Fall back to xterm-256color when the client TERM isn't on the server

dosh propagates the client's TERM, but a server lacking that terminal's
terminfo (e.g. xterm-ghostty, xterm-kitty) made tmux/vim fail with
"missing or unsuitable terminal". build_shell_command now keeps the
requested TERM only when its terminfo exists on this host, otherwise falls
back to xterm-256color so remote apps always work. Affects both the
in-process and holder-spawned shells.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/pty.rs

@@ -2,7 +2,7 @@ use anyhow::{Context, Result};
 use portable_pty::{Child, CommandBuilder, MasterPty, NativePtySystem, PtySize, PtySystem};
 use std::io::{Read, Write};
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 use std::thread;
 use tokio::sync::mpsc;
@@ -148,16 +148,65 @@ pub fn spawn_pty_session(
     })
 }
 
+/// Whether this host has a terminfo entry for `term`, searching the standard
+/// ncurses directories. Both the legacy single-letter (`x/xterm`) and the
+/// hashed (`78/xterm`) subdirectory layouts are checked.
+fn terminfo_available(term: &str) -> bool {
+    let Some(first) = term.chars().next() else {
+        return false;
+    };
+    let letter = first.to_string();
+    let hashed = format!("{:x}", first as u32);
+    let mut dirs: Vec<PathBuf> = Vec::new();
+    if let Ok(t) = std::env::var("TERMINFO")
+        && !t.is_empty()
+    {
+        dirs.push(PathBuf::from(t));
+    }
+    if let Some(home) = dirs::home_dir() {
+        dirs.push(home.join(".terminfo"));
+    }
+    if let Ok(td) = std::env::var("TERMINFO_DIRS") {
+        for d in td.split(':').filter(|d| !d.is_empty()) {
+            dirs.push(PathBuf::from(d));
+        }
+    }
+    for d in [
+        "/etc/terminfo",
+        "/lib/terminfo",
+        "/usr/share/terminfo",
+        "/usr/lib/terminfo",
+        "/usr/share/lib/terminfo",
+    ] {
+        dirs.push(PathBuf::from(d));
+    }
+    dirs.iter()
+        .any(|dir| dir.join(&letter).join(term).exists() || dir.join(&hashed).join(term).exists())
+}
+
 /// Build the [`CommandBuilder`] for a dosh shell, identically for the in-process
 /// `spawn_pty_session` and the out-of-process holder, so a persistent session's
 /// environment matches a non-persistent one.
 pub fn build_shell_command(shell: &str, env: &[(String, String)]) -> CommandBuilder {
     let mut cmd = CommandBuilder::new(shell);
-    cmd.env("TERM", "xterm-256color");
     cmd.env("COLORTERM", "truecolor");
     for (name, value) in env {
         cmd.env(name, value);
     }
+    // The client's TERM is propagated, but a server that lacks that terminal's
+    // terminfo entry (e.g. xterm-ghostty, xterm-kitty) breaks ncurses apps like
+    // tmux/vim with "missing or unsuitable terminal". Keep the requested TERM
+    // only when this host actually has its terminfo; otherwise fall back to a
+    // universally available entry so remote apps always work.
+    let term = match env
+        .iter()
+        .find(|(n, _)| n == "TERM")
+        .map(|(_, v)| v.as_str())
+    {
+        Some(requested) if terminfo_available(requested) => requested.to_string(),
+        _ => "xterm-256color".to_string(),
+    };
+    cmd.env("TERM", term);
     cmd.env("SHELL", shell);
     if let Some(home) = dirs::home_dir() {
         cmd.env("HOME", home.as_os_str());
@@ -235,3 +284,17 @@ fn spawn_reader_thread(
         .context("spawn pty reader")?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn terminfo_available_detects_known_and_unknown() {
+        // A near-universal entry should be present on any host with ncurses.
+        assert!(terminfo_available("xterm") || terminfo_available("xterm-256color"));
+        // Bogus / empty names must report missing so we fall back.
+        assert!(!terminfo_available("definitely-not-a-real-terminal-xyz"));
+        assert!(!terminfo_available(""));
+    }
+}