kavin525zhang/llm_reasoning_framework/terminal-tools-pty-sessions

Persistent PTY sessions

PTY sessions are how you talk to interactive programs — programs that detect a terminal (isatty()) and behave differently when they don't see one. Use a session when:

• You need state to persist across calls (cd, env vars, sourced scripts)
• You're driving a REPL (python -i, mysql, psql, node, irb)
• A program demands an interactive prompt (sudo, ssh, npm login, gh auth login)

For everything else, terminal_exec is simpler. Sessions cost more (per-session bash process, ring buffer, idle-reaping bookkeeping) and have a hard cap (TERMINAL_TOOLS_MAX_PTY, default 8).

Why PTY (and not subprocess pipes)

Subprocess pipes break on every interactive program. The moment a program calls isatty() and sees False, it disables prompts, color, line-editing, password masking, progress bars — sometimes refuses to start. PTY makes us look like a real terminal so these programs work the same as in your shell.

The cost: PTY output includes terminal escape codes (cursor moves, color codes). The session captures them as-is; if you need clean text, strip ANSI escapes in your processing layer.

Bash on macOS — by deliberate policy

terminal_pty_open always invokes /bin/bash, regardless of the user's $SHELL. macOS users: yes, even when zsh is your interactive default. This is the terminal-tools-foundations policy applied to PTYs.

Reasons:

• zsh has command/builtin classes (zmodload, =cmd expansion, zpty, ztcp) that bypass bash-shaped security checks
• One shell behavior across platforms eliminates "works on Linux, breaks on macOS" surprises
• Bash is universal: any shell you've used will accept the bash subset

The bash invocation uses --norc --noprofile so user dotfiles don't leak in. PS1 is set to a unique sentinel for prompt detection. PS2 is empty. PROMPT_COMMAND is empty.

Three modes of terminal_pty_run

1. Default: send command, wait for prompt sentinel

terminal_pty_run(session_id, command="ls -la")
  → { output, prompt_after: True, ... }

The session writes ls -la\n, waits for the sentinel that its custom PS1 emits, returns the slice between submission and prompt. One in-flight call per session — a concurrent call returns a "session busy" error.

2. raw_send: send raw input, no waiting

terminal_pty_run(session_id, command="print('hi')\n", raw_send=True)
  → { bytes_sent: 12 }

For REPLs, vim keystrokes, password prompts. The session writes the bytes and returns immediately — it doesn't wait for a prompt (REPLs don't print bash's prompt; they print their own).

After a raw_send, you typically follow with:

3. read_only: drain currently-buffered output

terminal_pty_run(session_id, read_only=True, timeout_sec=2)
  → { output: "hi\n", more: False, ... }

Reads whatever the session has accumulated since the last drain, with a brief settle window. Use after raw_send to capture the REPL's response.

Custom prompt detection (expect)

When the command launches a program with its own prompt (Python REPL's >>> , mysql's mysql> , sudo's password prompt), the bash sentinel won't appear until the program exits. Override:

terminal_pty_run(session_id, command="python3", expect=r">>>\s*$", timeout_sec=10)
  → output up to and including ">>>", then control returns

For sudo:

terminal_pty_run(session_id, command="sudo -k && sudo whoami", expect=r"[Pp]assword:")
terminal_pty_run(session_id, command="<password>", raw_send=True, command="<password>\n")
terminal_pty_run(session_id, read_only=True, timeout_sec=5)

(Treat passwords carefully — they end up in the ring buffer.)

Always close

terminal_pty_close(session_id)

Leaked sessions count against TERMINAL_TOOLS_MAX_PTY (default 8). Idle reaping happens lazily on every _open call (sessions inactive longer than idle_timeout_sec, default 1800s, are dropped) — but don't rely on it. Close when you're done.

For unresponsive sessions, force=True skips the graceful "exit" attempt and goes straight to SIGTERM/SIGKILL.

Common patterns

Stateful navigation

sid = terminal_pty_open(cwd="/")
terminal_pty_run(sid, command="cd /var/log")
terminal_pty_run(sid, command="ls -la *.log | head")
terminal_pty_close(sid)

Python REPL

sid = terminal_pty_open()
terminal_pty_run(sid, command="python3", expect=r">>>\s*$")
terminal_pty_run(sid, command="x = 42", raw_send=True)
terminal_pty_run(sid, command="print(x*x)\n", raw_send=True)
result = terminal_pty_run(sid, read_only=True)  # → "1764\n>>> "
terminal_pty_run(sid, command="exit()", raw_send=True)
terminal_pty_close(sid)

ssh with host-key prompt

sid = terminal_pty_open()
terminal_pty_run(sid, command="ssh user@new-host", expect=r"\(yes/no.*\)\?")
terminal_pty_run(sid, command="yes\n", raw_send=True)
terminal_pty_run(sid, read_only=True, timeout_sec=10)  # password prompt or login