Summary

psmux's argv parser does not correctly handle the standard Windows CommandLineToArgvW encoding for an argument whose value ends in a backslash and contains spaces. When such a value appears in tmux new-session -e KEY=VAL, all subsequent -e flags are swallowed into the first value's content.

This is independent of #264 (which we just filed for paste-buffer). Both surface only on Windows, both block the same downstream consumer (CAO).

Environment

• psmux: 3.3.4 (psmux -V → psmux 3.3.4), installed via scoop
• Host: Windows 11 Pro 26200, ARM
• Caller: Python 3 subprocess.run(cmd_list, ...) — uses list2cmdline to encode argv per Microsoft's documented rules

Minimal repro

# Run from a non-psmux shell (e.g., Git Bash with PSMUX_SESSION unset)
python -c "
import subprocess
cmd = ['tmux', 'new-session', '-s', 'bsrepro', '-n', 'w', '-c', r'C:\Users\YOU', '-d',
       '-e', r'TRAILING_BS=C:\Program Files\Foo Bar\plugins' + chr(92),
       '-e', 'NEXT_VAR=should_survive',
       '-e', 'CAO_TERMINAL_ID=test-id-12345']
subprocess.run(cmd)
"

tmux show-environment -t bsrepro

Observed

TRAILING_BS=C:\Program Files\Foo Bar\plugins" -e NEXT_VAR=should_survive -e CAO_TERMINAL_ID=test-id-12345

TRAILING_BS is set to the entire concatenation. NEXT_VAR and CAO_TERMINAL_ID are completely missing from the session env.

Expected

TRAILING_BS=C:\Program Files\Foo Bar\plugins\
NEXT_VAR=should_survive
CAO_TERMINAL_ID=test-id-12345

Real tmux on POSIX, and Windows tooling that uses CommandLineToArgvW, parse this correctly.

Why it's mis-parsed

Python's subprocess.list2cmdline correctly encodes MOZ_PLUGIN_PATH=...\plugins\ (value ending in \, value containing spaces) as:

"MOZ_PLUGIN_PATH=...\plugins\\"

Per Microsoft's argv parsing rules:

• \\" → 1 literal backslash + close-quote (the \ is N backslashes where N=2; 2N/2 = 1 literal backslash; the " closes the string).

psmux's parser appears to treat \\" as if it were \" (i.e., escaped-quote-keep-going), so the string never closes and consumes the following -e NEXT_VAR=... and -e CAO_TERMINAL_ID=....

Trigger conditions

The bug fires when an env value:

1. Contains whitespace (forces Python to wrap in quotes), AND
2. Ends with \.

On Windows this is common — MOZ_PLUGIN_PATH, some *_PATH vars, and various Program Files\…\ entries can end in trailing backslash.

Impact on CAO

CAO sets CAO_TERMINAL_ID last in the env dict. Any earlier env var matching the trigger silently swallows it, leaving the spawned agent without its terminal identifier — breaking assign/handoff MCP tools.

CAO-side workaround

We're stripping trailing backslashes from env values before passing as -e KEY=VAL (lossless for path-like vars: C:\Foo and C:\Foo\ are equivalent on Windows). Will remove once this is fixed in psmux.

Reference

Filed downstream of awslabs/cli-agent-orchestrator#207. See also #264 for paste-buffer (separate issue).