diff --git a/src/a2a/types.py b/src/a2a/types.py
index 918a06b5e..cc1cbcb16 100644
--- a/src/a2a/types.py
+++ b/src/a2a/types.py
@@ -34,7 +34,7 @@ class APIKeySecurityScheme(A2ABaseModel):
     """
     An optional description for the security scheme.
     """
-    in_: In
+    in_: In = Field(..., alias='in')
     """
     The location of the API key.
     """
diff --git a/src/a2a/utils/telemetry.py b/src/a2a/utils/telemetry.py
index c73d2ac92..22d1e8730 100644
--- a/src/a2a/utils/telemetry.py
+++ b/src/a2a/utils/telemetry.py
@@ -61,6 +61,8 @@ def internal_method(self):
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any
 
+from typing_extensions import Self
+
 
 if TYPE_CHECKING:
     from opentelemetry.trace import SpanKind as SpanKindType
@@ -86,7 +88,7 @@ class _NoOp:
         def __call__(self, *args: Any, **kwargs: Any) -> Any:
             return self
 
-        def __enter__(self) -> '_NoOp':
+        def __enter__(self) -> Self:
             return self
 
         def __exit__(self, *args: object, **kwargs: Any) -> None:
diff --git a/tests/server/apps/rest/test_rest_serialization.py b/tests/server/apps/rest/test_rest_serialization.py
new file mode 100644
index 000000000..b76761643
--- /dev/null
+++ b/tests/server/apps/rest/test_rest_serialization.py
@@ -0,0 +1,66 @@
+from unittest import mock
+
+import pytest
+
+from httpx import ASGITransport, AsyncClient
+
+from a2a.server.apps.rest.fastapi_app import A2ARESTFastAPIApplication
+from a2a.types import (
+    APIKeySecurityScheme,
+    AgentCapabilities,
+    AgentCard,
+    In,
+    SecurityScheme,
+)
+
+
+@pytest.fixture
+def agent_card_with_api_key() -> AgentCard:
+    api_key_scheme_data = {
+        'type': 'apiKey',
+        'name': 'X-API-KEY',
+        'in': 'header',
+    }
+    api_key_scheme = APIKeySecurityScheme.model_validate(api_key_scheme_data)
+
+    return AgentCard(
+        name='APIKeyAgent',
+        description='An agent that uses API Key auth.',
+        url='http://example.com/apikey-agent',
+        version='1.0.0',
+        capabilities=AgentCapabilities(),
+        default_input_modes=['text/plain'],
+        default_output_modes=['text/plain'],
+        skills=[],
+        security_schemes={'api_key_auth': SecurityScheme(root=api_key_scheme)},
+        security=[{'api_key_auth': []}],
+    )
+
+
+@pytest.mark.anyio
+async def test_rest_agent_card_with_api_key_scheme_alias(
+    agent_card_with_api_key: AgentCard,
+):
+    """Ensures REST agent card serialization uses the 'in' alias."""
+    handler = mock.AsyncMock()
+    app_instance = A2ARESTFastAPIApplication(agent_card_with_api_key, handler)
+    app = app_instance.build(
+        agent_card_url='/.well-known/agent.json', rpc_url=''
+    )
+
+    async with AsyncClient(
+        transport=ASGITransport(app=app), base_url='http://test'
+    ) as client:
+        response = await client.get('/.well-known/agent.json')
+
+    assert response.status_code == 200
+    response_data = response.json()
+
+    security_scheme_json = response_data['securitySchemes']['api_key_auth']
+    assert 'in' in security_scheme_json
+    assert security_scheme_json['in'] == 'header'
+    assert 'in_' not in security_scheme_json
+
+    parsed_card = AgentCard.model_validate(response_data)
+    parsed_scheme_wrapper = parsed_card.security_schemes['api_key_auth']
+    assert parsed_scheme_wrapper.root.in_ == In.header
diff --git a/tests/test_types.py b/tests/test_types.py
index 73e6af7bb..563bf5e2f 100644
--- a/tests/test_types.py
+++ b/tests/test_types.py
@@ -182,13 +182,23 @@
 # --- Test Functions ---
 
 
-def test_security_scheme_valid():
-    scheme = SecurityScheme.model_validate(MINIMAL_AGENT_SECURITY_SCHEME)
+@pytest.mark.parametrize('in_field_name', ['in', 'in_'])
+def test_security_scheme_in_field_handling(in_field_name: str) -> None:
+    scheme_data = {
+        'type': 'apiKey',
+        'name': 'X-API-KEY',
+        in_field_name: 'header',
+    }
+    scheme = SecurityScheme.model_validate(scheme_data)
     assert isinstance(scheme.root, APIKeySecurityScheme)
     assert scheme.root.type == 'apiKey'
     assert scheme.root.in_ == In.header
     assert scheme.root.name == 'X-API-KEY'
 
+    serialized_data = scheme.model_dump(mode='json', exclude_none=True)
+    assert serialized_data.get('in') == 'header'
+    assert 'in_' not in serialized_data
+
 
 def test_security_scheme_invalid():
     with pytest.raises(ValidationError):