diff --git a/tests/test_dependency_parallelization.py b/tests/test_dependency_parallelization.py
new file mode 100644
index 000000000..32f0b3c96
--- /dev/null
+++ b/tests/test_dependency_parallelization.py
@@ -0,0 +1,517 @@
+import asyncio
+import time
+import warnings
+
+import pytest
+from starlette.requests import Request
+from contextlib import AsyncExitStack
+from fastapi import Depends, FastAPI
+from fastapi.testclient import TestClient
+from fastapi.dependencies.utils import get_dependant, solve_dependencies, silence_future_exception
+
+
+PAR_LOW, PAR_HIGH = 0.08, 0.25
+SEQPAR_LOW, SEQPAR_HIGH = 0.18, 0.35
+OVERLAP_EPS = 0.05
+
+
+def assert_duration_between(value: float, low: float, high: float) -> None:
+    assert low <= value <= high, f"duration {value:.4f} not in [{low:.2f}, {high:.2f}]"
+
+
+def assert_duration_with_sequential_and_parallel_deps(value: float) -> None:
+    assert_duration_between(value, SEQPAR_LOW, SEQPAR_HIGH)
+
+
+def assert_duration_with_parallel_deps(value: float) -> None:
+    assert_duration_between(value, PAR_LOW, PAR_HIGH)
+
+
+def get(client: TestClient, path: str):
+    start = time.perf_counter()
+    r = client.get(path)
+    return r, time.perf_counter() - start
+
+
+def new_timings() -> dict:
+    return {}
+
+
+def t_start(ts: dict, key: str) -> None:
+    ts.setdefault(key, {})["start"] = time.perf_counter()
+
+
+def t_end(ts: dict, key: str) -> None:
+    ts[key]["end"] = time.perf_counter()
+
+
+def assert_overlaps(ts: dict, a: str, b: str, *, min_overlap: float = OVERLAP_EPS) -> None:
+    overlap = min(ts[a]["end"], ts[b]["end"]) - max(ts[a]["start"], ts[b]["start"])
+    assert overlap > min_overlap, f"no sufficient overlap between {a} and {b}: {overlap:.4f}s"
+
+
+def make_async_timed_dep(ts: dict, key: str, *, delay: float = 0.1, value: int = 1, order: list[str] | None = None):
+    async def dep():
+        t_start(ts, key)
+        await asyncio.sleep(delay)
+        t_end(ts, key)
+        if order is not None:
+            order.append(key)
+        return value
+
+    return dep
+
+
+def make_sync_timed_dep(ts: dict, key: str, *, delay: float = 0.1, value: int = 1):
+    def dep():
+        t_start(ts, key)
+        time.sleep(delay)
+        t_end(ts, key)
+        return value
+
+    return dep
+
+
+def make_security_counter_dep(calls: dict, ts: dict | None = None, key_factory=None, *, delay: float = 0.1):
+    counter = {"i": 0}
+
+    async def dep():
+        calls["n"] += 1
+        cur = calls["n"]
+        k = None
+        if ts is not None:
+            counter["i"] += 1
+            k = key_factory(counter["i"]) if key_factory else f"call{counter['i']}"
+            t_start(ts, k)
+        await asyncio.sleep(delay)
+        if ts is not None and k is not None:
+            t_end(ts, k)
+        return cur
+
+    return dep
+
+
+def test_global_parallel_opt_in_with_per_dep_opt_out():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    order = []
+    ts = new_timings()
+
+    async def seq_dep():
+        await asyncio.sleep(0.1)
+        order.append("seq")
+        return 1
+
+    par1 = make_async_timed_dep(ts, "p1", order=order)
+    par2 = make_async_timed_dep(ts, "p2", order=order)
+
+    @app.get("/measure1")
+    async def measure(
+        _: int = Depends(seq_dep, parallelizable=False),
+        __: int = Depends(par1),
+        ___: int = Depends(par2),
+    ):
+        return {"ok": True}
+
+    client = TestClient(app)
+
+    r, elapsed = get(client, "/measure1")
+
+    assert r.status_code == 200
+    assert_duration_with_sequential_and_parallel_deps(elapsed)
+    assert_overlaps(ts, "p1", "p2")
+    assert set(order) == {"seq", "p1", "p2"}
+
+
+def test_global_default_false_with_per_dep_enable_parallel():
+    app = FastAPI(depends_default_parallelizable=False)
+
+    order = []
+    ts = new_timings()
+
+    async def seq_dep():
+        await asyncio.sleep(0.1)
+        order.append("seq")
+        return 1
+
+    par1 = make_async_timed_dep(ts, "p1", order=order)
+    par2 = make_async_timed_dep(ts, "p2", order=order)
+
+    @app.get("/measure2")
+    async def measure(
+        _: int = Depends(seq_dep),  # uses app default (False) -> sequential
+        __: int = Depends(par1, parallelizable=True),
+        ___: int = Depends(par2, parallelizable=True),
+    ):
+        return {"ok": True}
+
+    client = TestClient(app)
+
+    r, elapsed = get(client, "/measure2")
+
+    assert r.status_code == 200
+    assert_duration_with_sequential_and_parallel_deps(elapsed)
+    assert_overlaps(ts, "p1", "p2")
+    assert set(order) == {"seq", "p1", "p2"}
+
+
+def test_parallel_cache_only_called_once():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    call_count = {"n": 0}
+
+    async def shared():
+        call_count["n"] += 1
+        await asyncio.sleep(0.1)
+        return call_count["n"]
+
+    @app.get("/cache")
+    async def measure(a: int = Depends(shared), b: int = Depends(shared)):
+        return {"a": a, "b": b, "calls": call_count["n"]}
+
+    client = TestClient(app)
+
+    r, elapsed = get(client, "/cache")
+
+    assert r.status_code == 200
+    data = r.json()
+    assert data["a"] == data["b"] == 1
+    assert data["calls"] == 1
+    assert_duration_with_parallel_deps(elapsed)
+
+
+def test_parallel_exception_silences_future_warning_and_raises_once():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    class BoomError(RuntimeError):
+        pass
+
+    async def failing():
+        await asyncio.sleep(0.01)
+        raise BoomError("boom")
+
+    @app.get("/fail1")
+    async def fail_endpoint(a: int = Depends(failing), b: int = Depends(failing)):
+        return {"ok": True} # pragma: no cover
+
+    client = TestClient(app)
+
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        with pytest.raises(BoomError):
+            client.get("/fail1")
+        messages = [str(x.message) for x in w]
+    assert not any("Future exception was never retrieved" in m for m in messages)
+
+
+def test_dependency_overrides_preserve_parallelizable():
+    app = FastAPI(depends_default_parallelizable=False)
+
+    async def original():
+        await asyncio.sleep(0.1) # pragma: no cover
+        return "original" # pragma: no cover
+
+    async def override():
+        await asyncio.sleep(0.1)
+        return "override"
+
+    @app.get("/override")
+    async def ep(x: str = Depends(original, parallelizable=True), y: str = Depends(original, parallelizable=True)):
+        return {"x": x, "y": y}
+
+    app.dependency_overrides[original] = override
+    client = TestClient(app)
+
+    r, elapsed = get(client, "/override")
+
+    assert r.status_code == 200
+    assert r.json() == {"x": "override", "y": "override"}
+    assert_duration_with_parallel_deps(elapsed)
+
+
+def test_security_parallel_and_cache_same_scope():
+    from fastapi import Security
+
+    app = FastAPI(depends_default_parallelizable=True)
+    calls = {"n": 0}
+    ts = new_timings()
+
+    sec_dep = make_security_counter_dep(calls, ts)
+
+    @app.get("/sec-same")
+    async def ep(
+        a: int = Security(sec_dep, scopes=["s1"]),
+        b: int = Security(sec_dep, scopes=["s1"]),
+    ):
+        return {"a": a, "b": b, "calls": calls["n"]}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/sec-same")
+
+    assert r.status_code == 200
+    data = r.json()
+    assert data["a"] == data["b"]
+    assert data["calls"] == 1
+    assert_duration_with_parallel_deps(elapsed)
+
+
+def test_security_parallel_and_no_cache_diff_scope():
+    from fastapi import Security
+
+    app = FastAPI(depends_default_parallelizable=True)
+    calls = {"n": 0}
+    ts = new_timings()
+
+    sec_dep = make_security_counter_dep(calls, ts, key_factory=lambda i: f"call{i}")
+
+    @app.get("/sec-diff")
+    async def ep(
+        a: int = Security(sec_dep, scopes=["s1"]),
+        b: int = Security(sec_dep, scopes=["s2"]),
+    ):
+        return {"a": a, "b": b, "calls": calls["n"]}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/sec-diff")
+
+    assert r.status_code == 200
+    data = r.json()
+    assert data["a"] != data["b"]
+    assert data["calls"] == 2
+    assert_overlaps(ts, "call1", "call2")
+
+
+def test_security_per_dep_enable_with_global_false():
+    from fastapi import Security
+
+    app = FastAPI(depends_default_parallelizable=False)
+
+    ts = new_timings()
+
+    sec_dep = make_security_counter_dep({"n": 0}, ts)
+
+    @app.get("/sec-per-dep")
+    async def ep(
+        a: int = Security(sec_dep, parallelizable=True),
+        b: int = Security(sec_dep, parallelizable=True),
+    ):
+        return {"ok": True}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/sec-per-dep")
+    assert r.status_code == 200
+    assert_duration_with_parallel_deps(elapsed)
+
+
+def test_generator_dep_forces_sequential_even_if_parallelizable():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    async def gen_dep():
+        await asyncio.sleep(0.1)
+        try:
+            yield 1
+        finally:
+            pass
+
+    async def par():
+        await asyncio.sleep(0.1)
+        return 2
+
+    @app.get("/gen-seq")
+    async def ep(a: int = Depends(gen_dep, parallelizable=True), b: int = Depends(par)):
+        return {"ok": True}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/gen-seq")
+    assert r.status_code == 200
+    assert_duration_with_sequential_and_parallel_deps(elapsed)
+
+
+def test_context_sensitivity_bubbles_up_from_subdeps():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    async def gen_dep():
+        await asyncio.sleep(0.1)
+        try:
+            yield 1
+        finally:
+            pass
+
+    async def a_dep(_: int = Depends(gen_dep)):
+        return 10
+
+    async def par():
+        await asyncio.sleep(0.1)
+        return 2
+
+    @app.get("/bubble")
+    async def ep(a: int = Depends(a_dep, parallelizable=True), b: int = Depends(par)):
+        return {"ok": True}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/bubble")
+    assert r.status_code == 200
+    assert_duration_with_sequential_and_parallel_deps(elapsed)
+
+
+def test_background_tasks_created_and_parallel_siblings():
+    from starlette.background import BackgroundTasks
+
+    app = FastAPI(depends_default_parallelizable=True)
+
+    flag = {"ran": False}
+
+    def bg():
+        flag["ran"] = True
+
+    async def needs_bg(tasks: BackgroundTasks):
+        tasks.add_task(bg)
+        return 1
+
+    async def par():
+        await asyncio.sleep(0.1)
+        return 2
+
+    @app.get("/bg")
+    async def ep(_: int = Depends(needs_bg), __: int = Depends(par)):
+        return {"ok": True}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/bg")
+    assert r.status_code == 200
+    assert_duration_with_parallel_deps(elapsed)
+    time.sleep(0.02)
+    assert flag["ran"] is True
+
+
+def test_exception_order_sequential_then_parallel():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    class SeqErr(RuntimeError):
+        pass
+
+    class ParErr(RuntimeError):
+        pass
+
+    async def fail_seq():
+        raise SeqErr("seq first")
+
+    async def fail_par():
+        await asyncio.sleep(0.01)
+        raise ParErr("par later")
+
+    @app.get("/ex-order-a")
+    async def ep(_: int = Depends(fail_seq), __: int = Depends(fail_par)):
+        return {"ok": True} # pragma: no cover
+
+    client = TestClient(app)
+    with pytest.raises(SeqErr):
+        client.get("/ex-order-a")
+
+
+def test_exception_order_with_parallel_siblings():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    class AErr(RuntimeError):
+        pass
+
+    class BErr(RuntimeError):
+        pass
+
+    async def fail_a():
+        await asyncio.sleep(0.01)
+        raise AErr("a")
+
+    async def fail_b():
+        await asyncio.sleep(0.01)
+        raise BErr("b")
+
+    @app.get("/ex-order-b")
+    async def ep(_: int = Depends(fail_a), __: int = Depends(fail_b)):
+        return {"ok": True} # pragma: no cover
+
+    client = TestClient(app)
+    with pytest.raises(AErr):
+        client.get("/ex-order-b")
+
+
+def test_threadpool_parallelization_for_sync_functions():
+    app = FastAPI(depends_default_parallelizable=True)
+
+    ts = new_timings()
+
+    s1 = make_sync_timed_dep(ts, "s1")
+    s2 = make_sync_timed_dep(ts, "s2")
+
+    @app.get("/sync")
+    def ep(_: int = Depends(s1), __: int = Depends(s2)):
+        return {"ok": True}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/sync")
+    assert r.status_code == 200
+    assert_duration_with_parallel_deps(elapsed)
+    assert_overlaps(ts, "s1", "s2")
+
+
+def test_security_scopes_cache_key_multiple():
+    from fastapi import Security
+
+    app = FastAPI(depends_default_parallelizable=True)
+    calls = {"n": 0}
+    ts = new_timings()
+
+    sec_dep = make_security_counter_dep(calls, ts, key_factory=lambda i: f"call{i}")
+
+    @app.get("/sec-multi")
+    async def ep(
+        a: int = Security(sec_dep, scopes=["a"]),
+        b: int = Security(sec_dep, scopes=["a"]),
+        c: int = Security(sec_dep, scopes=["b"]),
+    ):
+        return {"a": a, "b": b, "c": c, "calls": calls["n"]}
+
+    client = TestClient(app)
+    r, elapsed = get(client, "/sec-multi")
+    assert r.status_code == 200
+    data = r.json()
+    assert data["a"] == data["b"]
+    assert data["c"] != data["a"]
+    assert data["calls"] == 2
+    assert_overlaps(ts, "call1", "call2")
+
+def test_cached_value_converted_to_future_and_awaited():
+    app = FastAPI()
+
+    async def low() -> int:
+        return 999 # pragma: no cover
+
+    async def wrapper(x: int = Depends(low)) -> int:
+        return x # pragma: no cover
+
+    @app.get("/cov")
+    async def cov(request: Request):
+        async with AsyncExitStack() as astack:
+            dep = get_dependant(path="/cov", call=wrapper)
+            # Pre-populate cache with a non-Future raw value
+            dependency_cache = {(low, ()): 777}
+            solved = await solve_dependencies(
+                request=request,
+                dependant=dep,
+                dependency_cache=dependency_cache,
+                async_exit_stack=astack,
+                embed_body_fields=False,
+            )
+            return {"x": solved.values["x"]}
+
+    client = TestClient(app)
+    r = client.get("/cov")
+    assert r.status_code == 200
+    assert r.json() == {"x": 777}
+
+
+def test_silence_future_exception_handles_exception_path() -> None:
+    class Dummy:
+        def exception(self):
+            raise Exception("boom")
+    silence_future_exception(Dummy())