tmux for Agents

The Bash tool runs commands synchronously with a timeout
and cannot interact with programs that require a persistent TTY.
tmux provides programmatic session control:
create detached sessions, send keystrokes, and capture output,
all through non-interactive Bash commands.

Use tmux when you need to:

• Run interactive programs (REPLs, debuggers, database shells)
• Monitor long-running processes (servers, builds, watchers)
• Send input to a running process over time

Do not use tmux for simple one-shot commands.
The Bash tool handles those directly.

Session Setup

Socket Isolation

Never use the default tmux server.
Always use a dedicated socket to avoid interfering with user sessions.

TMUX_TMPDIR=$(mktemp -d)
TMUX="tmux -L agent"

Set TMUX_TMPDIR before any tmux command
so the socket file is created in an isolated directory.
Use TMUX_TMPDIR and -L agent consistently in every command.

Creating Sessions

TMUX_TMPDIR=$(mktemp -d) $TMUX new-session -d -s myrepl -x 200 -y 50

Required flags:

• -d - detached (never omit; agents cannot attach)
• -s <name> - named session (required for all subsequent commands)
• -x 200 -y 50 - explicit window size (prevents default 80x24 truncation)

Name sessions descriptively: python-repl, dev-server, debug-gdb.

Listing Sessions

$TMUX list-sessions

Check for existing sessions before creating new ones
to avoid duplicates.

Sending Input

Literal Text

Use -l (literal) mode to prevent key-name interpretation:

$TMUX send-keys -t myrepl -l 'print("hello world")'
$TMUX send-keys -t myrepl Enter

Always send Enter as a separate command after literal text.
Without -l, tmux interprets strings like C-c as control sequences.

Control Keys

Send control keys without -l:

$TMUX send-keys -t myrepl C-c      # interrupt
$TMUX send-keys -t myrepl C-d      # EOF
$TMUX send-keys -t myrepl C-l      # clear screen

Multi-Line Input

For multi-line input,
send each line separately with Enter between them:

$TMUX send-keys -t myrepl -l 'def greet(name):'
$TMUX send-keys -t myrepl Enter
$TMUX send-keys -t myrepl -l '    return f"Hello, {name}"'
$TMUX send-keys -t myrepl Enter
$TMUX send-keys -t myrepl Enter

Never paste large blocks at once.
REPLs may misinterpret rapid multi-line input.

Reading Output

Capture the current pane content:

$TMUX capture-pane -t myrepl -p -S -50

Flags:

• -p - print to stdout (required; without it, output goes to a paste buffer)
• -S -50 - start capture 50 lines back from cursor (negative = history)
• -J - join wrapped lines (add when output has long lines)

To capture the entire scrollback:

$TMUX capture-pane -t myrepl -p -S -

Always limit with -S -N when you only need recent output.
Full scrollback can be very large.

Synchronization

After sending a command,
wait for the program to produce output or return to a prompt.
Poll with a timeout:

for i in $(seq 1 30); do
  output=$($TMUX capture-pane -t myrepl -p -S -5)
  if echo "$output" | grep -q '>>> \|In \['; then
    break
  fi
  sleep 1
done

Adapt the grep pattern to match the expected prompt:

Program	Prompt pattern
Python	>>> |\\.\\.\\.
IPython	In \\[
Node.js	>
PostgreSQL	=# |=>
GDB	(gdb)
Shell	\\$

For long-running processes without a prompt (servers, builds),
poll for specific output strings:

for i in $(seq 1 60); do
  output=$($TMUX capture-pane -t myrepl -p -S -10)
  if echo "$output" | grep -q 'Listening on port'; then
    break
  fi
  sleep 2
done

Always set a finite upper bound on poll iterations.
Never poll indefinitely.

User Notification

After creating any tmux session,
always print the monitoring commands so the user can observe:

Session 'dev-server' started. Monitor with:
  TMUX_TMPDIR=/tmp/tmp.xxxx tmux -L agent attach -t dev-server

Include the full command with TMUX_TMPDIR path,
since the user needs it to connect to the isolated socket.

Cleanup

Kill sessions when they are no longer needed:

$TMUX kill-session -t myrepl

Kill the entire agent server when all work is done:

$TMUX kill-server

Clean up the socket directory:

rm -rf "$TMUX_TMPDIR"

Cleanup obligations:

• Kill sessions before creating replacements
• Kill the server when all sessions are done
• Always clean up on task completion, even if errors occurred
• Never leave orphaned sessions running

Common Issues

Python REPL needs extra Enter for blocks:
After indented blocks (def, if, for),
send an extra empty Enter to execute the block.

Stale output after send-keys:
Always poll or sleep before capture-pane.
send-keys returns immediately;
the program needs time to process input.

Socket not found errors:
Verify TMUX_TMPDIR is set to the same directory
used during session creation.
The socket lives at $TMUX_TMPDIR/agent.

Session name collisions:
Use list-sessions to check before creating.
Kill stale sessions rather than using duplicate names.

Output truncated at 80 columns:
Set explicit window dimensions with -x 200 -y 50
when creating the session.

Examples

Python REPL

TMUX_TMPDIR=$(mktemp -d) tmux -L agent new-session -d -s python -x 200 -y 50 python3

# Wait for prompt
for i in $(seq 1 15); do
  output=$(TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent capture-pane -t python -p -S -3)
  if echo "$output" | grep -q '>>>'; then break; fi
  sleep 1
done

# Send a command
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t python -l 'import sys; print(sys.version)'
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t python Enter

sleep 1
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent capture-pane -t python -p -S -5

Long-Running Server

TMUX_TMPDIR=$(mktemp -d) tmux -L agent new-session -d -s server -x 200 -y 50 'npm run dev'

# Wait for server ready
for i in $(seq 1 30); do
  output=$(TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent capture-pane -t server -p -S -10)
  if echo "$output" | grep -q 'ready\|listening\|started'; then break; fi
  sleep 2
done

echo "Session 'server' started. Monitor with:"
echo "  TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent attach -t server"

Debugger Session

TMUX_TMPDIR=$(mktemp -d) tmux -L agent new-session -d -s gdb -x 200 -y 50 'gdb ./myprogram'

# Wait for GDB prompt
for i in $(seq 1 15); do
  output=$(TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent capture-pane -t gdb -p -S -3)
  if echo "$output" | grep -q '(gdb)'; then break; fi
  sleep 1
done

# Set breakpoint and run
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t gdb -l 'break main'
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t gdb Enter
sleep 1
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t gdb -l 'run'
TMUX_TMPDIR=$TMUX_TMPDIR tmux -L agent send-keys -t gdb Enter