diff --git a/docs/explore-notes/conpty-sessions.md b/docs/explore-notes/conpty-sessions.md
index e3824ea5..2aecb7c3 100644
--- a/docs/explore-notes/conpty-sessions.md
+++ b/docs/explore-notes/conpty-sessions.md
@@ -46,6 +46,21 @@ except for a fresh task session with a brief, where `--add-dir <sessionDir>` mus
 `--model` is deliberately **NOT** forced on an interactive session — the user can still switch
 models in the TUI.
 
+### ⚠️ Gotcha: no directory arg may end in a separator
+
+`PtyTerminalSession` hands `Args` to `TerminalControl.Args`, which the library flattens into ONE
+Windows command line with each token quoted. Windows argv rules read `\"` as an *escaped* quote,
+so a token like `"C:\repo\"` never closes and every following argument is swallowed by the
+preceding **variadic** flag. A list working dir stored as `C:\Dev\Repos\Bandel.Hub\` therefore
+fed `--add-dir` the repo, `--append-system-prompt-file`, its value **and** the positional kickoff:
+the CLI warned `brief.md is not a directory` and the session opened with no prompt at all
+(2026-08-06). `BuildForMergeHelperAsync`/`BuildForMergeHelperHandoffAsync` now run the repo
+through `TrimTrailingSeparator`; session dirs the worker builds never carry one.
+
+Diagnosing this from code or a PowerShell repro is a dead end — PowerShell quotes correctly, so
+every repro passes. Read the real command line instead:
+`Get-CimInstance Win32_Process -Filter "Name = 'claude.exe'"`.
+
 ## Resuming a task session (`TaskEntity.InteractiveSessionId`)
 
 `claude --session-id <uuid>` lets the caller pre-assign a conversation's session id instead of