diff --git a/fastapi_utils/injected.py b/fastapi_utils/injected.py
new file mode 100644
index 00000000..fd68d1a1
--- /dev/null
+++ b/fastapi_utils/injected.py
@@ -0,0 +1,114 @@
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Annotated, Any, NoReturn, TypeVar, overload
+
+from fastapi import Depends, FastAPI
+from fastapi.dependencies.models import Dependant
+from fastapi.routing import APIRoute
+
+_AnyType = TypeVar("_AnyType")
+
+
+class _DependsPlaceholder:
+    """A placeholder for dependencies that will be bound in main.py / conftest.py
+    because FastAPI greedily resolves dependencies when routes are added.
+
+    It compares/hashes as the type it wraps so that FastAPI.dependency_overrides works.
+
+    The idea is that we can bind dependencies in main.py / conftest.py and then
+    check that every dependency was bound before starting the app so that we fail
+    at startup instead of at runtime or import time.
+    """
+
+    def __init__(self, t: Any) -> None:
+        self.t = t
+
+    def __eq__(self, __value: object) -> bool:
+        return __value == self.t
+
+    def __hash__(self) -> int:
+        return hash(self.t)
+
+    async def __call__(self) -> NoReturn:  # pragma: no cover
+        raise RuntimeError(
+            f"No dependency override found for {self.t}."
+            " Did you add it to app.dependency_overrides in main.create_app?"
+        )
+
+
+class InjectedTracker:
+    def __class_getitem__(cls, item: _AnyType) -> _AnyType:
+        return Annotated[item, Depends(_DependsPlaceholder(item))]  # type: ignore
+
+
+if TYPE_CHECKING:
+    # The Injected type is used to mark types that must be overridden by the FastAPI app by way of
+    # app.dependency_overrides[MyType] = wrap_dep_in_async_closure(...) in logfire_backend.api.create_app
+    Injected = Annotated[
+        _AnyType, ...
+    ]  # Injected[T] will be recognized by type checkers as T
+    """A dependency that is injected by FastAPI's dependency injection system.
+    These are all bound in `main.create_app` so that they can be overridden in tests.
+    """
+else:
+    Injected = InjectedTracker
+
+
+def validate_injections(app: FastAPI) -> None:
+    """Validate that all Injected types have been updated in app.dependency_overrides"""
+    dependencies: list[Dependant] = []
+    for route in app.routes:
+        if isinstance(route, APIRoute):
+            dependencies.append(route.dependant)
+            if route.dependency_overrides_provider is not app:
+                raise RuntimeError(
+                    "Route dependency overrides must be provided by the app itself"
+                )
+    # flatten
+    stack = dependencies
+    dependencies = []
+    while stack:
+        dep = stack.pop()
+        for subdep in dep.dependencies:
+            stack.append(subdep)
+        dependencies.append(dep)
+
+    calls = {
+        dep.call.t for dep in dependencies if isinstance(dep.call, _DependsPlaceholder)
+    }
+    overrides = set(app.dependency_overrides.keys())
+
+    missing_injections = list(calls - overrides)
+
+    if missing_injections:
+        raise RuntimeError(
+            f"Missing dependency overrides for {missing_injections}."
+            " Did you update `app.dependency_overrides` in `main.create_app`?"
+        )
+
+
+_T = TypeVar("_T")
+
+def wrap_dep_in_async_closure(dep: Any) -> Any:
+    """FastAPI runs sync deps in a thread so this creates an async lambda to wrap them."""
+
+    async def wrapper() -> Any:
+        return dep
+
+    return wrapper
+
+
+@overload
+def bind(app: FastAPI, dep_type: Injected[_T], dep: _T) -> None:
+    ...
+
+
+@overload
+def bind(app: FastAPI, dep_type: Any, dep: Any) -> None:
+    ...
+
+
+def bind(app: FastAPI, dep_type: type[_T], dep: _T) -> None:
+    """Bind a dependency to the app for use in FastAPI's dependency injection system."""
+    app.dependency_overrides[dep_type] = wrap_dep_in_async_closure(dep)
diff --git a/tests/test_injected.py b/tests/test_injected.py
new file mode 100644
index 00000000..b8dd1e0c
--- /dev/null
+++ b/tests/test_injected.py
@@ -0,0 +1,69 @@
+import pytest
+from fastapi import APIRouter, FastAPI
+from fastapi.testclient import TestClient
+
+from fastapi_utils.injected import Injected, bind, validate_injections
+
+
+class Thing:
+    def __init__(self, x: int) -> None:
+        self.x = x
+
+
+def test_injection() -> None:
+    app = FastAPI()
+
+    @app.get('/')
+    async def get_root(thing: Injected[Thing]) -> int:
+        return thing.x
+
+    bind(app, Thing, Thing(123))
+
+    validate_injections(app)
+
+    client = TestClient(app)
+
+    assert client.get('/').json() == 123
+
+
+def test_injectino_router() -> None:
+    router = APIRouter()
+
+    @router.get('/')
+    async def get_root(thing: Injected[Thing]) -> int:
+        return thing.x
+
+    app = FastAPI()
+    app.include_router(router)
+    bind(app, Thing, Thing(123))
+
+    validate_injections(app)
+
+    client = TestClient(app)
+
+    assert client.get('/').json() == 123
+
+
+def test_missing_injection() -> None:
+    app = FastAPI()
+
+    @app.get('/')
+    async def get_root(thing: Injected[Thing]) -> int:
+        return thing.x
+    
+    with pytest.raises(RuntimeError, match='Missing dependency'):
+        validate_injections(app)
+
+
+def test_missing_injections_router() -> None:
+    router = APIRouter()
+
+    @router.get('/')
+    async def get_root(thing: Injected[Thing]) -> int:
+        return thing.x
+
+    app = FastAPI()
+    app.include_router(router)
+    
+    with pytest.raises(RuntimeError, match='Missing dependency'):
+        validate_injections(app)