diff --git a/.github/workflows/release-canary.yml b/.github/workflows/release-canary.yml
index 239f49eced..c25381909a 100644
--- a/.github/workflows/release-canary.yml
+++ b/.github/workflows/release-canary.yml
@@ -193,6 +193,8 @@ jobs:
 
   ubuntu-snap-system-docker:
     name: Ubuntu Snap with system Docker
+    env:
+      OPENSHELL_INSTALL_SNAP: "1"
     if: ${{ github.event_name == 'workflow_dispatch' || github.event.workflow_run.conclusion == 'success' }}
     runs-on: ubuntu-latest
     timeout-minutes: 20
@@ -260,6 +262,8 @@ jobs:
 
   ubuntu-snap-docker-preflight:
     name: Ubuntu Snap Docker preflight
+    env:
+      OPENSHELL_INSTALL_SNAP: "1"
     if: ${{ github.event_name == 'workflow_dispatch' || github.event.workflow_run.conclusion == 'success' }}
     runs-on: ubuntu-latest
     timeout-minutes: 20
diff --git a/install.sh b/install.sh
index 60bfd6f64c..b482054d3b 100755
--- a/install.sh
+++ b/install.sh
@@ -61,23 +61,25 @@ ENVIRONMENT VARIABLES:
     OPENSHELL_ACK_BREAKING_UPGRADE
                         Set to 1 only after backing up and cleaning up a
                         pre-v0.0.37 or non-snap installation.
+    OPENSHELL_INSTALL_SNAP
+                        Set to 1 to install the OpenShell snap on Linux.
 
 NOTES:
     When OPENSHELL_VERSION is unset, this resolves the latest tagged release
     from ${GITHUB_URL}/releases/latest.
 
-    On Linux, the installer uses the OpenShell snap when the snap command is
-    available and OPENSHELL_VERSION is unset or dev. Snap installs use
-    latest/stable by default and latest/edge for dev. Explicit release tags
-    and prereleases use Debian or RPM packages. The OpenShell snap requires a
-    running Docker Engine installed from a system package or Docker's package
-    repository. The Docker snap is not currently compatible with OpenShell.
-
-    For explicit versions or without snap, Linux installs the Debian package
-    on amd64/arm64 or the RPM packages on x86_64/aarch64, depending on the
-    host package manager.
+    Linux installs the Debian package on amd64/arm64 or the RPM packages on
+    x86_64/aarch64, depending on the host package manager.
     macOS installs the release Homebrew formula on Apple Silicon and starts a
     brew services-backed local gateway.
+
+    The installer uses the OpenShell snap only when OPENSHELL_INSTALL_SNAP=1 is
+    set or an OpenShell snap is already installed, the snap command is
+    available, and OPENSHELL_VERSION is unset or dev. Snap installs use
+    latest/stable by default and latest/edge for dev. The OpenShell snap
+    requires a running Docker Engine installed from a system package or
+    Docker's package repository. The Docker snap is not currently compatible
+    with OpenShell.
 EOF
 }
 
@@ -657,10 +659,18 @@ local_gateway_endpoint() {
   esac
 }
 
+openshell_snap_installed() {
+  snap list openshell >/dev/null 2>&1
+}
+
+# The snap gateway runs as root without client authentication, so new installs
+# use it only on explicit opt-in. Existing snap installs keep refreshing rather
+# than gaining a second gateway on the same port.
 linux_package_method() {
   case "${OPENSHELL_VERSION:-}" in
     '' | dev)
-      if has_cmd snap; then
+      if has_cmd snap \
+        && { [ "${OPENSHELL_INSTALL_SNAP:-0}" = "1" ] || openshell_snap_installed; }; then
         echo "snap"
         return 0
       fi
@@ -1340,6 +1350,8 @@ Install Docker Engine from a system package or Docker's package repository, then
   info "using existing Docker installation"
   wait_for_docker_daemon
 
+  warn "the OpenShell snap gateway allows unauthenticated access from any local user or process"
+
   _channel="$(openshell_snap_channel)"
   if snap list openshell >/dev/null 2>&1; then
     info "refreshing OpenShell snap from ${_channel}..."
diff --git a/tasks/scripts/test-install-sh.sh b/tasks/scripts/test-install-sh.sh
index 8cc6102148..5ecdb55c31 100755
--- a/tasks/scripts/test-install-sh.sh
+++ b/tasks/scripts/test-install-sh.sh
@@ -103,22 +103,30 @@ assert_glibc_preflight_fails \
 assert_linux_package_method() {
   local name=$1
   local requested_version=$2
-  local snap_present=$3
+  local snap_mode=$3
   local dpkg_present=$4
   local rpm_present=$5
   local expected=$6
   local actual
 
+  # snap_mode: 0 = no snap command, 1 = snap command only,
+  # optin = OPENSHELL_INSTALL_SNAP=1, existing = OpenShell snap installed.
   actual="$(
     export OPENSHELL_VERSION="$requested_version"
+    if [ "$snap_mode" = "optin" ]; then
+      export OPENSHELL_INSTALL_SNAP=1
+    else
+      unset OPENSHELL_INSTALL_SNAP
+    fi
     has_cmd() {
       case "$1" in
-        snap) [ "$snap_present" = "1" ] ;;
+        snap) [ "$snap_mode" != "0" ] ;;
         dpkg) [ "$dpkg_present" = "1" ] ;;
         rpm) [ "$rpm_present" = "1" ] ;;
         *) return 1 ;;
       esac
     }
+    openshell_snap_installed() { [ "$snap_mode" = "existing" ]; }
     linux_package_method
   )"
   if [ "$actual" != "$expected" ]; then
@@ -127,13 +135,19 @@ assert_linux_package_method() {
   fi
 }
 
-assert_linux_package_method "snap takes precedence over deb and rpm" "" 1 1 1 snap
-assert_linux_package_method "dev uses snap" dev 1 1 1 snap
-assert_linux_package_method "pre uses deb despite snap" pre 1 1 1 deb
-assert_linux_package_method "numbered prerelease uses deb despite snap" v0.1.0-pre.3 1 1 1 deb
-assert_linux_package_method "pre uses rpm despite snap" pre 1 0 1 rpm
-assert_linux_package_method "pinned stable uses deb despite snap" v1.2.3 1 1 1 deb
-assert_linux_package_method "pinned stable uses rpm despite snap" v1.2.3 1 0 1 rpm
+assert_linux_package_method "deb is the default despite snap" "" 1 1 1 deb
+assert_linux_package_method "rpm is the default despite snap" "" 1 0 1 rpm
+assert_linux_package_method "dev uses deb despite snap" dev 1 1 1 deb
+assert_linux_package_method "opt-in selects snap" "" optin 1 1 snap
+assert_linux_package_method "opt-in dev selects snap" dev optin 1 1 snap
+assert_linux_package_method "existing snap install keeps refreshing" "" existing 1 1 snap
+assert_linux_package_method "existing snap install keeps refreshing dev" dev existing 1 1 snap
+assert_linux_package_method "opt-in without snap uses deb" "" 0 1 1 deb
+assert_linux_package_method "pre uses deb despite snap opt-in" pre optin 1 1 deb
+assert_linux_package_method "numbered prerelease uses deb despite snap opt-in" v0.1.0-pre.3 optin 1 1 deb
+assert_linux_package_method "pre uses rpm despite snap opt-in" pre optin 0 1 rpm
+assert_linux_package_method "pinned stable uses deb despite existing snap" v1.2.3 existing 1 1 deb
+assert_linux_package_method "pinned stable uses rpm despite existing snap" v1.2.3 existing 0 1 rpm
 assert_linux_package_method "deb is selected without snap" "" 0 1 1 deb
 assert_linux_package_method "dev uses deb without snap" dev 0 1 1 deb
 assert_linux_package_method "rpm is selected without snap or deb" "" 0 0 1 rpm