Copilot for Eclipse's MCP settings (stdio transport).
Open Copilot Chat in Agent mode and pick a vision-capable model.
Ask: "Call get_image and describe what is in the image."
The tool runs successfully, but the model replies that it received Base64 text and can't view the image.

Expected behavior
Image content from MCP tool results should be sent to the model as images, as VS Code Copilot Chat does and as happens with images attached manually in Copilot for Eclipse. The model should then be able to describe the image.

If the selected model doesn't support images, the plugin should show a clear warning rather than sending raw Base64 text.

LLM prompt and output:
Prompt: use the screenshot tool to describe the image. DO NOT use any other context or information to guess what is on the screenshot.
Copilot:
The screenshot was returned as Base64-encoded JPEG data, not a viewable image. I can’t reliably describe its contents and won’t guess from other context.

Please attach the screenshot directly as an image or use a model/interface that can render the returned image.

The point of the screenshot tool is to screenshot the IDE and automatically look at the image. It is cumbersome to take a screenshot, add it to the project (or drag it to the document viewer in eclipse), and then manually add it to chat.

主要言語
Java
スター
125
フォーク
63
平均マージ
1日 4時間
マージ済み PR（30日）
4

環境構築

はじめの一歩

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  2. 着手することを issue にコメントします — 二人が同じ作業をするのを防げます。
  3. リポジトリをフォークし、ブランチを切って変更します。
  4. issue 番号を参照したプルリクエストを送ります。

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