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fix(openapi): preserve anyOf unions in MCP tool input schemas #316

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fix(openapi): preserve anyOf unions in MCP tool input schemas #316

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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
69 changes: 62 additions & 7 deletions tests/test_openapi_conversion.py
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Original file line number Diff line number Diff line change
Expand Up @@ -150,6 +150,57 @@ def test_schema_utils():
assert get_single_param_type_from_schema({"type": "string"}) == "string"
assert get_single_param_type_from_schema({"type": "array", "items": {"type": "string"}}) == "array"


def test_union_anyof_schema_preserves_anyof_without_injected_type():
openapi_schema = {
"openapi": "3.1.0",
"info": {"title": "Test", "version": "0.1.0"},
"paths": {
"/save": {
"post": {
"operationId": "save",
"summary": "Save tags",
"requestBody": {
"required": True,
"content": {
"application/json": {
"schema": {
"type": "object",
"properties": {
"tags": {
"anyOf": [
{
"type": "object",
"additionalProperties": {
"type": "array",
"items": {"type": "string"},
},
},
{"type": "array", "items": {"type": "string"}},
],
}
},
"required": ["tags"],
}
}
},
},
"responses": {"200": {"description": "OK"}},
}
}
},
}

tools, operation_map = convert_openapi_to_mcp_tools(openapi_schema)

save_tool = next(tool for tool in tools if tool.name == "save")
tags_schema = save_tool.inputSchema["properties"]["tags"]

assert "anyOf" in tags_schema
assert "type" not in tags_schema
assert len(tags_schema["anyOf"]) == 2
assert operation_map["save"]["method"] == "post"

array_schema = {"type": "array", "items": {"type": "string", "enum": ["red", "green", "blue"]}}
array_example = generate_example_from_schema(array_schema)
assert isinstance(array_example, list)
Expand All @@ -176,16 +227,19 @@ 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"]
assert "type" not in properties["category"]
assert properties["category"]["anyOf"][0].get("type") == "string"
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"]
assert properties["min_price"]["anyOf"][0].get("type") == "number"
assert "description" in properties["min_price"]
assert "Minimum price filter" in properties["min_price"]["description"]
if "minimum" in properties["min_price"]:
assert properties["min_price"]["minimum"] > 0 # gt=0 in Query param
if "exclusiveMinimum" in properties["min_price"]["anyOf"][0]:
assert properties["min_price"]["anyOf"][0]["exclusiveMinimum"] == 0.0

assert "in_stock_only" in properties
assert properties["in_stock_only"].get("type") == "boolean"
Expand All @@ -204,7 +258,8 @@ 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"]
assert properties["tag"]["anyOf"][0].get("type") == "array"

required = list_products_tool.inputSchema.get("required", [])
assert "page" not in required # Has default value
Expand Down Expand Up @@ -416,8 +471,8 @@ 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
assert "anyOf" in properties["notes"]
assert "type" not in properties["notes"]

if "items" in properties:
item_props = properties["items"]["items"]["properties"]
Expand Down