diff --git a/packages/runtime-sdk/src/workers/utils.py b/packages/runtime-sdk/src/workers/utils.py
index 6b0fc410..ea4018d2 100644
--- a/packages/runtime-sdk/src/workers/utils.py
+++ b/packages/runtime-sdk/src/workers/utils.py
@@ -1,4 +1,5 @@
-from collections.abc import Iterator, Sequence
+import http.client
+from collections.abc import Iterator, Mapping, Sequence
 from contextlib import contextmanager
 from typing import Any
 
@@ -201,22 +202,19 @@ def _to_js_headers(headers):
         raise TypeError("Received unexpected type for headers argument")
 
 
+class HTTPMessageMapping(http.client.HTTPMessage, Mapping):
+    pass
+
+
 def _js_headers_to_http_message(
     js_headers: dict[str, str],
 ):
-    # `http.client` is imported here because it costs a lot of CPU time when imported at the
-    # top-level. At least it does when we do so in our validator tests, doesn't seem to cause
-    # trouble in production. So as a workaround we do the import here.
-    #
-    # TODO(later): when dedicated snapshots are default we can move this import to the top-level.
-    import http.client
-
     # Newer Pyodide versions already expose headers as an http.client.HTTPMessage,
     # in which case there is nothing to convert.
-    if isinstance(js_headers, http.client.HTTPMessage):
+    if isinstance(js_headers, HTTPMessageMapping):
         return js_headers
 
-    result = http.client.HTTPMessage()
+    result = HTTPMessageMapping()
     if not get_compat_flag("python_request_headers_preserve_commas"):
         for key, val in js_headers:
             result[key] = val.strip()
diff --git a/packages/runtime-sdk/tests/test_flask.py b/packages/runtime-sdk/tests/test_flask.py
new file mode 100644
index 00000000..ce8cb3ef
--- /dev/null
+++ b/packages/runtime-sdk/tests/test_flask.py
@@ -0,0 +1,26 @@
+"""Tests for Flask running against a live pywrangler dev server."""
+
+from pathlib import Path
+
+import pytest
+from conftest import COMPAT_CONFIGS, CompatConfig, register_in_worker_suites
+
+WEB_FRAMEWORKS_DIR = Path(__file__).parent / "web-frameworks-test" / "flask-tests"
+WEB_FRAMEWORKS_SRC_DIR = WEB_FRAMEWORKS_DIR / "src"
+
+
+@pytest.fixture(scope="module")
+def worker_project_dir() -> Path:
+    return WEB_FRAMEWORKS_DIR
+
+
+@pytest.fixture(
+    scope="module",
+    params=COMPAT_CONFIGS,
+    ids=[config.python_version for config in COMPAT_CONFIGS],
+)
+def compat_config(request: pytest.FixtureRequest) -> CompatConfig:
+    return request.param
+
+
+register_in_worker_suites(globals(), WEB_FRAMEWORKS_SRC_DIR)
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/pyproject.toml b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/pyproject.toml
new file mode 100644
index 00000000..6f0188c4
--- /dev/null
+++ b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/pyproject.toml
@@ -0,0 +1,5 @@
+[project]
+name = "flask-tests"
+version = "0.1.0"
+requires-python = ">=3.12"
+dependencies = ["flask", "pytest"]
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/test_response.py b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/test_response.py
new file mode 100644
index 00000000..30cb93b7
--- /dev/null
+++ b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/test_response.py
@@ -0,0 +1,13 @@
+from flask import Response as FlaskResponse
+
+from workers import FetchResponse, Response
+
+
+def test_fetch_response_headers():
+    fetch_response = Response(headers={"X-Workers-Test": "value"})
+
+    assert isinstance(fetch_response, FetchResponse)
+
+    flask_response = FlaskResponse(headers=fetch_response.headers)
+
+    assert flask_response.headers["X-Workers-Test"] == "value"
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/worker.py b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/worker.py
new file mode 100644
index 00000000..18d843e8
--- /dev/null
+++ b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/src/worker.py
@@ -0,0 +1,89 @@
+import importlib.util
+from urllib.parse import urlparse
+
+import pytest
+from flask import Flask
+
+from workers import Response, WorkerEntrypoint, wsgi
+
+app = Flask(__name__)
+
+
+@app.get("/health")
+def health():
+    return {"ok": True}
+
+
+class ResultCollector:
+    def __init__(self):
+        self.results = {}
+
+    @staticmethod
+    def _key(item):
+        name = item.name
+        return name[len("test_") :] if name.startswith("test_") else name
+
+    @pytest.hookimpl(hookwrapper=True)
+    def pytest_runtest_makereport(self, item, call):
+        outcome = yield
+        report = outcome.get_result()
+        key = self._key(item)
+
+        if report.when == "call":
+            if report.passed:
+                self.results[key] = {"status": "passed"}
+            elif report.skipped:
+                self.results[key] = {
+                    "status": "skipped",
+                    "reason": str(report.longrepr),
+                }
+            elif report.failed:
+                excinfo = call.excinfo
+                if excinfo is not None and excinfo.errisinstance(AssertionError):
+                    self.results[key] = {
+                        "status": "failed",
+                        "error": str(excinfo.value),
+                    }
+                else:
+                    self.results[key] = {
+                        "status": "error",
+                        "error": f"{excinfo.typename}: {excinfo.value}"
+                        if excinfo is not None
+                        else "unknown error",
+                        "traceback": report.longreprtext,
+                    }
+        elif report.when in ("setup", "teardown") and report.skipped:
+            self.results[key] = {
+                "status": "skipped",
+                "reason": str(report.longrepr),
+            }
+        elif report.when in ("setup", "teardown") and report.failed:
+            self.results[key] = {
+                "status": "error",
+                "error": report.longreprtext,
+                "traceback": report.longreprtext,
+            }
+
+
+class Default(WorkerEntrypoint):
+    async def fetch(self, request):
+        path = urlparse(request.url).path
+        if path.startswith("/run-tests/"):
+            return self._run_suite(path[len("/run-tests/") :])
+        return await wsgi.fetch(app, request, self.env, self.ctx)
+
+    @staticmethod
+    def _run_suite(suite_name):
+        module = f"test_{suite_name}"
+        if importlib.util.find_spec(module) is None:
+            return Response.json(
+                {"error": f"Unknown suite '{suite_name}' (no module '{module}')"},
+                status=404,
+            )
+
+        collector = ResultCollector()
+        pytest.main(
+            ["--pyargs", module, "-p", "no:cacheprovider"],
+            plugins=[collector],
+        )
+        return Response.json(collector.results)
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/wrangler.jsonc b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/wrangler.jsonc
new file mode 100644
index 00000000..1718baac
--- /dev/null
+++ b/packages/runtime-sdk/tests/web-frameworks-test/flask-tests/wrangler.jsonc
@@ -0,0 +1,6 @@
+{
+    "name": "flask-tests",
+    "main": "src/worker.py",
+    "compatibility_date": "%COMPAT_DATE",
+    "compatibility_flags": ["python_workers"]
+}