diff --git a/README.md b/README.md
index 3061a8e..20507c1 100644
--- a/README.md
+++ b/README.md
@@ -141,6 +141,33 @@ with InvisiblePlaywright(proxy=proxy) as browser:
 
 Schemes supported: `socks5`, `socks4`, `http`, `https`. Auth works on all of them (SOCKS5 via patched `nsProtocolProxyService.cpp`, HTTP/HTTPS via Playwright). DNS is routed through the proxy by default, no local leak.
 
+### Timezone
+
+The browser timezone follows `timezone=`:
+
+```python
+# default: with a proxy, the timezone is auto-derived from the proxy egress IP
+with InvisiblePlaywright(proxy=proxy) as browser:
+    ...
+
+# explicit IANA zone always wins
+with InvisiblePlaywright(proxy=proxy, timezone="America/New_York") as browser:
+    ...
+
+# opt out and keep the host timezone even behind a proxy
+with InvisiblePlaywright(proxy=proxy, timezone="host") as browser:
+    ...
+```
+
+| `timezone=` | with proxy | without proxy |
+|---|---|---|
+| `""` (default) | auto-derived from egress IP | host timezone |
+| `"auto"` | auto-derived from egress IP | host timezone |
+| `"Area/City"` | that zone | that zone |
+| `"host"` / `"local"` | host timezone | host timezone |
+
+A proxy in a different country paired with the host timezone is the classic `timezone_mismatch` signal, so a proxy with no explicit timezone now resolves automatically. The egress IP is looked up through the proxy and mapped to its IANA zone with an offline database ([`daijro/geoip-all-in-one`](https://github.com/daijro/geoip-all-in-one)), downloaded and cached on first use. If a proxy is set but the zone can't be resolved, the launch raises rather than silently falling back to the host zone — pass an explicit `timezone=` or `timezone="host"` to override. Point `STEALTHFOX_GEOIP_MMDB` at your own `.mmdb` to skip the download.
+
 ### Pinning specific fingerprint fields
 
 By default everything comes from `seed`. To force specific values while the rest stays seed-derived:
diff --git a/pyproject.toml b/pyproject.toml
index 4800e6f..d08f552 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,7 +22,9 @@ classifiers = [
 dependencies = [
     "playwright>=1.40",
     "platformdirs>=4",
-    "requests>=2.31",
+    "requests[socks]>=2.31",
+    "maxminddb>=2.2",
+    "tzdata>=2024.1",
     "tqdm>=4.66",
     "pywin32>=306; sys_platform == 'win32'",
 ]
diff --git a/src/invisible_playwright/__init__.py b/src/invisible_playwright/__init__.py
index f93def7..0871021 100644
--- a/src/invisible_playwright/__init__.py
+++ b/src/invisible_playwright/__init__.py
@@ -17,7 +17,8 @@ Quickstart:
 """
 from .config import get_default_args, get_default_stealth_prefs
 from .constants import BINARY_VERSION, FIREFOX_UPSTREAM_VERSION
-from .download import ensure_binary
+from ._geo import GeoTimezoneError, resolve_session_timezone
+from .download import ensure_binary, ensure_geoip_mmdb
 from .launcher import InvisiblePlaywright
 
 from importlib.metadata import PackageNotFoundError, version as _pkg_version
@@ -32,8 +33,11 @@ except PackageNotFoundError:
 __all__ = [
     "InvisiblePlaywright",
     "ensure_binary",
+    "ensure_geoip_mmdb",
     "get_default_stealth_prefs",
     "get_default_args",
+    "resolve_session_timezone",
+    "GeoTimezoneError",
     "BINARY_VERSION",
     "FIREFOX_UPSTREAM_VERSION",
     "__version__",
diff --git a/src/invisible_playwright/_geo.py b/src/invisible_playwright/_geo.py
new file mode 100644
index 0000000..95ab3c5
--- /dev/null
+++ b/src/invisible_playwright/_geo.py
@@ -0,0 +1,160 @@
+"""Resolve the session timezone from the proxy egress IP (``timezone="auto"``).
+
+Approach B: discover the egress IP with one HTTP request routed *through the
+configured proxy*, then map IP → IANA timezone with an offline mmdb
+(``daijro/geoip-all-in-one``, downloaded + cached by ``download.py``).
+
+Precedence (see ``resolve_session_timezone``):
+
+    "host" / "local"   → ""        force host TZ (escape hatch)
+    explicit IANA      → unchanged  explicit always wins
+    "" + no proxy      → ""        host TZ (default, unchanged behaviour)
+    "" + proxy         → egress    NEW default: a proxy with no timezone is
+                                   exactly the timezone_mismatch trap, so we
+                                   auto-resolve it.
+    "auto" + no proxy  → ""        nothing to resolve, fall back to host TZ
+    "auto" + proxy     → egress
+
+When a proxy IS set we fail loudly rather than silently fall back to the host
+TZ — a foreign proxy paired with the host timezone is the precise signal
+detectors flag as ``timezone_mismatch``.
+"""
+from __future__ import annotations
+
+import ipaddress
+from typing import Any, Dict, Optional
+from urllib.parse import quote
+
+import requests
+
+
+class GeoTimezoneError(RuntimeError):
+    """Raised when ``timezone="auto"`` cannot resolve a valid IANA zone."""
+
+
+# Plain-text IP echo endpoints (each returns just the caller's public IP).
+_IP_ECHO_ENDPOINTS = (
+    "https://api.ipify.org",
+    "https://icanhazip.com",
+    "https://checkip.amazonaws.com",
+)
+
+_SOCKS_SCHEMES = ("socks5://", "socks4://", "socks://")
+
+
+def _proxy_is_set(proxy: Optional[Dict[str, str]]) -> bool:
+    if not proxy:
+        return False
+    server = (proxy.get("server") or "").strip()
+    return bool(server) and server.lower() != "direct://"
+
+
+def _proxies_for_requests(proxy: Dict[str, str]) -> Dict[str, str]:
+    """Translate our proxy dict into a ``requests`` proxies mapping.
+
+    SOCKS5 uses the ``socks5h`` scheme so DNS is resolved proxy-side (matches
+    ``network.proxy.socks_remote_dns=True`` in the Firefox path). HTTP/HTTPS
+    pass through unchanged. Credentials are URL-encoded.
+    """
+    server = (proxy.get("server") or "").strip()
+    low = server.lower()
+    if low.startswith("socks5://") or low.startswith("socks://"):
+        scheme = "socks5h"
+    elif low.startswith("socks4://"):
+        scheme = "socks4"
+    elif low.startswith("https://"):
+        scheme = "https"
+    else:
+        scheme = "http"
+
+    host_port = server.split("://", 1)[1] if "://" in server else server
+    user = proxy.get("username") or ""
+    pwd = proxy.get("password") or ""
+    if user:
+        auth = f"{quote(user, safe='')}:{quote(pwd, safe='')}@"
+    else:
+        auth = ""
+    url = f"{scheme}://{auth}{host_port}"
+    return {"http": url, "https": url}
+
+
+def discover_egress_ip(
+    proxy: Dict[str, str], *, timeout: float = 10.0
+) -> str:
+    """Return the public IP seen when routing through ``proxy``.
+
+    Tries each echo endpoint in turn; raises :class:`GeoTimezoneError` if none
+    return a valid IP (SOCKS support requires ``requests[socks]`` / PySocks).
+    """
+    proxies = _proxies_for_requests(proxy)
+    last_err: Optional[Exception] = None
+    for url in _IP_ECHO_ENDPOINTS:
+        try:
+            resp = requests.get(url, proxies=proxies, timeout=timeout)
+            resp.raise_for_status()
+            ip = resp.text.strip()
+            ipaddress.ip_address(ip)  # validate (raises ValueError if not an IP)
+            return ip
+        except Exception as exc:  # noqa: BLE001 - try the next endpoint
+            last_err = exc
+            continue
+    raise GeoTimezoneError(
+        f"could not discover the proxy egress IP via {len(_IP_ECHO_ENDPOINTS)} "
+        f"endpoints (last error: {last_err!r}). For SOCKS proxies make sure "
+        f"requests[socks] / PySocks is installed."
+    )
+
+
+def ip_to_timezone(ip: str, mmdb_path: Any) -> str:
+    """Map ``ip`` to its IANA timezone using the offline mmdb.
+
+    Reads the standard MaxMind ``location.time_zone`` field and validates it
+    against the system tz database. Raises :class:`GeoTimezoneError` if the IP
+    is absent from the DB or the zone is missing / not a valid IANA name.
+    """
+    import maxminddb
+
+    with maxminddb.open_database(str(mmdb_path)) as reader:
+        record = reader.get(ip)
+    if not record:
+        raise GeoTimezoneError(f"egress IP {ip} not present in the geoip database")
+    tz = ((record.get("location") or {}) if isinstance(record, dict) else {}).get(
+        "time_zone"
+    )
+    if not tz:
+        raise GeoTimezoneError(f"no timezone for egress IP {ip} in the geoip database")
+    from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+
+    try:
+        ZoneInfo(tz)
+    except (ZoneInfoNotFoundError, ValueError) as exc:
+        raise GeoTimezoneError(
+            f"geoip returned an invalid IANA zone {tz!r} for {ip}: {exc}"
+        ) from exc
+    return tz
+
+
+def resolve_session_timezone(
+    timezone: str, proxy: Optional[Dict[str, str]]
+) -> str:
+    """Map the user's ``timezone`` setting to a concrete IANA zone (or ``""``).
+
+    See the module docstring for the full precedence table. Raises
+    :class:`GeoTimezoneError` when a proxy is set but the egress timezone
+    cannot be resolved (fail-early — never silently use the host TZ behind a
+    foreign proxy).
+    """
+    tz = (timezone or "").strip()
+    if tz.lower() in ("host", "local"):
+        return ""
+    if tz and tz.lower() != "auto":
+        return tz  # explicit IANA wins
+    if not _proxy_is_set(proxy):
+        return ""  # "" / "auto" without a proxy → host TZ
+    # proxy set, tz is "" (new default) or "auto" → resolve from egress.
+    assert proxy is not None
+    from .download import ensure_geoip_mmdb
+
+    ip = discover_egress_ip(proxy)
+    mmdb = ensure_geoip_mmdb()
+    return ip_to_timezone(ip, mmdb)
diff --git a/src/invisible_playwright/async_api.py b/src/invisible_playwright/async_api.py
index 6b04ea1..70a7aeb 100644
--- a/src/invisible_playwright/async_api.py
+++ b/src/invisible_playwright/async_api.py
@@ -9,6 +9,7 @@ from typing import Any, Dict, Optional, Union
 from playwright.async_api import Browser, BrowserContext, Playwright, async_playwright
 
 from ._fpforge import Profile, generate_profile
+from ._geo import resolve_session_timezone
 from ._headless import make_virtual_display
 from ._proxy import configure_proxy as _configure_proxy_shared
 from .download import ensure_binary
@@ -75,6 +76,13 @@ class InvisiblePlaywright:
 
     async def __aenter__(self) -> Union[Browser, BrowserContext]:
         import sys as _sys
+        # Resolve timezone="auto" (and the proxy-set-but-unset default) to a
+        # concrete IANA zone before anything reads self._timezone. Run the
+        # blocking geo lookup off the event loop. Fail-early if a proxy is set
+        # but the egress zone can't be resolved.
+        self._timezone = await asyncio.to_thread(
+            resolve_session_timezone, self._timezone, self._proxy
+        )
         executable = self._binary_path or ensure_binary()
         prefs = translate_profile_to_prefs(
             self._profile,
diff --git a/src/invisible_playwright/constants.py b/src/invisible_playwright/constants.py
index c7e2242..d2cc16b 100644
--- a/src/invisible_playwright/constants.py
+++ b/src/invisible_playwright/constants.py
@@ -46,3 +46,19 @@ BINARY_ENTRY_REL = {
 RELEASE_URL_TEMPLATE = (
     "https://github.com/feder-cr/invisible_playwright/releases/download/{tag}/{asset}"
 )
+
+# ─────────────────────────────────────────────────────────────────────────
+#  GeoIP database (timezone="auto" → resolve IANA zone from proxy egress IP)
+# ─────────────────────────────────────────────────────────────────────────
+# daijro/geoip-all-in-one merges IP2Location LITE + GeoLite2 + DB-IP into a
+# single mmdb (country ISO + coordinates + IANA timezone via tzfpy), rebuilt
+# weekly. GPL-3.0, so we DOWNLOAD it at runtime into the user cache (like the
+# Firefox binary) rather than bundling it into this MIT package. Pinned to a
+# known-good weekly tag; bump to refresh. The `-all` variant covers IPv4+IPv6.
+GEOIP_REPO: str = "daijro/geoip-all-in-one"
+GEOIP_MMDB_VERSION: str = "2026.06.03"
+GEOIP_ASSET: str = "geoip-aio-all.mmdb.zip"
+GEOIP_MMDB_NAME: str = "geoip-aio-all.mmdb"
+GEOIP_RELEASE_URL_TEMPLATE: str = (
+    "https://github.com/daijro/geoip-all-in-one/releases/download/{tag}/{asset}"
+)
diff --git a/src/invisible_playwright/download.py b/src/invisible_playwright/download.py
index 58a5e8f..3dbb8e1 100644
--- a/src/invisible_playwright/download.py
+++ b/src/invisible_playwright/download.py
@@ -18,6 +18,10 @@ from .constants import (
     ARCHIVE_NAME,
     BINARY_ENTRY_REL,
     BINARY_VERSION,
+    GEOIP_ASSET,
+    GEOIP_MMDB_NAME,
+    GEOIP_MMDB_VERSION,
+    GEOIP_RELEASE_URL_TEMPLATE,
     RELEASE_URL_TEMPLATE,
 )
 
@@ -151,3 +155,49 @@ def ensure_binary(version: str = BINARY_VERSION) -> Path:
     if not entry.exists():
         raise RuntimeError(f"binary not found after extraction: {entry}")
     return entry
+
+
+# ─────────────────────────────────────────────────────────────────────────
+#  GeoIP mmdb (used by timezone="auto" to map proxy egress IP → IANA zone)
+# ─────────────────────────────────────────────────────────────────────────
+def geoip_mmdb_path(version: str = GEOIP_MMDB_VERSION) -> Path:
+    """Cache location for the extracted geoip mmdb."""
+    return cache_root() / "geoip" / version / GEOIP_MMDB_NAME
+
+
+def ensure_geoip_mmdb(version: str = GEOIP_MMDB_VERSION) -> Path:
+    """Return a path to the geoip mmdb, downloading + caching it if needed.
+
+    Set ``STEALTHFOX_GEOIP_MMDB`` to point at a user-supplied mmdb (or a test
+    fixture) to skip the download entirely. Otherwise the pinned weekly build
+    of ``daijro/geoip-all-in-one`` is fetched from GitHub Releases (public, no
+    token) into the user cache and unzipped once.
+    """
+    override = os.environ.get("STEALTHFOX_GEOIP_MMDB")
+    if override:
+        p = Path(override)
+        if not p.exists():
+            raise RuntimeError(
+                f"STEALTHFOX_GEOIP_MMDB points to a missing file: {p}"
+            )
+        return p
+
+    dst = geoip_mmdb_path(version)
+    if dst.exists():
+        return dst
+
+    url = GEOIP_RELEASE_URL_TEMPLATE.format(tag=version, asset=GEOIP_ASSET)
+    dst.parent.mkdir(parents=True, exist_ok=True)
+    with tempfile.TemporaryDirectory() as td:
+        archive = Path(td) / GEOIP_ASSET
+        _download_file(url, archive)
+        _extract(archive, dst.parent)
+
+    if dst.exists():
+        return dst
+    # The asset name inside the zip may differ from GEOIP_MMDB_NAME — fall
+    # back to the first .mmdb the archive produced.
+    candidates = sorted(dst.parent.glob("*.mmdb"))
+    if candidates:
+        return candidates[0]
+    raise RuntimeError(f"geoip mmdb not found after extraction in {dst.parent}")
diff --git a/src/invisible_playwright/launcher.py b/src/invisible_playwright/launcher.py
index 32ac02f..d277711 100644
--- a/src/invisible_playwright/launcher.py
+++ b/src/invisible_playwright/launcher.py
@@ -8,6 +8,7 @@ from typing import Any, Dict, Optional, Union
 from playwright.sync_api import Browser, BrowserContext, Playwright, sync_playwright
 
 from ._fpforge import Profile, generate_profile
+from ._geo import resolve_session_timezone
 from ._headless import make_virtual_display
 from ._proxy import configure_proxy as _configure_proxy_shared
 from .download import ensure_binary
@@ -178,6 +179,10 @@ class InvisiblePlaywright:
         self._virtual_display: Any = None
 
     def __enter__(self) -> Union[Browser, BrowserContext]:
+        # Resolve timezone="auto" (and the proxy-set-but-unset default) to a
+        # concrete IANA zone before anything reads self._timezone. Fail-early
+        # if a proxy is set but the egress zone can't be resolved.
+        self._timezone = resolve_session_timezone(self._timezone, self._proxy)
         executable = self._binary_path or ensure_binary()
         prefs = self._build_prefs()
         playwright_proxy = _configure_proxy_shared(self._proxy, prefs)
diff --git a/tests/test_geo.py b/tests/test_geo.py
new file mode 100644
index 0000000..17d448f
--- /dev/null
+++ b/tests/test_geo.py
@@ -0,0 +1,263 @@
+"""Unit tests for `invisible_playwright._geo` (timezone="auto" resolution).
+
+Covers: the precedence policy (resolve_session_timezone), proxy→requests
+translation, egress IP discovery (mocked HTTP), and IP→IANA mapping (mocked
+mmdb). No real network or mmdb is touched.
+"""
+import sys
+import types
+
+import pytest
+
+from invisible_playwright import _geo
+from invisible_playwright._geo import (
+    GeoTimezoneError,
+    _proxies_for_requests,
+    _proxy_is_set,
+    discover_egress_ip,
+    ip_to_timezone,
+    resolve_session_timezone,
+)
+
+SOCKS = {"server": "socks5://gw.example:1080", "username": "u", "password": "p"}
+HTTP = {"server": "http://gw.example:8080", "username": "u", "password": "p"}
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  _proxy_is_set
+# ──────────────────────────────────────────────────────────────────────
+@pytest.mark.unit
+@pytest.mark.parametrize(
+    "proxy,expected",
+    [
+        (None, False),
+        ({}, False),
+        ({"server": ""}, False),
+        ({"server": "   "}, False),
+        ({"server": "direct://"}, False),
+        ({"server": "DIRECT://"}, False),
+        ({"server": "socks5://h:1"}, True),
+        ({"server": "http://h:8080"}, True),
+    ],
+)
+def test_proxy_is_set(proxy, expected):
+    assert _proxy_is_set(proxy) is expected
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  _proxies_for_requests — scheme + credential translation
+# ──────────────────────────────────────────────────────────────────────
+@pytest.mark.unit
+def test_proxies_socks5_uses_socks5h_remote_dns():
+    out = _proxies_for_requests(SOCKS)
+    assert out["http"] == "socks5h://u:p@gw.example:1080"
+    assert out["https"] == out["http"]
+
+
+@pytest.mark.unit
+def test_proxies_socks4_scheme():
+    out = _proxies_for_requests({"server": "socks4://gw:1080"})
+    assert out["http"] == "socks4://gw:1080"
+
+
+@pytest.mark.unit
+def test_proxies_http_and_https_schemes():
+    assert _proxies_for_requests(HTTP)["http"] == "http://u:p@gw.example:8080"
+    out = _proxies_for_requests({"server": "https://gw:8443"})
+    assert out["https"] == "https://gw:8443"
+
+
+@pytest.mark.unit
+def test_proxies_no_scheme_defaults_to_http():
+    out = _proxies_for_requests({"server": "gw.example:3128"})
+    assert out["http"] == "http://gw.example:3128"
+
+
+@pytest.mark.unit
+def test_proxies_credentials_are_url_encoded():
+    out = _proxies_for_requests(
+        {"server": "socks5://gw:1080", "username": "user@x", "password": "p:w/d"}
+    )
+    # '@', ':' and '/' in creds must be percent-encoded so they don't break
+    # the proxy URL parsing.
+    assert "user%40x:p%3Aw%2Fd@gw:1080" in out["http"]
+
+
+@pytest.mark.unit
+def test_proxies_no_credentials_has_no_auth_prefix():
+    out = _proxies_for_requests({"server": "socks5://gw:1080"})
+    assert out["http"] == "socks5h://gw:1080"
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  discover_egress_ip — mocked requests
+# ──────────────────────────────────────────────────────────────────────
+class _FakeResp:
+    def __init__(self, text, status=200):
+        self.text = text
+        self._status = status
+
+    def raise_for_status(self):
+        if self._status >= 400:
+            raise RuntimeError(f"HTTP {self._status}")
+
+
+@pytest.mark.unit
+def test_discover_egress_ip_first_endpoint_wins(monkeypatch):
+    calls = []
+
+    def fake_get(url, **kw):
+        calls.append(url)
+        return _FakeResp("203.0.113.7\n")
+
+    monkeypatch.setattr(_geo.requests, "get", fake_get)
+    assert discover_egress_ip(SOCKS) == "203.0.113.7"
+    assert len(calls) == 1  # stopped at the first success
+
+
+@pytest.mark.unit
+def test_discover_egress_ip_falls_through_to_next_on_error(monkeypatch):
+    seq = iter([_FakeResp("junk-not-an-ip"), _FakeResp("198.51.100.42")])
+
+    def fake_get(url, **kw):
+        return next(seq)
+
+    monkeypatch.setattr(_geo.requests, "get", fake_get)
+    assert discover_egress_ip(HTTP) == "198.51.100.42"
+
+
+@pytest.mark.unit
+def test_discover_egress_ip_all_fail_raises(monkeypatch):
+    def fake_get(url, **kw):
+        raise OSError("connection refused")
+
+    monkeypatch.setattr(_geo.requests, "get", fake_get)
+    with pytest.raises(GeoTimezoneError):
+        discover_egress_ip(SOCKS)
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  ip_to_timezone — mocked mmdb reader
+# ──────────────────────────────────────────────────────────────────────
+class _FakeReader:
+    def __init__(self, record):
+        self._record = record
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *a):
+        return False
+
+    def get(self, ip):
+        return self._record
+
+
+def _install_fake_maxminddb(monkeypatch, record):
+    mod = types.ModuleType("maxminddb")
+    mod.open_database = lambda path: _FakeReader(record)
+    monkeypatch.setitem(sys.modules, "maxminddb", mod)
+
+
+@pytest.mark.unit
+def test_ip_to_timezone_reads_location_time_zone(monkeypatch):
+    _install_fake_maxminddb(monkeypatch, {"location": {"time_zone": "Europe/Rome"}})
+    assert ip_to_timezone("1.2.3.4", "x.mmdb") == "Europe/Rome"
+
+
+@pytest.mark.unit
+def test_ip_to_timezone_ip_absent_raises(monkeypatch):
+    _install_fake_maxminddb(monkeypatch, None)
+    with pytest.raises(GeoTimezoneError):
+        ip_to_timezone("1.2.3.4", "x.mmdb")
+
+
+@pytest.mark.unit
+def test_ip_to_timezone_missing_zone_raises(monkeypatch):
+    _install_fake_maxminddb(monkeypatch, {"location": {}})
+    with pytest.raises(GeoTimezoneError):
+        ip_to_timezone("1.2.3.4", "x.mmdb")
+
+
+@pytest.mark.unit
+def test_ip_to_timezone_invalid_iana_raises(monkeypatch):
+    _install_fake_maxminddb(monkeypatch, {"location": {"time_zone": "Not/AZone"}})
+    with pytest.raises(GeoTimezoneError):
+        ip_to_timezone("1.2.3.4", "x.mmdb")
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  resolve_session_timezone — the precedence policy
+# ──────────────────────────────────────────────────────────────────────
+@pytest.fixture
+def stub_egress(monkeypatch):
+    """Make egress resolution deterministic + offline; record if it ran."""
+    state = {"called": False}
+
+    def fake_discover(proxy, **kw):
+        state["called"] = True
+        return "203.0.113.7"
+
+    monkeypatch.setattr(_geo, "discover_egress_ip", fake_discover)
+    monkeypatch.setattr(_geo, "ip_to_timezone", lambda ip, mmdb: "America/New_York")
+    # ensure_geoip_mmdb is imported from .download at call time
+    import invisible_playwright.download as dl
+
+    monkeypatch.setattr(dl, "ensure_geoip_mmdb", lambda *a, **k: "fake.mmdb")
+    return state
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize("sentinel", ["host", "local", "HOST", "Local"])
+def test_resolve_host_sentinel_forces_host_tz(sentinel, stub_egress):
+    # Even with a proxy set, "host"/"local" force the host TZ and never resolve.
+    assert resolve_session_timezone(sentinel, SOCKS) == ""
+    assert stub_egress["called"] is False
+
+
+@pytest.mark.unit
+def test_resolve_explicit_iana_wins_over_proxy(stub_egress):
+    assert resolve_session_timezone("Asia/Tokyo", SOCKS) == "Asia/Tokyo"
+    assert stub_egress["called"] is False  # no resolution when explicit
+
+
+@pytest.mark.unit
+def test_resolve_empty_no_proxy_is_host(stub_egress):
+    assert resolve_session_timezone("", None) == ""
+    assert stub_egress["called"] is False
+
+
+@pytest.mark.unit
+def test_resolve_auto_no_proxy_is_host(stub_egress):
+    assert resolve_session_timezone("auto", None) == ""
+    assert stub_egress["called"] is False
+
+
+@pytest.mark.unit
+def test_resolve_empty_with_proxy_defaults_to_auto(stub_egress):
+    # NEW default: a proxy with no timezone auto-resolves from the egress.
+    assert resolve_session_timezone("", SOCKS) == "America/New_York"
+    assert stub_egress["called"] is True
+
+
+@pytest.mark.unit
+def test_resolve_auto_with_proxy_resolves(stub_egress):
+    assert resolve_session_timezone("auto", HTTP) == "America/New_York"
+    assert stub_egress["called"] is True
+
+
+@pytest.mark.unit
+def test_resolve_direct_proxy_treated_as_no_proxy(stub_egress):
+    assert resolve_session_timezone("auto", {"server": "direct://"}) == ""
+    assert stub_egress["called"] is False
+
+
+@pytest.mark.unit
+def test_resolve_fail_early_propagates(monkeypatch):
+    # With a proxy set, a discovery failure must raise — never silent host TZ.
+    def boom(proxy, **kw):
+        raise GeoTimezoneError("no egress")
+
+    monkeypatch.setattr(_geo, "discover_egress_ip", boom)
+    with pytest.raises(GeoTimezoneError):
+        resolve_session_timezone("auto", SOCKS)