How to fix scp: Received message too long in SSH

Run a non-interactive remote command and watch for any output. A clean account should print nothing:

ssh user@host /bin/true

If you see any text, spaces, or banner output, your login scripts are writing to the stream that SFTP needs to keep clean. That stray output is the root cause.

For a non-login interactive shell, bash reads ~/.bashrc. For a login shell, it reads /etc/profile then the first of ~/.bash_profile, ~/.bash_login, ~/.profile. Inspect them and the system-wide files for anything that produces output:

cat ~/.bashrc
cat ~/.bash_profile
cat ~/.profile
cat /etc/profile
cat /etc/bash.bashrc
ls /etc/profile.d/

Look for echo, printf, cat, fortune, neofetch, motd, banner, or any command whose output is not redirected. On csh/tcsh accounts check ~/.cshrc and ~/.login.

Wrap any output-producing commands so they only run for interactive sessions. The most reliable guard checks bash's own interactivity flag; you can additionally require a TTY on stdout:

# In ~/.bashrc — skip everything below for non-interactive shells
case $- in
  *i*) ;;      # interactive: continue
  *) return ;; # non-interactive (SFTP/SCP/remote command): stop here
esac

# From here on it is safe to print
if [ -t 1 ]; then
  echo "Welcome to my server"
  fortune
fi

case $- in *i*) tests whether the shell is interactive; [ -t 1 ] tests whether stdout is a terminal. Either guard prevents output from leaking into the SFTP byte stream.

If a command must run during non-interactive sessions but you cannot suppress its chatter, redirect that chatter to stderr so it does not corrupt the SFTP data stream on stdout:

some_setup_command >&2

For purely cosmetic or diagnostic output (debug prints, fortune, banners), the cleaner fix is to delete it or move it inside the interactive guard from the previous step. As a rule of thumb, put login-only setup (run once per login) in ~/.bash_profile and interactive-shell tweaks (prompt, aliases) in ~/.bashrc, but the interactive guard — not the file location — is what actually keeps SFTP working.

Verify the stream is clean, then confirm SFTP connects:

ssh user@host /bin/true   # should print nothing
sftp user@host            # should connect with no error

If you administer the server and cannot rely on every user's startup files being clean, configure OpenSSH to use its built-in SFTP server. internal-sftp is handled inside sshd and does not spawn a login shell, so startup-file output can no longer corrupt the stream.

Edit /etc/ssh/sshd_config:

Subsystem sftp internal-sftp

Validate the config, then reload (reload re-reads config without dropping existing sessions):

sudo sshd -t
sudo systemctl reload ssh    # Debian/Ubuntu; use sshd on RHEL/Fedora

Note this fixes SFTP but not plain scp when scp uses its legacy shell-based mode; modern scp (OpenSSH 9+) defaults to the SFTP protocol and benefits from this change.