PrefectHQ/fastmcp · error · ValueError

No image data available

Error message

No image data available

What it means

Image._get_data base64-encodes the image from its path or in-memory bytes. If both self.path and self.data are somehow empty at read time (the constructor normally prevents this, but direct instantiation via __new__/deserialization or a falsy path can bypass it), it raises ValueError('No image data available'). This means the object holds no usable image source.

Source

Thrown at fastmcp_slim/fastmcp/utilities/types.py:291

        if self.path:
            # Workaround for WEBP in Py3.10
            mimetypes.add_type("image/webp", ".webp")
            resp = mimetypes.guess_type(self.path, strict=False)
            if resp and resp[0] is not None:
                return resp[0]
            return "application/octet-stream"
        return "image/png"  # default for raw binary data

    def _get_data(self) -> str:
        """Get raw image data as base64-encoded string."""
        if self.path:
            with open(self.path, "rb") as f:
                data = base64.b64encode(f.read()).decode()
        elif self.data is not None:
            data = base64.b64encode(self.data).decode()
        else:
            raise ValueError("No image data available")
        return data

    def to_image_content(
        self,
        mime_type: str | None = None,
        annotations: Annotations | None = None,
    ) -> mcp_types.ImageContent:
        """Convert to MCP ImageContent."""
        data = self._get_data()

        return mcp_types.ImageContent(
            type="image",
            data=data,
            mime_type=mime_type or self._mime_type,
            annotations=annotations or self.annotations,
        )

    def to_data_uri(self, mime_type: str | None = None) -> str:

View on GitHub (pinned to 1f02114297)

Solutions

1. Recreate the Image with a valid path or non-empty data
2. Check img.path / img.data before calling to_image_content/to_data_uri
3. Verify the path is a non-empty string and the file exists

Example fix

// before
img = Image(path='')  # falsy path, no data
content = img.to_image_content()  # raises
// after
img = Image(path='img.png')
content = img.to_image_content()

def to_content(img: Image):
    if not img.path and img.data is None:
        raise ValueError("Image has no path or data; rebuild it")
    return img.to_image_content()

def has_image_data(img: Image) -> bool:
    return bool(img.path) or img.data is not None

try:
    content = img.to_image_content()
except ValueError:
    content = None  # or rebuild the Image from the original source

When it happens

Trigger: Calling to_image_content() or to_data_uri() on an Image whose path and data are both unset — typically an object created through bypassed validation, unpickled state, or path='' (falsy) with data=None.

Common situations: Deserializing/serializing Image objects where the path attribute was dropped; constructing with an empty-string path that passes neither-check loosely; mutating .path/.data after construction.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Either path or data must be provided
• Only one of path or data can be provided
• No audio data available
• Cannot resolve tool reference: {fn!r}
• Cannot determine tool name for {fn!r}

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/6dd99143c8851d9f. Report an issue: GitHub.