tmux OSC 52 clipboard silently fails after reattaching session to terminal without Ms capability

There is currently no native tmux mechanism to detect terminal capabilities at runtime during a client attach event, nor is there a built-in hook to trigger warnings when the Ms terminfo capability is missing. Because tmux 3.2–3.4 evaluates the Ms capability statically based on the TERM variable of the attaching client, the failure is silent by design.

Likely Explanation

When set-clipboard external is active, tmux checks the terminfo database for the Ms (set clipboard) capability. If you attach from a terminal where the TERM environment variable points to a definition lacking Ms, tmux assumes the terminal cannot handle OSC 52 and drops the sequence to avoid printing garbage characters to the screen. Since this check happens at the client-attach level and not as a continuous negotiation, the session remains unaware that the clipboard is effectively disabled until a manual check is performed.

Resolution Steps

Since tmux lacks a runtime callback for this, you must implement a wrapper or a manual verification step to handle fallbacks.

1. Manual Capability Verification

To determine if your current terminal environment is the cause of the failure, run the following command on the host machine:

infocmp | grep Ms

If this returns no output, the terminfo for your current TERM does not advertise OSC 52 support, and tmux will block the sequences.

2. Bypass tmux for Verification

To confirm the terminal emulator itself supports OSC 52 regardless of the terminfo database, send the sequence directly:

printf "\e]52;c;$(printf "test-clipboard" | base64)\a"

If the text "test-clipboard" appears in your system clipboard, the issue is strictly the Ms terminfo entry.

3. Implementation of Fallbacks

Because you cannot detect this automatically within tmux, the most reliable approach is to use a shell script for clipboard operations that checks for a known "safe" terminal or environment variable before deciding between OSC 52 and a local tool (like xclip or wl-copy).

4. Nested Session Configuration

If you are using nested tmux sessions, ensure the following is set in both the inner and outer sessions (available in tmux 3.3+):

set -g allow-passthrough on

Diagnostic Requirement

To provide a more specific fallback script, please specify if you are using a consistent TERM variable across all clients or if the TERM changes based on the emulator used (e.g., xterm-256color vs kitty).