diff --git a/src/mcp/server/auth/routes.py b/src/mcp/server/auth/routes.py
index 08f735f36..9a10ac57f 100644
--- a/src/mcp/server/auth/routes.py
+++ b/src/mcp/server/auth/routes.py
@@ -31,19 +31,15 @@ def validate_issuer_url(url: AnyHttpUrl):
         ValueError: If the issuer URL is invalid
     """
 
-    # RFC 8414 requires HTTPS, but we allow localhost HTTP for testing
-    if (
-        url.scheme != "https"
-        and url.host != "localhost"
-        and (url.host is not None and not url.host.startswith("127.0.0.1"))
-    ):
-        raise ValueError("Issuer URL must be HTTPS")  # pragma: no cover
+    # RFC 8414 requires HTTPS, but we allow loopback/localhost HTTP for testing
+    if url.scheme != "https" and url.host not in ("localhost", "127.0.0.1", "[::1]"):
+        raise ValueError("Issuer URL must be HTTPS")
 
     # No fragments or query parameters allowed
     if url.fragment:
-        raise ValueError("Issuer URL must not have a fragment")  # pragma: no cover
+        raise ValueError("Issuer URL must not have a fragment")
     if url.query:
-        raise ValueError("Issuer URL must not have a query string")  # pragma: no cover
+        raise ValueError("Issuer URL must not have a query string")
 
 
 AUTHORIZATION_PATH = "/authorize"
diff --git a/tests/server/auth/test_routes.py b/tests/server/auth/test_routes.py
new file mode 100644
index 000000000..3d13b5ba5
--- /dev/null
+++ b/tests/server/auth/test_routes.py
@@ -0,0 +1,47 @@
+import pytest
+from pydantic import AnyHttpUrl
+
+from mcp.server.auth.routes import validate_issuer_url
+
+
+def test_validate_issuer_url_https_allowed():
+    validate_issuer_url(AnyHttpUrl("https://example.com/path"))
+
+
+def test_validate_issuer_url_http_localhost_allowed():
+    validate_issuer_url(AnyHttpUrl("http://localhost:8080/path"))
+
+
+def test_validate_issuer_url_http_127_0_0_1_allowed():
+    validate_issuer_url(AnyHttpUrl("http://127.0.0.1:8080/path"))
+
+
+def test_validate_issuer_url_http_ipv6_loopback_allowed():
+    validate_issuer_url(AnyHttpUrl("http://[::1]:8080/path"))
+
+
+def test_validate_issuer_url_http_non_loopback_rejected():
+    with pytest.raises(ValueError, match="Issuer URL must be HTTPS"):
+        validate_issuer_url(AnyHttpUrl("http://evil.com/path"))
+
+
+def test_validate_issuer_url_http_127_prefix_domain_rejected():
+    """A domain like 127.0.0.1.evil.com is not loopback."""
+    with pytest.raises(ValueError, match="Issuer URL must be HTTPS"):
+        validate_issuer_url(AnyHttpUrl("http://127.0.0.1.evil.com/path"))
+
+
+def test_validate_issuer_url_http_127_prefix_subdomain_rejected():
+    """A domain like 127.0.0.1something.example.com is not loopback."""
+    with pytest.raises(ValueError, match="Issuer URL must be HTTPS"):
+        validate_issuer_url(AnyHttpUrl("http://127.0.0.1something.example.com/path"))
+
+
+def test_validate_issuer_url_fragment_rejected():
+    with pytest.raises(ValueError, match="fragment"):
+        validate_issuer_url(AnyHttpUrl("https://example.com/path#frag"))
+
+
+def test_validate_issuer_url_query_rejected():
+    with pytest.raises(ValueError, match="query"):
+        validate_issuer_url(AnyHttpUrl("https://example.com/path?q=1"))