diff --git a/fastapi_startkit/tests/inertia/__init__.py b/fastapi_startkit/tests/inertia/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/fastapi_startkit/tests/inertia/test_inertia_context.py b/fastapi_startkit/tests/inertia/test_inertia_context.py
new file mode 100644
index 00000000..acbafa57
--- /dev/null
+++ b/fastapi_startkit/tests/inertia/test_inertia_context.py
@@ -0,0 +1,85 @@
+"""Tests for Inertia context and middleware header behaviour (task #13).
+
+Covers: current_request ContextVar, X-Inertia header presence,
+non-Inertia passthrough, InertiaResponse.__call__ without middleware.
+"""
+
+import unittest
+from unittest.mock import MagicMock
+
+from fastapi import FastAPI, Request
+from fastapi.testclient import TestClient
+
+from fastapi_startkit.inertia.constant import Header
+from fastapi_startkit.inertia.context import current_request
+from fastapi_startkit.inertia.inertia import Inertia, InertiaResponse
+from fastapi_startkit.inertia.middleware import InertiaMiddleware
+
+
+class TestCurrentRequestContextVar(unittest.TestCase):
+    def test_default_value_is_none(self):
+        token = current_request.set(None)
+        assert current_request.get() is None
+        current_request.reset(token)
+
+    def test_set_and_get_request(self):
+        mock_req = MagicMock(spec=Request)
+        token = current_request.set(mock_req)
+        assert current_request.get() is mock_req
+        current_request.reset(token)
+
+    def test_reset_restores_previous(self):
+        token = current_request.set(None)
+        mock_req = MagicMock(spec=Request)
+        token2 = current_request.set(mock_req)
+        current_request.reset(token2)
+        assert current_request.get() is None
+        current_request.reset(token)
+
+
+class TestInertiaResponseCallWithoutMiddleware(unittest.IsolatedAsyncioTestCase):
+    async def test_call_without_middleware_raises_runtime_error(self):
+        # Ensure context has no request set
+        token = current_request.set(None)
+        try:
+            response = InertiaResponse(component="Test", shared_props={}, props={})
+            with self.assertRaisesRegex(RuntimeError, "InertiaResponse requires InertiaMiddleware"):
+                await response(scope={}, receive=None, send=None)
+        finally:
+            current_request.reset(token)
+
+
+class TestMiddlewareHeaderBehaviour(unittest.TestCase):
+    def setUp(self):
+        self.app = FastAPI()
+        self.app.add_middleware(InertiaMiddleware)
+
+        @self.app.get("/ping")
+        def ping():
+            return {"ok": True}
+
+        self.client = TestClient(self.app)
+        Inertia._instance = None
+
+    def test_non_inertia_request_passes_through(self):
+        response = self.client.get("/ping")
+        assert response.status_code == 200
+        assert response.json() == {"ok": True}
+
+    def test_inertia_request_gets_vary_header(self):
+        response = self.client.get("/ping", headers={Header.INERTIA: "true"})
+        assert response.headers.get("Vary") == Header.INERTIA
+
+    def test_non_inertia_request_gets_vary_header(self):
+        # Vary is set on all responses regardless of X-Inertia presence
+        response = self.client.get("/ping")
+        assert response.headers.get("Vary") == Header.INERTIA
+
+    def test_x_inertia_header_name_constant(self):
+        assert Header.INERTIA == "X-Inertia"
+
+    def test_x_inertia_version_header_name_constant(self):
+        assert Header.INERTIA_VERSION == "X-Inertia-Version"
+
+    def test_x_inertia_location_header_name_constant(self):
+        assert Header.INERTIA_LOCATION == "X-Inertia-Location"
diff --git a/fastapi_startkit/tests/inertia/test_inertia_props.py b/fastapi_startkit/tests/inertia/test_inertia_props.py
new file mode 100644
index 00000000..fb8ad1a6
--- /dev/null
+++ b/fastapi_startkit/tests/inertia/test_inertia_props.py
@@ -0,0 +1,194 @@
+"""Tests for InertiaProps evaluation (task #13).
+
+Covers: OptionalProp (lazy) vs eager props, shared props merging,
+full response structure, and InertiaResponse.with_() builder.
+"""
+
+import json
+import unittest
+from unittest.mock import MagicMock
+
+from fastapi import Request
+
+from fastapi_startkit.inertia.constant import Header
+from fastapi_startkit.inertia.inertia import Inertia, InertiaResponse, OptionalProp, ResponseFactory
+
+
+class TestOptionalProp(unittest.TestCase):
+    def test_optional_prop_calls_callback(self):
+        called = []
+
+        def cb():
+            called.append(True)
+            return "value"
+
+        prop = OptionalProp(cb)
+        result = prop()
+        assert result == "value"
+        assert called
+
+    def test_optional_prop_is_callable(self):
+        prop = OptionalProp(lambda: 42)
+        assert callable(prop)
+
+    def test_optional_prop_wraps_any_callable(self):
+        prop = OptionalProp(lambda: {"key": "val"})
+        assert prop() == {"key": "val"}
+
+
+class TestEagerProps(unittest.IsolatedAsyncioTestCase):
+    def setUp(self):
+        self.request = MagicMock(spec=Request)
+        self.request.headers = {Header.INERTIA: "true"}
+        self.request.url = "http://localhost/page"
+
+    async def test_eager_props_always_included(self):
+        response = InertiaResponse(
+            component="Page",
+            shared_props={},
+            props={"eager": "value"},
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert data["props"]["eager"] == "value"
+
+    async def test_lazy_prop_excluded_from_full_load(self):
+        evaluated = []
+
+        def expensive():
+            evaluated.append(True)
+            return "expensive"
+
+        response = InertiaResponse(
+            component="Page",
+            shared_props={},
+            props={"lazy": OptionalProp(expensive)},
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert "lazy" not in data["props"]
+        assert not evaluated
+
+    async def test_lazy_prop_included_in_partial_reload(self):
+        evaluated = []
+
+        def expensive():
+            evaluated.append(True)
+            return "computed"
+
+        self.request.headers = {
+            Header.INERTIA: "true",
+            Header.INERTIA_PARTIAL_COMPONENT: "Page",
+            "X-Inertia-Partial-Data": "lazy",
+        }
+        response = InertiaResponse(
+            component="Page",
+            shared_props={},
+            props={"lazy": OptionalProp(expensive)},
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert data["props"]["lazy"] == "computed"
+        assert evaluated
+
+
+class TestSharedPropsmerging(unittest.IsolatedAsyncioTestCase):
+    def setUp(self):
+        self.request = MagicMock(spec=Request)
+        self.request.headers = {Header.INERTIA: "true"}
+        self.request.url = "http://localhost/"
+
+    async def test_shared_props_merged_into_response(self):
+        response = InertiaResponse(
+            component="Dashboard",
+            shared_props={"auth": {"user": "alice"}},
+            props={"count": 5},
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert data["props"]["auth"] == {"user": "alice"}
+        assert data["props"]["count"] == 5
+
+    async def test_page_props_override_shared_on_conflict(self):
+        response = InertiaResponse(
+            component="Dashboard",
+            shared_props={"key": "shared"},
+            props={"key": "page"},
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        # page props win because all_props = {**shared, **props}
+        assert data["props"]["key"] == "page"
+
+    async def test_full_response_structure(self):
+        response = InertiaResponse(
+            component="Users/Index",
+            shared_props={"app": "Startkit"},
+            props={"users": []},
+            version="v1.0",
+        )
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert "component" in data
+        assert "props" in data
+        assert "url" in data
+        assert "version" in data
+        assert data["component"] == "Users/Index"
+        assert data["version"] == "v1.0"
+
+
+class TestInertiaResponseBuilder(unittest.IsolatedAsyncioTestCase):
+    def setUp(self):
+        self.request = MagicMock(spec=Request)
+        self.request.headers = {Header.INERTIA: "true"}
+        self.request.url = "http://localhost/users"
+
+    async def test_with_adds_props(self):
+        response = InertiaResponse(
+            component="Users",
+            shared_props={},
+            props={"initial": True},
+        )
+        response.with_("extra", "data")
+
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert data["props"]["extra"] == "data"
+        assert data["props"]["initial"] is True
+
+    async def test_with_dict_merges_props(self):
+        response = InertiaResponse(component="Users", shared_props={}, props={})
+        response.with_({"a": 1, "b": 2})
+
+        actual = await response.to_response(self.request)
+        data = json.loads(actual.body)
+        assert data["props"]["a"] == 1
+        assert data["props"]["b"] == 2
+
+    async def test_with_root_view_changes_view(self):
+        response = InertiaResponse(component="Page", shared_props={}, props={})
+        result = response.with_root_view("custom.html")
+        assert result.root_view == "custom.html"
+
+
+class TestResponseFactory(unittest.TestCase):
+    def setUp(self):
+        Inertia._instance = None
+
+    def test_factory_renders_inertia_response(self):
+        factory = ResponseFactory()
+        factory.share("global", True)
+        resp = factory.render("Home", {"title": "Hello"})
+        assert isinstance(resp, InertiaResponse)
+        assert resp.component == "Home"
+        assert resp.props == {"title": "Hello"}
+        assert resp.shared_props == {"global": True}
+
+    def test_callable_version_resolved(self):
+        factory = ResponseFactory()
+        factory.set_version(lambda: "3.0")
+        assert factory.get_version() == "3.0"
+
+    def test_none_version_returns_none(self):
+        factory = ResponseFactory()
+        assert factory.get_version() is None