diff --git a/python/packages/devui/agent_framework_devui/_server.py b/python/packages/devui/agent_framework_devui/_server.py
index bc94dee9a21..aa4248967c7 100644
--- a/python/packages/devui/agent_framework_devui/_server.py
+++ b/python/packages/devui/agent_framework_devui/_server.py
@@ -137,6 +137,24 @@ def _loopback_allowed_hosts(self) -> frozenset[str] | None:
             return None
         return self._LOOPBACK_HOSTS
 
+    @staticmethod
+    def _hostname_from_host_header(host_header: str) -> str:
+        """Return the host name part of a Host header, lowercased and without any port.
+
+        IPv6 literals keep their brackets, so "[::1]:8080" and "[::1]" both yield "[::1]".
+        Splitting on the first colon instead would turn both into "[", which no allowlist
+        member matches.
+        """
+        host = host_header.strip().lower()
+        if host.startswith("["):
+            # IPv6 literal, optionally followed by ":port".
+            end = host.find("]")
+            return host[: end + 1] if end != -1 else host
+        if host.count(":") == 1:
+            # "example.com:8080". A bare IPv6 literal holds more than one colon and has no port.
+            return host.rsplit(":", 1)[0]
+        return host
+
     @classmethod
     def _resolve_auth_token(cls, host: str, auth_enabled: bool, auth_token: str | None) -> str | None:
         """Resolve the active Bearer token. Returns None when auth is disabled."""
@@ -472,7 +490,7 @@ async def auth_middleware(request: Request, call_next: Callable[[Request], Await
             @app.middleware("http")
             async def host_header_middleware(request: Request, call_next: Callable[[Request], Awaitable[Any]]) -> Any:
                 host_header = request.headers.get("host", "")
-                hostname = host_header.split(":", 1)[0].lower()
+                hostname = self._hostname_from_host_header(host_header)
                 if hostname and hostname not in expected_hosts:
                     return JSONResponse(
                         status_code=400,
diff --git a/python/packages/devui/tests/devui/test_server.py b/python/packages/devui/tests/devui/test_server.py
index 56055fd137a..c4989491f64 100644
--- a/python/packages/devui/tests/devui/test_server.py
+++ b/python/packages/devui/tests/devui/test_server.py
@@ -883,6 +883,46 @@ def test_loopback_bind_rejects_non_allowlisted_host_header(monkeypatch):
         assert ok_localhost.status_code == 200
 
 
+@pytest.mark.parametrize(
+    ("host_header", "expected_hostname"),
+    [
+        pytest.param("127.0.0.1", "127.0.0.1", id="ipv4"),
+        pytest.param("127.0.0.1:8080", "127.0.0.1", id="ipv4-with-port"),
+        pytest.param("localhost:8080", "localhost", id="hostname-with-port"),
+        pytest.param("LOCALHOST:8080", "localhost", id="hostname-is-lowercased"),
+        pytest.param("[::1]", "[::1]", id="ipv6-literal"),
+        pytest.param("[::1]:8080", "[::1]", id="ipv6-literal-with-port"),
+        pytest.param("[::FFFF:127.0.0.1]:8080", "[::ffff:127.0.0.1]", id="ipv6-mapped-ipv4"),
+        pytest.param("::1", "::1", id="bare-ipv6-has-no-port-to-strip"),
+        pytest.param("", "", id="empty"),
+    ],
+)
+def test_hostname_from_host_header(host_header: str, expected_hostname: str) -> None:
+    """The Host header parser strips a port only when the host is not an IPv6 literal."""
+    assert DevServer._hostname_from_host_header(host_header) == expected_hostname
+
+
+@pytest.mark.parametrize("bind", ["127.0.0.1", "::1", "[::1]"])
+def test_loopback_bind_accepts_ipv6_loopback_host_header(monkeypatch, bind: str) -> None:
+    """A browser addresses an IPv6 loopback bind as `[::1]` or `[::1]:<port>`.
+
+    Those two forms used to be reduced to `"["` by a split on the first colon, so every
+    request to an IPv6 loopback bind was answered with `host_not_allowed`.
+    """
+    monkeypatch.delenv("DEVUI_AUTH_TOKEN", raising=False)
+
+    server = DevServer(host=bind, auth_token="s3cret")
+    app = server.get_app()
+
+    with TestClient(app, base_url="http://127.0.0.1") as client:
+        for host_header in ("[::1]", "[::1]:8080"):
+            response = client.get("/health", headers={"Host": host_header})
+            assert response.status_code == 200, f"bind {bind} rejected Host {host_header!r}"
+
+        rebound = client.get("/health", headers={"Host": "evil.example:8080"})
+        assert rebound.status_code == 400
+
+
 def test_serve_defaults_to_auth_enabled():
     """`serve()`'s public signature must default to auth_enabled=True."""
     sig = inspect.signature(agent_framework_devui.serve)