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Question

Request_size_mismatch error with FilesManager.CreateNewVersionUploadSessionAsync()

  • May 22, 2025
  • 2 replies
  • 25 views

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I'm trying to implement chunked uploads for new file versions, and unfortunately I'm stumped at the very first step!

 

When calling the following code to start the session, I get the below error:

 

var boxFileUploadSession = await this.Client.FilesManager.CreateNewVersionUploadSessionAsync(fileId, fileSize.ToString());

 

 

{"code":"request_size_mismatch","message":"Request body size didn't match Content-Length 19","request_id":"f9b14c82069f3bcc885970545782058b"}

 

In Fiddler I can see the request looks fine:

POST https://upload.box.com/api/2.1/files/***number removed for privacy***422/upload_sessions HTTP/1.1
User-Agent: Box Windows SDK v2.15.6
Authorization: Bearer [removed]
Content-Type: application/x-www-form-urlencoded
Host: upload.box.com
Cookie: box_visitor_id=[removed]; site_preference=desktop
Content-Length: 19
Accept-Encoding: gzip, deflate

file_size=***number removed for privacy***

 

As you can see the body content is actually 19 bytes.

 

Am I doing something wrong? I don't have any similar issues creating a normal session with CreateUploadSessionAsync().

 

 

Thanks,

Martin

 

2 replies

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Did you ever figure this out?

 

I'm getting the same error in PHP, but not in Postman.


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We resolved the issue.  The Content-Type needs to be application/json in the request header ("Content-Type: application/json").