Ensure openapi path params are handled properly

src/fastmcp/server/openapi.py

@@ -171,6 +171,24 @@ class OpenAPITool(Tool):
             raise ToolError(f"Missing required path parameters: {missing_params}")
 
         for param_name, param_value in path_params.items():
+            # Handle array path parameters with style 'simple' (comma-separated)
+            # In OpenAPI, 'simple' is the default style for path parameters
+            # and explode=False behavior for arrays
+            param_info = next(
+                (p for p in self._route.parameters if p.name == param_name), None
+            )
+
+            if param_info and isinstance(param_value, list):
+                # Check if schema indicates an array type
+                schema = param_info.schema_
+                is_array = schema.get("type") == "array"
+
+                if is_array:
+                    # Format array values as comma-separated string
+                    # This follows the OpenAPI 'simple' style (default for path)
+                    # and explode=False behavior for arrays
+                    param_value = ",".join(str(item) for item in param_value)
+
             path = path.replace(f"{{{param_name}}}", str(param_value))
 
         # Prepare query parameters - filter out None and empty strings

tests/server/test_openapi_array_params.py

@@ -0,0 +1,90 @@
+from unittest.mock import AsyncMock, MagicMock
+
+import httpx
+import pytest
+
+from fastmcp.server.openapi import OpenAPITool
+from fastmcp.utilities.openapi import HTTPRoute, ParameterInfo
+
+
+@pytest.fixture
+def mock_client():
+    """Create a mock httpx.AsyncClient."""
+    client = AsyncMock(spec=httpx.AsyncClient)
+    # Set up a mock response
+    mock_response = MagicMock()
+    mock_response.json.return_value = {"result": "success"}
+    mock_response.raise_for_status.return_value = None
+    client.request.return_value = mock_response
+    return client
+
+
+@pytest.mark.asyncio
+async def test_array_path_parameter_handling(mock_client):
+    """Test how array path parameters are handled."""
+    # Create a simple route with array path parameter
+    route = HTTPRoute(
+        path="/select/{days}",
+        method="PUT",
+        operation_id="test-operation",
+        parameters=[
+            ParameterInfo(
+                name="days",
+                location="path",
+                required=True,
+                schema={
+                    "type": "array",
+                    "items": {
+                        "type": "string",
+                        "enum": [
+                            "monday",
+                            "tuesday",
+                            "wednesday",
+                            "thursday",
+                            "friday",
+                            "saturday",
+                            "sunday",
+                        ],
+                    },
+                },
+            )
+        ],
+    )
+
+    # Create the tool
+    tool = OpenAPITool(
+        client=mock_client,
+        route=route,
+        name="test-operation",
+        description="Test operation",
+        parameters={},
+    )
+
+    # Test with a single value
+    await tool._execute_request(days=["monday"])
+
+    # Check that the path parameter is formatted correctly
+    # This is where the bug is: it should be '/select/monday' not '/select/[\'monday\']'
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday",  # This is the expected format
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )
+    mock_client.request.reset_mock()
+
+    # Test with multiple values
+    await tool._execute_request(days=["monday", "tuesday"])
+
+    # Check that the path parameter is formatted correctly
+    # It should be '/select/monday,tuesday' not '/select/[\'monday\', \'tuesday\']'
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday,tuesday",  # This is the expected format
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )

tests/server/test_openapi_path_parameters.py

@@ -0,0 +1,190 @@
+from unittest.mock import AsyncMock, MagicMock
+
+import httpx
+import pytest
+
+from fastmcp import FastMCP
+from fastmcp.server.openapi import OpenAPITool
+from fastmcp.utilities.openapi import HTTPRoute, ParameterInfo
+
+
+@pytest.fixture
+def array_path_spec():
+    """Load a minimal OpenAPI spec with an array path parameter."""
+    return {
+        "openapi": "3.1.0",
+        "info": {"title": "Test API", "version": "1.0.0"},
+        "paths": {
+            "/select/{days}": {
+                "put": {
+                    "operationId": "test-operation",
+                    "parameters": [
+                        {
+                            "name": "days",
+                            "in": "path",
+                            "required": True,
+                            "style": "simple",
+                            "explode": False,
+                            "schema": {
+                                "type": "array",
+                                "items": {
+                                    "type": "string",
+                                    "enum": [
+                                        "monday",
+                                        "tuesday",
+                                        "wednesday",
+                                        "thursday",
+                                        "friday",
+                                        "saturday",
+                                        "sunday",
+                                    ],
+                                },
+                            },
+                        }
+                    ],
+                    "responses": {
+                        "200": {
+                            "description": "Success",
+                            "content": {
+                                "application/json": {
+                                    "schema": {
+                                        "type": "object",
+                                        "properties": {"result": {"type": "string"}},
+                                        "required": ["result"],
+                                    }
+                                }
+                            },
+                        }
+                    },
+                }
+            }
+        },
+    }
+
+
+@pytest.fixture
+def mock_client():
+    """Create a mock httpx.AsyncClient."""
+    client = AsyncMock(spec=httpx.AsyncClient)
+    # Set up a mock response
+    mock_response = MagicMock()
+    mock_response.json.return_value = {"result": "success"}
+    mock_response.raise_for_status.return_value = None
+    client.request.return_value = mock_response
+    return client
+
+
+async def test_fastmcp_from_openapi(array_path_spec, mock_client):
+    """Test creating FastMCP from OpenAPI spec with array path parameter."""
+    # Create FastMCP from the spec
+    mcp = FastMCP.from_openapi(array_path_spec, client=mock_client)
+
+    # Verify the tool was created using the MCP protocol method
+    tools_result = await mcp.get_tools()
+    tool_names = [tool.name for tool in tools_result.values()]
+    assert "test-operation" in tool_names
+
+
+@pytest.mark.asyncio
+async def test_array_path_parameter_handling(mock_client):
+    """Test how array path parameters are handled."""
+    # Create a simple route with array path parameter
+    route = HTTPRoute(
+        path="/select/{days}",
+        method="PUT",
+        operation_id="test-operation",
+        parameters=[
+            ParameterInfo(
+                name="days",
+                location="path",
+                required=True,
+                schema={
+                    "type": "array",
+                    "items": {
+                        "type": "string",
+                        "enum": [
+                            "monday",
+                            "tuesday",
+                            "wednesday",
+                            "thursday",
+                            "friday",
+                            "saturday",
+                            "sunday",
+                        ],
+                    },
+                },
+            )
+        ],
+    )
+
+    # Create the tool
+    tool = OpenAPITool(
+        client=mock_client,
+        route=route,
+        name="test-operation",
+        description="Test operation",
+        parameters={},
+    )
+
+    # Test with a single value
+    await tool._execute_request(days=["monday"])
+
+    # Check that the path parameter is formatted correctly
+    # This is where the bug is: it should be '/select/monday' not '/select/[\'monday\']'
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday",  # This is the expected format
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )
+    mock_client.request.reset_mock()
+
+    # Test with multiple values
+    await tool._execute_request(days=["monday", "tuesday"])
+
+    # Check that the path parameter is formatted correctly
+    # It should be '/select/monday,tuesday' not '/select/[\'monday\', \'tuesday\']'
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday,tuesday",  # This is the expected format
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )
+
+
+@pytest.mark.asyncio
+async def test_integration_array_path_parameter(array_path_spec, mock_client):
+    """Integration test for array path parameters."""
+    # Create FastMCP from the spec
+    mcp = FastMCP.from_openapi(array_path_spec, client=mock_client)
+
+    # Call the tool with a single value
+    await mcp._mcp_call_tool("test-operation", {"days": ["monday"]})
+
+    # Check the request was made correctly
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday",
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )
+    mock_client.request.reset_mock()
+
+    # Call the tool with multiple values
+    await mcp._mcp_call_tool("test-operation", {"days": ["monday", "tuesday"]})
+
+    # Check the request was made correctly
+    mock_client.request.assert_called_with(
+        method="PUT",
+        url="/select/monday,tuesday",
+        params={},
+        headers={},
+        json=None,
+        timeout=None,
+    )