diff --git a/app/core/i18n.py b/app/core/i18n.py
index 1b3eab7..39364fc 100644
--- a/app/core/i18n.py
+++ b/app/core/i18n.py
@@ -5,9 +5,18 @@
 
 1. URL path-prefix (``/de/...`` or ``/en/...``)
 2. Query-param (``?lang=de|en``)
-3. Cookie (``fm_lang``)
-4. ``Accept-Language`` header (``de*`` | ``en*``)
-5. Default ``de`` (Hamburg-based operator, see plan)
+3. ``Accept-Language`` header (``de*`` | ``en*``)
+4. Default ``de`` (Hamburg-based operator, see plan)
+
+The app stores **no client-side locale preference** (no cookie, no
+localStorage). The URL is the single source of truth: a user who clicks
+the EN switcher lands on ``/en/...`` and stays there as long as
+in-app links propagate ``current_prefix``. A return visit to an
+unprefixed URL falls through to ``Accept-Language`` and the operator
+default. This keeps the published privacy guarantee ("no cookies")
+intact and makes shared URLs deterministic. Logged-in users get a
+sticky preference via ``User.preferred_lang`` (PR-i18n-3) — server-side,
+device-portable, strictly better than a cookie.
 
 The resolved locale lands on ``request.state.locale`` via
 :class:`LocaleMiddleware`. Routes that render templates pull it through
@@ -36,8 +45,6 @@
 
 SUPPORTED_LOCALES: tuple[str, ...] = ("de", "en")
 DEFAULT_LOCALE: str = "de"
-COOKIE_NAME: str = "fm_lang"
-COOKIE_MAX_AGE: int = 30 * 24 * 60 * 60  # 30 days
 LOCALE_DIR: Path = Path(__file__).resolve().parent.parent.parent / "locale"
 
 # Module-level translation cache. Populated lazily on first lookup.
@@ -134,15 +141,11 @@ def resolve_locale(request: Request) -> str:
     q = request.query_params.get("lang")
     if q in SUPPORTED_LOCALES:
         return q
-    # 3. Sticky cookie
-    c = request.cookies.get(COOKIE_NAME)
-    if c in SUPPORTED_LOCALES:
-        return c
-    # 4. Best-effort Accept-Language
+    # 3. Best-effort Accept-Language
     al = _accept_language_locale(request.headers.get("accept-language"))
     if al:
         return al
-    # 5. Operator default (DE unless overridden)
+    # 4. Operator default (DE unless overridden)
     return _effective_default()
 
 
@@ -155,19 +158,6 @@ async def get_locale(request: Request) -> str:
     return getattr(request.state, "locale", None) or resolve_locale(request)
 
 
-def is_explicit_locale_signal(request: Request) -> bool:
-    """True when the caller's URL or query carries an unambiguous locale.
-
-    The cookie is only set when the signal was *explicit* — otherwise the
-    cookie would race the URL on every page load and surprise users who
-    hit a different prefix from a bookmark.
-    """
-    return (
-        path_prefix_locale(request.url.path) is not None
-        or request.query_params.get("lang") in SUPPORTED_LOCALES
-    )
-
-
 def localized_context(request: Request, **extra) -> dict:
     """Build a context dict carrying the per-request translator + locale.
 
diff --git a/app/main.py b/app/main.py
index 5f03fb9..abd82d3 100644
--- a/app/main.py
+++ b/app/main.py
@@ -32,11 +32,8 @@
     normalize_classification as _normalize_data_classification,
 )
 from app.core.i18n import (
-    COOKIE_MAX_AGE as _LOCALE_COOKIE_MAX_AGE,
-    COOKIE_NAME as _LOCALE_COOKIE_NAME,
     SUPPORTED_LOCALES,
     base_path,
-    is_explicit_locale_signal,
     localized_context,
     localized_url,
     resolve_locale,
@@ -279,26 +276,15 @@ def _build_csp_header(api_base_url: str) -> str:
 
 @app.middleware("http")
 async def locale_resolver(request: Request, call_next):
-    """Resolve the active locale, stash on request.state, persist via cookie.
+    """Resolve the active locale and stash it on ``request.state``.
 
-    The cookie is only written when the locale was *explicit* (URL prefix
-    or ``?lang=`` param) — otherwise we'd race the cookie against the URL
-    on every page load and lock visitors out of Accept-Language detection.
+    No cookie is written. The URL is the single source of truth — see
+    ``app/core/i18n.py`` module docstring for the rationale and the
+    privacy-policy commitment ("FileMorph sets no cookies on its own
+    domain", ``app/templates/privacy.html`` §6).
     """
-    locale = resolve_locale(request)
-    request.state.locale = locale
-    response = await call_next(request)
-    if is_explicit_locale_signal(request) and request.cookies.get(_LOCALE_COOKIE_NAME) != locale:
-        response.set_cookie(
-            _LOCALE_COOKIE_NAME,
-            value=locale,
-            max_age=_LOCALE_COOKIE_MAX_AGE,
-            samesite="lax",
-            secure=request.url.scheme == "https",
-            httponly=False,
-            path="/",
-        )
-    return response
+    request.state.locale = resolve_locale(request)
+    return await call_next(request)
 
 
 @app.middleware("http")
diff --git a/tests/test_i18n.py b/tests/test_i18n.py
index 4c24197..5fea0e0 100644
--- a/tests/test_i18n.py
+++ b/tests/test_i18n.py
@@ -1,15 +1,21 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """i18n infrastructure tests — pin the locale-resolution contract.
 
-Covers the five-step resolution chain in `app/core/i18n.py::resolve_locale`,
-the unknown-locale fallback, the cookie persistence rule (only-on-explicit-
-signal), the hreflang presence on every page, and the `<html lang>` /
-`og:locale` propagation.
+Covers the four-step resolution chain in
+``app/core/i18n.py::resolve_locale`` (URL-prefix → query-param →
+Accept-Language → operator default), the unknown-locale fallback, the
+hreflang presence on every page, the ``<html lang>`` / ``og:locale``
+propagation, and the **no-cookie** regression-guard.
+
+The app intentionally writes no client-side locale cookie — the
+published privacy policy (``app/templates/privacy.html`` §6) commits to
+"no cookies on its own domain", and the URL is the single source of
+truth for locale. ``test_no_locale_cookie_set_on_any_route`` is the
+programmatic guard against accidental reintroduction.
 
 These tests do not depend on any DE translations existing — PR-i18n-1
 ships infrastructure with empty .po files, so all rendered text stays
-EN. The tests assert *behaviour* (URL-prefix wins, cookie persists, etc.),
-not translated copy.
+EN. The tests assert *behaviour*, not translated copy.
 """
 
 from __future__ import annotations
@@ -29,14 +35,13 @@
 
 @pytest.fixture(autouse=True)
 def _clear_cookies(client):
-    """Reset the session-scoped TestClient's cookie jar before each i18n test.
-
-    The shared ``client`` fixture in ``conftest.py`` is session-scoped, so
-    cookies from earlier i18n tests (e.g. a `/de/` visit setting
-    ``fm_lang=de``) would leak into later tests and steal priority over
-    the resolution chain we're trying to assert. Clearing per-test gives
-    each case a clean cookie jar without sacrificing the speed of a
-    shared TestClient.
+    """Defensive: reset the session-scoped TestClient's cookie jar per test.
+
+    The app no longer sets any locale cookie, but the session-scoped
+    ``client`` from ``conftest.py`` accumulates whatever any other test
+    in the suite happens to set. Clearing here pins each i18n test to a
+    clean jar so unrelated suite ordering can't leak state into the
+    resolution-chain assertions.
     """
     client.cookies.clear()
     yield
@@ -121,16 +126,8 @@ def test_query_param_lang_overrides_default(client):
     assert m and m.group(1) == "en"
 
 
-def test_cookie_overrides_default(client):
-    """A returning visitor with `fm_lang=en` cookie sees EN even on `/`."""
-    r = client.get("/", cookies={"fm_lang": "en"})
-    assert r.status_code == 200
-    m = re.search(r'<html lang="([^"]+)"', r.text)
-    assert m and m.group(1) == "en"
-
-
 def test_accept_language_en_falls_through(client):
-    """When no URL prefix / query / cookie, Accept-Language: en* wins over default DE."""
+    """When no URL prefix / query is present, Accept-Language: en* wins over default DE."""
     r = client.get("/", headers={"accept-language": "en-US,en;q=0.9"})
     assert r.status_code == 200
     m = re.search(r'<html lang="([^"]+)"', r.text)
@@ -139,9 +136,9 @@ def test_accept_language_en_falls_through(client):
 
 def test_unknown_locale_query_falls_back_to_default(client):
     """`?lang=fr` is not supported, and with no Accept-Language signal
-    the server falls back to step 5 (operator default = DE upstream).
+    the server falls back to the operator default (DE upstream).
 
-    We send an empty ``accept-language`` so the test pins step 5 in
+    We send an empty ``accept-language`` so the test pins the default in
     isolation; without it httpx's TestClient sometimes inherits a
     locale-shaped default that masks the fallback path.
     """
@@ -151,14 +148,6 @@ def test_unknown_locale_query_falls_back_to_default(client):
     assert m and m.group(1) == DEFAULT_LOCALE
 
 
-def test_url_prefix_beats_cookie(client):
-    """Path-prefix resolution wins over cookie — prevents stale-cookie surprises."""
-    r = client.get("/en/", cookies={"fm_lang": "de"})
-    assert r.status_code == 200
-    m = re.search(r'<html lang="([^"]+)"', r.text)
-    assert m and m.group(1) == "en"
-
-
 def test_url_prefix_beats_query_param(client):
     """Path-prefix wins over `?lang=` so deep-linked URLs stay deterministic."""
     r = client.get("/de/?lang=en")
@@ -167,35 +156,36 @@ def test_url_prefix_beats_query_param(client):
     assert m and m.group(1) == "de"
 
 
-# ── Cookie persistence ────────────────────────────────────────────────────────
-
-
-def test_explicit_url_prefix_sets_cookie(client):
-    """Visiting `/en/` should set `fm_lang=en` cookie for sticky preference."""
-    r = client.get("/en/")
-    assert r.status_code == 200
-    set_cookie = r.headers.get("set-cookie", "")
-    assert "fm_lang=en" in set_cookie
-
+# ── No-cookie regression-guard ────────────────────────────────────────────────
 
-def test_explicit_query_lang_sets_cookie(client):
-    """`?lang=en` is also explicit — cookie persists the choice."""
-    r = client.get("/?lang=en")
-    assert r.status_code == 200
-    set_cookie = r.headers.get("set-cookie", "")
-    assert "fm_lang=en" in set_cookie
 
+@pytest.mark.parametrize(
+    "url",
+    [
+        "/",
+        "/de/",
+        "/en/",
+        "/?lang=de",
+        "/?lang=en",
+        "/login",
+        "/de/login",
+        "/en/login",
+    ],
+)
+def test_no_locale_cookie_set_on_any_route(client, url):
+    """The app commits to "no cookies on its own domain" (privacy.html §6).
 
-def test_unprefixed_path_does_not_set_cookie(client):
-    """`/` with no explicit locale signal must NOT set fm_lang.
-
-    Otherwise a default-DE visitor's first hit would write `fm_lang=de`,
-    locking them out of any future Accept-Language detection.
+    This is the programmatic gate against accidental reintroduction of a
+    locale cookie. Hits the URL space that PR-i18n-1 originally wired the
+    cookie to (root, prefixed, query-param, and a non-trivial page) and
+    asserts the response carries no ``fm_lang`` Set-Cookie.
     """
-    r = client.get("/")
+    r = client.get(url, headers={"accept-language": ""})
     assert r.status_code == 200
     set_cookie = r.headers.get("set-cookie", "")
-    assert "fm_lang" not in set_cookie
+    assert "fm_lang" not in set_cookie, (
+        f"locale cookie regression on {url!r} — Set-Cookie was: {set_cookie!r}"
+    )
 
 
 # ── hreflang + canonical ──────────────────────────────────────────────────────