Add FASTMCP_TRANSPORT setting for default transport selection (#1796)

Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Co-authored-by: Joris Vandermeersch <joris.vandermeersch@vrt.be>

Author: 3 people

src/fastmcp/server/mixins/transport.py

@@ -53,7 +53,7 @@ async def run_async(
         if show_banner is None:
             show_banner = fastmcp.settings.show_server_banner
         if transport is None:
-            transport = "stdio"
+            transport = fastmcp.settings.transport
         if transport not in {"stdio", "http", "sse", "streamable-http"}:
             raise ValueError(f"Unknown transport: {transport}")
 

src/fastmcp/settings.py

@@ -227,6 +227,9 @@ def normalize_log_level(cls, v):
         ),
     ] = None
 
+    # Transport settings
+    transport: Literal["stdio", "http", "sse", "streamable-http"] = "stdio"
+
     # HTTP settings
     host: str = "127.0.0.1"
     port: int = 8000

tests/utilities/test_tests.py

@@ -1,4 +1,7 @@
+from unittest.mock import AsyncMock, patch
+
 import fastmcp
+from fastmcp import FastMCP
 from fastmcp.utilities.tests import temporary_settings
 
 
@@ -8,3 +11,31 @@ def test_temporary_settings(self):
         with temporary_settings(log_level="ERROR"):
             assert fastmcp.settings.log_level == "ERROR"
         assert fastmcp.settings.log_level == "DEBUG"
+
+
+class TestTransportSetting:
+    def test_transport_default_is_stdio(self):
+        assert fastmcp.settings.transport == "stdio"
+
+    def test_transport_setting_can_be_changed(self):
+        with temporary_settings(transport="http"):
+            assert fastmcp.settings.transport == "http"
+        assert fastmcp.settings.transport == "stdio"
+
+    async def test_run_async_uses_transport_setting(self):
+        mcp = FastMCP("test")
+        with temporary_settings(transport="http"):
+            with patch.object(
+                mcp, "run_http_async", new_callable=AsyncMock
+            ) as mock_http:
+                await mcp.run_async()
+                mock_http.assert_called_once()
+
+    async def test_run_async_explicit_transport_overrides_setting(self):
+        mcp = FastMCP("test")
+        with temporary_settings(transport="http"):
+            with patch.object(
+                mcp, "run_stdio_async", new_callable=AsyncMock
+            ) as mock_stdio:
+                await mcp.run_async(transport="stdio")
+                mock_stdio.assert_called_once()