HTTP How to Calculate Content-Length multipart/form-data Including Binary

I'm working with a very obstinate API with arguably horrendous documentation (which is often also incorrect). I'm fairly certain my Cloud Flow is failing to POST a request to it containing multipart/form-data because I'm not passing the Content-Length header, at least that's the only thing I can deduce comparing the trace output of a curl command that is able to make the request vs what I have in my HTTP action.

I've resorted to using the smallest jpg that I think is possible (107 bytes) to try to reduce the potential for error, however I still haven't found a way to accurately calculate what the Content-Length actually would be.

For context, the file is uploaded to a sharepoint site, and then needs to be sent to a third party API as binary. So the reference to "Body" inside the body is actually the output from a Sharepoint Get File Contents action that happens before the API request. Also the current Content-Length value is being set by a Compose step I've been using to try to calculate it.

Does anyone know how to calculate what the body would be including the binary that's in there?

BTW, this one is close, but I'm not sending it as Base64

https://community.powerplatform.com/forums/thread/details/?threadid=3724db2d-b006-469e-9ef1-6fd741a01e66

Categories:

Business Process Flows

@CU20030127-0

when dealing with multipart/form-data and binary uploads in Power Automate, especially to a poorly documented API, the lack of a Content-Length header can cause issues. Here's how you can approach this:

✅ Understanding `Content-Length` in Multipart Requests

The Content-Length header must reflect the total byte size of the entire HTTP body, including:

Boundary markers
Headers for each part
Binary content
Line breaks and formatting

This is not just the size of the binary file, but the entire formatted multipart body.

🛠️ How to Calculate `Content-Length` (Workaround)

Since Power Automate doesn’t expose the raw byte size of the full HTTP body, you can try this workaround:

Option 1: Use a Custom Azure Function or Logic App

Create an Azure Function that accepts the binary and builds the multipart body.
It can calculate the exact Content-Length and send the request.
Power Automate can call this function via HTTP.

Option 2: Use a Prebuilt Tool to Simulate the Request

Use Postman or curl to build the request manually.
Capture the full request body and calculate its byte size.
Use that value in Power Automate’s HTTP header.

Option 3: Estimate in Power Automate

If you must stay within Power Automate:

Use length(outputs('Get_file_content')) to get the binary size.
Add estimated overhead for headers and boundaries (e.g., 300–500 bytes).
Use a Compose action to build the header:
{

"Content-Length": "

}

⚠️ This is not precise, but may work if the API is lenient.

🔍 Additional Tips
Ensure your Content-Type header is set to:

multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

(or whatever boundary you're using)

Make sure the binary is passed as-is, not Base64-encoded unless the API expects it.
Use “Get file content using path” from SharePoint to ensure you get the binary stream.