diff --git a/src/marketdata/input_types/options.py b/src/marketdata/input_types/options.py
index 7da03ee..b15dba9 100644
--- a/src/marketdata/input_types/options.py
+++ b/src/marketdata/input_types/options.py
@@ -35,7 +35,9 @@ class OptionsChainInput(BaseInputType):
         description="The expiration date to filter by", default=None
     )
     days_to_expiration: int | None = Field(
-        description="The number of days to expiration to filter by", default=None
+        description="The number of days to expiration to filter by",
+        alias="dte",
+        default=None,
     )
     from_date: datetime.date | str | None = Field(
         description="The start date to fetch options chain for",
diff --git a/src/tests/test_input_types.py b/src/tests/test_input_types.py
index 6c5d317..5ce292c 100644
--- a/src/tests/test_input_types.py
+++ b/src/tests/test_input_types.py
@@ -1,15 +1,19 @@
 import datetime
+import importlib
+import pkgutil
 from pathlib import Path
 
 import pytest
 from pydantic import Field, model_validator
 
+import marketdata.input_types as input_types_pkg
 from marketdata.exceptions import MinMaxDateValidationError
 from marketdata.input_types.base import (
     BaseInputType,
     OutputFormat,
     UserUniversalAPIParams,
 )
+from marketdata.internal_settings import GLOBAL_EXCLUDED_PARAMS
 
 
 class DummyInput(BaseInputType):
@@ -22,6 +26,70 @@ def validate_input(self) -> "DummyInput":
         return self
 
 
+def _all_input_models() -> list[type[BaseInputType]]:
+    """Return every concrete BaseInputType subclass defined in the SDK.
+
+    Imports each input_types submodule first so all subclasses are registered.
+    """
+    for module_info in pkgutil.iter_modules(input_types_pkg.__path__):
+        importlib.import_module(f"{input_types_pkg.__name__}.{module_info.name}")
+
+    seen: set[type[BaseInputType]] = set()
+    stack: list[type[BaseInputType]] = [BaseInputType]
+    while stack:
+        for sub in stack.pop().__subclasses__():
+            if sub not in seen:
+                seen.add(sub)
+                stack.append(sub)
+    # Only audit models shipped in the SDK, not test-only helper subclasses.
+    return sorted(
+        (c for c in seen if c.__module__.startswith(input_types_pkg.__name__)),
+        key=lambda c: c.__name__,
+    )
+
+
+def _snake_case_fields() -> list[tuple[str, str, "object"]]:
+    """All (model_name, field_name, field) tuples for fields whose Python name
+    differs from a bare API parameter (i.e. contain an underscore)."""
+    cases = []
+    for model in _all_input_models():
+        for field_name, field in model.model_fields.items():
+            if "_" in field_name:
+                cases.append((model.__name__, field_name, field))
+    return cases
+
+
+_SNAKE_CASE_FIELDS = _snake_case_fields()
+
+
+def test_snake_case_field_discovery_is_not_empty():
+    # Guard: if discovery silently breaks, the parametrized test below would
+    # vacuously pass. days_to_expiration alone guarantees at least one case.
+    assert _SNAKE_CASE_FIELDS
+
+
+@pytest.mark.parametrize(
+    "model_name, field_name, field",
+    _SNAKE_CASE_FIELDS,
+    ids=[f"{m}.{f}" for m, f, _ in _SNAKE_CASE_FIELDS],
+)
+def test_snake_case_input_fields_have_api_alias(model_name, field_name, field):
+    """Every multi-word input field must be sent under an explicit API alias.
+
+    The URL builder serializes with ``by_alias=True``; a snake_case field
+    without an alias would leak its Python name to the wire (see issue #30,
+    ``days_to_expiration`` -> ``dte``). Fields that are never serialized to the
+    query string are exempted via GLOBAL_EXCLUDED_PARAMS.
+    """
+    if field_name in GLOBAL_EXCLUDED_PARAMS:
+        return
+
+    assert field.alias and field.alias != field_name, (
+        f"{model_name}.{field_name} has no API alias; it would be sent to the "
+        f"API as '{field_name}'. Add an alias or exclude it."
+    )
+
+
 def test_base_input_type_min_max_validation():
     with pytest.raises(MinMaxDateValidationError):
         DummyInput(min_param="2025-01-01", max_param="2024-01-01")
diff --git a/src/tests/test_options_chain.py b/src/tests/test_options_chain.py
index a8477e0..8a680d5 100644
--- a/src/tests/test_options_chain.py
+++ b/src/tests/test_options_chain.py
@@ -334,3 +334,22 @@ def test_options_chain_strike_limit_is_int_on_wire(load_json, respx_mock, client
 
     params = respx_mock.calls.last.request.url.params
     assert params.get("strikeLimit") == "10"
+
+
+def test_options_chain_input_days_to_expiration_alias_on_wire(
+    load_json, respx_mock, client
+):
+    mock_data = load_json("options_chain_response_200")
+    respx_mock.get("https://api.marketdata.app/v1/options/chain/AAPL/").respond(
+        json=mock_data, status_code=200
+    )
+
+    client.options.chain(
+        "AAPL",
+        days_to_expiration=30,
+        output_format=OutputFormat.INTERNAL,
+    )
+
+    params = respx_mock.calls.last.request.url.params
+    assert params.get("dte") == "30"
+    assert params.get("days_to_expiration") is None