diff --git a/sdks/implementations/python/src/stack_auth/_token_store.py b/sdks/implementations/python/src/stack_auth/_token_store.py
index f9dbb9dfb..1f2415a00 100644
--- a/sdks/implementations/python/src/stack_auth/_token_store.py
+++ b/sdks/implementations/python/src/stack_auth/_token_store.py
@@ -197,8 +197,9 @@ def resolve_token_store(init: TokenStoreInit, project_id: str) -> TokenStore | N
             access_token=init.get("access_token"),
             refresh_token=init.get("refresh_token"),
         )
-    # Must be RequestLike (has .headers attribute)
-    return RequestTokenStore(init)  # type: ignore[arg-type]
+    if isinstance(init, RequestLike):
+        return RequestTokenStore(init)
+    raise TypeError(f"Invalid token store initializer: {type(init)}")
 
 
 # ---------------------------------------------------------------------------
diff --git a/sdks/implementations/python/src/stack_auth/_types.py b/sdks/implementations/python/src/stack_auth/_types.py
index db6bb5fd3..601140301 100644
--- a/sdks/implementations/python/src/stack_auth/_types.py
+++ b/sdks/implementations/python/src/stack_auth/_types.py
@@ -1,8 +1,9 @@
 from __future__ import annotations
 
-from typing import Mapping, Protocol
+from typing import Mapping, Protocol, runtime_checkable
 
 
+@runtime_checkable
 class RequestLike(Protocol):
     @property
     def headers(self) -> Mapping[str, str]: ...
diff --git a/sdks/implementations/python/tests/test_token_store.py b/sdks/implementations/python/tests/test_token_store.py
index dfa566f9c..5539fa42c 100644
--- a/sdks/implementations/python/tests/test_token_store.py
+++ b/sdks/implementations/python/tests/test_token_store.py
@@ -215,6 +215,39 @@ class TestTokenStoreRegistry:
     def test_resolve_none(self) -> None:
         assert resolve_token_store(None, "proj") is None
 
+    def test_resolve_raises_type_error_for_invalid_input(self) -> None:
+        with pytest.raises(TypeError, match="Invalid token store initializer"):
+            resolve_token_store(12345, "proj")  # type: ignore[arg-type]
+
+
+# ---------------------------------------------------------------------------
+# RequestLike runtime_checkable isinstance checks
+# ---------------------------------------------------------------------------
+
+class TestRequestLikeProtocol:
+    def test_isinstance_works_on_conforming_object(self) -> None:
+        """RequestLike should be runtime_checkable so isinstance works."""
+        from stack_auth._types import RequestLike
+
+        request = _FakeRequest({"x-stack-auth": "{}"})
+        assert isinstance(request, RequestLike)
+
+    def test_isinstance_rejects_non_conforming_object(self) -> None:
+        """Objects without a .headers property should not pass isinstance."""
+        from stack_auth._types import RequestLike
+
+        assert not isinstance("a string", RequestLike)
+        assert not isinstance(42, RequestLike)
+        assert not isinstance({}, RequestLike)
+
+    def test_resolve_token_store_returns_request_token_store_for_request_like(self) -> None:
+        """resolve_token_store should detect RequestLike via isinstance and return RequestTokenStore."""
+        header_value = json.dumps({"accessToken": "at", "refreshToken": "rt"})
+        request = _FakeRequest({"x-stack-auth": header_value})
+        store = resolve_token_store(request, "proj")
+        assert isinstance(store, RequestTokenStore)
+        assert store.get_stored_access_token() == "at"
+
 
 # ---------------------------------------------------------------------------
 # Helper functions: _is_fresh_enough, _is_expired, _decode_jwt_payload