py-screen

A terminal multiplexer in Python, in the family of GNU screen and tmux: windows and splits that keep running when you detach, and a server you can restart, with new code or a new config, without losing a single shell.

With AI agents: pys-agents shows which window runs which Claude Code session, and after pys kill (or a crash) pys-sessions restore opens them all again, each with its session and folder (For AI agents):

A terminal workspace of my own, in Python: not a clone of screen or tmux, built for fun, made more powerful and improved by daily use. The design is in spec/; the current version is spec/spec.v3.md.

uvx py-screen                     # try it: no install (the command is py-screen or pys)
uv tool install py-screen         # the command is pys; uv fetches Python 3.14 if needed
pys                               # attach, starting the server on first use

No uv or Python? Each release has a ready-to-use bundle per system, with its own Python inside: unpack it and run bin/pys.

Why py-screen

• Restart without losing anything. A tiny keeper owns every pty; the brain (screens, keys, rendering) is a separate process. pys reload (or C-a R) starts a new brain with new code or config, and your shells, editors and builds never notice.
• Nothing is dropped under a flood. Every byte a window writes is parsed, in order, shown or hidden, with backpressure instead of dropping. Apps that redraw by diff, such as AI coding agents, stay correct. With its libvterm engine, seq 1 5000000 takes 1.6 s against GNU screen's 13.4 s.
• Made to be driven by scripts and agents. pys ls --json, pys send, pys capture and pys title (a window names itself, and a rename by hand still wins) make every window readable and writable from outside. An app's OSC 52 copy reaches your clipboard. For Claude Code there is more: pys-agents shows which window runs which session, and pys-sessions brings them all back after pys died (see For AI agents).
• Configured in Python. ~/.config/pys/config.py adds status widgets and key bindings (pys sample-config prints a starting point).
• Linux, macOS and Windows (ConPTY), with wheels for six systems, each one tested on its own system before a release.

Quick start

Status: milestone M1, used every day by its author on Linux; macOS and Windows are tested on CI. It is young: tmux and GNU screen do much more (sessions by name, many clients with their own views, a huge set of options). See Known gaps.

Use

uv tool install -e .       # puts `pys` in ~/.local/bin, running this checkout (edits apply on reload)
pys                        # attach (starts the server on first use)
pys ls [--json]     # windows
pys new [--select] [-- cmd …]
pys send 0 'make test' --enter
pys capture 0       # the window's screen as text
pys title 'release · kit · #12 libfuse'   # name the window this runs in ($PYS_WINDOW)
pys title --wid 3 --clear                 # or another one; no text: print it
pys title --owner $PPID 'review · pys'    # dropped when that process ends
pys attn ask --why 'needs a password'     # mark this window: it wants you (red tab)
pys attn done --wid 3                     # its work is done, it waits (green); --clear
pys reload          # new brain, same shells
pys kill            # end everything

A window shows the first title that is set, in this order: one set by hand (prefix ,, or a BIND entry, saved with the layout), then one set by a program with pys title (also kept across a reload), then the app's own (OSC 0/2), then the command's name. So an agent can name its window, the app's spinner titles don't cover it, and a rename by hand still wins; renaming to nothing shows the agent's title again. pys ls --json gives all three per window (title, agent_title, app_title) next to the name it shows. A pys title is one line of printable text, at most 200 characters.

An agent title outlives the agent that set it unless it names an owner: with --owner PID the brain drops the title once that process is gone (checked every 2 s; on Linux by the process's start time too, so a reused PID doesn't count). In Claude Code's Bash tool $PPID is claude, so --owner $PPID ties the title to the session. Without --owner it stays until cleared or the window closes. The owner is a PID as the brain sees it, so it means nothing from inside a PID namespace (a sandbox with its own PIDs).

A window that wants you gets a mark on its tab, so you see it from any other window: 7! on red, ask: it can't go on without you (a permission prompt, a question); 7! on green, done: it finished and waits. Its app marks it ask by itself with a bell (BEL) or a notification (OSC 9, as iTerm2 takes it; OSC 777;notify, as urxvt, VTE and Ghostty do; OSC 9 followed by a number is ConEmu's progress bar or folder, so it isn't one); a program or a hook marks it with pys attn. Switching to the window clears the mark (you saw it), and the window in front of you is never marked. Prefix ! goes to the next marked window, ask ones first. A marked window whose whole tab doesn't fit on the status line still shows, as a short 7!; the window list marks it too. pys ls --json gives attn, attn_why and attn_at per window. For Claude Code, pys setup adds the hooks that do it (below).

Keys: the prefix is C-a, or C-] when you run pys inside GNU screen ($STY set), so the two don't fight. Override with PREFIX in the config or PYS_PREFIX.

Without the prefix: Alt-0…Alt-9 select, F2 opens the list, Alt-Tab / Alt-Shift-Tab go to the next / previous window. Desktops usually keep Alt-Tab for themselves (GNOME: org.gnome.desktop.wm.keybindings switch-applications), so it reaches py-screen only where the desktop lets it through, e.g. over ssh from another machine. Inside GNU screen, screen's own bindkey lines may also take the Alt keys first.

Mouse

On by default, like tmux's mouse on:

Every report is routed to the pane under the pointer and rewritten in that pane's coordinates, in the encoding the app asked for (SGR 1006, or the old X10 form). A press grabs its pane: the drag and the release go there even if the pointer leaves it. MOUSE = False in the config gives plain drags back to the terminal; the mouse is then pys's only while the focused app asks for it (still translated per pane).

Scrollback

Each window keeps the rows that scroll off the top of its screen (SCROLLBACK, 5000 by default; the alternate screen of full-screen apps keeps none, as in xterm). The wheel or a touch drag (a phone's terminal sends it as wheel reports), or Shift-PgUp / Shift-PgDn, scroll the pane back and forth through them; the caption shows [rows back/rows kept]. The view is only a view: the app keeps running and drawing (Claude Code's input box included), and output that arrives meanwhile doesn't move the rows you are reading. Any key that goes to the app takes the pane back to live. ESC [ 3 J (what clear sends) empties the history. pys capture --history prints it.

Copy mode

Prefix [ (or prefix Esc) puts a cursor on the focused pane, over its history and screen, as GNU screen and tmux do; the caption says [copy back/kept] and the status line lists the keys. The pane is frozen as it was at the start, as in tmux: the app keeps running and its output is kept, but it shows only when copy mode ends, so an app that redraws in place (a spinner, top, Claude Code's fullscreen mode) doesn't change the text under the cursor. An app on the alternate screen (alt in pys ls --json) keeps no history in pys: copy mode reaches only its one screen, and its own history is the app's to scroll (Claude Code: PgUp / PgDn, the wheel).

A copy is kept for prefix ], which types it into the focused window (as a bracketed paste when the app asked for one), and is sent to your terminal as OSC 52, which puts it on the system clipboard in terminals that allow it (kitty, ghostty, WezTerm, iTerm2, Windows Terminal, foot; not GNOME Terminal, and not through a GNU screen or tmux around pys). It goes through ssh too, since it is only bytes on the way to your terminal.

An app can copy the same way: an OSC 52 from any window, shown or not, is treated as a copy, so what Claude Code or vim (over ssh, with a plugin) copies gets to the terminal you are attached from. An app can't read the clipboard: pys doesn't answer the OSC 52 query. A copy over about 750 KB is dropped whole. Shift+drag is another path: your terminal selects by itself and pys never sees it, so copy with your terminal's own key.

pys ls --json says per window whether it is in copy mode, how far it is scrolled, how much history it keeps, and which mouse mode (1000/1002/1003, 0 = none) and screen (alt) its app asked for: an app with a mouse mode gets the wheel itself.

A row costs about 36 bytes per cell up to its last visible one: a window full of 100-column lines holds ~18 MB at 5000 rows. Keeping rows has a price on floods too: about 0.2 µs a line (see bench/README.md).

Config

~/.config/pys/config.py (plain Python, read on every brain start, so C-a R applies an edit): status widgets, the caption line, extra key bindings. See src/py_screen/config.py for the helpers.

pys doctor                 # where it is, and a check: it loads it as the brain does, runs each
                           # widget and the CAPTION once (text, time, error), and finds typos
                           # (WIDGET for WIDGETS: pys never reads it), an ON_CHANGE not on PATH
pys doctor --init          # write the sample there; never over yours
pys doctor --reset         # keep yours as config.py.<YYYYMMDD-HHMMSS>, then write the sample

Drop-ins: ~/.config/pys/config.d/*.py run after config.py, in name order, in the same namespace: a file there sets a name (PREFIX = "C-b") or changes what came before (WIDGETS.append(...), BIND["x"] = ...), so a machine's own lines, or a tool's, live apart from the config you share or replace. pys setup writes its ON_CHANGE to config.d/50-pys-setup.py and never edits config.py. A drop-in that fails is named by pys doctor (with its line) and left out; the others still count. Next to a $PYS_CONFIG of another name, the folder is <its name>.d.

The caption line (the line under each pane: its number and name, then its folder) takes CAPTION in place of the folder: any object with .every and async update(ctx) -> str, run for each window on screen (ctx: cwd, wid, pid), at most 5 s per update. caption() from py_screen.agents.widget shows where the work is: the folder and its git branch and changes, and for a window with an agent in it, the folder it started in first, then the repo it last wrote in (✎) when that is elsewhere:

from py_screen.agents.widget import caption
CAPTION = caption()        # " 3 build fix  ~/src/app ✎ ~/src/app-fix  fix-build wt 1 mod "

It reads: started in ~/src/app (its Bash goes back there, and a resume runs there), works in ~/src/app-fix (✎), on branch fix-build, a linked worktree (wt), 1 tracked file modified. Also: 3 new untracked files, ↑2 ↓5 commits ahead of / behind the upstream (as of the last fetch), @1f0e2c1 a detached HEAD; nothing after the branch: clean.

WIDGETS = [cmd(["git", "-C", "{cwd}", "branch", "--show-current"], every=10), load(), clock()]
BIND = {"T": ("top", ["htop"])}   # C-a T opens a window running htop
MOUSE = True                      # False: plain drags select text in the terminal
SCROLLBACK = 5000                 # rows of history per window; 0 = none

pys sample-config prints a fuller sample to start from (pys sample-config > ~/.config/pys/config.py): CPU, memory and network (down/up in kB/s) widgets and a clock that ticks on each second, written in the config itself, and branch() from py_screen.agents.widget: the focused window's git branch, worktree and changes, and for a window with an agent in it, the repo the agent last wrote in (✎, see For AI agents). A widget is any object with .every, .text and async update(ctx) -> str, so new ones need no change to pys; ctx is the focused window: cwd, wid and pid (its process). CPU, memory, network and disk (read/write in kB/s) read /proc on Linux; on macOS and Windows they use psutil, which pys installs there.

The layout (splits, focus, renamed titles) is saved on every change, so a reload keeps it.

What the windows were: ON_CHANGE and the server record

pys kill and prefix Q end every window and delete the layout. Two things keep what they were, so a tool can bring them back:

• ON_CHANGE in the config, a command (an argv list), run about 2 s after a change (a window opens or exits, a title changes, the brain starts) with PYS_EVENT=change, and once before pys kill / Q with PYS_EVENT=kill, while every window still runs; the kill waits for it at most KILL_HOOK_TIMEOUT seconds (5), then ends as before. One run at a time (changes during a run give one more run after it); PYS_DIR is set, so pys ls --json inside it reaches this session; its output goes to server.log.
• The server record: on the same changes and at a kill, the brain writes $XDG_STATE_HOME/pys/servers/<start YYYYMMDD-HHMMSS>-<server pid>.json (default ~/.local/state, a session with its own PYS_DIR too, since the runtime dir is gone at the next boot): the server's pid, started, updated, ended (set when it ends by a kill or Q, with the windows it ended; by its last window, with none; null while it runs or if it died), and each window's wid, pid, cmd, cwd (its shell's current directory), name, title, agent_title, agent_owner. One file per server life, rewritten whole (tmp + rename), never deleted.

ON_CHANGE = ["pys-sessions", "save", "--quiet"]   # comes with pys (below), or a script of
                                                  # your own: an argv list
KILL_HOOK_TIMEOUT = 5

Sockets live in $XDG_RUNTIME_DIR/pys/ (PYS_DIR overrides; keep it short: an AF_UNIX path is limited to 108 bytes). Server log: server.log in that dir.

Each window gets PYS_WINDOW (its number) and PYS_SESSION (its server's dir). pys typed in a window of the same session refuses to attach: the session would show inside itself and the keys would loop between the window and the server. Another session (PYS_DIR=<dir> pys) may be nested; give it its own prefix (PYS_PREFIX).

For AI agents

Agents such as Claude Code often run in pys windows, several at a time. pys comes with two commands for them and two skills that teach an agent to use pys well. They are built only on the commands above, so the core knows nothing of them. Claude Code for now (Codex later); Linux, macOS and Windows (processes: /proc on Linux, psutil elsewhere).

All of it in one step, the same for uvx, pip, a uv tool and the bundle:

pys setup          # what it would do
pys setup --yes    # do it; running it again is safe

1. a config: the sample when you have none, with ON_CHANGE snapshots on (pys-sessions save, below); a config of your own is kept, a copy first, and ON_CHANGE is added only when it has none;
2. the skills pys-drive and pys-restore, linked into Claude Code's skills folder;
3. a SessionStart hook in Claude Code's settings.json (a copy of it first): every claude that starts in a pys window is told, in a few lines, that it is in pys, which window, how to name it, how to look before it types, never to pys kill, and how to run pys from this install; outside pys it says nothing. python -m py_screen.agents.instructions --show prints the text. A skill is read when a task calls for it; these lines are always there.
4. hooks for Notification, Stop, UserPromptSubmit and SessionEnd (python -m py_screen.agents.attn): a claude's tab turns red when it asks you (a permission prompt), green when its turn ends, and back when you answer. Claude's idle reminder is left out: the turn's end marked it already, and once you looked it stays seen. Outside pys, or with no server, the hook does nothing and never holds Claude up.

(The agents in the demo are stand-ins that write what Claude Code writes; it is recorded in a container with docs/record-demo-agents.sh.)

pys-agents: every running claude, the window it runs in, and how it is doing:

$ pys-agents
WIN  TITLE              STATE  CTX   COMPACT  MODEL     NAME      FOLDER
0    review PR 42       idle   237k  0        opus-5-5  reviewer  ~/src/app
2    build fix          busy   844k  1        opus-5-5  builder   ~/src/app
-    -                  idle   12k   0        opus-5-5  -         ~/notes

CTX is what the last call held (input, cache read and written, from the transcript), COMPACT the compactions so far; a claude outside pys shows window -. --json gives one object per claude (window, pid, session, name, state, cwd, context_tokens, model, compactions, and titled_by_it when it named its window with pys title --owner).

pys-agents where: every window, agent or not, with the git state of its folder:

$ pys-agents where
WIN  TITLE         FOLDER              BRANCH                       STATE        AGENT
0    review PR 42  ~/src/app           main @a96cd56                clean ↑2     claude reviewer (idle)
1    build fix     ✎ ~/src/app-fix     fix-build @1f0e2c1 (wt)      1 mod 3 new  claude builder (busy)
2    -             ~/notes             -                            -            -
✎ = the repo of the agent's latest writes (its transcript), not the folder it started in
(wt) = a linked worktree (git worktree add) of that repo

An agent's folder is not where it works: Claude Code keeps the folder it was started in, its tools take absolute paths anywhere, and its Bash shell goes back to the start folder after every command. So for an agent, where looks at what it did: the files it last wrote (Write, Edit) and the cd <dir> && … its Bash commands start with, newest first, and shows the first one inside a git repo (✎). A Read is not work, and a folder outside any repo is skipped. --json keeps both: cwd (where it started) and works_in.

STATE is git status against the upstream (↑ ahead, ↓ behind, as of the last fetch); git runs with --no-optional-locks, so it never takes the index lock from under you. --json gives git: {repo, top, common, worktree, branch, head, changed, untracked, ahead, behind} per window.

pys-sessions: keep what every window is, and open it all again after pys died (a kill, a crash, a reboot):

# ~/.config/pys/config.py
ON_CHANGE = ["pys-sessions", "save", "--quiet"]   # a snapshot at each change and before a kill
SESSIONS_KEEP_ENV = []    # variables of a claude's environment its resume gets back

Snapshots go to ~/.local/state/pys-sessions/<YYYYMMDD-HHMMSS>/ (PYS_SESSIONS_DIR) and are never deleted or overwritten.

Skills. pys-agents skills --install links them into ~/.claude/skills (one of the same name that is already there is left alone; a link to a pys install that is gone is renewed; on Windows, without the right to make a symlink, a directory junction). A link points into the install that ran the command, so run it from one you keep (uv tool install, pip, a bundle): a uvx run lives in uv's cache, which uv cache clean empties.

• pys-drive: name your window, find the other sessions, read a window, type into one only when it is idle (text, then Enter on its own: an Enter inside a paste is a new line), never pys kill, test pys on a second server.
• pys-restore: bring the windows back after pys died, step by step, showing the plan first.

Both commands read Claude Code's own files (~/.claude/sessions/<pid>.json, the transcripts under ~/.claude/projects/; CLAUDE_CONFIG_DIR moves them). Those are not a documented interface, so the reads are defensive: after a Claude Code update a field may show - until pys catches up.

How it works

client (pys attach) ──► brain (Python, restartable) ──► keeper (tiny, stable) ──► ptys

• keeper owns every pty and child process and is the only reader of their output (a byte pump with a ring buffer per window). It restarts the brain when it exits.
• brain holds a pyte screen per window, renders the current one plus a status line, handles keys and answers RPCs. pys reload restarts it; shells never notice, because the ptys live in the keeper. The new brain replays the ring buffers, then nudges each pty (SIGWINCH) so full-screen apps repaint.
• client puts the terminal in raw mode, forwards keys and size, prints frames, and reconnects by itself across a brain reload.

Floods: the screen model is lossless

Every byte a window writes is parsed, in order, whether the window is shown or hidden, attached or not. That matters most for apps that draw by diff, like Claude Code: they write only the cells they think changed and never rewrite a cell they think is right, so a single lost byte leaves wrong cells for good (and a lost mode switch, such as the alternate screen or bracketed paste, breaks input too). Until 2026-11-01 py-screen parsed only the tail of a flood and dropped a hidden window's output past 128 KB; that is what left stray characters in Claude's status line.

Parsing is the slow part (pyte in Python; py-screen's fast paths for plain text, line feeds and CSI sequences take it from 0.85 to 1.7 MB/s of scrolling text), so:

• Backpressure, not dropping. The keeper gives each window a credit of 256 KB un-acked output and stops reading that pty past it. The brain acks bytes only once it has parsed them, so a flooding app simply waits on its write, as under GNU screen. One window can't fill the shared stream, so another window's echo never waits behind it.
• Small steps. The brain parses in steps of 2 ms (2 KB per window per turn, shown windows first, the others round robin), then goes back to keys and frames.
• Keys first. For 100 ms after a key, only shown windows are parsed: nothing else stands between a key and its echo (about 2 ms median while a hidden window floods).

A flood runs at the parser's speed. With the vterm engine (libvterm, in C) that is faster than GNU screen: seq 1 5000000 takes 1.6 s against screen's 13.4 s, a 50 MB coloured log 0.9 s against 8.1 s (bench/README.md has the history).

Screen engines. Each window's screen is a term.Term; PYS_TERM picks the engine:

PYS_TERM=vterm makes a missing extension an error instead of a quiet fallback.

A sequence that pyte can't apply (an extra parameter, a ? where none is defined, an unknown erase value) is ignored, as terminals do; the bytes after it are still parsed.

Releases

A tag is a release. Pushing a tag <YYYYMMDD>.<N> on a commit of main publishes that version on PyPI as py-screen and as a GitHub release, once, never replaced:

git tag 20261106.1 <sha> && git push origin 20261106.1

Every push, on every branch, and every pull request runs the same tests, in ci.yml, and publishes nothing. A release runs them again, then builds, in release.yml, and must pass:

• the suite on Linux and macOS, and the RPC smoke test on Windows, on both screen engines (vterm and pyte);
• the vterm engine built with ASan + UBSan and fuzzed against libvterm's own screen (a long run every night, in sanitize.yml);
• a parse-speed check: vterm's lead over pyte must keep at least half of the newest recorded run's (release/bench_gate.py, against bench/results/parse-*.json);
• each wheel, built by cibuildwheel from the sdist, installed and smoke-tested on the vterm engine on its own system: Linux x86_64 and aarch64 (manylinux), macOS arm64 and x86_64, Windows x86_64 and ARM64;
• a ready-to-use bundle per system (below), built from that wheel, unpacked into another folder and smoke-tested there with no uv or Python on PATH.

Install with uv, which fetches Python 3.14 if needed and picks the wheel for your system:

uv tool install py-screen          # the command is pys

The import is py_screen and the command pys. Until 20261109.2 the package was py-screen-mux (an older pyscreen held the name); that name gets no more releases, so uv tool upgrade py-screen-mux stays at 20261109.2. To move to the new name: uv tool uninstall py-screen-mux && uv tool install py-screen, then pys setup --yes (it points the skills and the hook at the new install).

Or take the ready-to-use bundle for your system from the release: pys-<version>-<os>-<arch>.tar.gz (.zip on Windows) holds its own Python 3.14, pys and its dependencies (pyte, psutil), so there is nothing to install and no network needed. Unpack it anywhere and run bin/pys (pys.exe on Windows; pys-agents and pys-sessions beside it); the folder still works after you move it. release/bundle.py builds it: python-build-standalone's CPython (the one uv installs) with the release's wheel installed into it, a launcher in place of pip's scripts (those hold an absolute path), and Tk, IDLE and the C headers left out (about 35 MB packed). On macOS, a browser download is quarantined: xattr -dr com.apple.quarantine pys-* before the first start.

Elsewhere, the sdist builds with a C compiler, or runs the pure-Python pyte engine without one.

Test and measure

uv run pytest -s                                      # M0 done-criteria (spec §6)
uv run python bench/m0_speed.py --work <scratch dir>  # speed vs GNU screen (spec §8.1)

Results of each run are kept in bench/results/, one file per run, and bench/README.md tells how the speed evolved, one row per run.

Platforms

Everything that differs between systems lives in src/py_screen/platform/ (spec §5.2); the keeper, brain and client import nothing else that is OS-specific.

CI: every push runs the suite on Linux and macOS and tests/test_rpc_smoke.py on Windows. spike/win_check.py runs too when the platform layer changes, or by hand (platform workflow).

Known gaps (M1)

• A mouse report split across two reads is passed on as keys (rare; the outer terminal writes each report at once).
• After a brain restart the screens are rebuilt from the keeper's last 2 MB per window; if that ring wrapped, older state is gone and full-screen apps get a SIGWINCH to repaint. Keeping the screens across a planned reload is open.
• Floods are bounded by pyte's speed (see "Floods").
• The scrollback isn't rewrapped on a resize; copy mode selects character-wise only (no line or block selection), and the paste buffer is lost on a brain reload.
• Windows: pys attach needs a real console (Windows Terminal or conhost, not mintty); a new window doesn't inherit the focused window's directory (it isn't read from the child yet), and the load widget is empty. The pty harness tests are POSIX-only, so Windows CI runs the RPC smoke test.
• Windows named pipes keep the default DACL (other local users may open them); instead every connection must first send a per-session key from the user-private runtime dir. A per-user DACL would close the pipe itself; still to do.

Contributing

Issues with a way to reproduce them are welcome, and so are pull requests. To work on it:

git clone https://github.com/congzhangzh/py-screen && cd py-screen
uv tool install -e .     # pys runs this checkout; `pys reload` loads your edits
uv run pytest            # the suite, about a minute

Every push and pull request runs the suite on Linux and macOS and the RPC smoke test on Windows (ci.yml). A commit's subject says what changed for a user; it is what the release notes list. The demo above is recorded from docs/demo.tape with vhs: docs/record-demo.sh runs it in a clean environment, as a made-up user in a session of its own. A new take gets a new version (demo.v3.gif) beside the old one.

License

MIT (LICENSE). The bundled libvterm is MIT as well (deps/libvterm/LICENSE).