diff --git a/.changeset/quiet-mice-skip-dms.md b/.changeset/quiet-mice-skip-dms.md
new file mode 100644
index 000000000..9ba569d9f
--- /dev/null
+++ b/.changeset/quiet-mice-skip-dms.md
@@ -0,0 +1,5 @@
+---
+"fnm": patch
+---
+
+guard the documented shell setup snippets (and the installer-appended ones) behind an interactive-shell check so non-interactive logins, such as display managers scraping env vars at session start, no longer leak a stale `fnm_multishells` path into `$PATH`. Closes #1551.
diff --git a/.ci/install.sh b/.ci/install.sh
index 9a9647748..e89be5572 100755
--- a/.ci/install.sh
+++ b/.ci/install.sh
@@ -177,7 +177,10 @@ setup_shell() {
       if [ "$USE_HOMEBREW" != "true" ]; then
         echo '  export PATH="$FNM_PATH:$PATH"'
       fi
-      echo '  eval "$(fnm env --shell zsh)"'
+      echo '  # only initialize for interactive shells; non-interactive logins'
+      echo '  # (e.g. display managers scraping env vars) would otherwise leak a'
+      echo '  # stale multishell into the session PATH.'
+      echo '  [[ $- == *i* ]] && eval "$(fnm env --shell zsh)"'
       echo 'fi'
     } | tee -a "$CONF_FILE"
 
@@ -193,7 +196,10 @@ setup_shell() {
       if [ "$USE_HOMEBREW" != "true" ]; then
         echo '  set PATH "$FNM_PATH" $PATH'
       fi
-      echo '  fnm env --shell fish | source'
+      echo '  # only initialize for interactive shells; non-interactive logins'
+      echo '  # (e.g. display managers scraping env vars) would otherwise leak a'
+      echo '  # stale multishell into the session PATH.'
+      echo '  status is-interactive; and fnm env --shell fish | source'
       echo 'end'
     } | tee -a "$CONF_FILE"
 
@@ -213,7 +219,10 @@ setup_shell() {
       if [ "$USE_HOMEBREW" != "true" ]; then
         echo '  export PATH="$FNM_PATH:$PATH"'
       fi
-      echo '  eval "$(fnm env --shell bash)"'
+      echo '  # only initialize for interactive shells; non-interactive logins'
+      echo '  # (e.g. display managers scraping env vars) would otherwise leak a'
+      echo '  # stale multishell into the session PATH.'
+      echo '  [[ $- == *i* ]] && eval "$(fnm env --shell bash)"'
       echo 'fi'
     } | tee -a "$CONF_FILE"
 
diff --git a/.ci/test_installation_script.sh b/.ci/test_installation_script.sh
index cee4b323f..025247f86 100755
--- a/.ci/test_installation_script.sh
+++ b/.ci/test_installation_script.sh
@@ -15,12 +15,14 @@ echo "---"
 echo "PATH=$PATH"
 echo "---"
 
-$SHELL_TO_RUN -c "
+# Use an interactive shell (-i) so the appended fnm setup, which is now guarded
+# by an interactive-shell check, actually runs when the profile is sourced.
+$SHELL_TO_RUN -i -c "
   . $PROFILE_FILE
   fnm --version
 "
 
-$SHELL_TO_RUN -c "
+$SHELL_TO_RUN -i -c "
   . $PROFILE_FILE
   fnm install 12.5.0
   fnm ls | grep 12.5.0
@@ -28,7 +30,7 @@ $SHELL_TO_RUN -c "
   echo 'fnm ls worked.'
 "
 
-$SHELL_TO_RUN -c "
+$SHELL_TO_RUN -i -c "
   . $PROFILE_FILE
   fnm use 12.5.0
   node --version | grep 12.5.0
diff --git a/README.md b/README.md
index f5dbf9320..ed370bdc8 100644
--- a/README.md
+++ b/README.md
@@ -158,7 +158,9 @@ Check out the following guides for the shell you use:
 Add the following to your `.bashrc` profile:
 
 ```bash
-eval "$(fnm env --use-on-cd --shell bash)"
+# only run for interactive shells; non-interactive logins (e.g. display managers
+# scraping env vars) would otherwise leave a stale multishell in the session PATH.
+[[ $- == *i* ]] && eval "$(fnm env --use-on-cd --shell bash)"
 ```
 
 #### Zsh
@@ -166,7 +168,9 @@ eval "$(fnm env --use-on-cd --shell bash)"
 Add the following to your `.zshrc` profile:
 
 ```zsh
-eval "$(fnm env --use-on-cd --shell zsh)"
+# only run for interactive shells; non-interactive logins (e.g. display managers
+# scraping env vars) would otherwise leave a stale multishell in the session PATH.
+[[ $- == *i* ]] && eval "$(fnm env --use-on-cd --shell zsh)"
 ```
 
 #### Fish shell
@@ -174,7 +178,9 @@ eval "$(fnm env --use-on-cd --shell zsh)"
 Create `~/.config/fish/conf.d/fnm.fish` and add this line to it:
 
 ```fish
-fnm env --use-on-cd --shell fish | source
+# only run for interactive shells; non-interactive logins (e.g. display managers
+# scraping env vars) would otherwise leave a stale multishell in the session PATH.
+status is-interactive; and fnm env --use-on-cd --shell fish | source
 ```
 
 #### PowerShell