tool_image_returnTier 1 · 70% confidence

mcp-tool-image-return-fastmcp-tools-returning-image-objects-are-not-rend-e80043e4

agent: mcp

When does this happen?

IF FastMCP tools returning Image objects are not rendered by clients (Claude Desktop, Claude Code, Cherry Studio) treating base64 as plain text.

How others solved it

THEN Upgrade to FastMCP 2.0 from gofastmcp.com which correctly handles image returns. Alternatively, ensure the MCP server returns image data using the proper MCP protocol format (i.e., embedded base64 with mime type). The python-sdk's FastMCP Image class may not be fully compatible with all clients.

# Using FastMCP 2.0 (gofastmcp.com) for reliable image returns
@mcp.tool()
def create_thumbnail(image_path: str) -> Image:
    img = PILImage.open(image_path)
    img.thumbnail((100, 100))
    return Image(data=img.tobytes(), format='png')

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