diff --git a/config/prometheus/alert_rules.yml b/config/prometheus/alert_rules.yml
index d3c04d651..74bb851f7 100644
--- a/config/prometheus/alert_rules.yml
+++ b/config/prometheus/alert_rules.yml
@@ -200,3 +200,62 @@ groups:
             A previously-failed-deploy color has been kept alive for
             24+ hours. Either complete forensics + run cleanup-failed,
             or the next deploy will recycle it automatically.
+
+  # v1.0.9 W5 Day 24 : synthetic monitoring (blackbox exporter).
+  # Each parcours is probed every 5 min ; the 10m `for:` window means
+  # an alert fires after 2 consecutive failures (per the roadmap
+  # acceptance gate). `parcours` label carries the human-readable
+  # name from blackbox_targets.yml so dashboards group cleanly.
+  - name: veza_synthetic
+    rules:
+      - alert: SyntheticParcoursDown
+        # probe_success is 0 when blackbox couldn't complete the probe.
+        # The metric is emitted per (instance, parcours) so the alert
+        # fires per-parcours, letting the on-call see exactly which
+        # journey is broken without grepping logs.
+        expr: probe_success{probe_kind="synthetic"} == 0
+        for: 10m
+        labels:
+          severity: warning
+          page: "false"
+        annotations:
+          summary: "Synthetic parcours {{ $labels.parcours }} failing for 10m"
+          description: |
+            Blackbox exporter has been unable to complete the
+            {{ $labels.parcours }} parcours against {{ $labels.instance }}
+            for 10 minutes (≥ 2 consecutive failures). End-user impact
+            is likely real — investigate the underlying component
+            BEFORE the related per-component alert fires.
+          runbook_url: "https://veza.fr/runbooks/synthetic-parcours-down"
+
+      - alert: SyntheticAuthLoginDown
+        # Login is the gate for everything else ; a single 10m blip
+        # is critical. Pages.
+        expr: probe_success{parcours="auth_login"} == 0
+        for: 10m
+        labels:
+          severity: critical
+          page: "true"
+        annotations:
+          summary: "Synthetic auth_login down — login surface is broken"
+          description: |
+            The auth_login synthetic parcours has failed for 10+ minutes.
+            Real users cannot log in. Page now.
+          runbook_url: "https://veza.fr/runbooks/synthetic-parcours-down"
+
+      - alert: SyntheticProbeSlow
+        # Probe latency budget : 5s for HTTP, 8s for the heavier ones.
+        # When real-user latency degrades, blackbox is the canary.
+        expr: probe_duration_seconds{probe_kind="synthetic"} > 8
+        for: 15m
+        labels:
+          severity: warning
+          page: "false"
+        annotations:
+          summary: "Synthetic parcours {{ $labels.parcours }} > 8s for 15m"
+          description: |
+            Probe duration exceeded 8 seconds for the past 15 minutes.
+            Real users are likely seeing visible latency. Cross-check
+            the SLO burn-rate alerts ; if those are quiet but this
+            fires, the issue is in the synthetic-only path (DNS,
+            external dependency).
diff --git a/config/prometheus/blackbox_targets.yml b/config/prometheus/blackbox_targets.yml
new file mode 100644
index 000000000..a106f60f2
--- /dev/null
+++ b/config/prometheus/blackbox_targets.yml
@@ -0,0 +1,89 @@
+# Prometheus blackbox scrape config — synthetic monitoring of the
+# 6 parcours from v1.0.9 W5 Day 24.
+#
+# Probed every 5 minutes ; alerts fire after 2 consecutive failures.
+# This file is sourced by the main prometheus.yml :
+#
+#   scrape_configs:
+#     - job_name: 'blackbox'
+#       file_sd_configs:
+#         - files:
+#           - /etc/prometheus/blackbox_targets.yml
+#       metrics_path: /probe
+#       relabel_configs:
+#         - source_labels: [__address__]
+#           target_label: __param_target
+#         - source_labels: [__param_target]
+#           target_label: instance
+#         - target_label: __address__
+#           replacement: blackbox-exporter.lxd:9115
+#
+# Each entry below carries a `module` label that maps to a
+# blackbox.yml module name AND a `parcours` label so Grafana can
+# group / filter. Prometheus passes module + target through the
+# query string when it scrapes blackbox.
+
+# Parcours 1 — register / verify / login
+# (Reachability of the auth surface ; multi-step register-then-verify
+# requires a synthetic-client binary, tracked as follow-up.)
+- targets:
+    - https://staging.veza.fr/api/v1/auth/login
+  labels:
+    module: http_status_envelope
+    parcours: auth_login
+    probe_kind: synthetic
+
+# Parcours 2 — login → search → play first
+- targets:
+    - https://staging.veza.fr/api/v1/search?q=test
+  labels:
+    module: http_search
+    parcours: search
+    probe_kind: synthetic
+
+# Parcours 3 — login → upload tiny audio → poll status
+# Approximated by reaching the upload-config endpoint ; the actual
+# upload requires auth + file body which blackbox can't model.
+- targets:
+    - https://staging.veza.fr/api/v1/upload/config
+  labels:
+    module: http_2xx
+    parcours: upload_init
+    probe_kind: synthetic
+
+# Parcours 4 — login → browse marketplace → add to cart
+# Approximated by reaching the marketplace listing endpoint.
+- targets:
+    - https://staging.veza.fr/api/v1/marketplace/products?limit=5
+  labels:
+    module: http_marketplace
+    parcours: marketplace_list
+    probe_kind: synthetic
+
+# Parcours 5 — WebSocket chat connect + send message
+# TCP-only probe : confirms the listener is up. The full handshake +
+# auth + send round-trip needs the synthetic-client binary.
+- targets:
+    - staging.veza.fr:443
+  labels:
+    module: tcp_websocket
+    parcours: chat_websocket
+    probe_kind: synthetic
+
+# Parcours 6 — live stream metadata fetch
+- targets:
+    - https://staging.veza.fr/api/v1/streams/active
+  labels:
+    module: http_2xx
+    parcours: live_streams
+    probe_kind: synthetic
+
+# Bonus — public status page health (covers the /api/v1/status
+# response shape so a Cachet/statuspage.io consumer doesn't depend
+# on a hand-pinged check).
+- targets:
+    - https://staging.veza.fr/api/v1/status
+  labels:
+    module: http_status_envelope
+    parcours: status_endpoint
+    probe_kind: synthetic
diff --git a/infra/ansible/inventory/lab.yml b/infra/ansible/inventory/lab.yml
index 2f9251b5b..f0ca36929 100644
--- a/infra/ansible/inventory/lab.yml
+++ b/infra/ansible/inventory/lab.yml
@@ -112,6 +112,14 @@ all:
       vars:
         ansible_connection: community.general.incus
         ansible_python_interpreter: /usr/bin/python3
+    # v1.0.9 W5 Day 24 — synthetic monitoring runner. Should sit on a
+    # host external to the prod cluster ; lab phase-1 colocates it.
+    blackbox_exporter:
+      hosts:
+        blackbox-exporter:
+      vars:
+        ansible_connection: community.general.incus
+        ansible_python_interpreter: /usr/bin/python3
     # v1.0.9 W3 Day 12: distributed MinIO with EC:2. 4 Incus containers,
     # each providing one drive ; single erasure set tolerates 2 simultaneous
     # node failures.
diff --git a/infra/ansible/playbooks/blackbox_exporter.yml b/infra/ansible/playbooks/blackbox_exporter.yml
new file mode 100644
index 000000000..082811d25
--- /dev/null
+++ b/infra/ansible/playbooks/blackbox_exporter.yml
@@ -0,0 +1,56 @@
+# Synthetic monitoring playbook — provisions the blackbox-exporter
+# Incus container and lays down the role.
+#
+# v1.0.9 W5 Day 24.
+#
+# IMPORTANT : the blackbox exporter SHOULD run on a host that is
+# externally-routed (separate from the prod cluster) so a probe
+# failure reflects what an external user sees. v1.0 lab keeps it on
+# the same Incus host for simplicity ; phase-2 moves it off-box.
+#
+# Run with:
+#   ansible-galaxy collection install community.general
+#   ansible-playbook -i inventory/lab.yml playbooks/blackbox_exporter.yml
+---
+- name: Provision Incus container for blackbox exporter
+  hosts: incus_hosts
+  become: true
+  gather_facts: true
+  tasks:
+    - name: Launch blackbox-exporter container
+      ansible.builtin.shell:
+        cmd: |
+          set -e
+          if ! incus info blackbox-exporter >/dev/null 2>&1; then
+            incus launch images:ubuntu/22.04 blackbox-exporter
+            for _ in $(seq 1 30); do
+              if incus exec blackbox-exporter -- cloud-init status 2>/dev/null | grep -q "status: done"; then
+                break
+              fi
+              sleep 1
+            done
+            incus exec blackbox-exporter -- apt-get update
+            incus exec blackbox-exporter -- apt-get install -y python3 python3-apt
+          fi
+      args:
+        executable: /bin/bash
+      register: provision_result
+      changed_when: "'incus launch' in provision_result.stdout"
+      tags: [blackbox, provision]
+
+    - name: Refresh inventory
+      ansible.builtin.meta: refresh_inventory
+
+- name: Apply common baseline
+  hosts: blackbox_exporter
+  become: true
+  gather_facts: true
+  roles:
+    - common
+
+- name: Install + configure blackbox exporter
+  hosts: blackbox_exporter
+  become: true
+  gather_facts: true
+  roles:
+    - blackbox_exporter
diff --git a/infra/ansible/roles/blackbox_exporter/README.md b/infra/ansible/roles/blackbox_exporter/README.md
new file mode 100644
index 000000000..64182cd46
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/README.md
@@ -0,0 +1,93 @@
+# `blackbox_exporter` role — synthetic monitoring runner
+
+Single Incus container running Prometheus' `blackbox_exporter`. Probed by Prometheus every 5 minutes against the 6 user parcours from v1.0.9 W5 Day 24. Alerts fire after 2 consecutive failures (`for: 10m` × 5-min scrape = 2 cycles).
+
+## Topology
+
+```
+                               Prometheus :9090
+                                     │ scrape every 5m
+                                     ▼
+                       ┌─────────────────────────────┐
+                       │ blackbox-exporter.lxd:9115  │
+                       │  (this role)                │
+                       └────────────┬────────────────┘
+                                    │ probes (HTTP / TCP)
+              ┌─────────────────────┼─────────────────────┐
+              ▼                     ▼                     ▼
+  staging.veza.fr/api/v1/auth/login  /api/v1/search?q=test   /api/v1/marketplace/products
+              ...                                            ...
+```
+
+The exporter SHOULD run on a host **external** to the prod cluster so probe failures reflect what an external user sees, not what an already-broken internal service hides. v1.0 lab phase-1 colocates it for simplicity ; phase-2 moves the container off-box.
+
+## Probe modules (defined in `templates/blackbox.yml.j2`)
+
+| Module                 | Used by parcours       | What it asserts                                        |
+| ---------------------- | ---------------------- | ------------------------------------------------------ |
+| `http_2xx`             | upload_init, live_streams | Status code 200 or 204, TLS valid                  |
+| `http_status_envelope` | auth_login, status_endpoint | Body matches `"success":\s*true`                  |
+| `http_search`          | search                 | Body matches `"tracks"` (seed data must include hits) |
+| `http_marketplace`     | marketplace_list       | 200 (no body assertion ; an empty array is valid)     |
+| `tcp_websocket`        | chat_websocket         | TLS-wrapped TCP handshake completes                   |
+
+Multi-step parcours that need session state (Register → Verify → Login, Login → Search → Play first result) are **out of scope** for blackbox. Tracked as a follow-up : a small Go binary that runs as a CronJob, walks the steps, and writes textfile-collector metrics to `/var/lib/node_exporter/textfile_collector/veza_synthetic.prom`.
+
+## Defaults
+
+| variable                   | default                       | meaning                                  |
+| -------------------------- | ----------------------------- | ---------------------------------------- |
+| `blackbox_version`         | `0.25.0`                      | Prometheus blackbox_exporter release     |
+| `blackbox_listen_port`     | `9115`                        | Prometheus default                       |
+| `blackbox_target_base_url` | `https://staging.veza.fr`     | base URL the probes hit                  |
+
+## Prometheus scrape config
+
+`config/prometheus/blackbox_targets.yml` carries the 7 file-SD entries (6 parcours + status-endpoint bonus). Wire it in `prometheus.yml` :
+
+```yaml
+scrape_configs:
+  - job_name: blackbox
+    file_sd_configs:
+      - files: [/etc/prometheus/blackbox_targets.yml]
+    metrics_path: /probe
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+      - source_labels: [__param_target]
+        target_label: instance
+      - source_labels: [module]
+        target_label: __param_module
+      - target_label: __address__
+        replacement: blackbox-exporter.lxd:9115
+```
+
+## Alert rules
+
+`config/prometheus/alert_rules.yml` group `veza_synthetic` :
+
+- `SyntheticParcoursDown` — any parcours fails for 10 m → warning.
+- `SyntheticAuthLoginDown` — auth_login fails for 10 m → critical (page).
+- `SyntheticProbeSlow` — probe duration > 8 s for 15 m → warning.
+
+## Operations
+
+```bash
+# Service status :
+sudo systemctl status blackbox_exporter
+
+# One-off probe (dev / debug) :
+curl 'http://blackbox-exporter.lxd:9115/probe?target=https://staging.veza.fr/api/v1/health&module=http_status_envelope'
+
+# Live probe latency tail :
+curl -s http://blackbox-exporter.lxd:9115/metrics | grep probe_duration
+
+# Tail the exporter log :
+sudo journalctl -u blackbox_exporter -f
+```
+
+## What this role does NOT cover
+
+- **Multi-step parcours.** Blackbox can't carry session cookies across probes ; the Register-then-Verify-then-Login flow needs a custom synthetic client. Tracked for v1.0.10.
+- **Status page.** Cachet/statuspage.io is a separate operator decision per the roadmap. The `/api/v1/status` endpoint is consumable by both.
+- **Off-box deploy.** Lab phase-1 runs the container on the same Incus host as the things it's probing. Phase-2 moves it off-cluster.
diff --git a/infra/ansible/roles/blackbox_exporter/defaults/main.yml b/infra/ansible/roles/blackbox_exporter/defaults/main.yml
new file mode 100644
index 000000000..3a99caa40
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/defaults/main.yml
@@ -0,0 +1,20 @@
+# blackbox_exporter defaults — synthetic monitoring runner.
+# v1.0.9 W5 Day 24.
+#
+# Sits OUTSIDE the prod network (separate Incus host or off-box) so a
+# probe failure reflects what an external user sees, not what an
+# already-broken internal service hides. Six parcours per the roadmap,
+# probed every 5 min by Prometheus.
+---
+blackbox_version: "0.25.0"
+blackbox_arch: amd64
+
+# Listener — Prometheus scrapes this on port 9115 (the blackbox_exporter
+# default).
+blackbox_listen_port: 9115
+
+# Probe targets. The 6 parcours from the roadmap are mapped to simpler
+# blackbox probes here (HTTP 2xx) ; the multi-step parcours that need
+# session state (Register → Login → Search) are out of scope for
+# blackbox itself and tracked as a follow-up (synthetic-client binary).
+blackbox_target_base_url: "https://staging.veza.fr"
diff --git a/infra/ansible/roles/blackbox_exporter/handlers/main.yml b/infra/ansible/roles/blackbox_exporter/handlers/main.yml
new file mode 100644
index 000000000..15016f20f
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/handlers/main.yml
@@ -0,0 +1,6 @@
+---
+- name: Restart blackbox_exporter
+  ansible.builtin.systemd:
+    name: blackbox_exporter
+    state: restarted
+    daemon_reload: true
diff --git a/infra/ansible/roles/blackbox_exporter/tasks/main.yml b/infra/ansible/roles/blackbox_exporter/tasks/main.yml
new file mode 100644
index 000000000..44667a764
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/tasks/main.yml
@@ -0,0 +1,89 @@
+# blackbox_exporter role — installs the Prometheus blackbox exporter
+# from the official tarball, drops the systemd unit, renders the probe
+# config. Idempotent.
+---
+- name: Ensure /opt/blackbox_exporter exists
+  ansible.builtin.file:
+    path: /opt/blackbox_exporter
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+  tags: [blackbox, install]
+
+- name: Check installed blackbox_exporter version
+  ansible.builtin.stat:
+    path: "/opt/blackbox_exporter/blackbox_exporter-{{ blackbox_version }}"
+  register: blackbox_installed
+  tags: [blackbox, install]
+
+- name: Download blackbox_exporter tarball
+  ansible.builtin.get_url:
+    url: "https://github.com/prometheus/blackbox_exporter/releases/download/v{{ blackbox_version }}/blackbox_exporter-{{ blackbox_version }}.linux-{{ blackbox_arch }}.tar.gz"
+    dest: "/tmp/blackbox_exporter-{{ blackbox_version }}.tar.gz"
+    mode: "0644"
+  when: not blackbox_installed.stat.exists
+  tags: [blackbox, install]
+
+- name: Extract blackbox_exporter into versioned slot
+  ansible.builtin.unarchive:
+    src: "/tmp/blackbox_exporter-{{ blackbox_version }}.tar.gz"
+    dest: /opt/blackbox_exporter
+    remote_src: true
+    creates: "/opt/blackbox_exporter/blackbox_exporter-{{ blackbox_version }}.linux-{{ blackbox_arch }}"
+  when: not blackbox_installed.stat.exists
+  tags: [blackbox, install]
+
+- name: Symlink /usr/local/bin/blackbox_exporter → versioned binary
+  ansible.builtin.file:
+    src: "/opt/blackbox_exporter/blackbox_exporter-{{ blackbox_version }}.linux-{{ blackbox_arch }}/blackbox_exporter"
+    dest: /usr/local/bin/blackbox_exporter
+    state: link
+    force: true
+  notify: Restart blackbox_exporter
+  tags: [blackbox, install]
+
+- name: Create blackbox system user
+  ansible.builtin.user:
+    name: blackbox
+    system: true
+    shell: /usr/sbin/nologin
+    create_home: false
+  tags: [blackbox, install]
+
+- name: Ensure /etc/blackbox_exporter exists
+  ansible.builtin.file:
+    path: /etc/blackbox_exporter
+    state: directory
+    owner: root
+    group: blackbox
+    mode: "0750"
+  tags: [blackbox, config]
+
+- name: Render blackbox.yml
+  ansible.builtin.template:
+    src: blackbox.yml.j2
+    dest: /etc/blackbox_exporter/blackbox.yml
+    owner: root
+    group: blackbox
+    mode: "0640"
+  notify: Restart blackbox_exporter
+  tags: [blackbox, config]
+
+- name: Render systemd unit
+  ansible.builtin.template:
+    src: blackbox_exporter.service.j2
+    dest: /etc/systemd/system/blackbox_exporter.service
+    owner: root
+    group: root
+    mode: "0644"
+  notify: Restart blackbox_exporter
+  tags: [blackbox, service]
+
+- name: Enable + start blackbox_exporter
+  ansible.builtin.systemd:
+    name: blackbox_exporter
+    state: started
+    enabled: true
+    daemon_reload: true
+  tags: [blackbox, service]
diff --git a/infra/ansible/roles/blackbox_exporter/templates/blackbox.yml.j2 b/infra/ansible/roles/blackbox_exporter/templates/blackbox.yml.j2
new file mode 100644
index 000000000..1a76544a3
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/templates/blackbox.yml.j2
@@ -0,0 +1,61 @@
+# Managed by Ansible — do not edit by hand.
+# Probe modules used by Prometheus' blackbox scrape config.
+# v1.0.9 W5 Day 24.
+
+modules:
+  # http_2xx — vanilla HTTP probe, accepts any 2xx response.
+  http_2xx:
+    prober: http
+    timeout: 5s
+    http:
+      preferred_ip_protocol: ip4
+      valid_status_codes: [200, 204]
+      method: GET
+      no_follow_redirects: false
+      fail_if_ssl: false
+      fail_if_not_ssl: true   # synthetic monitoring runs against staging w/ TLS
+
+  # http_status_envelope — accept the {success: true, ...} body shape.
+  # Used for /api/v1/health which wraps the verdict.
+  http_status_envelope:
+    prober: http
+    timeout: 5s
+    http:
+      preferred_ip_protocol: ip4
+      valid_status_codes: [200]
+      method: GET
+      fail_if_body_not_matches_regexp:
+        - '"success"\s*:\s*true'
+
+  # http_search — POST-less search probe. The synthetic user hits
+  # /api/v1/search?q=test ; staging seed data must include something
+  # for that query to return non-empty.
+  http_search:
+    prober: http
+    timeout: 8s
+    http:
+      preferred_ip_protocol: ip4
+      valid_status_codes: [200]
+      method: GET
+      fail_if_body_not_matches_regexp:
+        - '"tracks"'
+
+  # http_marketplace — same shape, different endpoint.
+  http_marketplace:
+    prober: http
+    timeout: 8s
+    http:
+      preferred_ip_protocol: ip4
+      valid_status_codes: [200]
+      method: GET
+
+  # tcp_websocket — bare TCP connect to the WS port to verify the
+  # listener is alive. Doesn't speak the WS protocol — for that the
+  # synthetic-client binary (out of scope for this role) handles
+  # connect+send+receive.
+  tcp_websocket:
+    prober: tcp
+    timeout: 5s
+    tcp:
+      preferred_ip_protocol: ip4
+      tls: true
diff --git a/infra/ansible/roles/blackbox_exporter/templates/blackbox_exporter.service.j2 b/infra/ansible/roles/blackbox_exporter/templates/blackbox_exporter.service.j2
new file mode 100644
index 000000000..663725345
--- /dev/null
+++ b/infra/ansible/roles/blackbox_exporter/templates/blackbox_exporter.service.j2
@@ -0,0 +1,27 @@
+# Managed by Ansible — do not edit by hand.
+[Unit]
+Description=Prometheus Blackbox Exporter
+Documentation=https://github.com/prometheus/blackbox_exporter
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=blackbox
+Group=blackbox
+ExecStart=/usr/local/bin/blackbox_exporter \
+  --config.file=/etc/blackbox_exporter/blackbox.yml \
+  --web.listen-address=:{{ blackbox_listen_port }}
+Restart=on-failure
+RestartSec=5s
+LimitNOFILE=65535
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+PrivateTmp=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true
+
+[Install]
+WantedBy=multi-user.target