fix: FastAPI TestClient compatibility and lifespan re-initialization (#3736)

tests/server/test_fastapi_testclient_compat.py

@@ -0,0 +1,57 @@
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from fastmcp import FastMCP
+
+
+def test_fastapi_testclient_multiple_runs():
+    """Test that TestClient can be used multiple times with a mounted FastMCP app.
+
+    This verifies that the StreamableHTTPSessionManager is correctly recreated
+    for each lifespan cycle.
+    """
+    mcp = FastMCP("test")
+    mcp_app = mcp.http_app(path="/mcp")
+
+    @mcp.tool
+    def add(a: int, b: int) -> int:
+        return a + b
+
+    @asynccontextmanager
+    async def combined_lifespan(app: FastAPI):
+        # Trigger the sub-app's lifespan
+        async with mcp_app.router.lifespan_context(mcp_app):
+            yield
+
+    app = FastAPI(lifespan=combined_lifespan)
+    app.mount("/analytics", mcp_app)
+
+    # First test run
+    with TestClient(app) as client:
+        # We use analytics prefix since it's mounted there
+        client.get("/analytics/mcp")  # Would raise RuntimeError before fix
+
+    # Second test run - this would fail before the fix
+    with TestClient(app) as client:
+        client.get("/analytics/mcp")
+
+
+def test_fastapi_testclient_nested_lifespan():
+    """Test that TestClient works with custom combined lifespans and multiple iterations."""
+    mcp = FastMCP("test")
+    mcp_app = mcp.http_app(path="/mcp")
+
+    @asynccontextmanager
+    async def combined_lifespan(app: FastAPI):
+        async with mcp_app.router.lifespan_context(mcp_app):
+            yield
+
+    app = FastAPI(lifespan=combined_lifespan)
+    app.mount("/analytics", mcp_app)
+
+    # Multiple runs with custom lifespan
+    for _ in range(3):
+        with TestClient(app) as client:
+            client.get("/analytics/mcp")