fix(middleware): Add handling for Middleware wrapped app in FastAPI

Author: Punit Maheshwari

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 > Use [this search for a list of all CHANGELOG.md files in this repo](https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-python-contrib+path%3A**%2FCHANGELOG.md&type=code).
 
 ## Unreleased
+- `opentelemetry-instrumentation-fastapi` Support for Middleware Wrapped FastAPI Application [#4041](https://github.com/open-telemetry/opentelemetry-python-contrib/issues/4031)
 
 ## Version 1.39.0/0.60b0 (2025-12-03)
 

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -265,6 +265,8 @@ def instrument_app(
             http_capture_headers_sanitize_fields: Optional list of HTTP headers to sanitize.
             exclude_spans: Optionally exclude HTTP `send` and/or `receive` spans from the trace.
         """
+        # unwraps any middleware to get to the FastAPI or Starlette app
+        app = _unwrap_middleware(app)
         if not hasattr(app, "_is_instrumented_by_opentelemetry"):
             app._is_instrumented_by_opentelemetry = False
 
@@ -391,6 +393,12 @@ async def __call__(
                     app=otel_middleware,
                 )
 
+            # add check if the app object has build_middleware_stack method
+            if not hasattr(app, "build_middleware_stack"):
+                _logger.error(
+                    "Skipping FastAPI instrumentation due to missing build_middleware_stack method on app object."
+                )
+                return
             app._original_build_middleware_stack = app.build_middleware_stack
             app.build_middleware_stack = types.MethodType(
                 functools.wraps(app.build_middleware_stack)(
@@ -409,6 +417,9 @@ async def __call__(
 
     @staticmethod
     def uninstrument_app(app: fastapi.FastAPI):
+        # Unwraps any middleware to get to the FastAPI or Starlette app
+        app = _unwrap_middleware(app)
+
         original_build_middleware_stack = getattr(
             app, "_original_build_middleware_stack", None
         )
@@ -514,3 +525,17 @@ def _get_default_span_details(scope):
     else:  # fallback
         span_name = method
     return span_name, attributes
+
+
+def _unwrap_middleware(app):
+    """
+    Unwraps the middleware stack to find the underlying FastAPI or Starlette app.
+
+    Args:
+        app: The ASGI application potentially wrapped in middleware.
+    Returns:
+        The unwrapped FastAPI or Starlette application.
+    """
+    while hasattr(app, "app"):
+        app = app.app
+    return app

instrumentation/opentelemetry-instrumentation-fastapi/tests/test_fastapi_instrumentation.py

@@ -25,6 +25,7 @@
 
 import fastapi
 import pytest
+from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.httpsredirect import HTTPSRedirectMiddleware
 from fastapi.responses import JSONResponse, PlainTextResponse
 from fastapi.routing import APIRoute
@@ -1487,6 +1488,38 @@ def test_mark_span_internal_in_presence_of_span_from_other_framework(self):
         )
 
 
+class TestMiddlewareWrappedApplication(TestBase):
+    def setUp(self):
+        super().setUp()
+        self.fastapi_app = fastapi.FastAPI()
+
+        @self.fastapi_app.get("/foobar")
+        async def _():
+            return {"message": "hello world"}
+
+        self.app = CORSMiddleware(self.fastapi_app, allow_origins=["*"])
+
+        otel_fastapi.FastAPIInstrumentor().instrument_app(self.app)
+        self.client = TestClient(self.app)
+
+    def tearDown(self) -> None:
+        super().tearDown()
+        with self.disable_logging():
+            otel_fastapi.FastAPIInstrumentor().uninstrument_app(self.app)
+
+    def test_instrumentation_with_existing_middleware(self):
+        resp = self.client.get("/foobar")
+        self.assertEqual(200, resp.status_code)
+
+        span_list = self.memory_exporter.get_finished_spans()
+        self.assertEqual(len(span_list), 3)
+
+        server_span = [
+            span for span in span_list if span.kind == trace.SpanKind.SERVER
+        ][0]
+        self.assertEqual(server_span.name, "GET /foobar")
+
+
 class TestFastAPIGarbageCollection(unittest.TestCase):
     def test_fastapi_app_is_collected_after_instrument(self):
         app = fastapi.FastAPI()