diff --git a/python/freetoken/server/api_server.py b/python/freetoken/server/api_server.py
index 3e2acc854..94b3e10ef 100644
--- a/python/freetoken/server/api_server.py
+++ b/python/freetoken/server/api_server.py
@@ -33,7 +33,7 @@
     init_logger,
     load_generation_sampling,
 )
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from .args import ServerArgs
 from .anthropic_api import register_anthropic_routes
@@ -494,7 +494,7 @@ class CacheRebuildRequest(BaseModel):
     # is deferred (needs the drain-gate machinery); constraining the Literal makes an
     # unsupported value fail fast with a 422 at the API layer instead of a generic 503.
     mode: Literal["if_idle"] = "if_idle"
-    timeout: float = 300.0
+    timeout: float = Field(default=300.0, gt=0, le=3600)
 
 
 async def dispatch_rebuild(
diff --git a/python/freetoken/server/generation.py b/python/freetoken/server/generation.py
index be05d908a..a60119ecf 100644
--- a/python/freetoken/server/generation.py
+++ b/python/freetoken/server/generation.py
@@ -15,6 +15,7 @@
 
 import asyncio
 import json
+import math
 import time
 from collections.abc import AsyncIterator
 from dataclasses import dataclass, field
@@ -177,12 +178,21 @@ def pick(value, key, framework):
     # non-positive value is a client error.
     if max_tokens is not None and max_tokens < 1:
         raise ValueError(f"max_tokens must be at least 1, got {max_tokens}")
+    resolved_temperature = pick(temperature, "temperature", 0.0)
+    if not math.isfinite(resolved_temperature) or resolved_temperature < 0:
+        raise ValueError(f"temperature must be a finite number >= 0, got {resolved_temperature}")
+    resolved_top_p = pick(top_p, "top_p", 1.0)
+    if not math.isfinite(resolved_top_p) or not 0 < resolved_top_p <= 1:
+        raise ValueError(f"top_p must be in (0, 1], got {resolved_top_p}")
+    resolved_top_k = pick(top_k, "top_k", -1)
+    if resolved_top_k != -1 and resolved_top_k < 1:
+        raise ValueError(f"top_k must be -1 (disabled) or >= 1, got {resolved_top_k}")
     return SamplingParams(
         ignore_eos=ignore_eos,
         max_tokens=DEFAULT_MAX_OUTPUT_TOKENS if max_tokens is None else max_tokens,
-        temperature=pick(temperature, "temperature", 0.0),
-        top_k=pick(top_k, "top_k", -1),
-        top_p=pick(top_p, "top_p", 1.0),
+        temperature=resolved_temperature,
+        top_k=resolved_top_k,
+        top_p=resolved_top_p,
         stop_strs=[s for s in stop_list if s],  # drop empty strings (would match everything)
     )
 
diff --git a/tests/server/test_openai_api.py b/tests/server/test_openai_api.py
index facd469b3..f6808a7d3 100644
--- a/tests/server/test_openai_api.py
+++ b/tests/server/test_openai_api.py
@@ -676,3 +676,66 @@ def test_minimax_http_non_stream_forces_implicit_reasoning_without_request_knob(
     message = response["choices"][0]["message"]
     assert message["reasoning_content"] == "private thought"
     assert message["content"] == "visible answer"
+
+
+import pytest  # noqa: E402
+
+
+@pytest.mark.parametrize(
+    ("field", "value"),
+    [
+        ("temperature", -0.5),
+        ("top_p", 0.0),
+        ("top_k", 0),
+    ],
+)
+def test_chat_completion_rejects_invalid_sampling(field, value):
+    app = FastAPI()
+    state = FakeState([])
+
+    @app.post("/v1/chat/completions")
+    async def chat_completion(req: ChatCompletionRequest):
+        return await handle_chat_completion(req, request=None, state=state, model_sampling={})
+
+    body = {
+        "model": "unit-model",
+        "messages": [{"role": "user", "content": "hi"}],
+        "stream": True,
+        field: value,
+    }
+    response = TestClient(app).post("/v1/chat/completions", json=body)
+
+    assert response.status_code == 400
+    error = response.json()["error"]
+    assert error["type"] == "invalid_request_error"
+    assert field in error["message"]
+    assert state.sent is None
+
+
+@pytest.mark.parametrize(
+    "sampling",
+    [
+        {"temperature": 0.0, "top_p": 1.0, "top_k": -1},
+        {"temperature": 0.0, "top_p": 1.0, "top_k": 1},
+    ],
+)
+def test_chat_completion_accepts_sampling_boundaries(sampling):
+    app = FastAPI()
+    state = FakeState([])
+
+    @app.post("/v1/chat/completions")
+    async def chat_completion(req: ChatCompletionRequest):
+        return await handle_chat_completion(req, request=None, state=state, model_sampling={})
+
+    response = TestClient(app).post(
+        "/v1/chat/completions",
+        json={
+            "model": "unit-model",
+            "messages": [{"role": "user", "content": "hi"}],
+            "stream": True,
+            **sampling,
+        },
+    )
+
+    assert response.status_code == 200
+    assert state.sent is not None
diff --git a/tests/server/test_rebuild_maintenance.py b/tests/server/test_rebuild_maintenance.py
index a1d10a7e5..0df3d6bfe 100644
--- a/tests/server/test_rebuild_maintenance.py
+++ b/tests/server/test_rebuild_maintenance.py
@@ -327,3 +327,18 @@ def test_cache_rebuild_request_rejects_unknown_mode():
     assert CacheRebuildRequest(mode="if_idle").mode == "if_idle"
     with pytest.raises(ValidationError):
         CacheRebuildRequest(mode="drain")
+
+
+from pydantic import ValidationError  # noqa: E402
+
+from freetoken.server.api_server import CacheRebuildRequest  # noqa: E402
+
+
+def test_rebuild_timeout_bounds():
+    import pytest
+
+    for bad in (0, -5, 3601, float("nan")):
+        with pytest.raises(ValidationError):
+            CacheRebuildRequest(timeout=bad)
+    assert CacheRebuildRequest(timeout=3600).timeout == 3600
+    assert CacheRebuildRequest().timeout == 300.0