Error on fetch with stream in nextjs dev with --experimental-flag

[el-frontend]

Summary

Hi guys, I have a scenario where I need to create a route handler for file uploads and send the request using a stream. Something like this:

    const response = await fetch(url, {
      headers: {
        "Content-Type": "application/octet-stream",
        "X-File-Size": String(file.size),
      },
      method: "POST",
      body: file.stream(),
      // TS doesn’t know this yet; add it anyway:
      // @ts-expect-error duplex is required for streaming requests
      duplex: "half",
    });

The browser blocks the request with the following error: net::ERR_ALPN_NEGOTIATION_FAILED (see image).
I’m running my application with the --experimental-https flag to enable HTTPS so that streaming requests can work with fetch.

pnpm run dev --experimental-https

When I run:

curl -Iv --http2 https://localhost:300

I get this response:

* Host localhost:3000 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:3000...
* Connected to localhost (::1) port 3000
* ALPN: curl offers h2,http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
*  CAfile: /etc/ssl/cert.pem
*  CApath: none
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* (304) (IN), TLS handshake, CERT verify (15):
* (304) (IN), TLS handshake, Finished (20):
* (304) (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / AEAD-AES256-GCM-SHA384 / [blank] / UNDEF
* ALPN: server accepted http/1.1
* Server certificate:
*  subject: O=mkcert development certificate; [email protected]
*  start date: Oct 17 18:27:53 2025 GMT
*  expire date: Jan 17 19:27:53 2028 GMT
*  subjectAltName: host "localhost" matched cert's "localhost"
*  issuer: O=mkcert development CA; [email protected]; CN=mkcert [email protected]
*  SSL certificate verify ok.
* using HTTP/1.x
> HEAD / HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/8.7.1
> Accept: */*
> 

It looks like the certificate is fine. Any idea what happened here? I’m using Chrome.

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