Next15 - Large file upload with FormData drops binary data on request body.

[hkdnzk]

We are having a problem with file uploads using server actions and FormData.
While there seems to be no problem with version 15.4.x after upgrading to 15.5.x, we started having the following problem with file uploads.

Browser devtools shows that the file upload POST request (vie server action) was successful with status code 200 and FormData has the field file: (binary) on the body. But on the server side next just receives an empty body.

After testing we found out:

1. When there is other data in the request body they are successfully transferred to next server. (only one 'file' field from the body on the request is dropped, even when extra file fields (file1, file2, ...) with same file are sent they are also received by the server but only one file field (seemingly random) is dropped)
2. Problem does not always reproduce but with larger files it is more likely to trigger it.
3. Disabling all the middleware and setting serverActions.bodySizeLimit to a lot higher (>4x) than the file does not solve the problem.
4. Problem is only triggered with production builds. (Not during development)

Is there anyone else having this problem after upgrading to next 15.5.x ?

[icyJoseph]

What bodySizeLimit do you have? What kind of sizes are we talking about for files that disappear? Is perhaps the total file upload size being capped, leading to some being dropped if they'd push the total over the limit? Just making assumptions at this point. I did a quick try and it works fine in 15.5.x build+start locally.

[hkdnzk]

bodySizeLimit is set to 35mb and when trying with a 10kb file this problem never occurs. With a 150kb file about 50% of the time triggers the problem and with a 5mb file 100% of tries failed in our case.

[icyJoseph]

I can't repro right now, w/ 15.5.7:

I was actually testing with gb sizes before, cuz there's some issue with Node.js and files over 2gb.

Is this only observed in your deploy environment? or can you replicate locally w/ build+start? if so could you try to make a small repro?

[mehmetkursataydin]

@icyJoseph Hello, if you haven’t deleted your attempt, could you please try this with the middleware runtime set to Node.js?

[icyJoseph]

Mmm is that because of:

• https://nextjs.org/docs/15/app/api-reference/config/next-config-js/middlewareClientMaxBodySize or https://nextjs.org/docs/app/api-reference/config/next-config-js/proxyClientMaxBodySize (for proxy)?