From 1b637522d166d40ad41346997d2fda74bc886934 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 30 Dec 2025 17:51:57 -0500
Subject: [PATCH] Fix base_url fallback when url is not set (#2776) (#2782)

Co-authored-by: Taisei Mima <bhbstar.me@gmail.com>
fix for httpx.URL("") being truthy but stringifying to empty string.
---
 src/fastmcp/server/openapi/components.py   |  6 ++--
 tests/server/openapi/test_comprehensive.py | 32 ++++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/src/fastmcp/server/openapi/components.py b/src/fastmcp/server/openapi/components.py
index 40577cfa7..496a23327 100644
--- a/src/fastmcp/server/openapi/components.py
+++ b/src/fastmcp/server/openapi/components.py
@@ -64,10 +64,8 @@ class OpenAPITool(Tool):
         try:
             # Get base URL from client
             base_url = (
-                str(self._client.base_url)
-                if hasattr(self._client, "base_url") and self._client.base_url
-                else "http://localhost"
-            )
+                str(self._client.base_url) if hasattr(self._client, "base_url") else ""
+            ) or "http://localhost"
 
             # Get Headers from client
             cli_headers = (
diff --git a/tests/server/openapi/test_comprehensive.py b/tests/server/openapi/test_comprehensive.py
index e7602c06c..777cd54c8 100644
--- a/tests/server/openapi/test_comprehensive.py
+++ b/tests/server/openapi/test_comprehensive.py
@@ -498,6 +498,38 @@ class TestOpenAPIComprehensive:
             assert "123" in str(request.url)
             assert "users/123" in str(request.url)
 
+    async def test_request_uses_localhost_fallback_when_no_base_url(
+        self, comprehensive_openapi_spec
+    ):
+        """Test that tool uses localhost fallback when client has no base_url."""
+        mock_client = Mock(spec=httpx.AsyncClient)
+        mock_client.base_url = httpx.URL("")  # Empty URL, same as httpx default
+        mock_client.headers = None
+
+        mock_response = Mock(spec=Response)
+        mock_response.status_code = 200
+        mock_response.json.return_value = {
+            "id": 123,
+            "name": "Test User",
+            "email": "test@example.com",
+        }
+        mock_response.raise_for_status = Mock()
+
+        mock_client.send = AsyncMock(return_value=mock_response)
+
+        server = FastMCPOpenAPI(
+            openapi_spec=comprehensive_openapi_spec,
+            client=mock_client,
+        )
+
+        async with Client(server) as mcp_client:
+            await mcp_client.call_tool("get_user", {"id": 123})
+
+        # Verify request was made to localhost fallback
+        mock_client.send.assert_called_once()
+        request = mock_client.send.call_args[0][0]
+        assert str(request.url).startswith("http://localhost")
+
     async def test_complex_request_with_body_and_parameters(
         self, comprehensive_openapi_spec
     ):