diff --git a/src/mcp/server/fastmcp/server.py b/src/mcp/server/fastmcp/server.py
index e915a12bfd..9888832268 100644
--- a/src/mcp/server/fastmcp/server.py
+++ b/src/mcp/server/fastmcp/server.py
@@ -1087,6 +1087,13 @@ async def get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -
             raise ValueError(str(e))
 
 
+# `Settings.lifespan` forward-references `FastMCP`, which is defined after `Settings`,
+# so the annotation is unresolved when the class body executes. Rebuild the model now
+# that `FastMCP` exists, otherwise settings sources may fail to resolve the field and
+# pydantic-settings >= 2.15 warns about the incomplete definition on instantiation.
+Settings.model_rebuild()
+
+
 class StreamableHTTPASGIApp:
     """
     ASGI application for Streamable HTTP server transport.
diff --git a/tests/server/fastmcp/test_server.py b/tests/server/fastmcp/test_server.py
index b134489bc5..2f471c90bc 100644
--- a/tests/server/fastmcp/test_server.py
+++ b/tests/server/fastmcp/test_server.py
@@ -183,6 +183,21 @@ async def test_add_resource_decorator_incorrect_usage(self):
             def get_data(x: str) -> str:  # pragma: no cover
                 return f"Data: {x}"
 
+    def test_settings_model_is_fully_defined_after_import(self):
+        """Regression test for #3294: `Settings.lifespan` forward-references `FastMCP`,
+        which is defined later in the module, so `Settings` must be rebuilt once
+        `FastMCP` exists. An unresolved forward reference leaves the model incomplete:
+        settings sources may fail to resolve the field, and pydantic-settings >= 2.15
+        warns (`IncompleteFieldDefinitionWarning`) on every `FastMCP()` construction.
+        """
+        from typing import ForwardRef
+
+        from mcp.server.fastmcp.server import Settings
+
+        assert Settings.__pydantic_complete__ is True
+        lifespan = Settings.model_fields["lifespan"]
+        assert not isinstance(lifespan.annotation, ForwardRef)
+
 
 class TestDnsRebindingProtection:
     """Tests for automatic DNS rebinding protection on localhost."""