diff --git a/src/core/schemas.py b/src/core/schemas.py
index aa81055a8..458d05fa0 100644
--- a/src/core/schemas.py
+++ b/src/core/schemas.py
@@ -76,6 +76,23 @@ def model_dump(self, **kwargs):
             kwargs["exclude_none"] = True
         return super().model_dump(**kwargs)
 
+    def model_dump_json(self, **kwargs):
+        """Dump model to JSON string with AdCP-compliant defaults.
+
+        By default, excludes None values to match AdCP spec where optional fields
+        should be omitted rather than set to null. This prevents JSON validation
+        errors from AdCP consumers that use "additionalProperties": false and don't
+        allow null for optional fields.
+
+        Examples:
+            response = ListAuthorizedPropertiesResponse(publisher_domains=["example.com"])
+            # Only includes publisher_domains, omits all None-valued optional fields
+            json_str = response.model_dump_json()  # exclude_none=True by default
+        """
+        if "exclude_none" not in kwargs:
+            kwargs["exclude_none"] = True
+        return super().model_dump_json(**kwargs)
+
 
 class TaskStatus(str, Enum):
     """Standardized task status enum per AdCP MCP Status specification.
diff --git a/tests/unit/test_adcp_json_serialization.py b/tests/unit/test_adcp_json_serialization.py
new file mode 100644
index 000000000..b9b7169d5
--- /dev/null
+++ b/tests/unit/test_adcp_json_serialization.py
@@ -0,0 +1,88 @@
+"""Test that AdCP responses correctly exclude None values in JSON serialization.
+
+This is critical for client compatibility - the AdCP client (adcp/client npm package)
+validates responses against JSON schemas that don't allow null for optional fields.
+
+Issue: PR #xxx - list_authorized_properties returned null for optional fields
+Fix: AdCPBaseModel.model_dump_json() now defaults to exclude_none=True
+"""
+
+import json
+
+from src.core.schema_adapters import (
+    GetProductsResponse,
+    ListAuthorizedPropertiesResponse,
+    ListCreativeFormatsResponse,
+)
+
+
+def test_list_authorized_properties_excludes_none_in_json():
+    """Test that model_dump_json() excludes None values by default.
+
+    This prevents schema validation errors in the AdCP client which expects
+    optional fields to be omitted (not set to null).
+    """
+    # Create response with only required field (all optional fields will be None)
+    response = ListAuthorizedPropertiesResponse(publisher_domains=["example.com"])
+
+    # Serialize to JSON
+    json_str = response.model_dump_json()
+    parsed = json.loads(json_str)
+
+    # Verify None fields are not present in JSON
+    assert "publisher_domains" in parsed
+    assert "primary_channels" not in parsed  # Should be excluded (None)
+    assert "primary_countries" not in parsed  # Should be excluded (None)
+    assert "portfolio_description" not in parsed  # Should be excluded (None)
+    assert "advertising_policies" not in parsed  # Should be excluded (None)
+    assert "last_updated" not in parsed  # Should be excluded (None)
+    assert "errors" not in parsed  # Should be excluded (None)
+
+
+def test_adcp_response_includes_explicit_values():
+    """Test that explicitly set values are included in JSON."""
+    response = ListAuthorizedPropertiesResponse(
+        publisher_domains=["example.com"],
+        primary_channels=["display", "video"],
+        advertising_policies="No tobacco or alcohol",
+    )
+
+    json_str = response.model_dump_json()
+    parsed = json.loads(json_str)
+
+    # Verify explicitly set fields are included
+    assert parsed["publisher_domains"] == ["example.com"]
+    assert parsed["primary_channels"] == ["display", "video"]
+    assert parsed["advertising_policies"] == "No tobacco or alcohol"
+
+    # Verify unset fields are still excluded
+    assert "primary_countries" not in parsed
+    assert "portfolio_description" not in parsed
+
+
+def test_model_dump_also_excludes_none():
+    """Test that model_dump() (dict) also excludes None by default."""
+    response = ListAuthorizedPropertiesResponse(publisher_domains=["example.com"])
+
+    dump = response.model_dump()
+
+    # Verify None fields are not present
+    assert "publisher_domains" in dump
+    assert "primary_channels" not in dump
+    assert "primary_countries" not in dump
+    assert "portfolio_description" not in dump
+
+
+def test_other_responses_also_exclude_none():
+    """Verify all AdCP response types exclude None values."""
+    # GetProductsResponse
+    products_resp = GetProductsResponse(products=[])
+    products_json = json.loads(products_resp.model_dump_json())
+    assert "products" in products_json
+    # Should not have None-valued optional fields
+
+    # ListCreativeFormatsResponse
+    formats_resp = ListCreativeFormatsResponse(formats=[])
+    formats_json = json.loads(formats_resp.model_dump_json())
+    assert "formats" in formats_json
+    # Should not have None-valued optional fields