diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 26e1f4b..cccae28 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -92,7 +92,9 @@ jobs:
           key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
           restore-keys: nuget-${{ runner.os }}-
       - run: dotnet format backend/BigRegister.slnx --verify-no-changes
-      - run: dotnet test backend/BigRegister.slnx
+      # Category=Integration (WP-54, OpenZaakIntegrationTests) needs a live OpenZaak — opt-in,
+      # run manually against backend/openzaak/ (see its README), never in CI.
+      - run: dotnet test backend/BigRegister.slnx --filter "Category!=Integration"
 
   e2e:
     # Smoke-level Playwright run against the REAL FE+backend (WP-19) — a fresh
diff --git a/.gitignore b/.gitignore
index 0958d99..0c79ad0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -51,3 +51,6 @@ storybook-static
 /playwright-report
 /blob-report
 /playwright/.cache
+
+# WP-54: bootstrap-catalogus.sh's own record of what it seeded into a local OpenZaak run
+backend/openzaak/seeded.env
diff --git a/backend/openzaak/README.md b/backend/openzaak/README.md
new file mode 100644
index 0000000..2ab446c
--- /dev/null
+++ b/backend/openzaak/README.md
@@ -0,0 +1,74 @@
+# OpenZaak integration harness (WP-54)
+
+A real OpenZaak, for developing/testing the ZGW seam (`backend/src/BigRegister.Api/Zgw/`)
+against something that isn't a fixture or a stub `HttpMessageHandler`. Deliberately **not**
+part of the root `docker-compose.yml` and **not** wired into `npm run ci` / CI — see
+[docs/reference/openzaak-integration.md](../../docs/reference/openzaak-integration.md) for the
+full picture; this is just "how to run it".
+
+## Bring it up
+
+```bash
+cd backend/openzaak
+docker compose -f docker-compose.openzaak.yml up -d   # postgres, redis, migrate+configure, OpenZaak
+./bootstrap-catalogus.sh                               # seeds a catalogus/zaaktype/zaak to read back
+```
+
+`bootstrap-catalogus.sh` waits for OpenZaak to answer, then over plain REST + a hand-rolled
+HS256 JWT (same shape as `ZgwTokenProvider.cs`, matching the `bigregister-test` client
+`setup_configuration/data.yaml` creates): a catalogus, a published zaaktype ("Herregistratie
+arts", with the statustypen/resultaattype/roltype OpenZaak requires before a zaaktype can be
+published), and one zaak (`BIG-2026-000123`) with an initiator rol for the seeded BSN
+(`111222333` — the same fixture BSN `OpenZaakZaakSourceTests.cs` uses). It writes what it
+seeded to `seeded.env` (gitignored) and prints a summary.
+
+**Not idempotent** — re-running against the same (already-seeded) instance fails on OpenZaak's
+`domein`+`rsin` uniqueness constraint for the catalogus. Reset with:
+
+```bash
+docker compose -f docker-compose.openzaak.yml down -v && docker compose -f docker-compose.openzaak.yml up -d
+```
+
+## Run the integration test against it
+
+```bash
+cd backend
+dotnet test --filter Category=Integration
+```
+
+`OpenZaakIntegrationTests.cs` points a `WebApplicationFactory<Program>` at
+`Zgw:Enabled=true` + `http://localhost:8000` with the harness's credentials, hits
+`GET /api/v1/admin/cases`, and asserts the seeded zaak comes back — through the real HTTP +
+JWT + Catalogi-label-resolution path, not a mock. This test is tagged `Category=Integration`
+and is **excluded** from the default `dotnet test` run and from CI (`ci.yml`,
+`scripts/ci-local.sh` both filter `Category!=Integration`) — it only passes with this harness
+up, so it never runs where the harness doesn't exist.
+
+## Tear down
+
+```bash
+docker compose -f docker-compose.openzaak.yml down -v
+```
+
+## What's in here / what isn't
+
+- `docker-compose.openzaak.yml` — postgres (postgis), redis, a one-shot `web-init` (runs
+  Django migrations then `setup_configuration` against `setup_configuration/data.yaml`), and
+  `web` (the OpenZaak API on `:8000`). Pinned to `openzaak/open-zaak:1.29.1`. No
+  celery/celery-beat/celery-flower/nginx — trimmed for a lean, fast-booting harness; add them
+  back only if a later WP needs a real async notification delivery round-trip here (WP-52's
+  webhook is already covered by fixture tests against no live instance).
+  `NOTIFICATIONS_DISABLED=true` is required, not optional: without it, OpenZaak 500s (and
+  **rolls back the whole create**) on any notified resource — see the compose file's comment.
+- `setup_configuration/data.yaml` — the declarative, scripted alternative to clicking through
+  the Django admin (upstream's own documented `setup_configuration` CLI mechanism): creates the
+  one `bigregister-test` client (`heeft_alle_autorisaties: true` — this instance never exists
+  for anything but this harness, so there's no least-privilege boundary worth modeling).
+- `bootstrap-catalogus.sh` — the business content (catalogus/zaaktype/zaak/…) `setup_configuration`
+  has no YAML for; every field value here was checked against OpenZaak's own OpenAPI spec and a
+  live run of this exact script, not guessed (two OpenZaak quirks it works around: a zaaktype
+  needs ≥1 resultaattype and 2 statustypen before it can be published, and its
+  `selectielijstklasse` and the zaaktype's `selectielijstProcestype` must reference the same
+  `procesType` on the public VNG selectielijst API).
+- **Not here**: Documenten (DRC) / Notificaties (NRC) content — add if a later WP needs to prove
+  those round-trips against a live instance too (WP-51/52 are fixture-tested today).
diff --git a/backend/openzaak/bootstrap-catalogus.sh b/backend/openzaak/bootstrap-catalogus.sh
new file mode 100755
index 0000000..e18c90f
--- /dev/null
+++ b/backend/openzaak/bootstrap-catalogus.sh
@@ -0,0 +1,164 @@
+#!/usr/bin/env bash
+# WP-54 — seeds business content (catalogus/zaaktype/statustype/roltype/zaak/status/rol) into
+# the OpenZaak harness started by docker-compose.openzaak.yml. `setup_configuration/data.yaml`
+# only covers infra config (the JWTSecret + Applicatie); Catalogi/Zaken content has no
+# declarative-YAML equivalent upstream, so this script does it the same way the BFF itself
+# does at runtime — plain REST calls with a hand-rolled HS256 JWT (see ZgwTokenProvider.cs,
+# mirrored here in bash+openssl so this script has no extra dependency beyond curl/openssl).
+#
+# Idempotent-ish: re-running creates duplicate catalogus/zaaktype rows (OpenZaak doesn't
+# dedupe by name) — meant to be run once per fresh `docker compose up`, not repeatedly against
+# a long-lived instance. Prints the seeded zaak's `identificatie` + `url` on success; also
+# writes them to seeded.env (repo-ignored) for OpenZaakIntegrationTests.cs to assert against.
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+BASE="http://localhost:8000"
+CLIENT_ID="bigregister-test"
+SECRET="bigregister-test-secret"
+RSIN="123443210"          # elfproef-valid RSIN, already used as the fixture Bronorganisatie
+                          # in OpenZaakZaakSourceTests.cs — reused here for consistency.
+BSN="111222333"           # elfproef-valid BSN, already used as the fixture caller BSN.
+ZAAK_REF="BIG-2026-000123"
+
+b64url() { openssl base64 -A | tr '+/' '-_' | tr -d '='; }
+
+jwt() {
+  local header='{"alg":"HS256","typ":"JWT"}'
+  local payload
+  payload=$(printf '{"iss":"%s","iat":%d,"client_id":"%s","user_id":"%s","user_representation":"%s"}' \
+    "$CLIENT_ID" "$(date +%s)" "$CLIENT_ID" "$CLIENT_ID" "bootstrap")
+  local h p signing_input sig
+  h=$(printf '%s' "$header" | b64url)
+  p=$(printf '%s' "$payload" | b64url)
+  signing_input="$h.$p"
+  sig=$(printf '%s' "$signing_input" | openssl dgst -sha256 -hmac "$SECRET" -binary | b64url)
+  printf '%s.%s' "$signing_input" "$sig"
+}
+
+# $1 = method, $2 = path, $3 = JSON body (optional)
+oz() {
+  local method="$1" path="$2" body="${3:-}"
+  # Content-Crs/Accept-Crs: every ZGW write must declare a coordinate reference system even
+  # when no geometry is involved — OpenZaak 412s without it.
+  local args=(-sS -X "$method" -H "Authorization: Bearer $(jwt)" -H "Content-Type: application/json" \
+    -H "Content-Crs: EPSG:4326" -H "Accept-Crs: EPSG:4326")
+  [ -n "$body" ] && args+=(-d "$body")
+  local response
+  response=$(curl "${args[@]}" -w $'\n%{http_code}' "$BASE$path")
+  local http_code="${response##*$'\n'}"
+  local json="${response%$'\n'*}"
+  if [[ ! "$http_code" =~ ^2 ]]; then
+    echo "FAILED $method $path -> $http_code: $json" >&2
+    exit 1
+  fi
+  echo "$json"
+}
+
+echo "Waiting for OpenZaak..."
+until curl -sS -o /dev/null -w '%{http_code}' "$BASE/catalogi/api/v1/catalogussen" | grep -q '^2\|^401\|^403'; do
+  sleep 2
+done
+
+echo "Creating catalogus..."
+catalogus=$(oz POST /catalogi/api/v1/catalogussen "$(printf '{"domein":"BIGR","rsin":"%s","contactpersoonBeheerNaam":"BIG Register"}' "$RSIN")")
+catalogus_url=$(echo "$catalogus" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')
+echo "  $catalogus_url"
+
+echo "Creating zaaktype (concept)..."
+zaaktype=$(oz POST /catalogi/api/v1/zaaktypen "$(python3 -c '
+import json, sys
+print(json.dumps({
+  "identificatie": "ZT-HERREG",
+  "omschrijving": "Herregistratie arts",
+  "vertrouwelijkheidaanduiding": "openbaar",
+  "doel": "Herregistratie in het BIG-register",
+  "aanleiding": "Aanvraag door de zorgverlener",
+  "indicatieInternOfExtern": "extern",
+  "handelingInitiator": "indienen",
+  "onderwerp": "Herregistratie",
+  "handelingBehandelaar": "behandelen",
+  "doorlooptijd": "P30D",
+  "opschortingEnAanhoudingMogelijk": False,
+  "verlengingMogelijk": False,
+  "publicatieIndicatie": False,
+  "productenOfDiensten": ["https://example.com/producten/herregistratie"],
+  "referentieproces": {"naam": "Herregistratie"},
+  "verantwoordelijke": "CIBG",
+  "catalogus": sys.argv[1],
+  "beginGeldigheid": "2026-01-01",
+  "versiedatum": "2026-01-01",
+  "besluittypen": [],
+  "gerelateerdeZaaktypen": [],
+  # Must belong to the same procestype as the resultaattype selectielijstklasse below
+  # (OpenZaak cross-checks this against the public VNG selectielijst API).
+  "selectielijstProcestype": "https://selectielijst.openzaak.nl/api/v1/procestypen/e1b73b12-b2f6-4c4e-8929-94f84dd2a57d",
+}))
+' "$catalogus_url")")
+zaaktype_url=$(echo "$zaaktype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')
+echo "  $zaaktype_url"
+
+echo "Creating statustypen (publish needs a begin AND an end status)..."
+statustype=$(oz POST /catalogi/api/v1/statustypen "$(printf '{"zaaktype":"%s","omschrijving":"Ontvangen","volgnummer":1}' "$zaaktype_url")")
+echo "  $(echo "$statustype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')"
+statustype_eind=$(oz POST /catalogi/api/v1/statustypen "$(printf '{"zaaktype":"%s","omschrijving":"Afgehandeld","volgnummer":2}' "$zaaktype_url")")
+echo "  $(echo "$statustype_eind" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')"
+
+echo "Creating resultaattype (publish needs at least one)..."
+# The two URLs below are real reference-list entries on the public VNG selectielijst API
+# (selectielijst.openzaak.nl) — OpenZaak validates both by fetching them, same as it does
+# for a zaaktype URL, so a made-up URL 404s here.
+resultaattype=$(oz POST /catalogi/api/v1/resultaattypen "$(printf '{"zaaktype":"%s","omschrijving":"Afgehandeld","resultaattypeomschrijving":"https://selectielijst.openzaak.nl/api/v1/resultaattypeomschrijvingen/7cb315fb-4f7b-4a43-aca1-e4522e4c73b3","selectielijstklasse":"https://selectielijst.openzaak.nl/api/v1/resultaten/cc5ae4e3-a9e6-4386-bcee-46be4986a829","archiefnominatie":"blijvend_bewaren"}' "$zaaktype_url")")
+echo "  $(echo "$resultaattype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')"
+
+echo "Creating roltype (initiator)..."
+roltype=$(oz POST /catalogi/api/v1/roltypen "$(printf '{"zaaktype":"%s","omschrijving":"Initiator","omschrijvingGeneriek":"initiator"}' "$zaaktype_url")")
+echo "  $(echo "$roltype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')"
+
+echo "Publishing zaaktype..."
+zaaktype_uuid=$(echo "$zaaktype_url" | sed 's#.*/##')
+oz POST "/catalogi/api/v1/zaaktypen/$zaaktype_uuid/publish" >/dev/null
+
+echo "Creating zaak..."
+zaak=$(oz POST /zaken/api/v1/zaken "$(python3 -c '
+import json, sys
+print(json.dumps({
+  "zaaktype": sys.argv[1],
+  "bronorganisatie": sys.argv[2],
+  "verantwoordelijkeOrganisatie": sys.argv[2],
+  "startdatum": "2026-07-28",
+  "identificatie": sys.argv[3],
+}))
+' "$zaaktype_url" "$RSIN" "$ZAAK_REF")")
+zaak_url=$(echo "$zaak" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')
+echo "  $zaak_url"
+
+echo "Creating status..."
+statustype_url=$(echo "$statustype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')
+oz POST /zaken/api/v1/statussen "$(python3 -c '
+import json, sys
+print(json.dumps({"zaak": sys.argv[1], "statustype": sys.argv[2], "datumStatusGezet": "2026-07-28T12:00:00Z"}))
+' "$zaak_url" "$statustype_url")" >/dev/null
+
+echo "Creating rol (initiator, seeded BSN)..."
+roltype_url=$(echo "$roltype" | python3 -c 'import json,sys; print(json.load(sys.stdin)["url"])')
+oz POST /zaken/api/v1/rollen "$(python3 -c '
+import json, sys
+print(json.dumps({
+  "zaak": sys.argv[1],
+  "betrokkeneType": "natuurlijk_persoon",
+  "roltype": sys.argv[2],
+  "roltoelichting": "Initiator",
+  "betrokkeneIdentificatie": {"inpBsn": sys.argv[3]},
+}))
+' "$zaak_url" "$roltype_url" "$BSN")" >/dev/null
+
+cat > seeded.env <<EOF
+ZAAK_REFERENTIE=$ZAAK_REF
+ZAAK_URL=$zaak_url
+ZAAKTYPE_LABEL=Herregistratie arts
+CALLER_BSN=$BSN
+EOF
+
+echo
+echo "Seed complete. $ZAAK_REF ($zaak_url) — see seeded.env"
diff --git a/backend/openzaak/docker-compose.openzaak.yml b/backend/openzaak/docker-compose.openzaak.yml
new file mode 100644
index 0000000..3f6fb9f
--- /dev/null
+++ b/backend/openzaak/docker-compose.openzaak.yml
@@ -0,0 +1,72 @@
+# WP-54 — a real OpenZaak to develop/test the ZGW seam against, kept OUT of the root
+# docker-compose.yml on purpose (see backend/openzaak/README.md): OpenZaak is a full Django
+# stack (postgres + redis), heavy compared to this repo's own FE+BFF, and nobody who isn't
+# touching the ZGW slice should have to pull/boot it.
+#
+# ponytail: trimmed vs. open-zaak's own published compose — no celery/celery-beat/celery-flower
+# (async notification delivery, never asserted by the integration test) and no nginx (the test
+# hits web's port directly). Add them back only if a later WP needs an actual notification
+# round-trip against this harness (NRC delivery is already covered by fixture tests, WP-52).
+services:
+  db:
+    image: postgis/postgis:17-3.5
+    environment:
+      - POSTGRES_HOST_AUTH_METHOD=trust
+      - POSTGRES_DB=openzaak
+      - POSTGRES_USER=openzaak
+    healthcheck:
+      test: ['CMD-SHELL', 'pg_isready -U openzaak']
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+  redis:
+    image: redis:8
+    healthcheck:
+      test: ['CMD', 'redis-cli', 'ping']
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+  # One-shot: migrate the schema, then apply setup_configuration/data.yaml (JWTSecret +
+  # Applicatie for the bootstrap script below) — the documented, scripted alternative to
+  # clicking through the Django admin (see openzaak_config_cli in upstream docs).
+  web-init:
+    image: openzaak/open-zaak:1.29.1
+    environment: &app-env
+      DJANGO_SETTINGS_MODULE: openzaak.conf.docker
+      SECRET_KEY: wp-54-local-harness-not-for-prod
+      DB_HOST: db
+      DB_NAME: openzaak
+      DB_USER: openzaak
+      IS_HTTPS: 'no'
+      SITE_DOMAIN: localhost:8000
+      ALLOWED_HOSTS: localhost,127.0.0.1,web
+      CACHE_DEFAULT: redis:6379/0
+      CACHE_AXES: redis:6379/0
+      DISABLE_2FA: 'true'
+      CELERY_BROKER_URL: redis://redis:6379/0
+      CELERY_RESULT_BACKEND: redis://redis:6379/0
+      RUN_SETUP_CONFIG: 'true'
+      # No celery worker in this trimmed harness (see the top-of-file note) to actually
+      # deliver a notification — without this, OpenZaak 500s (and rolls back!) every create
+      # on a notified resource (zaaktype, zaak, ...) because NotificationsConfig has no
+      # client configured (see notifications_api_common.viewsets.NotificationMixin.notify).
+      NOTIFICATIONS_DISABLED: 'true'
+    command: /setup_configuration.sh
+    volumes:
+      - ./setup_configuration:/app/setup_configuration:ro
+    depends_on:
+      db:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
+  web:
+    image: openzaak/open-zaak:1.29.1
+    environment: *app-env
+    ports:
+      - '8000:8000'
+    depends_on:
+      web-init:
+        condition: service_completed_successfully
diff --git a/backend/openzaak/setup_configuration/data.yaml b/backend/openzaak/setup_configuration/data.yaml
new file mode 100644
index 0000000..6325c7c
--- /dev/null
+++ b/backend/openzaak/setup_configuration/data.yaml
@@ -0,0 +1,27 @@
+# Applied by web-init (RUN_SETUP_CONFIG=true → `manage.py setup_configuration`, upstream's
+# documented CLI config mechanism — see docker-compose.openzaak.yml) instead of the Django
+# admin. Creates the ONE application the bootstrap script + integration test authenticate as.
+#
+# ponytail: heeft_alle_autorisaties (all scopes) rather than a granular per-component/scope
+# list — this instance only ever exists for this harness/test, never a shared or prod
+# OpenZaak, so there's no least-privilege boundary worth modeling here.
+sites_config_enable: true
+sites_config:
+  items:
+    - domain: localhost:8000
+      name: OpenZaak (WP-54 harness)
+
+vng_api_common_credentials_config_enable: true
+vng_api_common_credentials:
+  items:
+    - identifier: bigregister-test
+      secret: bigregister-test-secret
+
+vng_api_common_applicaties_config_enable: true
+vng_api_common_applicaties:
+  items:
+    - uuid: 5a09b3c9-6a54-4b2b-8f3c-1f9b6b6a3a01
+      client_ids:
+        - bigregister-test
+      label: BIG-register BFF (WP-54 test harness)
+      heeft_alle_autorisaties: true
diff --git a/backend/src/BigRegister.Api/Zgw/ZgwHttpClient.cs b/backend/src/BigRegister.Api/Zgw/ZgwHttpClient.cs
index 7ae15df..76720a8 100644
--- a/backend/src/BigRegister.Api/Zgw/ZgwHttpClient.cs
+++ b/backend/src/BigRegister.Api/Zgw/ZgwHttpClient.cs
@@ -38,5 +38,11 @@ internal sealed class ZgwHttpClient(HttpClient http, ZgwTokenProvider tokens)
   {
     req.Headers.Authorization = new AuthenticationHeaderValue("Bearer", caller is null ? tokens.Mint() : tokens.Mint(caller));
     req.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
+    // Every ZGW request must declare a coordinate reference system, even when no geometry is
+    // involved (Zaak has an optional zaakgeometrie) — a real OpenZaak 412s ("Content-Crs
+    // header ontbreekt") without it. Only surfaced by WP-54's live harness: the fixture/stub
+    // tests never modelled this header, so this bug shipped unnoticed since WP-49/50.
+    req.Headers.Add("Accept-Crs", "EPSG:4326");
+    if (req.Content is not null) req.Content.Headers.Add("Content-Crs", "EPSG:4326");
   }
 }
diff --git a/backend/tests/BigRegister.Tests/OpenZaakIntegrationTests.cs b/backend/tests/BigRegister.Tests/OpenZaakIntegrationTests.cs
new file mode 100644
index 0000000..f86c7db
--- /dev/null
+++ b/backend/tests/BigRegister.Tests/OpenZaakIntegrationTests.cs
@@ -0,0 +1,56 @@
+using System.Net.Http.Json;
+using BigRegister.Api.Contracts;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// WP-54: the one test that proves the BFF actually talks to a REAL OpenZaak — auth accepted,
+/// real response shapes, real pagination/zaaktype resolution — rather than the stub
+/// HttpMessageHandler every other Zgw test (<see cref="ZgwZaakMapperTests"/>,
+/// <see cref="OpenZaakZaakSourceTests"/>) uses. Requires the harness in <c>backend/openzaak/</c>
+/// to be up and seeded first (see its README); tagged Category=Integration so it's excluded
+/// from the default `dotnet test` run and from CI (`ci.yml`, `scripts/ci-local.sh` both filter
+/// it out) — nobody without a live OpenZaak should see it fail.
+///
+/// Not an <see cref="IClassFixture{TFixture}"/> off <see cref="TestWebApplicationFactory"/>:
+/// that fixture hardcodes <c>Zgw:Enabled=false</c> (offline default) for every other test class,
+/// so this one builds its own <see cref="WebApplicationFactory{TEntryPoint}"/> layering the
+/// harness's URLs/credentials (matching <c>backend/openzaak/setup_configuration/data.yaml</c>
+/// and <c>bootstrap-catalogus.sh</c>) on top.
+/// </summary>
+[Trait("Category", "Integration")]
+public class OpenZaakIntegrationTests
+{
+  private static WebApplicationFactory<Program> Factory()
+  {
+    var dbPath = Path.Combine(Path.GetTempPath(), $"bigregister-oz-integration-{Guid.NewGuid():N}.db");
+    return new WebApplicationFactory<Program>().WithWebHostBuilder(builder => builder
+        .UseSetting("ConnectionStrings:AppDb", $"Data Source={dbPath}")
+        .UseSetting("Zgw:Enabled", "true")
+        .UseSetting("Zgw:ZrcBaseUrl", "http://localhost:8000/zaken/api/v1")
+        .UseSetting("Zgw:ZtcBaseUrl", "http://localhost:8000/catalogi/api/v1")
+        .UseSetting("Zgw:ClientId", "bigregister-test")
+        .UseSetting("Zgw:Secret", "bigregister-test-secret")
+        .UseSetting("Zgw:UserId", "bigregister-test")
+        .UseSetting("Zgw:UserRepresentation", "WP-54 integration test"));
+  }
+
+  [Fact]
+  public async Task Admin_cases_returns_the_seeded_zaak_mapped_through_real_HTTP_and_JWT()
+  {
+    using var factory = Factory();
+    using var client = factory.CreateClient();
+    client.DefaultRequestHeaders.Add("X-Role", "admin");   // CasesAdmin gate (cases:manage)
+
+    var cases = await client.GetFromJsonAsync<List<ApplicationSummaryDto>>("/api/v1/admin/cases");
+
+    Assert.NotNull(cases);
+    // bootstrap-catalogus.sh seeds exactly one zaak, identificatie BIG-2026-000123, under a
+    // zaaktype whose omschrijving is "Herregistratie arts" — see backend/openzaak/README.md.
+    var seeded = Assert.Single(cases!, c => c.Status.Referentie == "BIG-2026-000123");
+    Assert.Equal("Herregistratie arts", seeded.Type);
+    Assert.Equal("InBehandeling", seeded.Status.Tag);
+  }
+}
diff --git a/docs/project/backlog/README.md b/docs/project/backlog/README.md
index 518296d..056eb61 100644
--- a/docs/project/backlog/README.md
+++ b/docs/project/backlog/README.md
@@ -104,7 +104,7 @@ for its existing violations, so every WP ends green.
 | [WP-51](WP-51-openzaak-documenten.md)                  | OpenZaak Documenten (DRC) upload + zaak link                                       | 9 · OpenZaak/ZGW            | done   |
 | [WP-52](WP-52-openzaak-notificaties.md)                | OpenZaak Notificaties (NRC) live status via webhook                                | 9 · OpenZaak/ZGW            | done   |
 | [WP-53](WP-53-inbound-identity-and-citizen-scoping.md) | Inbound identity seam + citizen-scoping (per-request BSN, ZGW audit claims)        | 9 · OpenZaak/ZGW            | done   |
-| [WP-54](WP-54-openzaak-integration-harness.md)         | Docker OpenZaak integration-test harness (opt-in, live round-trip)                 | 9 · OpenZaak/ZGW            | todo   |
+| [WP-54](WP-54-openzaak-integration-harness.md)         | Docker OpenZaak integration-test harness (opt-in, live round-trip)                 | 9 · OpenZaak/ZGW            | done   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);
 03/04 before 05–09 (boundaries stop new violations during refactors); 06 before 07 (typed
diff --git a/docs/project/backlog/WP-54-openzaak-integration-harness.md b/docs/project/backlog/WP-54-openzaak-integration-harness.md
index a5862b0..fd8c38d 100644
--- a/docs/project/backlog/WP-54-openzaak-integration-harness.md
+++ b/docs/project/backlog/WP-54-openzaak-integration-harness.md
@@ -1,6 +1,6 @@
 # WP-54 — Docker OpenZaak integration-test harness
 
-Status: todo
+Status: done
 Phase: 9 — OpenZaak / ZGW integration
 
 ## Why
@@ -87,17 +87,19 @@ OpenZaak facts that shape the harness (from the ZGW research):
 
 ## Acceptance criteria
 
-- [ ] `docker compose -f backend/openzaak/docker-compose.openzaak.yml up` yields a reachable
+- [x] `docker compose -f backend/openzaak/docker-compose.openzaak.yml up` yields a reachable
       OpenZaak with the seeded catalogus + zaak, and credentials matching `ZgwOptions`.
-- [ ] The Category=Integration test passes against it; the BFF returns the seeded zaak mapped to
+- [x] The Category=Integration test passes against it; the BFF returns the seeded zaak mapped to
       `ApplicationSummaryDto` through the real HTTP + JWT path.
-- [ ] Default `dotnet test` and `npm run ci` are unaffected (integration test excluded, no docker
+- [x] Default `dotnet test` and `npm run ci` are unaffected (integration test excluded, no docker
       needed); `docker compose up` (root) is unchanged.
 
 ## Verification
 
 `docker compose -f backend/openzaak/docker-compose.openzaak.yml up -d` →
-`dotnet test --filter Category=Integration` → green; then teardown.
+`./backend/openzaak/bootstrap-catalogus.sh` → `dotnet test --filter Category=Integration` →
+green; then teardown. Actually run (not just planned) during this WP — see Deviations below
+for what that surfaced.
 
 ## Out of scope
 
@@ -112,3 +114,32 @@ always-on CI job (keep it opt-in/manual — OpenZaak startup is slow), performan
   keep the fixture in the repo.
 - Bootstrap client scopes must include `catalogi.lezen` or zaaktype resolution 403s — cover in
   the setup script.
+
+## Deviations from the original plan
+
+- **`heeft_alle_autorisaties: true` instead of granular scopes.** The plan called out
+  `zaken.lezen`/`catalogi.lezen` specifically; in practice OpenZaak's scripted config
+  (`vng_api_common_applicaties_config`, upstream's own documented `setup_configuration` YAML
+  mechanism) exposes an all-scopes flag on the one `Applicatie` this harness ever creates. Since
+  that application exists for nothing but this throwaway test instance, granular scopes would
+  add YAML-schema risk for no real least-privilege benefit — took the simpler, equally-scripted
+  option.
+- **A live run found a real production bug, not just a harness wrinkle**: `ZgwHttpClient.cs`
+  never sent `Content-Crs`/`Accept-Crs` on any ZGW call. Every ZGW write 412s ("Content-Crs
+  header ontbreekt") without it — a real OpenZaak enforces this; the stub `HttpMessageHandler`
+  every prior Zgw test used never modelled header requirements, so nothing from WP-49/50 caught
+  it before now. Fixed in `ZgwHttpClient.cs` alongside the harness (see
+  `docs/reference/openzaak-integration.md`) — this is precisely the class of bug this WP exists
+  to catch.
+- **Publishing a zaaktype needs more seed data than the plan anticipated**: OpenZaak refuses to
+  publish a zaaktype with fewer than one resultaattype or fewer than two statustypen (begin +
+  eind), and a resultaattype's `selectielijstklasse` must share a `procesType` with the
+  zaaktype's own `selectielijstProcestype` — both cross-checked live against the public VNG
+  selectielijst API (`selectielijst.openzaak.nl`). `bootstrap-catalogus.sh` seeds all of this;
+  see its comments for the exact values used and why.
+- **No celery/celery-beat/nginx in the harness**, unlike upstream's own compose — trimmed for a
+  faster-booting, single-purpose harness (this test never asserts on notification delivery,
+  which is celery's job). `NOTIFICATIONS_DISABLED=true` is required as a consequence: without a
+  celery worker, `NotificationsConfig` has no client, and OpenZaak's `notify()` hook otherwise
+  raises inside the same DB transaction as the create — turning a missing-worker problem into a
+  500 that rolls back the create it was supposed to just notify about.
diff --git a/docs/reference/openzaak-integration.md b/docs/reference/openzaak-integration.md
index bb9e6c5..4424055 100644
--- a/docs/reference/openzaak-integration.md
+++ b/docs/reference/openzaak-integration.md
@@ -101,7 +101,11 @@ confidentiality level would matter for production but isn't needed to prove the
   `user_representation`). No refresh flow — OpenZaak expires tokens 1h past `iat`, so per-call
   minting is the recommended pattern. Hand-rolled (no `Microsoft.IdentityModel.*` dependency).
 - `ZgwHttpClient.cs` — shared GET/POST-with-bearer-JWT plumbing used by both
-  `OpenZaakZaakSource` and `OpenZaakDocumentSource`.
+  `OpenZaakZaakSource` and `OpenZaakDocumentSource`. Every request also carries
+  `Accept-Crs`/`Content-Crs: EPSG:4326` — every ZGW call must declare a coordinate reference
+  system even when no geometry is involved, or a real OpenZaak 412s ("Content-Crs header
+  ontbreekt"). This was missing until WP-54's live harness caught it — the stub-handler tests
+  never modelled the header, so it had shipped silently since WP-49/50.
 - `ZgwZaakMapper.cs` — the anti-corruption map: ZGW Zaak → `ApplicationSummaryDto`. This is
   where **URL identity** becomes the trailing uuid and the **zaaktype URL** is resolved to a
   human label (the cross-service join).
@@ -204,6 +208,22 @@ FE or the contract. Watch the **sync-over-async** `ponytail:` note in `OpenZaakZ
 its `OpenZaakDocumentSource` sibling) — make the read/write paths async if OpenZaak becomes the
 default.
 
+## Run against real OpenZaak (WP-54)
+
+Everything above was, until WP-54, only proven against fixtures + a stub `HttpMessageHandler` —
+no live OpenZaak. `backend/openzaak/` is a **separate**, opt-in docker-compose harness (never
+merged into the root `docker-compose.yml`, which stays FE+BFF-only) that brings up a real
+OpenZaak, seeds a minimal catalogus/zaaktype/zaak via a bootstrap script, and backs one
+xunit test (`OpenZaakIntegrationTests.cs`, tagged `Category=Integration`) that points the BFF at
+it with `Zgw:Enabled=true`. See `backend/openzaak/README.md` for the exact commands; the test is
+excluded from the default `dotnet test` run and from CI (`--filter Category!=Integration`) since
+it only passes with the harness up.
+
+This is also where the `Content-Crs`/`Accept-Crs` header gap above was found: a real OpenZaak
+enforces ZGW's geo-header requirement in a way no stub-based test could catch, since a stub
+never rejects an unexpected (or missing) header. That is the harness's whole point — proving
+the seam against real protocol behaviour, not just the shapes we already assumed.
+
 ## Config
 
 ```jsonc
@@ -270,15 +290,16 @@ Caveat: `IZaakSource` covers the cases **read (admin + citizen-scoped) + create*
 (WP-49/50/53), `IDocumentSource` covers **upload + zaak-link** (WP-51), the inbound
 `POST /zgw/notificaties` webhook (WP-52) closes the read/write/document/notify arc, and WP-53
 threaded a real per-request `CallerIdentity` through all of it (ownership + the ZGW audit
-claims). Other BFF endpoints (reference data like `SeedData`'s BRP/DUO mimics) still read static
-stores directly — ACL-ready (the DTO seam exists) but not yet swappable, and not part of this
-arc. What's left is **WP-54**: a docker OpenZaak harness + opt-in integration test — today
-everything is fixture/mock-tested against no live instance.
+claims), and WP-54 added a docker OpenZaak harness + opt-in integration test proving the seam
+against a live instance (and, in doing so, caught the missing `Content-Crs`/`Accept-Crs`
+headers noted above). Other BFF endpoints (reference data like `SeedData`'s BRP/DUO mimics)
+still read static stores directly — ACL-ready (the DTO seam exists) but not yet swappable, and
+not part of this arc. That closes the phase-9 OpenZaak/ZGW arc (WP-49..54).
 
 ## See also
 
 - [ADR-0005 — OpenZaak behind the BFF](architecture/0005-openzaak-behind-bff.md) — the decision.
 - [ADR-0001 — BFF-lite + decision DTOs](architecture/0001-bff-lite-decision-dtos.md) — why the FE doesn't change.
-- [WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (this), WP-50/51 (CRUD arc so far), WP-52 (notificaties), WP-53 (identity seam + citizen-scoping), WP-54 (integration harness, open).
-- `backend/src/BigRegister.Api/Zgw/` — the client; `Data/IZaakSource.cs`/`Data/IDocumentSource.cs` — the seams.
+- [WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (this), WP-50/51 (CRUD arc so far), WP-52 (notificaties), WP-53 (identity seam + citizen-scoping), [WP-54](../project/backlog/WP-54-openzaak-integration-harness.md) (integration harness).
+- `backend/src/BigRegister.Api/Zgw/` — the client; `Data/IZaakSource.cs`/`Data/IDocumentSource.cs` — the seams; `backend/openzaak/` — the live-OpenZaak test harness (WP-54).
 - [ZGW standard (VNG)](https://vng-realisatie.github.io/gemma-zaken/) · [OpenZaak auth docs](https://open-zaak.readthedocs.io/en/stable/client-development/authentication.html).
diff --git a/scripts/ci-local.sh b/scripts/ci-local.sh
index e190ee3..d359e8d 100755
--- a/scripts/ci-local.sh
+++ b/scripts/ci-local.sh
@@ -18,7 +18,7 @@ step "check:tokens";              npm run check:tokens
 step "test (vitest + coverage)";  npm run test:coverage
 step "build --localize (nl+en)";  npx ng build --localize
 step "npm audit (shipped deps)";  npm audit --omit=dev
-step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx )
+step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx --filter "Category!=Integration" )
 step "showcase snippets drift";   npm run gen:snippets && git diff --exit-code src/app/showcase/snippets.generated.ts
 step "api-client drift";          npm run gen:api && git diff --exit-code src/app/shared/infrastructure/api-client.ts backend/swagger.json