diff --git a/docs/architecture/adr-0031-mfa-on-the-medewerker-realm.md b/docs/architecture/adr-0031-mfa-on-the-medewerker-realm.md
new file mode 100644
index 0000000..0f8c8b7
--- /dev/null
+++ b/docs/architecture/adr-0031-mfa-on-the-medewerker-realm.md
@@ -0,0 +1,49 @@
+# ADR-0031 — MFA on the medewerker realm, with a fixture TOTP secret
+
+- **Status:** Accepted
+- **Date:** 2026-09-03
+- **Slice:** S-15c (Gitea #132)
+
+## Context
+
+Staff (behandelaar, teamlead, beheerder) act on citizens' registrations and on the ACL's
+default-fill: the highest-privilege logins in the platform. The medewerker realm protected
+them with a password alone, while the citizen realms (digid, eherkenning, eidas) mock
+brokers that carry their own assurance levels. A reference application that demonstrates a
+government architecture should show MFA on the staff realm.
+
+Two things had to be decided: **how** to enforce OTP in a realm export, and **how the
+automated checks and a human demo obtain a code** — the e2e drives a real browser login and
+`make keycloak-smoke` drives a real password grant, so neither can scan a QR.
+
+## Decision
+
+**Enforce OTP by giving every seeded medewerker a TOTP credential**, rather than replacing
+Keycloak's browser flow with a copy whose OTP execution is `REQUIRED`.
+
+Keycloak's stock `browser` and `direct grant` flows both contain a *conditional OTP*
+subflow that fires when the user has an OTP credential. Seeding the credential therefore
+turns the challenge on for every seeded user, in both flows, without duplicating ~40 lines
+of flow JSON into the export. `CONFIGURE_TOTP` is additionally set as a **default required
+action**, so a medewerker created later must enrol before their first login.
+
+**The seeded secret is a fixed, committed fixture** (`BIGMEDEWERKEROTPSEED`) shared by all
+medewerkers. Codes are then computable: `infra/keycloak/check_realms.py` (Python, stdlib
+`hmac`) and `tests/e2e/medewerker-login.ts` (Node `crypto`) each implement RFC 6238 in
+about six lines — no OTP dependency on either side, and no enrolment step in the tests.
+
+## Consequences
+
+- A password alone no longer yields a token on the medewerker realm; `check_realms.py`
+  asserts that refusal, so the enforcement cannot silently regress.
+- Every medewerker login in the e2e goes through `loginMedewerker()`, which submits the OTP
+  form. New staff specs must use it.
+- **The secret is public.** It is a demo fixture and worthless outside this synthetic
+  stack, in the same class as the committed `test123` passwords and the mock DigiD broker.
+  A real deployment enrols per-user authenticators (or federates to DigiD Machtigen /
+  eHerkenning at the required assurance level) and seeds no credentials at all.
+- Enforcement is *effectively* realm-wide but *technically* per-user: the conditional
+  subflow is what fires. A medewerker whose OTP credential were removed would fall back to
+  the required action at next login (enrol, then challenge) rather than skipping MFA — an
+  acceptable equivalence for this purpose, and the reason the required action is set.
+- Reversal is a one-file edit: drop the `otp` credentials and the `requiredActions` block.
diff --git a/docs/demo-script.md b/docs/demo-script.md
index 25423aa..5172b33 100644
--- a/docs/demo-script.md
+++ b/docs/demo-script.md
@@ -140,7 +140,8 @@ zaaktype cache). Store is in-memory: an edit reverts to the configured env on re
 
 ```bash
 make up
-# 1. Log in as bram-beheerder / test123 → "Default-fill" tab → change a value → Opslaan.
+# 1. Log in as bram-beheerder / test123 + OTP (`python3 infra/keycloak/check_realms.py otp`)
+#    → "Default-fill" tab → change a value → Opslaan.
 open http://localhost:8143/default-fill
 #
 # 2. Automated: the ACL uses the current default-fill per zaak (unit) and the endpoints are behind the
@@ -161,7 +162,8 @@ directly (ADR-0025); managing the default-fill config (S-15b) and MFA (S-15c) co
 
 ```bash
 make up
-# 1. Log in as bram-beheerder / test123 → the catalogus lists the published zaaktypen.
+# 1. Log in as bram-beheerder / test123 + OTP (`python3 infra/keycloak/check_realms.py otp`)
+#    → the catalogus lists the published zaaktypen.
 open http://localhost:8143
 #
 # 2. Automated (a CI verify-stack e2e): a beheerder logs in and sees BIG-REGISTRATIE.
@@ -304,7 +306,8 @@ make verify-local     # → "OK — a fresh local stack completed the flow with
 
 # 3. Or by hand in the browser: log in at http://localhost:8140 (jan-burger / test123), submit +
 #    upload a PDF, then approve it in the werkbak at http://localhost:8142 (merel-behandelaar /
-#    test123); it shows as INGESCHREVEN in the openbaar register at http://localhost:8141.
+#    test123 + OTP, see S-15c); it shows as INGESCHREVEN in the openbaar register at
+#    http://localhost:8141.
 ```
 
 > The zaaktype is discovered by the ACL itself since S-27 (below); `local-seed`'s `acl.env` now
@@ -589,7 +592,7 @@ or **afwijzen** — which also completes the Beoordelen task so the process adva
 
 ```text
 # 1. Open the behandel portal and log in as a behandelaar (medewerker realm):
-#    http://localhost:8142/   →  merel-behandelaar / test123
+#    http://localhost:8142/   →  merel-behandelaar / test123 + OTP
 #
 # 2. The werkbak lists the registrations awaiting beoordeling (referentie / bsn / status).
 #    Find the reference from the submit confirmation and click "Goedkeuren" on that row.
@@ -812,3 +815,32 @@ make verify-domain     # → "the timed-out registration's zaak was cancelled to
 `POST /annuleringen` → ZGW `resultaten` + `statussen` (Geannuleerd); the aggregate then moves to
 `Verlopen`. The ACL cancels the zaak **before** the aggregate is expired, so a failed ZGW call leaves the
 job for redelivery rather than diverging the two (ADR-0019).
+
+---
+
+## S-15c — MFA on the medewerker realm (#132, ADR-0031)
+
+**Outcome:** staff logins (behandel + beheer portals) need a **second factor**. The medewerker realm
+seeds every medewerker with a TOTP credential, so Keycloak's conditional-OTP step challenges them in
+both the browser flow and the direct grant; a password alone no longer yields a token. `CONFIGURE_TOTP`
+is a default required action, so a medewerker added later must enrol first. Citizen realms (digid,
+eherkenning, eidas) are unchanged — they mock brokers that carry their own assurance.
+
+```bash
+# 1. Manual: log in to the behandel portal. After username + password Keycloak asks for a code.
+python3 infra/keycloak/check_realms.py otp   # a valid code, right now
+open http://localhost:8142                   # merel-behandelaar / test123 + that code
+#
+# 2. Automated: the realm smoke check asserts the password alone is REFUSED, then that
+#    password + TOTP succeeds and still carries the behandelaar role:
+make keycloak-smoke    # → "medewerker  merel-behandelaar  password-only login refused [OK]"
+#
+# 3. End-to-end: every staff login in the e2e goes through the OTP prompt (loginMedewerker):
+make verify-e2e        # → registration.spec (behandelaar approves), catalogus.spec, default-fill.spec
+```
+
+**The path:** the seeded `otp` credential in `infra/keycloak/realms/medewerker-realm.json` activates
+Keycloak's stock conditional-OTP subflow — no custom browser flow. The fixture secret is shared and
+committed on purpose so the checks can compute codes; a real deployment enrols per-user authenticators
+(ADR-0031).
+
diff --git a/docs/runbooks/keycloak.md b/docs/runbooks/keycloak.md
index df43d93..8d71bbf 100644
--- a/docs/runbooks/keycloak.md
+++ b/docs/runbooks/keycloak.md
@@ -23,6 +23,9 @@ login per realm and asserts the identifying claim:
 | eidas | pierre-dupont | `eidas_id` |
 | medewerker | merel-behandelaar | role `behandelaar` |
 
+The medewerker row also asserts that the password **alone** is refused — that realm
+enforces MFA (below).
+
 All test users / credentials are in [../synthetic-data.md](../synthetic-data.md).
 
 ## Notes
@@ -35,3 +38,36 @@ All test users / credentials are in [../synthetic-data.md](../synthetic-data.md)
 - **Image** pinned to `quay.io/keycloak/keycloak:26.1`.
 - Claims are injected by OIDC protocol mappers on `big-portal` (user attribute → token
   claim); `medewerker` roles come through `realm_access.roles`.
+
+## MFA on the medewerker realm (S-15c)
+
+Staff logins (behandel + beheer portals) need a second factor; citizen/company realms
+(digid, eherkenning, eidas) do not. Two halves in `medewerker-realm.json`:
+
+- Every seeded medewerker carries a **TOTP credential** with the fixture secret
+  `BIGMEDEWERKEROTPSEED`, so Keycloak's built-in *conditional OTP* step fires on every
+  login — browser flow (an `#otp` prompt after the password) and direct grant (a `totp`
+  form field) alike.
+- `CONFIGURE_TOTP` is a **default required action**, so any medewerker added later must
+  enrol an authenticator before the first login.
+
+See [../architecture/adr-0031-mfa-on-the-medewerker-realm.md](../architecture/adr-0031-mfa-on-the-medewerker-realm.md).
+
+### Getting a code
+
+```bash
+python3 infra/keycloak/check_realms.py otp   # prints a valid 6-digit code right now
+```
+
+Or enrol a phone once: the secret in base32 is `IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE`
+(`otpauth://totp/medewerker?secret=IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE`). The e2e computes its
+own code in `tests/e2e/medewerker-login.ts`.
+
+**A code is single-use.** Keycloak's `otpPolicyCodeReusable` defaults to false, so it refuses a
+code it has already accepted — a second login as the same medewerker inside the same 30-second
+window fails with `invalid_grant` / *Invalid user credentials*, even though the code is current.
+Nothing to fix in the realm: wait for the next window, or spend the following counter, which is
+what `nextUnusedCounter` in `tests/e2e/medewerker-login.ts` does for back-to-back specs.
+
+**Fixture only.** A shared, committed secret is a demo convenience, never a production
+posture — see the ADR's consequences.
diff --git a/docs/synthetic-data.md b/docs/synthetic-data.md
index ccdd538..853ba5b 100644
--- a/docs/synthetic-data.md
+++ b/docs/synthetic-data.md
@@ -19,6 +19,11 @@ All test users share the password **`test123`**.
 | `eidas` | eIDAS (EU) | `pierre-dupont` | `eidas_id` = `FR/NL/AB-1234-5678` |
 | `medewerker` | Internal staff | `merel-behandelaar` | role `behandelaar` |
 | `medewerker` | Internal staff | `tom-teamlead` | roles `behandelaar`, `teamlead` |
+| `medewerker` | Internal staff | `bram-beheerder` | role `beheerder` |
+
+`medewerker` users additionally need a **second factor**: that realm enforces MFA (S-15c,
+ADR-0031). All three share the fixture TOTP secret `BIGMEDEWERKEROTPSEED`; print a current
+code with `python3 infra/keycloak/check_realms.py otp`.
 
 The identifying claims are injected via OIDC protocol mappers on `big-portal`
 (user-attribute → token claim); `medewerker` roles appear in `realm_access.roles`.
@@ -32,5 +37,8 @@ curl -s -X POST \
   -d username=jan-burger -d password=test123 -d scope=openid | jq -r .access_token
 ```
 
+For a `medewerker` user, add `-d totp=$(python3 infra/keycloak/check_realms.py otp)` —
+without it the grant is refused with `invalid_grant`.
+
 Decode the JWT payload to see the `bsn` claim. `make keycloak-smoke` checks every realm
 automatically.
diff --git a/infra/keycloak/check_realms.py b/infra/keycloak/check_realms.py
index e338d29..240d22d 100644
--- a/infra/keycloak/check_realms.py
+++ b/infra/keycloak/check_realms.py
@@ -1,19 +1,25 @@
 #!/usr/bin/env python3
 """Smoke-check the Keycloak realms: each realm's OIDC login works (password grant)
-and returns its expected identifying claim. Stdlib only. Exits non-zero on failure.
+and returns its expected identifying claim. The medewerker realm additionally enforces
+MFA (S-15c), so its login must be refused without a TOTP code. Stdlib only.
+Exits non-zero on failure.
 """
-import base64, json, sys, urllib.error, urllib.parse, urllib.request
+import base64, hashlib, hmac, json, struct, sys, time, urllib.error, urllib.parse, urllib.request
 
 BASE = "http://localhost:8180"
 CLIENT = "big-portal"
 PWD = "test123"
 
-# realm, user, claim ("__roles__" => check realm_access.roles), expected-contains
+# Fixture TOTP secret seeded into every medewerker in infra/keycloak/realms/medewerker-realm.json.
+# Keycloak HMACs the raw secret bytes, so no base32 decoding is involved.
+OTP_SECRET = b"BIGMEDEWERKEROTPSEED"
+
+# realm, user, claim ("__roles__" => check realm_access.roles), expected-contains, mfa-enforced
 CHECKS = [
-    ("digid", "jan-burger", "bsn", "123456782"),
-    ("eherkenning", "acme-ondernemer", "kvk", "12345678"),
-    ("eidas", "pierre-dupont", "eidas_id", "FR/NL"),
-    ("medewerker", "merel-behandelaar", "__roles__", "behandelaar"),
+    ("digid", "jan-burger", "bsn", "123456782", False),
+    ("eherkenning", "acme-ondernemer", "kvk", "12345678", False),
+    ("eidas", "pierre-dupont", "eidas_id", "FR/NL", False),
+    ("medewerker", "merel-behandelaar", "__roles__", "behandelaar", True),
 ]
 
 
@@ -23,10 +29,17 @@ def decode(jwt):
     return json.loads(base64.urlsafe_b64decode(p))
 
 
-def grant(realm, user):
+def totp(secret=OTP_SECRET, period=30, digits=6):
+    """RFC 6238 code: HMAC-SHA1 over the 30-second counter, dynamically truncated."""
+    mac = hmac.new(secret, struct.pack(">Q", int(time.time()) // period), hashlib.sha1).digest()
+    o = mac[-1] & 0x0F
+    return str((struct.unpack(">I", mac[o:o + 4])[0] & 0x7FFFFFFF) % 10 ** digits).zfill(digits)
+
+
+def grant(realm, user, **extra):
     data = urllib.parse.urlencode({
         "grant_type": "password", "client_id": CLIENT,
-        "username": user, "password": PWD, "scope": "openid",
+        "username": user, "password": PWD, "scope": "openid", **extra,
     }).encode()
     req = urllib.request.Request(
         f"{BASE}/realms/{realm}/protocol/openid-connect/token", data=data,
@@ -35,11 +48,27 @@ def grant(realm, user):
         return json.loads(r.read())
 
 
+def second_factor_refused(realm, user):
+    """The password alone must not yield a token on an MFA-enforced realm."""
+    try:
+        grant(realm, user)
+    except urllib.error.HTTPError as e:
+        return e.code in (400, 401)
+    return False
+
+
 def main():
     ok = True
-    for realm, user, claim, expect in CHECKS:
+    for realm, user, claim, expect, mfa in CHECKS:
+        extra = {}
+        if mfa:
+            refused = second_factor_refused(realm, user)
+            ok = ok and refused
+            print(f"{realm:12} {user:18} password-only login refused "
+                  f"[{'OK' if refused else 'MFA NOT ENFORCED'}]")
+            extra = {"totp": totp()}
         try:
-            at = decode(grant(realm, user)["access_token"])
+            at = decode(grant(realm, user, **extra)["access_token"])
             if claim == "__roles__":
                 val = at.get("realm_access", {}).get("roles", [])
                 good = expect in val
@@ -57,4 +86,9 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    # `check_realms.py otp` prints a current code for the fixture secret — what a human demoing
+    # the medewerker portals types at Keycloak's OTP prompt (docs/runbooks/keycloak.md).
+    if len(sys.argv) > 1 and sys.argv[1] == "otp":
+        print(totp())
+    else:
+        main()
diff --git a/infra/keycloak/realms/medewerker-realm.json b/infra/keycloak/realms/medewerker-realm.json
index fa739bc..f298d45 100644
--- a/infra/keycloak/realms/medewerker-realm.json
+++ b/infra/keycloak/realms/medewerker-realm.json
@@ -2,6 +2,16 @@
   "realm": "medewerker",
   "enabled": true,
   "displayName": "Medewerkers",
+  "requiredActions": [
+    {
+      "alias": "CONFIGURE_TOTP",
+      "name": "Configure OTP",
+      "providerId": "CONFIGURE_TOTP",
+      "enabled": true,
+      "defaultAction": true,
+      "priority": 10
+    }
+  ],
   "roles": {
     "realm": [
       { "name": "behandelaar", "description": "Behandelt registratieaanvragen" },
@@ -43,7 +53,15 @@
       "lastName": "Behandelaar",
       "email": "merel@big.example.nl",
       "emailVerified": true,
-      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "credentials": [
+        { "type": "password", "value": "test123", "temporary": false },
+        {
+          "type": "otp",
+          "userLabel": "seeded TOTP (fixture)",
+          "secretData": "{\"value\":\"BIGMEDEWERKEROTPSEED\"}",
+          "credentialData": "{\"subType\":\"totp\",\"digits\":6,\"counter\":0,\"period\":30,\"algorithm\":\"HmacSHA1\"}"
+        }
+      ],
       "realmRoles": ["behandelaar"]
     },
     {
@@ -53,7 +71,15 @@
       "lastName": "Teamlead",
       "email": "tom@big.example.nl",
       "emailVerified": true,
-      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "credentials": [
+        { "type": "password", "value": "test123", "temporary": false },
+        {
+          "type": "otp",
+          "userLabel": "seeded TOTP (fixture)",
+          "secretData": "{\"value\":\"BIGMEDEWERKEROTPSEED\"}",
+          "credentialData": "{\"subType\":\"totp\",\"digits\":6,\"counter\":0,\"period\":30,\"algorithm\":\"HmacSHA1\"}"
+        }
+      ],
       "realmRoles": ["behandelaar", "teamlead"]
     },
     {
@@ -63,7 +89,15 @@
       "lastName": "Beheerder",
       "email": "bram@big.example.nl",
       "emailVerified": true,
-      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "credentials": [
+        { "type": "password", "value": "test123", "temporary": false },
+        {
+          "type": "otp",
+          "userLabel": "seeded TOTP (fixture)",
+          "secretData": "{\"value\":\"BIGMEDEWERKEROTPSEED\"}",
+          "credentialData": "{\"subType\":\"totp\",\"digits\":6,\"counter\":0,\"period\":30,\"algorithm\":\"HmacSHA1\"}"
+        }
+      ],
       "realmRoles": ["beheerder"]
     }
   ]
diff --git a/tests/e2e/catalogus.spec.ts b/tests/e2e/catalogus.spec.ts
index d7876c5..bb6e85f 100644
--- a/tests/e2e/catalogus.spec.ts
+++ b/tests/e2e/catalogus.spec.ts
@@ -1,4 +1,5 @@
 import { expect, test } from '@playwright/test';
+import { loginMedewerker } from './medewerker-login';
 
 // S-15a walking skeleton: a beheerder logs in to the beheer portal (medewerker realm) and sees the
 // read-only ZTC catalogus. The verify stack seeds and publishes the BIG-REGISTRATIE zaaktype (the
@@ -7,10 +8,9 @@ import { expect, test } from '@playwright/test';
 test('a beheerder sees the published zaaktypen in the catalogus', async ({ page }) => {
   await page.goto('http://beheer/');
 
-  // The beheer portal redirects to the Keycloak medewerker realm login (same realm as behandel).
-  await page.locator('#username').fill('bram-beheerder');
-  await page.locator('#password').fill('test123');
-  await page.locator('#kc-login').click();
+  // The beheer portal redirects to the Keycloak medewerker realm login (same realm as behandel),
+  // which enforces MFA: password, then a TOTP code.
+  await loginMedewerker(page, 'bram-beheerder');
 
   await expect(page.getByRole('heading', { name: /Catalogus/i })).toBeVisible();
 
diff --git a/tests/e2e/default-fill.spec.ts b/tests/e2e/default-fill.spec.ts
index 8ff8d03..637fe57 100644
--- a/tests/e2e/default-fill.spec.ts
+++ b/tests/e2e/default-fill.spec.ts
@@ -1,4 +1,5 @@
 import { expect, test } from '@playwright/test';
+import { loginMedewerker } from './medewerker-login';
 
 // S-15b: a beheerder edits the ACL default-fill in the beheer portal and gets a saved confirmation.
 // Runs against the shared verify stack; it edits + saves (the ACL store is in-memory, ADR-0026) and
@@ -6,10 +7,8 @@ import { expect, test } from '@playwright/test';
 test('a beheerder edits and saves the default-fill', async ({ page }) => {
   await page.goto('http://beheer/');
 
-  // Keycloak medewerker-realm login (same realm as behandel).
-  await page.locator('#username').fill('bram-beheerder');
-  await page.locator('#password').fill('test123');
-  await page.locator('#kc-login').click();
+  // Keycloak medewerker-realm login (same realm as behandel) — password + enforced TOTP.
+  await loginMedewerker(page, 'bram-beheerder');
 
   await expect(page.getByRole('heading', { name: /Catalogus/i })).toBeVisible();
 
diff --git a/tests/e2e/medewerker-login.spec.ts b/tests/e2e/medewerker-login.spec.ts
new file mode 100644
index 0000000..b4e83bb
--- /dev/null
+++ b/tests/e2e/medewerker-login.spec.ts
@@ -0,0 +1,14 @@
+import { expect, test } from '@playwright/test';
+import { OTP_PERIOD_MS, nextUnusedCounter } from './medewerker-login';
+
+// Pure check of the TOTP counter guard in loginMedewerker — no browser, no stack. Keycloak refuses
+// a code it has already accepted (its otpPolicyCodeReusable defaults to false), so two logins as
+// the same medewerker inside one 30-second window must not spend the same counter twice (#132).
+test('a login never spends a TOTP counter this medewerker already used', () => {
+  const now = 3 * OTP_PERIOD_MS + 1_000; // 1 second into counter 3
+
+  expect(nextUnusedCounter(now, -1)).toBe(3); // nothing spent yet → the current counter
+  expect(nextUnusedCounter(now, 3)).toBe(4); // the current counter is spent → the next one
+  expect(nextUnusedCounter(now, 4)).toBe(5); // two logins already in this window → the one after
+  expect(nextUnusedCounter(now + OTP_PERIOD_MS, 3)).toBe(4); // window moved on → current again
+});
diff --git a/tests/e2e/medewerker-login.ts b/tests/e2e/medewerker-login.ts
new file mode 100644
index 0000000..53ba23a
--- /dev/null
+++ b/tests/e2e/medewerker-login.ts
@@ -0,0 +1,57 @@
+import { createHmac } from 'node:crypto';
+import { readFileSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import type { Page } from '@playwright/test';
+
+// The medewerker realm enforces MFA (S-15c), so a staff login is two steps: password, then a TOTP
+// code. The realm export seeds every medewerker with this fixture secret — Keycloak HMACs the raw
+// secret bytes — so the e2e can compute a valid code instead of enrolling an authenticator.
+const OTP_SECRET = 'BIGMEDEWERKEROTPSEED';
+
+export const OTP_PERIOD_MS = 30_000;
+
+// RFC 6238 TOTP: HMAC-SHA1 over the 30-second counter, dynamically truncated to 6 digits.
+export function totp(secret = OTP_SECRET, at = Date.now()): string {
+  const counter = Buffer.alloc(8);
+  counter.writeBigUInt64BE(BigInt(Math.floor(at / OTP_PERIOD_MS)));
+  const mac = createHmac('sha1', secret).update(counter).digest();
+  const offset = mac[mac.length - 1] & 0x0f;
+  return String((mac.readUInt32BE(offset) & 0x7fffffff) % 1_000_000).padStart(6, '0');
+}
+
+// Keycloak refuses a TOTP code it has already accepted (its otpPolicyCodeReusable defaults to
+// false), so two logins as the same medewerker inside one 30-second window would both submit the
+// same code and the second is rejected. Spend the first counter this medewerker has left.
+export function nextUnusedCounter(now: number, spent: number): number {
+  return Math.max(Math.floor(now / OTP_PERIOD_MS), spent + 1);
+}
+
+// The spent counter lives on disk rather than in module state: Playwright starts a fresh worker
+// process for a retry, which would otherwise forget it and resubmit the rejected code.
+function spendCounter(username: string): number {
+  const file = join(tmpdir(), `otp-counter-${username}`);
+  let spent = -1;
+  try {
+    spent = Number(readFileSync(file, 'utf8')) || -1;
+  } catch {
+    // first login as this medewerker in this run
+  }
+  const counter = nextUnusedCounter(Date.now(), spent);
+  writeFileSync(file, String(counter));
+  return counter;
+}
+
+export async function loginMedewerker(page: Page, username: string): Promise<void> {
+  await page.locator('#username').fill(username);
+  await page.locator('#password').fill('test123');
+  await page.locator('#kc-login').click();
+
+  // Keycloak's conditional-OTP step. Wait out the rest of the window if the counter we may spend is
+  // still in the future; its lookAheadWindow would accept the code a moment early, but only by one
+  // counter — waiting keeps a third login in the same window valid too.
+  const counter = spendCounter(username);
+  await page.waitForTimeout(Math.max(0, counter * OTP_PERIOD_MS - Date.now()));
+  await page.locator('#otp').fill(totp(OTP_SECRET, counter * OTP_PERIOD_MS));
+  await page.locator('#kc-login').click();
+}
diff --git a/tests/e2e/registration.spec.ts b/tests/e2e/registration.spec.ts
index 27167e5..a7a9c0e 100644
--- a/tests/e2e/registration.spec.ts
+++ b/tests/e2e/registration.spec.ts
@@ -1,4 +1,5 @@
 import { expect, request, test } from '@playwright/test';
+import { loginMedewerker } from './medewerker-login';
 
 // Walking-skeleton happy path (S-08d + S-09 + S-09b + S-12 + S-10a + S-19b-2): a zorgprofessional
 // logs in via mock DigiD and submits through the self-service portal → BFF → domain; the entry
@@ -76,9 +77,8 @@ test('DigiD submit → public INGEDIEND → documenten → behandelaar goedkeurt
   // — the S-12 flow that replaces the temporary admin endpoint. The staff tab switches to the
   // medewerker realm (a different Keycloak realm than the citizen's digid session).
   await staff.goto('http://behandel/');
-  await staff.locator('#username').fill('merel-behandelaar');
-  await staff.locator('#password').fill('test123');
-  await staff.locator('#kc-login').click();
+  // That realm enforces MFA (S-15c), so the behandelaar logs in with password + TOTP.
+  await loginMedewerker(staff, 'merel-behandelaar');
 
   await expect(staff.getByRole('heading', { name: /Werkbak/i })).toBeVisible();