diff --git a/tools/setup/install.sh b/tools/setup/install.sh
index e23d17ef32..649d33aeb7 100755
--- a/tools/setup/install.sh
+++ b/tools/setup/install.sh
@@ -1,7 +1,8 @@
 #!/usr/bin/env bash
 #
 # tools/setup/install.sh — the one install script consumed three ways
-# (dev laptops, CI runners, devcontainer images) per GOVERNANCE.md §24.
+# (dev laptops, CI runners, devcontainer images) per GOVERNANCE.md §24,
+# plus iter-B-0857.2: NixOS-aware routing for cluster nodes.
 #
 # Safe to run repeatedly — detect-first-install-else-update. Safe to
 # run daily to keep tools fresh.
@@ -9,17 +10,88 @@
 # Usage:
 #   tools/setup/install.sh
 #
-# Exit 0 on success. Any failure is a dev-experience bug; the CI
-# `gate.yml` workflow asserts this script completes twice in sequence.
+# Exit codes:
+#   0 — success (install/runtime-setup completed on a routable environment)
+#   1 — error (unsupported OS, unrecognized Linux flavor, or downstream
+#       failure from setup/macos.sh or setup/linux.sh)
+#   2 — intentional routing guard (NixOS live-USB environment detected;
+#       script directs operator to zeta-install.sh — this is NOT a
+#       dev-experience bug, it's the live-USB-vs-installed routing per
+#       B-0857.2 below)
+#
+# The CI `gate.yml` workflow asserts this script completes with exit 0
+# twice in sequence in its tested environments (none of which are NixOS
+# live-USB), so exit 2 from the live-USB branch does NOT affect CI gate
+# assertions.
+#
+# B-0857.2 routing (added 2026-05-27):
+#   - macOS (uname -s = Darwin)              -> setup/macos.sh
+#   - Linux non-NixOS (no /etc/NIXOS)        -> setup/linux.sh
+#   - NixOS installed (/etc/NIXOS,           -> setup/linux.sh
+#     not docker, no /iso, no                   (NixOS-installed nodes get the same
+#     /run/initramfs)                            mise + bun + claude runtime setup
+#                                                as any Linux build machine; nixos-
+#                                                rebuild handles the NixOS-side
+#                                                declarative config)
+#   - NixOS docker test harness (/etc/NIXOS  -> setup/linux.sh (treated as installed;
+#     + /.dockerenv from B-0849 docker          discriminator-2 short-circuit)
+#     test harness)
+#   - NixOS live-USB (/etc/NIXOS + /iso OR   -> message pointing to zeta-install.sh
+#     /run/initramfs canonical installer-       (B-0857.3 will factor that body into
+#     ISO markers)                              a callable nixos-install-from-usb.sh;
+#                                                this routing stub lands first)
+#
+# Per B-0857 operator framing (2026-05-27): install.sh is the universal
+# Unix-like-OS install + self-update entry — "there is no distinction
+# between build machines and prod when prod can update itself." This
+# row's substrate scope is environment-routing dispatch.
 
 set -euo pipefail
 
 REPO_ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
 SETUP_DIR="$REPO_ROOT/tools/setup"
 
-echo "=== Zeta install — three-way parity script (GOVERNANCE.md §24) ==="
+echo "=== Zeta install — universal Unix-like-OS entry (GOVERNANCE.md §24 + B-0857.2) ==="
 echo "Repo root: $REPO_ROOT"
 
+# Detect-NixOS-and-mode helper (B-0857.2).
+#
+# Outputs one of: "nixos-live", "nixos-installed", "linux-non-nixos".
+# Caller decides routing.
+#
+# Discriminator priority (per B-0849 docker-test-harness composition):
+#   1. /etc/NIXOS marker → NixOS (else linux-non-nixos)
+#   2. /.dockerenv → installed (Docker container, e.g., the B-0849
+#      docker-nixos-install-sh-test harness, which manually creates
+#      /etc/NIXOS to exercise the NixOS userspace path; Docker uses
+#      overlayfs at root which would false-positive on the live-USB
+#      check, so the docker discriminator runs FIRST)
+#   3. /iso present OR /run/initramfs present → live-USB (the canonical
+#      NixOS-installer-ISO markers; these are what zeta-install.sh
+#      itself probes for in its boot-USB detection logic)
+#   4. Otherwise → installed (safer default; overlayfs-without-iso is
+#      more likely an unusual installed config than a live boot)
+detect_linux_flavor() {
+  if [ ! -f /etc/NIXOS ]; then
+    echo "linux-non-nixos"
+    return 0
+  fi
+  # Discriminator 2: Docker container short-circuits to installed
+  # (the B-0849 harness creates /etc/NIXOS manually but is not a
+  # live USB; its overlayfs root would otherwise false-positive).
+  if [ -f /.dockerenv ]; then
+    echo "nixos-installed"
+    return 0
+  fi
+  # Discriminator 3: canonical NixOS-installer-ISO markers.
+  if [ -d /iso ] || [ -d /run/initramfs ]; then
+    echo "nixos-live"
+    return 0
+  fi
+  # Discriminator 4 (default): installed.
+  echo "nixos-installed"
+}
+
 os="$(uname -s)"
 case "$os" in
   Darwin)
@@ -27,8 +99,61 @@ case "$os" in
     "$SETUP_DIR/macos.sh"
     ;;
   Linux)
-    echo "OS: Linux"
-    "$SETUP_DIR/linux.sh"
+    flavor="$(detect_linux_flavor)"
+    case "$flavor" in
+      linux-non-nixos)
+        echo "OS: Linux (non-NixOS)"
+        "$SETUP_DIR/linux.sh"
+        ;;
+      nixos-installed)
+        echo "OS: NixOS (installed; runtime setup via mise + bun + claude)"
+        echo "Note: NixOS-side declarative config managed via nixos-rebuild;"
+        echo "      this step only sets up the operator runtime tooling."
+        "$SETUP_DIR/linux.sh"
+        ;;
+      nixos-live)
+        # Resolve the absolute path to zeta-install.sh so the message
+        # works regardless of caller's cwd. install.sh is callable
+        # from any working directory (e.g., on a fresh live-USB boot
+        # the operator may invoke this via a symlink / absolute path /
+        # bash <(...) — relative path would fail in those cases).
+        ZETA_INSTALL_ABS="$REPO_ROOT/full-ai-cluster/usb-nixos-installer/zeta-install.sh"
+        INJECTION_POINTS_ABS="$REPO_ROOT/full-ai-cluster/INJECTION-POINTS.md"
+        cat >&2 <<EOF
+OS: NixOS live-USB
+
+This is a NixOS live-USB environment. To install Zeta onto a target
+disk, run the NixOS USB-disk installer (absolute path so it works
+regardless of cwd):
+
+  sudo bash $ZETA_INSTALL_ABS
+
+That script handles disk partitioning, nixos-install, and the
+operator-injection points (SSH pubkey, hostname, password) per:
+
+  $INJECTION_POINTS_ABS
+
+After the target machine reboots into installed-NixOS, run this
+install.sh script again on the installed system to set up the
+runtime tooling (mise + bun + claude). The same script entry —
+different routing.
+
+B-0857.3 (follow-up sub-row) will factor zeta-install.sh into a
+callable nixos-install-from-usb.sh that this script can dispatch
+to directly. Until then, this stub points operators to the
+existing entry.
+
+Exit 2 from this branch is the intentional routing guard (NOT a
+dev-experience bug — see exit-code documentation in the script
+header).
+EOF
+        exit 2
+        ;;
+      *)
+        echo "error: unrecognized Linux flavor '$flavor'" >&2
+        exit 1
+        ;;
+    esac
     ;;
   *)
     echo "error: unsupported OS '$os' (macOS + Linux only this round; Windows backlogged)"