diff --git a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/pyproject.toml b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/pyproject.toml
index 2ce9dd1a..e913f8a8 100644
--- a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/pyproject.toml
+++ b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/pyproject.toml
@@ -4,6 +4,7 @@ version = "0.1.0"
 requires-python = ">=3.12"
 dependencies = [
     "fastapi",
+    "python-multipart",
     "pytest",
     "pytest-asyncio<1.2.0",
 ]
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/_client.py b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/_client.py
index 644a7a44..35617ce2 100644
--- a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/_client.py
+++ b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/_client.py
@@ -6,11 +6,20 @@
 BASE_URL = "http://testserver"
 
 
+def _with_content_length(headers, body):
+    """Ensure Content-Length is present when a body is provided."""
+    hdrs = dict(headers or {})
+    if body is not None and not any(k.lower() == "content-length" for k in hdrs):
+        length = len(body.encode() if isinstance(body, str) else body)
+        hdrs["Content-Length"] = str(length)
+    return hdrs
+
+
 async def fetch(app, path, env=None, method="GET", headers=None, body=None):
     request = Request(
         f"{BASE_URL}{path}",
         method=method,
-        headers=dict(headers or {}),
+        headers=_with_content_length(headers, body),
         body=body,
     )
     return await asgi.fetch(app, request, env or {})
@@ -24,3 +33,34 @@ async def read_json(response):
 async def get_json(app, path, **kwargs):
     response = await fetch(app, path, **kwargs)
     return response, await read_json(response)
+
+
+async def post_json(app, path, data, **kwargs):
+    return await fetch(
+        app,
+        path,
+        method="POST",
+        headers={"Content-Type": "application/json"},
+        body=_json.dumps(data),
+        **kwargs,
+    )
+
+
+def build_multipart(files, boundary="----WebTestBoundary"):
+    """Build a multipart/form-data body from a list of (name, filename, content) tuples."""
+    lines = []
+    for name, filename, content in files:
+        if isinstance(content, bytes):
+            content = content.decode()
+        lines.append(f"--{boundary}")
+        lines.append(
+            f'Content-Disposition: form-data; name="{name}"; filename="{filename}"'
+        )
+        lines.append("Content-Type: application/octet-stream")
+        lines.append("")
+        lines.append(content)
+    lines.append(f"--{boundary}--")
+    lines.append("")
+    body = "\r\n".join(lines)
+    content_type = f"multipart/form-data; boundary={boundary}"
+    return body, content_type
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/test_sync_handlers.py b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/test_sync_handlers.py
new file mode 100644
index 00000000..be6c04a2
--- /dev/null
+++ b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/test_sync_handlers.py
@@ -0,0 +1,128 @@
+"""Tests for FastAPI features that rely on anyio.to_thread.run_sync.
+
+The workers runtime is single-threaded, so ``anyio.to_thread.run_sync`` is
+patched to run the callable inline.  Without the patch every test in this file
+would fail with ``RuntimeError: can't start new thread``.
+
+Covered call-sites:
+- Sync ``def`` route handlers (``starlette.routing`` / ``fastapi.routing``)
+- Sync dependencies (``fastapi.dependencies.utils``)
+- Sync ``BackgroundTask`` (``starlette.background``)
+- Sync-iterator ``StreamingResponse`` (``starlette.concurrency.iterate_in_threadpool``)
+- ``UploadFile.read`` (``starlette.datastructures``)
+"""
+
+import pytest
+from _client import build_multipart, fetch, get_json, read_json
+
+# -- sync route handlers -----------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_sync_handler_returns_json(fastapi_app):
+    """A plain ``def`` route handler returns a JSON response."""
+    resp, data = await get_json(fastapi_app, "/sync/hello")
+    assert resp.status == 200
+    assert data["message"] == "sync hello"
+
+
+@pytest.mark.asyncio
+async def test_sync_post_handler(fastapi_app):
+    """A sync POST handler receives the request and responds."""
+    resp = await fetch(fastapi_app, "/sync/echo", method="POST", body="hi")
+    assert resp.status == 200
+    data = await read_json(resp)
+    assert data["method"] == "POST"
+
+
+# -- sync dependency ----------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_sync_dependency(fastapi_app):
+    """An async handler that depends on a sync ``def`` dependency."""
+    resp, data = await get_json(fastapi_app, "/sync/dep")
+    assert resp.status == 200
+    assert data["greeting"] == "hello from sync dep"
+
+
+# -- sync background task -----------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_sync_background_task(fastapi_app):
+    """A sync function passed to ``BackgroundTask`` runs to completion."""
+    resp, data = await get_json(fastapi_app, "/sync/background/run")
+    assert resp.status == 200
+    assert data["submitted"] is True
+
+    # The background task runs inline (patched), so by the time the response
+    # is fully sent the side-effect should already be visible.
+    resp2, data2 = await get_json(fastapi_app, "/sync/background/check")
+    assert resp2.status == 200
+    assert data2["bg_ran"] is True
+
+
+# -- sync-iterator streaming response ----------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_sync_streaming_response(fastapi_app):
+    """StreamingResponse backed by a sync generator delivers all chunks."""
+    resp = await fetch(fastapi_app, "/sync/stream")
+    assert resp.status == 200
+    body = await resp.text()
+    for i in range(5):
+        assert f"chunk-{i}" in body
+
+
+@pytest.mark.asyncio
+async def test_sync_streaming_content_type(fastapi_app):
+    """StreamingResponse preserves the declared media type."""
+    resp = await fetch(fastapi_app, "/sync/stream")
+    assert resp.status == 200
+    assert "text/plain" in resp.headers.get("content-type")
+
+
+# -- file upload (UploadFile) -------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_single_file_upload(fastapi_app):
+    """A single file upload is received and its content is echoed back."""
+    body, content_type = build_multipart([("file", "greet.txt", "hello world")])
+    resp = await fetch(
+        fastapi_app,
+        "/upload/single",
+        method="POST",
+        headers={"Content-Type": content_type},
+        body=body,
+    )
+    assert resp.status == 200
+    data = await read_json(resp)
+    assert data["filename"] == "greet.txt"
+    assert data["size"] > 0
+    assert "hello world" in data["text"]
+
+
+@pytest.mark.asyncio
+async def test_multiple_file_upload(fastapi_app):
+    """Multiple files are received and their metadata is echoed back."""
+    body, content_type = build_multipart(
+        [
+            ("files", "one.txt", "first"),
+            ("files", "two.txt", "second"),
+        ]
+    )
+    resp = await fetch(
+        fastapi_app,
+        "/upload/multiple",
+        method="POST",
+        headers={"Content-Type": content_type},
+        body=body,
+    )
+    assert resp.status == 200
+    data = await read_json(resp)
+    assert len(data) == 2
+    assert [f["filename"] for f in data] == ["one.txt", "two.txt"]
+    assert all(f["size"] > 0 for f in data)
diff --git a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/worker.py b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/worker.py
index cad56eb8..cc0b7ff9 100644
--- a/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/worker.py
+++ b/packages/runtime-sdk/tests/web-frameworks-test/fastapi-tests/src/worker.py
@@ -3,11 +3,12 @@
 from pathlib import Path
 
 import pytest
-from fastapi import FastAPI, Request
-from fastapi.responses import FileResponse
+from fastapi import Depends, FastAPI, File, Request, UploadFile
+from fastapi.responses import FileResponse, JSONResponse, StreamingResponse
 from fastapi.responses import Response as FastAPIResponse
 from fastapi.staticfiles import StaticFiles
 from pyodide.webloop import WebLoop
+from starlette.background import BackgroundTask
 
 import asgi
 from workers import Response, WorkerEntrypoint
@@ -24,6 +25,16 @@ async def _noop(*args):
 
 app = FastAPI()
 
+# --------------------------------------------------------------------------- #
+# Shared mutable state used to verify side-effects (e.g. background tasks).
+# --------------------------------------------------------------------------- #
+_side_effects: dict = {}
+
+
+# --------------------------------------------------------------------------- #
+# Routes exercising the anyio.to_thread.run_sync patch
+# --------------------------------------------------------------------------- #
+
 
 @app.get("/api/hello")
 async def api_hello():
@@ -35,6 +46,87 @@ async def health():
     return {"ok": True}
 
 
+# -- sync route handler (dispatched via run_in_threadpool) -------------------
+@app.get("/sync/hello")
+def sync_hello():
+    """A plain `def` (non-async) route handler."""
+    return {"message": "sync hello"}
+
+
+@app.post("/sync/echo")
+def sync_echo(request: Request):
+    """Sync POST handler that reads the body."""
+    # request.body() is async; the sync handler can still return a dict.
+    return {"method": request.method}
+
+
+# -- sync dependency ---------------------------------------------------------
+def _get_greeting():
+    """A plain `def` dependency (not async)."""
+    return "hello from sync dep"
+
+
+@app.get("/sync/dep")
+async def sync_dep_route(greeting: str = Depends(_get_greeting)):
+    """Async handler with a sync dependency."""
+    return {"greeting": greeting}
+
+
+# -- sync background task ----------------------------------------------------
+def _bg_task():
+    """Sync function executed as a BackgroundTask."""
+    _side_effects["bg_ran"] = True
+
+
+@app.get("/sync/background/run")
+async def sync_background_run():
+    """Fire a sync background task and return immediately."""
+    _side_effects.pop("bg_ran", None)
+    return JSONResponse({"submitted": True}, background=BackgroundTask(_bg_task))
+
+
+@app.get("/sync/background/check")
+async def sync_background_check():
+    """Return whether the background task has executed."""
+    return {"bg_ran": _side_effects.get("bg_ran", False)}
+
+
+# -- sync iterator streaming response ---------------------------------------
+def _sync_chunks():
+    """A plain generator (not async) yielding text chunks."""
+    for i in range(5):
+        yield f"chunk-{i}\n"
+
+
+@app.get("/sync/stream")
+async def sync_stream():
+    """StreamingResponse backed by a sync iterator."""
+    return StreamingResponse(_sync_chunks(), media_type="text/plain")
+
+
+# -- file upload (UploadFile.read/write/seek go through run_in_threadpool) ---
+@app.post("/upload/single")
+async def upload_single(file: UploadFile = File(...)):  # noqa: B008
+    """Accept a single file upload and echo its metadata + content."""
+    content = await file.read()
+    return {
+        "filename": file.filename,
+        "content_type": file.content_type,
+        "size": len(content),
+        "text": content.decode(errors="replace"),
+    }
+
+
+@app.post("/upload/multiple")
+async def upload_multiple(files: list[UploadFile] = File(...)):  # noqa: B008
+    """Accept multiple file uploads and echo their metadata."""
+    result = []
+    for f in files:
+        data = await f.read()
+        result.append({"filename": f.filename, "size": len(data)})
+    return result
+
+
 @app.get("/native-file")
 async def native_file():
     """Serve a single bundled file with FastAPI's native FileResponse."""