diff --git a/docs/HARNESS.md b/docs/HARNESS.md
index 1bf392a..00d240e 100644
--- a/docs/HARNESS.md
+++ b/docs/HARNESS.md
@@ -47,6 +47,7 @@ Three regulation categories:
 | Arch. | `no-os-getenv-home` | L1 | `internal/archtest/envhome_test.go` |
 | Arch. | `dryrun` — destructive ops must check `DryRun` | L1 | `internal/archtest/dryrun_test.go` |
 | Arch. | `no-raw-fmtprint` — UI output via `ui.*` helpers, not raw `fmt.Print*` | L1 | `internal/archtest/fmtprint_test.go` |
+| Arch. | `install.sh` must not prompt on stdin — under `curl \| bash` stdin is the script | L1 | `internal/archtest/installsh_test.go` |
 | Behav. | L1 unit + integration + contract (faked runners *and* real brew/git/npm in temp dirs) | pre-push, CI | `make test-unit` |
 | Behav. | L2 contract schema (against openboot-contract repo) | CI | `.github/workflows/test.yml` `contract` job |
 | Behav. | L3 e2e binary | release | `make test-e2e` |
@@ -74,6 +75,7 @@ When you observe a recurring issue, decide where to encode the fix:
 | "Agent doesn't know about preset X." | Update `internal/config/data/presets.yaml`. Source of truth, not docs. |
 | "Agent introduced a new lint failure that golangci-lint should have caught." | Enable the relevant linter in `.golangci.yml`. |
 | "Agent broke a behaviour that has no test." | Write the test at the right tier — L1 covers both faked-runner units in `internal/<pkg>/` and real-subprocess integration in `test/integration/`. |
+| "A shipped release reached nobody: `install.sh`'s already-installed branch prompted on stdin, which under `curl \| bash` is the script itself, so it always took the don't-upgrade default." | Already handled by the `installsh` archtest. The wider lesson the sensor does *not* cover: `curl-bash-smoke` is gated `if: github.event_name != 'pull_request'` and only exercises the mock-server path, so the real `scripts/install.sh` brew branch has no behavioural test. Upgrade-over-existing-install is the case to add. |
 | "Agent missed a CLAUDE.md rule we keep restating." | Make it a hard or soft archtest rule (a docs rule that doesn't fail is a docs rule that drifts). |
 | "Agent did something safe but suboptimal." | Add to CLAUDE.md "Project-specific conventions" and consider whether it's encodable. |
 | "Agent guessed at an API contract." | Update `openboot-contract` repo + fixtures; CI already runs schema validation. |
diff --git a/internal/archtest/installsh_test.go b/internal/archtest/installsh_test.go
new file mode 100644
index 0000000..8261027
--- /dev/null
+++ b/internal/archtest/installsh_test.go
@@ -0,0 +1,72 @@
+package archtest
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// bareReadRe matches a `read` prompt that takes no explicit input redirect.
+// `read ... </dev/tty` and `read ... <<<"$x"` are fine; a plain `read` is not.
+var bareReadRe = regexp.MustCompile(`(^|\s|;|\||&&)read\s`)
+
+// TestInstallShHasNoBareRead enforces the rule that scripts/install.sh must
+// never prompt on stdin.
+//
+// Its headline use is:
+//
+//	curl -fsSL openboot.dev/install.sh | bash
+//
+// There, stdin is the pipe carrying the script itself — not the user's
+// keyboard. A bare `read` consumes the script's own next bytes as the answer,
+// so it both mangles the script and takes a branch the user never chose.
+//
+// This is not hypothetical. The "OpenBoot is already installed. Reinstall?
+// (y/N)" prompt defaulted to No, and via the documented curl|bash it always
+// took that default — silently keeping every existing user on their old
+// version, then exec'ing it. Releases went out that nobody could receive, and
+// the symptom (`openboot version` reporting yesterday's build after a
+// successful-looking install) pointed nowhere near the cause.
+//
+// Prompts must read from /dev/tty via the script's ask_tty helper, which also
+// supplies a default for when there is no terminal at all (CI, piped shells).
+func TestInstallShHasNoBareRead(t *testing.T) {
+	path := filepath.Join("..", "..", "scripts", "install.sh")
+	src, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read install.sh: %v", err)
+	}
+
+	var bad []string
+	for i, line := range strings.Split(string(src), "\n") {
+		code, _, _ := strings.Cut(line, "#") // ignore prose in comments
+		if !bareReadRe.MatchString(code) {
+			continue
+		}
+		// The helper's own read is the sanctioned one: it redirects </dev/tty.
+		if strings.Contains(code, "/dev/tty") {
+			continue
+		}
+		bad = append(bad, "scripts/install.sh:"+itoa(i+1)+": "+strings.TrimSpace(line))
+	}
+
+	if len(bad) > 0 {
+		t.Errorf("install.sh prompts on stdin, which under `curl | bash` is the script itself:\n  %s\n\n"+
+			"Fix: ask via the ask_tty helper (reads /dev/tty, falls back to a default with no terminal).",
+			strings.Join(bad, "\n  "))
+	}
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	var b []byte
+	for n > 0 {
+		b = append([]byte{byte('0' + n%10)}, b...)
+		n /= 10
+	}
+	return string(b)
+}
diff --git a/scripts/install.sh b/scripts/install.sh
index 3ae119a..30b4b5b 100644
--- a/scripts/install.sh
+++ b/scripts/install.sh
@@ -7,6 +7,27 @@ BINARY_NAME="openboot"
 TAP_NAME="openbootdotdev/tap"
 DRY_RUN="${OPENBOOT_DRY_RUN:-false}"
 
+# This script's headline use is `curl -fsSL openboot.dev/install.sh | bash`,
+# where stdin is the pipe carrying the script itself — NOT the user's keyboard.
+# A bare `read` there consumes the script's own next bytes as the answer, which
+# both mangles the script and silently takes a branch the user never chose.
+# Every prompt must therefore read from the controlling terminal, and must have
+# a safe answer for when there isn't one (piped into a CI job, no tty).
+has_tty() { [[ -e /dev/tty ]] && (: >/dev/tty) 2>/dev/null; }
+
+# ask_tty <prompt> <default> [read-opts...] — echoes the reply, or <default>
+# when no terminal is available to ask.
+ask_tty() {
+    local prompt="$1" default="$2"; shift 2
+    if ! has_tty; then
+        echo "$default"
+        return
+    fi
+    local reply=""
+    read -r "$@" -p "$prompt" reply </dev/tty || reply="$default"
+    echo "${reply:-$default}"
+}
+
 install_xcode_clt() {
     if xcode-select -p &>/dev/null; then
         return 0
@@ -18,7 +39,7 @@ install_xcode_clt() {
     echo "Xcode Command Line Tools need to be installed."
     echo "A dialog will appear - please click 'Install' and enter your password."
     echo ""
-    read -p "Press Enter to launch installer..." -r
+    ask_tty "Press Enter to launch installer..." "" >/dev/null
     echo ""
 
     xcode-select --install 2>/dev/null || true
@@ -161,29 +182,36 @@ main() {
     fi
 
     if brew list openboot &>/dev/null 2>&1; then
-        echo "OpenBoot is already installed via Homebrew."
+        echo "OpenBoot is already installed — updating..."
         echo ""
-        read -p "Reinstall? (y/N) " -n 1 -r
-        echo
-        
-        if [[ $REPLY =~ ^[Yy]$ ]]; then
-            echo "Reinstalling OpenBoot..."
+
+        # Refresh the tap before upgrading. Homebrew only auto-updates it every
+        # HOMEBREW_AUTO_UPDATE_SECS (24h by default), so a release published
+        # since the last refresh is invisible to `upgrade`, which then reports
+        # success while leaving the old binary in place.
+        brew update >/dev/null 2>&1 || true
+        if ! brew upgrade ${TAP_NAME}/openboot 2>/dev/null; then
             brew reinstall ${TAP_NAME}/openboot
-            echo ""
-            echo "✓ OpenBoot reinstalled!"
-        else
-            echo "Using existing installation."
         fi
+
+        echo ""
+        echo "✓ OpenBoot updated!"
     else
         echo "Installing OpenBoot via Homebrew..."
         echo ""
-        
+
         brew install ${TAP_NAME}/openboot
-        
+
         echo ""
         echo "✓ OpenBoot installed!"
     fi
 
+    # Always state the version we ended up on. Running the installer and
+    # silently getting yesterday's binary is the failure this whole path is
+    # guarding against; printing it makes that impossible to miss.
+    hash -r 2>/dev/null || true
+    echo "  $(openboot version 2>/dev/null || echo 'version unavailable')"
+
     echo ""
     echo "Starting OpenBoot setup..."
     echo ""