diff --git a/docs/design/server_contracts/openai.md b/docs/design/server_contracts/openai.md
index 5cd5812ed..10c04fac5 100644
--- a/docs/design/server_contracts/openai.md
+++ b/docs/design/server_contracts/openai.md
@@ -50,13 +50,15 @@ highest first:
 2. **`ServeConfig.default_request` (operator-explicit)** — projected via
    [`explicit_request_updates()`](https://github.com/hao-ai-lab/FastVideo/blob/main/fastvideo/api/translation.py);
    only fields the operator actually wrote into the YAML count as
-   defaults. Every other field inherits the schema default rather than
-   being pinned.
+   defaults (an explicit `null` counts as unset). Every other sampling
+   field stays `None` — "inherit the model preset" — and other sections
+   keep their schema defaults without being pinned.
 3. **Hardcoded fallback** — e.g. `fps = 24`.
 
-The gate matters: both surfaces carry schema defaults. Without
-`model_fields_set` / explicit-path tracking, schema defaults would
-masquerade as intent and silently shadow the other side.
+The gate matters: the Pydantic surface carries schema defaults and the
+dataclass surface carries non-None defaults outside `sampling`. Without
+`model_fields_set` / explicit-path tracking, defaults would masquerade
+as intent and silently shadow the other side.
 
 See [`video_api.py::_build_generation_kwargs`](https://github.com/hao-ai-lab/FastVideo/blob/main/fastvideo/entrypoints/openai/video_api.py)
 for the canonical implementation; the per-request assembly lives there,
diff --git a/examples/inference/gradio/serving/ray_serve_backend.py b/examples/inference/gradio/serving/ray_serve_backend.py
index 3dfbd2980..684db9e24 100644
--- a/examples/inference/gradio/serving/ray_serve_backend.py
+++ b/examples/inference/gradio/serving/ray_serve_backend.py
@@ -167,7 +167,9 @@ def prepare_generation_request(video_request: VideoGenerationRequest, image_path
     seed = (video_request.seed if not video_request.randomize_seed
             else torch.randint(0, SEED_RANGE_MAX, (1,)).item())
 
-    negative_prompt = video_request.negative_prompt if video_request.use_negative_prompt else None
+    # "" explicitly clears the model preset's negative prompt (None would
+    # inherit it, changing this demo's long-standing behavior).
+    negative_prompt = video_request.negative_prompt if video_request.use_negative_prompt else ""
 
     request = GenerationRequest(
         prompt=video_request.prompt,
diff --git a/fastvideo/api/parser.py b/fastvideo/api/parser.py
index 969dc8576..366f9f353 100644
--- a/fastvideo/api/parser.py
+++ b/fastvideo/api/parser.py
@@ -37,15 +37,34 @@ def parse_config(config_type: type[T], raw: Mapping[str, Any] | T) -> T:
     if not isinstance(raw, Mapping):
         raise ConfigValidationError("", f"expected mapping for {config_type.__name__}")
     parsed = _SchemaParser().parse_dataclass(config_type, raw, "")
+    # None is the schema-wide sentinel for "not specified, inherit the
+    # model preset", so an explicit YAML/JSON null must not bind as an
+    # explicit path — otherwise it would stomp preset values with None.
     if config_type is GenerationRequest:
-        return bind_generation_request_raw(parsed, raw)
+        return bind_generation_request_raw(parsed, drop_none_leaves(raw))
     if config_type is RunConfig:
-        return bind_run_config_raw(parsed, raw)
+        return bind_run_config_raw(parsed, drop_none_leaves(raw))
     if config_type is ServeConfig:
-        return bind_serve_config_raw(parsed, raw)
+        return bind_serve_config_raw(parsed, drop_none_leaves(raw))
     return parsed
 
 
+def drop_none_leaves(raw: Any) -> Any:
+    """Prune None leaves — and dicts emptied by the pruning — from a raw
+    config mapping, so they are never recorded as explicit paths."""
+    if not isinstance(raw, dict):
+        return raw
+    pruned: dict[str, Any] = {}
+    for key, value in raw.items():
+        if value is None:
+            continue
+        value = drop_none_leaves(value)
+        if isinstance(value, dict) and not value:
+            continue
+        pruned[key] = value
+    return pruned
+
+
 def config_to_dict(config: Any) -> Any:
     """Serialize a typed config object into plain Python containers."""
     if dataclasses.is_dataclass(config) and not isinstance(config, type):
diff --git a/fastvideo/api/schema.py b/fastvideo/api/schema.py
index 5931bb7a7..df837ad6a 100644
--- a/fastvideo/api/schema.py
+++ b/fastvideo/api/schema.py
@@ -136,19 +136,27 @@ class InputConfig:
 
 @dataclass
 class SamplingConfig:
-    num_videos_per_prompt: int = 1
-    seed: int = 1024
-    num_frames: int = 125
-    height: int = 720
-    width: int = 1280
-    height_sr: int = 1072
-    width_sr: int = 1920
-    fps: int = 24
-    num_inference_steps: int = 50
-    num_inference_steps_sr: int = 50
-    guidance_scale: float = 1.0
+    """Sampling knobs for a generation request.
+
+    ``None`` means "not specified": the value is inherited from the
+    model's preset (``SamplingParam.from_pretrained``) at generate time.
+    Set a field to override the preset. There is deliberately no way to
+    express "explicitly the schema default" — pass the concrete value
+    you want instead.
+    """
+    num_videos_per_prompt: int | None = None
+    seed: int | None = None
+    num_frames: int | None = None
+    height: int | None = None
+    width: int | None = None
+    height_sr: int | None = None
+    width_sr: int | None = None
+    fps: int | None = None
+    num_inference_steps: int | None = None
+    num_inference_steps_sr: int | None = None
+    guidance_scale: float | None = None
     guidance_scale_2: float | None = None
-    guidance_rescale: float = 0.0
+    guidance_rescale: float | None = None
     true_cfg_scale: float | None = None
     boundary_ratio: float | None = None
     sigmas: list[float] | None = None
@@ -194,6 +202,8 @@ class GenerationPlan:
 class GenerationRequest:
     prompt: str | list[str] | None = None
     negative_prompt: str | None = None
+    """``None`` inherits the model preset's negative prompt; pass ``""``
+    to explicitly clear it."""
     inputs: InputConfig = field(default_factory=InputConfig)
     sampling: SamplingConfig = field(default_factory=SamplingConfig)
     runtime: RequestRuntimeConfig = field(default_factory=RequestRuntimeConfig)
@@ -261,9 +271,10 @@ class ServeConfig:
     the incoming body as the operator-pinned baseline.
 
     Important nuance: only fields the operator **explicitly wrote** in the
-    serve YAML/JSON count as defaults. Although the in-memory object is
-    fully populated (schema defaults fill every unset field), the merge
-    walks ``_fastvideo_explicit_paths`` — populated during parse — so
+    serve YAML/JSON count as defaults. Unset sampling fields stay ``None``
+    ("inherit the model preset"), other sections keep their schema
+    defaults in memory — but the merge walks ``_fastvideo_explicit_paths``
+    (populated during parse; explicit ``null`` is treated as unset), so
     unset fields are *not* forced onto requests. Per-request precedence:
 
         body (client-explicit) > default_request (operator-explicit)
diff --git a/fastvideo/api/translation.py b/fastvideo/api/translation.py
index a3913a4bd..6484d6872 100644
--- a/fastvideo/api/translation.py
+++ b/fastvideo/api/translation.py
@@ -26,7 +26,12 @@ from pathlib import Path
 from typing import Any
 
 from fastvideo.api.overrides import apply_overrides, normalize_overrides
-from fastvideo.api.parser import config_to_dict, load_raw_config, parse_config
+from fastvideo.api.parser import (
+    config_to_dict,
+    drop_none_leaves,
+    load_raw_config,
+    parse_config,
+)
 from fastvideo.api.request_metadata import (
     EXPLICIT_PATHS_ATTR,
     bind_generation_request_raw,
@@ -332,8 +337,12 @@ def normalize_generation_request(request: GenerationRequest | Mapping[str, Any],
 
     if not hasattr(normalized, EXPLICIT_PATHS_ATTR):
         # Request wasn't bound through the parser (e.g. constructed
-        # directly). Treat every currently-set field as explicit.
-        bind_generation_request_raw(normalized, _serialize_generation_request(normalized))
+        # directly). Bind only non-None leaves as explicit: None is the
+        # schema-wide sentinel for "not specified, inherit the model
+        # preset", so treating it as explicit would stomp preset values
+        # with schema defaults. (parse_config applies the same pruning
+        # to YAML/JSON nulls.)
+        bind_generation_request_raw(normalized, drop_none_leaves(_serialize_generation_request(normalized)))
     return normalized
 
 
@@ -382,15 +391,13 @@ def request_to_sampling_param(
     updates = explicit_request_updates(request)
 
     for key, value in updates.items():
+        if key == "return_state":
+            # Already translated to return_continuation_state above.
+            continue
         if hasattr(sampling_param, key):
             setattr(sampling_param, key, deepcopy(value))
         elif key in _REQUEST_PIPELINE_OVERRIDE_FIELDS:
             continue
-        elif value == _SCHEMA_DEFAULT_UPDATES.get(key, _MISSING):
-            # Schema-default field that isn't on SamplingParam; tolerated
-            # because direct GenerationRequest(...) construction has no
-            # way to distinguish "user set" from "schema default".
-            continue
         else:
             raise ValueError(f"Request field {key!r} is not supported by sampling params for {model_path}")
 
@@ -494,7 +501,9 @@ def explicit_request_updates(request: GenerationRequest) -> dict[str, Any]:
     This is what makes ``ServeConfig.default_request`` work as an
     operator-pinned baseline rather than a full override: a YAML with just
     ``sampling.seed: 42`` yields ``{"seed": 42}``, not the full sampling
-    config with its 15 schema defaults.
+    config. (Sampling fields default to ``None`` = "inherit the model
+    preset", so for directly-constructed requests only non-None leaves
+    are bound as explicit.)
 
     Precondition: the request must carry ``_fastvideo_explicit_paths`` —
     populated by :func:`fastvideo.api.parser.parse_config` or
@@ -597,8 +606,6 @@ def _serialize_generation_request(request: GenerationRequest) -> dict[str, Any]:
     return deepcopy(config_to_dict(request))
 
 
-_SCHEMA_DEFAULT_UPDATES = _extract_request_updates(config_to_dict(GenerationRequest()))
-
 _KNOWN_CONTINUATION_KINDS: set[str] = set()
 
 
diff --git a/fastvideo/entrypoints/openai/video_api.py b/fastvideo/entrypoints/openai/video_api.py
index 7b2ce645a..4fd330ba1 100644
--- a/fastvideo/entrypoints/openai/video_api.py
+++ b/fastvideo/entrypoints/openai/video_api.py
@@ -60,12 +60,13 @@ def _build_generation_kwargs(
          defaults on the dataclass are *not* treated as defaults here.
       3. Hardcoded fallback (e.g. ``fps=24`` when neither side set it).
 
-    Why gate on ``model_fields_set`` / explicit paths? Both the request
-    Pydantic model and the ``GenerationRequest`` dataclass carry schema
-    defaults (e.g. ``seed=1024``, ``num_frames=125``). Without the gate
-    those would masquerade as intent and shadow the other side — the
-    gate preserves "operator pinned it" vs. "dataclass happened to have
-    that default."
+    Why gate on ``model_fields_set`` / explicit paths? The request
+    Pydantic model carries schema defaults, and the ``GenerationRequest``
+    dataclass carries non-None defaults outside ``sampling`` (e.g.
+    ``output_path``; sampling fields default to None = "inherit the
+    model preset"). Without the gate those would masquerade as intent
+    and shadow the other side — the gate preserves "operator pinned it"
+    vs. "dataclass happened to have that default."
     """
     kwargs: dict[str, Any] = {}
     if default_request is not None:
diff --git a/fastvideo/entrypoints/streaming/mock_server.py b/fastvideo/entrypoints/streaming/mock_server.py
index 9d8ff2daa..b7029d788 100644
--- a/fastvideo/entrypoints/streaming/mock_server.py
+++ b/fastvideo/entrypoints/streaming/mock_server.py
@@ -48,10 +48,14 @@ class MockGenerator:
     def generate(self, request: GenerationRequest) -> dict[str, Any]:
         if self.sleep_ms:
             time.sleep(self.sleep_ms / 1000.0)
-        width = max(16, request.sampling.width)
-        height = max(16, request.sampling.height)
-        num_frames = max(1, request.sampling.num_frames)
-        frames = [_gradient_frame(height, width, idx, seed=request.sampling.seed) for idx in range(num_frames)]
+        # Requests carry None for unset sampling fields (normally resolved
+        # against the model preset); the mock has no model, so fall back
+        # to fixed values here.
+        width = max(16, request.sampling.width or 16)
+        height = max(16, request.sampling.height or 16)
+        num_frames = max(1, request.sampling.num_frames or 1)
+        seed = request.sampling.seed if request.sampling.seed is not None else 1024
+        frames = [_gradient_frame(height, width, idx, seed=seed) for idx in range(num_frames)]
         state = ContinuationState(
             kind="ltx2.v1",
             payload={
@@ -90,6 +94,9 @@ def build_mock_app(*, sleep_ms: float = 0.0):
         width=256,
         fps=24,
         num_inference_steps=1,
+        # The mock renders frames from the seed directly (no model, no
+        # preset resolution), so it must be concrete here.
+        seed=1024,
     )
     return build_app(serve_config, MockGenerator(sleep_ms=sleep_ms))
 
diff --git a/fastvideo/entrypoints/streaming/server.py b/fastvideo/entrypoints/streaming/server.py
index cabecf614..5d7ac8d4b 100644
--- a/fastvideo/entrypoints/streaming/server.py
+++ b/fastvideo/entrypoints/streaming/server.py
@@ -95,6 +95,19 @@ class ServerState:
     session_store: SessionStore
 
 
+def _validate_default_sampling_pins(serve_config: ServeConfig) -> None:
+    """The WS protocol consumes these before any model-preset resolution
+    happens (segment frames, MP4 encoder dims), so the operator must pin
+    them; SamplingConfig defaults them to None = "inherit preset", which
+    has no meaning at this layer."""
+    required = ("num_frames", "height", "width", "fps", "num_inference_steps")
+    missing = [name for name in required if getattr(serve_config.default_request.sampling, name) is None]
+    if missing:
+        raise ValueError("Streaming server requires explicit default_request.sampling values for: " +
+                         ", ".join(missing) + ". Set them in the serve config YAML "
+                         "(e.g. `default_request: {sampling: {num_frames: 121, ...}}`).")
+
+
 def build_app(
     serve_config: ServeConfig,
     generator: _GeneratorProto | None = None,
@@ -118,6 +131,8 @@ def build_app(
     if (generator is None) == (pool is None):
         raise ValueError("build_app requires exactly one of `generator` or `pool`")
 
+    _validate_default_sampling_pins(serve_config)
+
     store = session_store or InMemorySessionStore()
     if pool is None:
         assert generator is not None
@@ -183,6 +198,8 @@ def run_server(serve_config: ServeConfig, *, generator: _GeneratorProto | None =
     if serve_config.streaming is None:
         raise ValueError("ServeConfig.streaming must be set to launch the streaming server; "
                          "got None. Add a `streaming:` block to your serve config.")
+    # Fail on config mistakes before paying the multi-minute model boot.
+    _validate_default_sampling_pins(serve_config)
 
     import uvicorn
 
@@ -490,7 +507,9 @@ def _build_generation_request(
     }
     request = GenerationRequest(
         prompt=message.prompt,
-        negative_prompt=message.negative_prompt or base.negative_prompt,
+        # None inherits (base, then model preset); an explicit "" from
+        # the client clears the negative prompt and must survive.
+        negative_prompt=(message.negative_prompt if message.negative_prompt is not None else base.negative_prompt),
         inputs=InputConfig(image_path=session.metadata.get("session_init_image"), ),
         sampling=SamplingConfig(**sampling_kwargs),
         output=OutputConfig(save_video=False, return_frames=True, return_state=True),
diff --git a/fastvideo/entrypoints/video_generator.py b/fastvideo/entrypoints/video_generator.py
index d4f83e425..61d0b6af7 100644
--- a/fastvideo/entrypoints/video_generator.py
+++ b/fastvideo/entrypoints/video_generator.py
@@ -314,7 +314,12 @@ class VideoGenerator:
         import asyncio
 
         normalized = normalize_generation_request(request)
-        total_steps = max(1, normalized.sampling.num_inference_steps)
+        steps = normalized.sampling.num_inference_steps
+        if steps is None:
+            # Unset = inherit the model preset; resolve it here so the
+            # advertised total matches what the run will actually do.
+            steps = SamplingParam.from_pretrained(self.fastvideo_args.model_path).num_inference_steps
+        total_steps = max(1, steps or 1)
         yield VideoProgressEvent(step=0, total_steps=total_steps, stage="denoise")
 
         if log_queue:
diff --git a/fastvideo/tests/api/test_configs.py b/fastvideo/tests/api/test_configs.py
index d3082741a..96ddaae56 100644
--- a/fastvideo/tests/api/test_configs.py
+++ b/fastvideo/tests/api/test_configs.py
@@ -37,5 +37,6 @@ def test_serve_config_includes_server_and_default_request_defaults() -> None:
         "port": 8000,
         "output_dir": "outputs/",
     }
-    assert dumped["default_request"]["sampling"]["fps"] == 24
+    # Sampling fields default to None = "inherit the model preset".
+    assert dumped["default_request"]["sampling"]["fps"] is None
     assert dumped["default_request"]["runtime"]["enable_teacache"] is False
diff --git a/fastvideo/tests/api/test_parser.py b/fastvideo/tests/api/test_parser.py
index 91a2bbfe4..4133832f3 100644
--- a/fastvideo/tests/api/test_parser.py
+++ b/fastvideo/tests/api/test_parser.py
@@ -169,19 +169,21 @@ def test_load_run_config_supports_yaml_roundtrip(tmp_path) -> None:
                 "stage1_video": None,
             },
             "sampling": {
-                "num_videos_per_prompt": 1,
-                "seed": 1024,
+                # None = inherit the model preset at generate time; only
+                # the YAML-set num_frames carries a value.
+                "num_videos_per_prompt": None,
+                "seed": None,
                 "num_frames": 16,
-                "height": 720,
-                "width": 1280,
-                "height_sr": 1072,
-                "width_sr": 1920,
-                "fps": 24,
-                "num_inference_steps": 50,
-                "num_inference_steps_sr": 50,
-                "guidance_scale": 1.0,
+                "height": None,
+                "width": None,
+                "height_sr": None,
+                "width_sr": None,
+                "fps": None,
+                "num_inference_steps": None,
+                "num_inference_steps_sr": None,
+                "guidance_scale": None,
                 "guidance_scale_2": None,
-                "guidance_rescale": 0.0,
+                "guidance_rescale": None,
                 "true_cfg_scale": None,
                 "boundary_ratio": None,
                 "sigmas": None,
diff --git a/fastvideo/tests/api/test_preset_inheritance.py b/fastvideo/tests/api/test_preset_inheritance.py
new file mode 100644
index 000000000..f0f0e9e29
--- /dev/null
+++ b/fastvideo/tests/api/test_preset_inheritance.py
@@ -0,0 +1,106 @@
+# SPDX-License-Identifier: Apache-2.0
+"""Regression tests for None-sentinel SamplingConfig semantics.
+
+A directly-constructed ``GenerationRequest`` must inherit the model
+preset (``SamplingParam.from_pretrained``) for every sampling field the
+caller did not set. Before the None-sentinel change, direct construction
+marked every schema default as explicit and stomped the preset —
+e.g. FastWan's 3 distilled steps silently became 50.
+"""
+from __future__ import annotations
+
+import pytest
+
+from fastvideo.api.sampling_param import SamplingParam
+from fastvideo.api.schema import GenerationRequest, SamplingConfig
+from fastvideo.api.translation import (
+    normalize_generation_request,
+    request_to_sampling_param,
+)
+
+
+def _resolve(request: GenerationRequest) -> SamplingParam:
+    # Mirror the production path: generate() normalizes (binding explicit
+    # paths) before translating to a SamplingParam.
+    return request_to_sampling_param(normalize_generation_request(request), model_path=MODEL)
+
+# Distilled preset whose values differ from every old schema default
+# (3 steps vs 50, gs 3.0 vs 1.0, 448x832 vs 720x1280, 61 frames vs 125,
+# fps 16 vs 24) — if inheritance breaks, every assertion below fails.
+MODEL = "FastVideo/FastWan2.1-T2V-1.3B-Diffusers"
+
+
+@pytest.fixture()
+def preset() -> SamplingParam:
+    return SamplingParam.from_pretrained(MODEL)
+
+
+class TestDirectConstructionInheritsPreset:
+
+    def test_bare_request_preserves_preset(self, preset: SamplingParam) -> None:
+        resolved = _resolve(GenerationRequest(prompt="a fox"))
+        assert resolved.num_inference_steps == preset.num_inference_steps == 3
+        assert resolved.guidance_scale == preset.guidance_scale == 3.0
+        assert (resolved.height, resolved.width) == (preset.height, preset.width) == (448, 832)
+        assert resolved.num_frames == preset.num_frames
+        assert resolved.fps == preset.fps == 16
+        assert resolved.negative_prompt == preset.negative_prompt
+        assert resolved.negative_prompt  # preset prompt is non-empty
+
+    def test_partial_sampling_overrides_only_set_fields(self, preset: SamplingParam) -> None:
+        resolved = _resolve(GenerationRequest(prompt="a fox", sampling=SamplingConfig(num_frames=81)))
+        assert resolved.num_frames == 81
+        assert resolved.num_inference_steps == preset.num_inference_steps
+        assert resolved.guidance_scale == preset.guidance_scale
+        assert (resolved.height, resolved.width) == (preset.height, preset.width)
+
+    def test_explicit_value_equal_to_old_schema_default_wins(self) -> None:
+        # gs=1.0 was the old schema default; it must still be honored
+        # when the caller sets it deliberately against a gs=3.0 preset.
+        resolved = _resolve(GenerationRequest(prompt="a fox", sampling=SamplingConfig(guidance_scale=1.0)))
+        assert resolved.guidance_scale == 1.0
+
+    def test_negative_prompt_none_inherits_empty_string_clears(self, preset: SamplingParam) -> None:
+        inherited = _resolve(GenerationRequest(prompt="x"))
+        assert inherited.negative_prompt == preset.negative_prompt
+        cleared = _resolve(GenerationRequest(prompt="x", negative_prompt=""))
+        assert cleared.negative_prompt == ""
+
+    def test_parsed_dict_request_also_inherits(self) -> None:
+        from fastvideo.api.parser import parse_config
+        resolved = _resolve(GenerationRequest(prompt="a fox"))
+        parsed_resolved = _resolve(parse_config(GenerationRequest, {"prompt": "a fox"}))
+        assert parsed_resolved.num_inference_steps == resolved.num_inference_steps
+        assert parsed_resolved.guidance_scale == resolved.guidance_scale
+
+    def test_explicitly_set_unsupported_field_still_raises(self) -> None:
+        # true_cfg_scale exists on SamplingConfig but not on Wan's
+        # SamplingParam; setting it explicitly must fail loudly (the old
+        # schema-default tolerance is gone).
+        with pytest.raises(ValueError, match="true_cfg_scale"):
+            _resolve(GenerationRequest(prompt="x", sampling=SamplingConfig(true_cfg_scale=2.0)))
+
+
+class TestParsedNullsInherit:
+    """YAML/JSON `null` must behave exactly like an unset field — it is
+    parsed into None but never bound as an explicit path."""
+
+    def test_parsed_sampling_null_inherits_preset(self, preset: SamplingParam) -> None:
+        from fastvideo.api.parser import parse_config
+        parsed = parse_config(GenerationRequest, {"prompt": "x", "sampling": {"num_frames": None, "height": None}})
+        resolved = _resolve(parsed)
+        assert resolved.num_frames == preset.num_frames
+        assert resolved.height == preset.height
+
+    def test_parsed_negative_prompt_null_inherits_preset(self, preset: SamplingParam) -> None:
+        from fastvideo.api.parser import parse_config
+        parsed = parse_config(GenerationRequest, {"prompt": "x", "negative_prompt": None})
+        assert _resolve(parsed).negative_prompt == preset.negative_prompt
+
+    def test_direct_none_valued_extension_is_dropped_not_resurrected(self) -> None:
+        # A None-valued extension entry is pruned from the explicit paths;
+        # it must not resurface via the live-attribute read in
+        # explicit_request_updates and raise as an unsupported field.
+        request = GenerationRequest(prompt="x", extensions={"bogus_key": None})
+        resolved = _resolve(request)
+        assert resolved.num_inference_steps == 3  # preset intact, no ValueError
diff --git a/fastvideo/tests/entrypoints/streaming/test_auxiliaries.py b/fastvideo/tests/entrypoints/streaming/test_auxiliaries.py
index 3be3ef47c..3140af8b6 100644
--- a/fastvideo/tests/entrypoints/streaming/test_auxiliaries.py
+++ b/fastvideo/tests/entrypoints/streaming/test_auxiliaries.py
@@ -224,6 +224,28 @@ class TestMockServer:
         app = build_mock_app()
         assert isinstance(app, FastAPI)
 
+    def test_build_app_rejects_unpinned_default_sampling(self):
+        import pytest
+
+        from fastvideo.entrypoints.streaming.mock_server import MockGenerator
+        from fastvideo.entrypoints.streaming.server import build_app
+        from fastvideo.api.schema import (
+            GeneratorConfig,
+            SamplingConfig,
+            ServeConfig,
+            StreamingConfig,
+        )
+
+        serve_config = ServeConfig(
+            generator=GeneratorConfig(model_path="/models/mock"),
+            streaming=StreamingConfig(),
+        )
+        # fps (among others) left None = "inherit preset" — meaningless to
+        # the WS encoder, so build_app must reject before serving.
+        serve_config.default_request.sampling = SamplingConfig(num_frames=24, height=256, width=256)
+        with pytest.raises(ValueError, match="fps"):
+            build_app(serve_config, MockGenerator())
+
     def test_mock_generator_produces_frames(self):
         from fastvideo.api.schema import GenerationRequest, SamplingConfig
         from fastvideo.entrypoints.streaming.mock_server import MockGenerator
diff --git a/fastvideo/tests/entrypoints/test_video_generator.py b/fastvideo/tests/entrypoints/test_video_generator.py
index dae6151fa..ce234c62c 100644
--- a/fastvideo/tests/entrypoints/test_video_generator.py
+++ b/fastvideo/tests/entrypoints/test_video_generator.py
@@ -261,7 +261,7 @@ def test_generate_uses_typed_request_path(monkeypatch):
     assert result.video_path == "outputs/test.mp4"
 
 
-def test_generate_preserves_schema_defaults_for_dataclass_request(monkeypatch):
+def test_generate_dataclass_request_inherits_preset_for_unset_fields(monkeypatch):
     generator = _new_runtime_video_generator()
     captured = {}
 
@@ -288,11 +288,18 @@ def test_generate_preserves_schema_defaults_for_dataclass_request(monkeypatch):
         )
     )
 
-    assert captured["sampling_param"].negative_prompt is None
+    # Explicitly-set fields win; None (negative_prompt) inherits the
+    # model preset instead of stomping it with the schema default.
+    assert captured["sampling_param"].negative_prompt == "model default"
     assert captured["sampling_param"].num_frames == 125
     assert captured["sampling_param"].height == 720
     assert captured["sampling_param"].width == 1280
 
+    generator.generate(GenerationRequest(prompt="hello world", negative_prompt=""))
+    assert captured["sampling_param"].negative_prompt == ""
+    assert captured["sampling_param"].num_frames == 61
+    assert captured["sampling_param"].height == 448
+
 
 def test_generate_mapping_request_preserves_model_defaults_for_omitted_fields(
     monkeypatch,