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fix: preserve anyOf nullable schemas without injecting type field #264

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fix: preserve anyOf nullable schemas without injecting type field #264

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6 changes: 3 additions & 3 deletions fastapi_mcp/openapi/convert.py
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Original file line number Diff line number Diff line change
Expand Up @@ -207,7 +207,7 @@ def convert_openapi_to_mcp_tools(
if param_desc:
properties[param_name]["description"] = param_desc

if "type" not in properties[param_name]:
if "type" not in properties[param_name] and "anyOf" not in properties[param_name]:
properties[param_name]["type"] = param_schema.get("type", "string")

if param_required:
Expand All @@ -224,7 +224,7 @@ def convert_openapi_to_mcp_tools(
if param_desc:
properties[param_name]["description"] = param_desc

if "type" not in properties[param_name]:
if "type" not in properties[param_name] and "anyOf" not in properties[param_name]:
properties[param_name]["type"] = get_single_param_type_from_schema(param_schema)

if "default" in param_schema:
Expand All @@ -244,7 +244,7 @@ def convert_openapi_to_mcp_tools(
if param_desc:
properties[param_name]["description"] = param_desc

if "type" not in properties[param_name]:
if "type" not in properties[param_name] and "anyOf" not in properties[param_name]:
properties[param_name]["type"] = get_single_param_type_from_schema(param_schema)

if "default" in param_schema:
Expand Down
85 changes: 80 additions & 5 deletions tests/test_openapi_conversion.py
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Original file line number Diff line number Diff line change
Expand Up @@ -176,12 +176,14 @@ def test_parameter_handling(complex_fastapi_app: FastAPI):
assert "product_id" not in properties # This is from get_product, not list_products

assert "category" in properties
assert properties["category"].get("type") == "string" # Enum converted to string
assert "anyOf" in properties["category"] # Nullable enum preserved as anyOf
assert "type" not in properties["category"] # No top-level type for nullable schemas
assert "description" in properties["category"]
assert "Filter by product category" in properties["category"]["description"]

assert "min_price" in properties
assert properties["min_price"].get("type") == "number"
assert "anyOf" in properties["min_price"] # Nullable number preserved as anyOf
assert "type" not in properties["min_price"] # No top-level type for nullable schemas
assert "description" in properties["min_price"]
assert "Minimum price filter" in properties["min_price"]["description"]
if "minimum" in properties["min_price"]:
Expand All @@ -204,7 +206,7 @@ def test_parameter_handling(complex_fastapi_app: FastAPI):
assert properties["size"]["maximum"] <= 100 # le=100 in Query param

assert "tag" in properties
assert properties["tag"].get("type") == "array"
assert "anyOf" in properties["tag"] # Nullable array preserved as anyOf

required = list_products_tool.inputSchema.get("required", [])
assert "page" not in required # Has default value
Expand Down Expand Up @@ -416,9 +418,82 @@ def test_body_params_edge_cases(complex_fastapi_app: FastAPI):
assert properties["customer_id"]["title"] == "customer_id"

assert "notes" in properties
assert "type" in properties["notes"]
assert properties["notes"]["type"] in ["string", "object"] # Default should be either string or object
# notes is nullable (anyOf with null), so it should have anyOf or type
assert "type" in properties["notes"] or "anyOf" in properties["notes"]

if "items" in properties:
item_props = properties["items"]["items"]["properties"]
assert "total" in item_props


def test_nullable_anyof_schema_preserved():
"""Test that anyOf with null type is preserved without injecting a type field."""
openapi_schema = {
"openapi": "3.1.0",
"info": {"title": "Test", "version": "0.1.0"},
"paths": {
"/items": {
"post": {
"operationId": "create_item",
"summary": "Create an item",
"requestBody": {
"content": {
"application/json": {
"schema": {
"type": "object",
"properties": {
"name": {"type": "string"},
"description": {
"anyOf": [{"type": "string"}, {"type": "null"}],
"title": "Description",
},
},
"required": ["name"],
}
}
},
"required": True,
},
"responses": {"200": {"description": "OK"}},
}
},
"/items/{item_id}": {
"get": {
"operationId": "get_item",
"summary": "Get an item",
"parameters": [
{
"name": "item_id",
"in": "path",
"required": True,
"schema": {"type": "string"},
},
{
"name": "fields",
"in": "query",
"required": False,
"schema": {
"anyOf": [{"type": "string"}, {"type": "null"}],
"title": "Fields",
},
},
],
"responses": {"200": {"description": "OK"}},
}
},
},
}

tools, _ = convert_openapi_to_mcp_tools(openapi_schema)

# Check body parameter with anyOf
create_tool = next(t for t in tools if t.name == "create_item")
desc_prop = create_tool.inputSchema["properties"]["description"]
assert "anyOf" in desc_prop
assert "type" not in desc_prop, "type field should not be injected when anyOf is present"

# Check query parameter with anyOf
get_tool = next(t for t in tools if t.name == "get_item")
fields_prop = get_tool.inputSchema["properties"]["fields"]
assert "anyOf" in fields_prop
assert "type" not in fields_prop, "type field should not be injected when anyOf is present"