False "Unsupported terminal (conhost)" warning under ConPTY hosts that don't set WT_SESSION (e.g. Herdr)

[dave-dm]

Summary

Devin CLI shows the "Unsupported terminal / Windows Console Host (conhost)" warning whenever it runs inside a ConPTY-hosted terminal that does not forward WT_SESSION, even though the host is fully VT-capable. Observed inside Herdr, a terminal multiplexer for coding agents, on Windows 11.

⚠︎ Unsupported terminal
Windows Console Host (conhost) has limited support. Switch to Windows Terminal or Git Bash for the best experience

Cause

The terminal detection appears to key on WT_SESSION (and TERM_PROGRAM). Herdr spawns panes through its own bundled ConPTY (OpenConsole.exe --headless) and does not pass WT_SESSION through, so the check falls back to "conhost". The pane environment is:

WT_SESSION=          (unset)
WT_PROFILE_ID={61c54bbd-...}
TERM_PROGRAM=        (unset)
TERM=xterm-256color

The same will affect any ConPTY host that isn't Windows Terminal itself (other multiplexers, IDE-embedded terminals, SSH into Windows, etc.). The warning is a false positive: the pane renders truecolor, mouse, and alternate-screen output correctly.

Expected behavior

Detect capability rather than vendor. Suggestions, any of which would fix this case:

• Treat TERM=xterm-256color (or any TERM other than unset/dumb) as a VT-capable terminal on Windows.
• Check whether the console output handle already has ENABLE_VIRTUAL_TERMINAL_PROCESSING, or whether SetConsoleMode with that flag succeeds, which is the reliable signal that a ConPTY is in front of the process.
• Only warn when the host is genuinely legacy conhost (no VT mode, TERM unset).

Reproduction

1. Windows 11, install Herdr (herdr.dev) and Devin CLI 3000.6.7.
2. Start herdr from Windows Terminal; in any pane run devin.
3. Observe the warning. Run devin directly in a Windows Terminal tab: no warning.
4. Workaround confirming the cause: $env:WT_SESSION = [guid]::NewGuid(); devin inside the pane suppresses the warning.

Environment

• Devin CLI: 3000.6.7 (260a97c8)
• OS: Windows 11 Pro 10.0.26200
• Host: Herdr 0.9.0-preview.2026-09-08 (ConPTY via OpenConsole.exe --headless), launched from Windows Terminal 1.24.11911.0
• Shell: pwsh 7 and Git Bash (same result)