deepset-ai/haystack · error · ValueError

At least one of 'mime_type_meta_field' or 'file_path_meta_fi

Error message

At least one of 'mime_type_meta_field' or 'file_path_meta_field' must be provided to determine MIME types.

What it means

DocumentTypeRouter.__init__ raises this ValueError when neither mime_type_meta_field nor file_path_meta_field is provided, because the router has no way to determine each document's MIME type.

Source

Thrown at haystack/components/routers/document_type_router.py:89

            (for example: `["text/plain", "audio/x-wav", "image/jpeg"]`).
        :param mime_type_meta_field:
            Optional name of the metadata field that holds the MIME type.
        :param file_path_meta_field:
            Optional name of the metadata field that holds the file path. Used to infer the MIME type if
            `mime_type_meta_field` is not provided or missing in a document.
        :param additional_mimetypes:
            Optional dictionary mapping MIME types to file extensions to enhance or override the standard
            `mimetypes` module. Useful when working with uncommon or custom file types.
            For example: `{"application/vnd.custom-type": ".custom"}`.

        :raises ValueError: If `mime_types` is empty or if both `mime_type_meta_field` and `file_path_meta_field` are
            not provided.
        """
        if not mime_types:
            raise ValueError("The list of mime types cannot be empty.")

        if mime_type_meta_field is None and file_path_meta_field is None:
            raise ValueError(
                "At least one of 'mime_type_meta_field' or 'file_path_meta_field' must be provided to determine MIME "
                "types."
            )
        self.mime_type_meta_field = mime_type_meta_field
        self.file_path_meta_field = file_path_meta_field

        if additional_mimetypes:
            for mime, ext in additional_mimetypes.items():
                mimetypes.add_type(mime, ext)

        self._mime_type_patterns = []
        for mime_type in mime_types:
            try:
                pattern = re.compile(mime_type)
            except re.error as e:
                raise ValueError(f"Invalid regex pattern '{mime_type}'.") from e
            self._mime_type_patterns.append(pattern)

View on GitHub (pinned to e318778c9b)

Solutions

1. Pass mime_type_meta_field='meta.mimetype' (or your metadata key) and/or file_path_meta_field='meta.file_path'
2. Ensure documents carry the referenced metadata key at runtime

Example fix

# before
router = DocumentTypeRouter(mime_types=["text/html"])
# after
router = DocumentTypeRouter(mime_types=["text/html"], mime_type_meta_field="meta.mimetype")

if mime_type_meta_field is None and file_path_meta_field is None:
    raise ValueError("Provide mime_type_meta_field or file_path_meta_field")

def has_meta_field(args: dict) -> bool:
    return args.get("mime_type_meta_field") is not None or args.get("file_path_meta_field") is not None

try:
    router = DocumentTypeRouter(mime_types=["text/html"])
except ValueError as e:
    if "must be provided" in str(e):
        router = DocumentTypeRouter(mime_types=["text/html"], mime_type_meta_field="meta.mimetype")
    else:
        raise

When it happens

Trigger: DocumentTypeRouter(mime_types=['text/html']) with both meta field arguments left as None.

Common situations: Forgetting that MIME type detection relies on document metadata (custom field or file path); migrating code that dropped one of the two fields.

Understand the failure class

Background: Missing required parameter errors: what 'X is required' and 'the required X param is missing' mean, and how to fix them — this error's family across 27 libraries.

Related errors

• The list of mime types cannot be empty.
• The list of mime types cannot be empty.
• Invalid template for condition: {condition_value}
• Invalid template for output: {output!r} (type: {type(output)
• Invalid template for output: {output}

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/e16053a2f0508e1c. Report an issue: GitHub.