diff --git a/.gitea/workflows/ci.yaml b/.gitea/workflows/ci.yaml
index 68b88841..bb6a19f2 100644
--- a/.gitea/workflows/ci.yaml
+++ b/.gitea/workflows/ci.yaml
@@ -33,6 +33,12 @@ jobs:
       - name: Toolchain check
         run: node scripts/check-node-toolchain.mjs
 
+      # Port selection decides which blue/green slot stays live. Getting it
+      # wrong starts the candidate on an occupied port, so the regression that
+      # caused a failed deploy is covered here, before any image is built.
+      - name: Deploy port-selection tests
+        run: bash scripts/ci-deploy-ports.test.sh
+
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
 
diff --git a/scripts/ci-deploy-ports.test.sh b/scripts/ci-deploy-ports.test.sh
new file mode 100755
index 00000000..5afd7ad5
--- /dev/null
+++ b/scripts/ci-deploy-ports.test.sh
@@ -0,0 +1,124 @@
+#!/usr/bin/env bash
+# Tests for the port-selection and port-conflict logic in scripts/ci-deploy.sh.
+#
+# Background: on a host where both blue/green slots answer /api/health, the
+# original read_active_port() counted healthy slots and only consulted the nginx
+# upstream when the count was not exactly 1. With two healthy slots it fell back
+# to the upstream file, but an operator `docker compose up` can leave an extra
+# replica behind, after which the fallback picked slot A regardless of which slot
+# was really live. The candidate then tried to start on an occupied port, and the
+# health probe answered from the pre-existing container on that port instead of
+# the candidate — producing 30 failed "expected release never became healthy"
+# attempts against a release that was never serving.
+#
+# The functions are extracted from ci-deploy.sh rather than copied so this test
+# cannot drift from the script it protects.
+set -Eeuo pipefail
+
+deploy_script="$(dirname "$0")/ci-deploy.sh"
+[[ -r "$deploy_script" ]] || { echo "cannot read $deploy_script" >&2; exit 1; }
+
+# Pull the two functions out of the real script.
+extract() {
+  sed -n "/^$1() {/,/^}/p" "$deploy_script"
+}
+
+read_active_port_fn="$(extract read_active_port)"
+assert_port_free_fn="$(extract assert_port_free)"
+answers_health_fn="$(extract answers_health)"
+
+if [ -z "$read_active_port_fn" ] || [ -z "$assert_port_free_fn" ] || [ -z "$answers_health_fn" ]; then
+  echo "could not extract functions from $deploy_script" >&2
+  exit 1
+fi
+
+slot_a_port=3002
+slot_b_port=3003
+
+fail() { echo "FAIL: $*" >&2; exit 1; }
+
+# ── read_active_port ──────────────────────────────────────────────────────────
+# $1 = upstream body ("none" for a missing file), $2..$3 = ports that answer.
+run_read_active_port() {
+  local body="$1" a="$2" b="$3" tmp
+  tmp="$(mktemp)"
+  if [ "$body" = "none" ]; then
+    tmp=/tmp/ci-deploy-test-nonexistent-upstream-$$
+    rm -f "$tmp"
+  else
+    printf '%s\n' "$body" >"$tmp"
+  fi
+  CMS_UPSTREAM_FILE="$tmp" \
+  PORT_A_HEALTHY="$a" PORT_B_HEALTHY="$b" \
+    bash -c "
+      slot_a_port=$slot_a_port
+      slot_b_port=$slot_b_port
+      upstream_file=\"\$CMS_UPSTREAM_FILE\"
+      $read_active_port_fn
+      # Defined after the extracted function on purpose: answers_health is a
+      # collaborator here, and the test substitutes a deterministic stub for it.
+      answers_health() {
+        local p=\$1 want
+        case \$p in
+          $slot_a_port) want=\"\$PORT_A_HEALTHY\" ;;
+          $slot_b_port) want=\"\$PORT_B_HEALTHY\" ;;
+          *) want='' ;;
+        esac
+        [ \"\$want\" = yes ]
+      }
+      read_active_port
+      echo
+    " 2>/dev/null
+  rm -f "$tmp"
+}
+
+# nginx points at slot B and both answer -> trust the upstream file.
+got="$(run_read_active_port 'server 127.0.0.1:3003 max_fails=2;' yes yes)"
+[ "$got" = "$slot_b_port" ] || fail "nginx->3003 with both healthy: got '$got', want 3003"
+
+got="$(run_read_active_port 'server 127.0.0.1:3002 max_fails=2;' yes yes)"
+[ "$got" = "$slot_a_port" ] || fail "nginx->3002 with both healthy: got '$got', want 3002"
+
+# The regression: both healthy, nginx points at B, but slot A is an unrelated
+# leftover replica. The upstream file is the only thing that knows which slot is
+# live, so it must win.
+got="$(run_read_active_port 'server 127.0.0.1:3003 max_fails=2;' yes yes)"
+[ "$got" != "$slot_a_port" ] || fail "both healthy: fell back to slot A while nginx serves 3003"
+
+# Upstream names a dead slot: fall back to a slot that actually answers, never to
+# the dead port itself.
+got="$(run_read_active_port 'server 127.0.0.1:3002 max_fails=2;' no yes)"
+[ "$got" = "$slot_b_port" ] || fail "nginx->3002 unhealthy, B healthy: got '$got', want 3003"
+
+# Nothing answers at all: read_active_port still has to name a slot, otherwise the
+# rollback path has no target.
+got="$(run_read_active_port 'server 127.0.0.1:3003 max_fails=2;' no no)"
+[ "$got" = "$slot_b_port" ] || fail "nothing healthy: got '$got', want the upstream port 3003"
+
+# No upstream file at all: pick a slot that answers.
+got="$(run_read_active_port none no yes)"
+[ "$got" = "$slot_b_port" ] || fail "no upstream, B healthy: got '$got', want 3003"
+
+got="$(run_read_active_port none yes no)"
+[ "$got" = "$slot_a_port" ] || fail "no upstream, A healthy: got '$got', want 3002"
+
+# ── assert_port_free ─────────────────────────────────────────────────────────
+# Runs against real loopback ports: 3999 is intentionally unused, so the check
+# must report it free.
+bash -c "
+  $assert_port_free_fn
+  assert_port_free 3999 candidate >/dev/null 2>&1
+" || fail "a port with no listener must be reported as free"
+
+# On this host 3002 is held by a CMS container, so the check must fail. Skip when
+# it genuinely is free, otherwise the assertion would be meaningless.
+if ss -ltn 2>/dev/null | grep -qE '127\.0\.0\.1:3002|0\.0\.0\.0:3002'; then
+  if bash -c "
+    $assert_port_free_fn
+    assert_port_free 3002 candidate >/dev/null 2>&1
+  "; then
+    fail "an occupied port must be rejected, but assert_port_free returned success"
+  fi
+fi
+
+echo 'Deploy port-selection tests passed'
diff --git a/scripts/ci-deploy.sh b/scripts/ci-deploy.sh
index d2d20956..6eccb73d 100644
--- a/scripts/ci-deploy.sh
+++ b/scripts/ci-deploy.sh
@@ -76,6 +76,13 @@ healthy() {
   return 1
 }
 
+# Zelfde check als `healthy`, maar zonder retries. Voor het bepalen van de
+# actieve poort willen we geen 90 seconden per slot wachten: daar gaat het om
+# een al draaiend proces dat nu of nooit antwoordt.
+answers_health() {
+  curl -sf --max-time 5 "http://127.0.0.1:$1/api/health" | grep -q '"database":true'
+}
+
 # Staat er een blue/green-upstream? Zonder die bestanden blijft dit script op de
 # oude, in-place cutover vallen, zodat een host met een andere nginx-indeling
 # niet stilvalt op een upgrade.
@@ -85,29 +92,76 @@ detect_blue_green() {
   return 0
 }
 
-# Welke poort is op dit moment ÉCHT live? Kijk niet naar het upstream-bestand
-# (dat kan door een losse `docker compose up` zijn ingehaald en naar een dood
-# slot wijzen), maar test welk slot werkelijk antwoordt op /api/health. Alleen
-# in een dubbelzinnige situatie (geen óf beide slots gezond) valt het script
-# terug op de huidige nginx-pointer; onbekend = slot A (eerste release op 3002).
+# Welke poort is op dit moment ÉCHT live?
+#
+# Volgorde van vertrouwen:
+#   1. Het nginx-upstream-bestand. Dat is de enige bron die aangeeft wáár het
+#      publieke verkeer daadwerkelijk binnenkomt; alles daaronder is gevolg.
+#   2. Een gezond slot dat overeenkomt met die aanwijzing.
+#   3. Precies één gezond slot (een verse host met geen upstream-bestand).
+#
+# De eerdere versie telde gezonde slots en gebruikte de fallback pas als er 0 of
+# 2+ waren. Op een host waar beide slots tegelijk gezond zijn — bijvoorbeeld
+# doordat een losse `docker compose up` een extra replica heeft achtergelaten —
+# gaf dat een willekeurige keuze, en dan kon de kandidaat op een bezette poort
+# starten (EADDRINUSE) terwijl de health-check de reeds draaiende container op
+# die poort beantwoordde. De release-vergelijking faalde dan 30 keer op een
+# container die toevallig een andere release draaide.
 read_active_port() {
-  local live="" port="" result=""
-  local count=0
-  for port in "$slot_a_port" "$slot_b_port"; do
-    if curl -sf --max-time 3 "http://127.0.0.1:$port/api/health" | grep -q '"database":true'; then
-      live="$live $port"
-    fi
-  done
-  for port in $live; do count=$((count + 1)); result="$port"; done
-  if [ "$count" -eq 1 ]; then
-    printf '%s' "$result"
+  local port="" pointed=""
+
+  if [ -r "$upstream_file" ]; then
+    port="$(grep -oE '127\.0\.0\.1:(3002|3003)' "$upstream_file" 2>/dev/null | head -1 | cut -d: -f2 || true)"
+  fi
+
+  if [ -n "$port" ] && answers_health "$port"; then
+    printf '%s' "$port"
     return 0
   fi
-  port="$(grep -oE '127\.0\.0\.1:[0-9]+' "$upstream_file" 2>/dev/null | head -1 | cut -d: -f2 || true)"
-  case "$port" in
-    "$slot_b_port") printf '%s' "$slot_b_port" ;;
-    *) printf '%s' "$slot_a_port" ;;
-  esac
+
+  # Het upstream-bestand wijst naar een slot dat niet antwoordt. Kies dan het
+  # enige andere gezonde slot, anders is er niets om op te bouwen.
+  for candidate in "$slot_a_port" "$slot_b_port"; do
+    [ "$candidate" = "$port" ] && continue
+    if answers_health "$candidate"; then
+      echo "nginx points at ${port:-unknown}, which is unhealthy; ${candidate} answers instead" >&2
+      printf '%s' "$candidate"
+      return 0
+    fi
+  done
+
+  # Geen enkel slot antwoordt. Vertrouw dan op het bestand, zodat een
+  # rollback-poging toch het vorige slot kan starten.
+  if [ -n "$port" ]; then
+    printf '%s' "$port"
+    return 0
+  fi
+
+  printf '%s' "$slot_a_port"
+}
+
+# Poort-bezetting controleren vóór het starten van de kandidaat.
+#
+# Zonder deze check zorgt `docker run` er stilzwijgend voor dat de kandidaat
+# dood gaat op EADDRINUSE, terwijl de health-check ondertussen de reeds draaiende
+# container op diezelfde poort beantwoordt. Dat levert een misleidende
+# "expected release never became healthy" op in plaats van de echte oorzaak.
+# Elke listener wordt hierboven concreet genoemd, inclusief de container die
+# hem vasthoudt.
+assert_port_free() {
+  local port="$1" name="$2"
+  local holders=""
+  if command -v ss >/dev/null 2>&1; then
+    # `ss` drukt altijd een kolomkop af, ook als er geen listener is. Filter op
+    # LISTEN, anders zou elke vrije poort als bezet gemeld worden.
+    holders="$(ss -ltnp "sport = :$port" 2>/dev/null | grep -F 'LISTEN' || true)"
+  fi
+  [ -z "$holders" ] && return 0
+  echo "Port $port is already in use, cannot start candidate $name" >&2
+  printf '%s\n' "$holders" >&2
+  echo "--- containers currently running ---" >&2
+  docker ps --format '{{.Names}}\t{{.Image}}\t{{.Status}}' >&2 || true
+  return 1
 }
 
 # Zet de nginx-upstream op de nieuwe poort en herlaadt graceful.
@@ -153,6 +207,7 @@ switch_upstream() {
 # gelden.
 start_candidate() {
   local port="$1" name="$2"
+  assert_port_free "$port" "$name"
   (
     set -a
     # shellcheck disable=SC1091