diff --git a/hypermedia/fastapi.py b/hypermedia/fastapi.py
index 00afda3..2988bbe 100644
--- a/hypermedia/fastapi.py
+++ b/hypermedia/fastapi.py
@@ -3,8 +3,8 @@
     Any,
     Callable,
     Coroutine,
-    ParamSpec,
     Protocol,
+    TypeAlias,
     TypeVar,
 )
 
@@ -19,29 +19,35 @@
         "Or `uv add hypermedia --extras fastapi`"
     ) from ie
 
-Param = ParamSpec("Param")
-ReturnType = TypeVar("ReturnType")
 
+T = TypeVar("T", bound="Element")
+LazyElement: TypeAlias = Callable[..., Element]
 
-class RequestPartialAndFull(Protocol):
-    """Requires, `request`, `partial` and `full` args on decorated function."""
+
+class PartialHTMXRequest(Protocol):
+    """Requires, `request`, `partial` args on decorated function."""
 
     def __call__(  # noqa: D102
-        self, request: Request, partial: Element, full: Element
+        self,
+        request: Request,
+        partial: Element,
     ) -> Coroutine[Any, Any, None]: ...
 
 
-class RequestAndPartial(Protocol):
-    """Requires, `request` and `partial` args on decorated function."""
+class FullHTMXRequest(Protocol):
+    """Requires, `request`, `partial` and `full` args on decorated function."""
 
     def __call__(  # noqa: D102
-        self, request: Request, partial: Element
+        self,
+        request: Request,
+        partial: Element,
+        full: LazyElement,
     ) -> Coroutine[Any, Any, None]: ...
 
 
 def htmx(
-    func: RequestPartialAndFull | RequestAndPartial,
-) -> Callable[..., str]:
+    func: PartialHTMXRequest | FullHTMXRequest,
+) -> PartialHTMXRequest | FullHTMXRequest:
     """Wrap a FastAPI endpoint, to enable partial and full rendering.
 
     The endpoint function _must_ have a partial render dependency, and
@@ -59,7 +65,7 @@ def htmx(
     @wraps(func)
     async def wrapper(
         *,
-        request: Any,
+        request: Request,
         partial: Element,
         full: None | Callable[..., Element] = None,
     ) -> str:
@@ -77,29 +83,25 @@ async def wrapper(
     return wrapper  # type: ignore
 
 
-def full(
-    func: Callable[Param, ReturnType],
-) -> Callable[Param, Coroutine[Any, Any, Callable[[], ReturnType]]]:
-    """Wrap the full page render dependency and makes it lazy."""
+def full(func: Callable[..., T]) -> Callable[..., Callable[..., T]]:
+    """Mark a function as a full renderer.
+
+    This will prevent the function from being evaluated before it is needed
+    """
 
     @wraps(func)
-    async def wrapper(
-        *args: Param.args,
-        **kwargs: Param.kwargs,
-    ) -> Callable[[], ReturnType]:
-        """Wrap function."""
+    def deferred_renderer(*args: Any, **kwargs: Any) -> Callable[..., T]:
         return lambda: func(*args, **kwargs)
 
-    return wrapper
+    return deferred_renderer
 
 
 def add_htmx_middleware(app: FastAPI) -> None:
-    """Instrument the app with middleware to add Vary: Accept header.
+    """Add middleware to the app that adds the Vary: Accept header.
 
     This allows the browser to cache the responses based on caller,
     which should prevent the browser from caching htmx responses as a full page
     """
-    # Check if we've already instrumented
     if getattr(app.state, "hypermedia_htmx_middleware", False):
         return
 
diff --git a/tests/fastapi/__init__.py b/tests/fastapi/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/fastapi/conftest.py b/tests/fastapi/conftest.py
index 6de3cc8..ed23bc3 100644
--- a/tests/fastapi/conftest.py
+++ b/tests/fastapi/conftest.py
@@ -1,9 +1,13 @@
+from typing import Annotated
+
 import pytest
-from fastapi import FastAPI
+from fastapi import Depends, FastAPI, Request
 from fastapi.responses import HTMLResponse
 from fastapi.testclient import TestClient
 
-from hypermedia.fastapi import add_htmx_middleware
+from hypermedia.fastapi import LazyElement, add_htmx_middleware, full, htmx
+from hypermedia.models import Element
+from tests.fastapi.index import render_index, render_index_partial
 
 
 @pytest.fixture
@@ -14,15 +18,31 @@ def app() -> FastAPI:
     )
 
     @_app.get("/", response_class=HTMLResponse)
-    async def root() -> str:
-        """Root."""
-        return "root"
+    @htmx
+    async def index(
+        request: Request,
+        partial: Annotated[Element, Depends(render_index_partial)],
+        full: Annotated[LazyElement, Depends(full(render_index))],
+    ) -> None:
+        """Return the index page."""
+        pass
+
+    @_app.get("/partial", response_class=HTMLResponse)
+    @htmx
+    async def index_partial(
+        request: Request,
+        partial: Annotated[Element, Depends(render_index_partial)],
+    ) -> None:
+        """Return the index page."""
+        pass
 
     return _app
 
 
 @pytest.fixture
-def client(app: FastAPI) -> TestClient:
+def client(
+    app: FastAPI,
+) -> TestClient:
     """Test client."""
     return TestClient(app)
 
diff --git a/tests/fastapi/index.py b/tests/fastapi/index.py
new file mode 100644
index 0000000..7ada2f7
--- /dev/null
+++ b/tests/fastapi/index.py
@@ -0,0 +1,17 @@
+from typing import Annotated
+
+from fastapi import Depends
+
+from hypermedia import Div, Element
+
+
+def render_index_partial() -> Element:
+    """Return partial HTML."""
+    return Div("partial")
+
+
+def render_index(
+    partial: Annotated[Element, Depends(render_index_partial)],
+) -> Element:
+    """Return full HTML."""
+    return Div("full", partial)
diff --git a/tests/fastapi/test_htmx_decorator.py b/tests/fastapi/test_htmx_decorator.py
new file mode 100644
index 0000000..d671f92
--- /dev/null
+++ b/tests/fastapi/test_htmx_decorator.py
@@ -0,0 +1,80 @@
+from unittest.mock import Mock
+
+from fastapi import FastAPI, status
+from fastapi.testclient import TestClient
+
+from hypermedia import Div
+from tests.fastapi.index import render_index, render_index_partial
+
+
+def test_full_html_returned(
+    client: TestClient,
+) -> None:
+    response = client.get("/")
+    assert response.status_code == status.HTTP_200_OK
+    assert "full" in response.text
+    assert "partial" in response.text
+
+
+def test_only_partial_html_returned(
+    client: TestClient,
+) -> None:
+    response = client.get("/", headers={"HX-Request": "true"})
+    print(response.text)
+    assert response.status_code == status.HTTP_200_OK
+    assert "full" not in response.text
+    assert "partial" in response.text
+
+
+def test_render_index_partial_called(
+    app: FastAPI,
+    client: TestClient,
+) -> None:
+    mock_partial = Mock(return_value=Div("mocked"))
+    app.dependency_overrides[render_index_partial] = lambda: mock_partial()
+
+    client.get("/", headers={"HX-Request": "true"})
+
+    mock_partial.assert_called_once()
+
+
+def test_render_index_not_called(
+    app: FastAPI,
+    client: TestClient,
+) -> None:
+    mock_full = Mock(return_value=Div("mocked"))
+    app.dependency_overrides[render_index] = lambda: mock_full()
+
+    client.get("/", headers={"HX-Request": "true"})
+
+    mock_full.assert_not_called()
+
+
+def test_render_index_partial_called_only_once_on_full(
+    app: FastAPI,
+    client: TestClient,
+) -> None:
+    mock_partial = Mock(return_value=Div("mocked"))
+    app.dependency_overrides[render_index_partial] = lambda: mock_partial()
+
+    client.get("/")
+
+    mock_partial.assert_called_once()
+
+
+def test_render_partial_data_only_when_no_full_available(
+    client: TestClient,
+) -> None:
+    response = client.get("/partial")
+
+    assert "full" not in response.text
+    assert "partial" in response.text
+
+
+def test_render_partial_data_only_when_no_full_available_htmx(
+    client: TestClient,
+) -> None:
+    response = client.get("/partial", headers={"HX-Request": "true"})
+
+    assert "full" not in response.text
+    assert "partial" in response.text