diff --git a/pyproject.toml b/pyproject.toml
index 750144cb..c16bce80 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -50,11 +50,13 @@ testpaths = ["tests"]
 python_files = "test_*.py"
 python_functions = "test_*"
 addopts = "-ra --strict-markers"
-asyncio_mode = "strict"
 markers = [
   "asyncio: mark a test as a coroutine that should be run by pytest-asyncio",
 ]
 
+[tool.pytest-asyncio]
+mode = "strict"
+
 [build-system]
 requires = ["hatchling", "uv-dynamic-versioning"]
 build-backend = "hatchling.build"
diff --git a/src/a2a/server/apps/jsonrpc/jsonrpc_app.py b/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
index ff6998cd..0b78cc1c 100644
--- a/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
+++ b/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
@@ -375,7 +375,10 @@ async def _handle_get_agent_card(self, request: Request) -> JSONResponse:
         # The public agent card is a direct serialization of the agent_card
         # provided at initialization.
         return JSONResponse(
-            self.agent_card.model_dump(mode='json', exclude_none=True)
+            self.agent_card.model_dump(
+                exclude_none=True,
+                by_alias=True,
+            )
         )
 
     async def _handle_get_authenticated_extended_agent_card(
@@ -392,7 +395,8 @@ async def _handle_get_authenticated_extended_agent_card(
         if self.extended_agent_card:
             return JSONResponse(
                 self.extended_agent_card.model_dump(
-                    mode='json', exclude_none=True
+                    exclude_none=True,
+                    by_alias=True,
                 )
             )
         # If supportsAuthenticatedExtendedCard is true, but no specific
diff --git a/tests/server/apps/jsonrpc/test_serialization.py b/tests/server/apps/jsonrpc/test_serialization.py
new file mode 100644
index 00000000..ea3da1c0
--- /dev/null
+++ b/tests/server/apps/jsonrpc/test_serialization.py
@@ -0,0 +1,94 @@
+from unittest import mock
+
+import pytest
+from starlette.testclient import TestClient
+
+from a2a.server.apps import A2AFastAPIApplication, A2AStarletteApplication
+from a2a.types import (
+    APIKeySecurityScheme,
+    AgentCapabilities,
+    AgentCard,
+    In,
+    SecurityScheme,
+)
+from pydantic import ValidationError
+
+
+@pytest.fixture
+def agent_card_with_api_key():
+    """Provides an AgentCard with an APIKeySecurityScheme for testing serialization."""
+    # This data uses the alias 'in', which is correct for creating the model.
+    api_key_scheme_data = {
+        'type': 'apiKey',
+        'name': 'X-API-KEY',
+        'in': 'header',
+    }
+    api_key_scheme = APIKeySecurityScheme.model_validate(api_key_scheme_data)
+
+    agent_card = AgentCard(
+        name='APIKeyAgent',
+        description='An agent that uses API Key auth.',
+        url='http://example.com/apikey-agent',
+        version='1.0.0',
+        capabilities=AgentCapabilities(),
+        defaultInputModes=['text/plain'],
+        defaultOutputModes=['text/plain'],
+        skills=[],
+        securitySchemes={'api_key_auth': SecurityScheme(root=api_key_scheme)},
+        security=[{'api_key_auth': []}],
+    )
+    return agent_card
+
+
+def test_starlette_agent_card_with_api_key_scheme_alias(
+    agent_card_with_api_key: AgentCard,
+):
+    """
+    Tests that the A2AStarletteApplication endpoint correctly serializes aliased fields.
+
+    This verifies the fix for `APIKeySecurityScheme.in_` being serialized as `in_` instead of `in`.
+    """
+    handler = mock.AsyncMock()
+    app_instance = A2AStarletteApplication(agent_card_with_api_key, handler)
+    client = TestClient(app_instance.build())
+
+    response = 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
+
+    try:
+        parsed_card = AgentCard.model_validate(response_data)
+        parsed_scheme_wrapper = parsed_card.securitySchemes['api_key_auth']
+        assert isinstance(parsed_scheme_wrapper.root, APIKeySecurityScheme)
+        assert parsed_scheme_wrapper.root.in_ == In.header
+    except ValidationError as e:
+        pytest.fail(
+            f"AgentCard.model_validate failed on the server's response: {e}"
+        )
+
+
+def test_fastapi_agent_card_with_api_key_scheme_alias(
+    agent_card_with_api_key: AgentCard,
+):
+    """
+    Tests that the A2AFastAPIApplication endpoint correctly serializes aliased fields.
+
+    This verifies the fix for `APIKeySecurityScheme.in_` being serialized as `in_` instead of `in`.
+    """
+    handler = mock.AsyncMock()
+    app_instance = A2AFastAPIApplication(agent_card_with_api_key, handler)
+    client = TestClient(app_instance.build())
+
+    response = 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 'in_' not in security_scheme_json
+    assert security_scheme_json['in'] == 'header'