Adding message to gmail error "Payload parts count different from expected" - gmail

Question

I am adding a message to a gmail folder using this (example) URL:
https://www.googleapis.com/gmail/v1/users/user#domain.com/messages/import?uploadType=multipart
The body of the request looks like this:
--test_abc123
Content-Type: application/json; charset=UTF-8
{
"labelIds": [ "Label_525" ],
"raw": "RnJvbTogIlNlY3RpZ28gQ2VydGlmaWNh..."
}
--test_abc123--
The raw data is a base64 encoded standard MIME message that looks normal to me. The result of this POST is http error 400 with the error response "Payload parts count different from expected 2. Request payload parts count: 1".
I can supply the original MIME text if that is helpful, but let me emphasize that I have been running this code for several years without problem. I've tried different messages to test this out, but it appears that Google has changed something to break my software.
Is Google objecting to my raw data, or something about the MIME encoding? Any ideas what the problem could be?
---- Addendum ----
I have gotten a few messages to work, they seem to all have image or data attachments. However I really don't see any problem with the messages that are failing - I can import them into Office 365 or Thunderbird or anything else and they render just fine. As a test, I tried importing the message below, which was taken from the MIME RFC. It fails with the same error. I think that Google has changed something to make their MIME parser very fussy, but I don't see how I can fix my input data.
From: Nathaniel Borenstein <nsb#bellcore.com>
To: Ned Freed <ned#innosoft.com>
Subject: Sample message
MIME-Version: 1.0
Content-type: multipart/mixed; boundary="simple boundary"
This is the preamble. It is to be ignored, though it
is a handy place for mail composers to include an
explanatory note to non-MIME compliant readers.
--simple boundary
This is implicitly typed plain ASCII text.
It does NOT end with a linebreak.
--simple boundary
Content-type: text/plain; charset=us-ascii
This is explicitly typed plain ASCII text.
It DOES end with a linebreak.
--simple boundary--
This is the epilogue. It is also to be ignored.
Addendum 2: I tried a simple upload (using content-type header message/rfc822) and it worked, except the message was unlabeled. How
would I specify what label I want applied to a message? I was originally trying to follow the documentation here
link
which tells me to create the json body that I gave above. This allows me to specify the label. But I cannot seem to use
this body in a simple upload. The content type is either invalid, or what Gmail imports is just literally the json body,
it does not parse out the raw data. If you could point me to a specific example showing the URI, message body, http headers
(not java code) that would be very useful to me.

Answer 1

OK never mind, I got it working by adding an empty message/rfc822 part to the body of the multipart upload. That satisfies Google, and the empty part is ignored in favor of the raw data.

Answer 2

You are doing a multipart upload,see here:
The body of the request is formatted as a multipart/related content
type [RFC2387] and contains exactly two parts. The parts are
identified by a boundary string, and the final boundary string is
followed by two hyphens.
This is why it works only for your messages with images or attachments, since your message
--test_abc123
is only one part.
In the past there was no check if this condition is fulfilled, so you might have gotten away with using multipart for 1-part-messages.
But now it's not possible anymore, so if have a single-part message, you should use Simple upload.
If you do not know in advance how many parts your message has, you can always try the multipart first, implementing a try...catch statement, and implement a simple upload request within catch in case of failure.