EC
http.ErrContentLength
GoERRORNotableHTTP

http: wrote more than the declared Content-Length

Quick Answer

Set Content-Length only after computing the exact response size, or omit it and let the server use chunked encoding.

What this means

Returned when a handler writes more bytes than specified in the Content-Length header. The extra bytes are silently discarded by the server.

Why it happens
  1. 1Handler sets Content-Length header to a value smaller than the actual body
  2. 2Reusing a ResponseWriter after content has already been flushed

Fix

Buffer response before setting header

Buffer response before setting header
var buf bytes.Buffer
fmt.Fprintf(&buf, "hello")
w.Header().Set("Content-Length",
    strconv.Itoa(buf.Len()))
w.Write(buf.Bytes())

Why this works

Computing length from the buffer ensures Content-Length matches the actual body.

Code examples
Detectgo
_, err := w.Write(extra)
if errors.Is(err, http.ErrContentLength) {
    log.Println("content-length mismatch")
}
Buffer firstgo
var b bytes.Buffer
fmt.Fprint(&b, data)
w.Header().Set("Content-Length",
    strconv.Itoa(b.Len()))
Omit headergo
// Let HTTP/1.1 use chunked encoding
fmt.Fprint(w, body)
Sources
Official documentation ↗

Go standard library

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