DecodingError when handling MCP tool calls - type 'mcp_call' not recognized

[VikashLoomba]

Bug Description

When using MCP tools with the CreateModelResponseQuery API, the library throws a DecodingError when processing MCP tool call events.

Error Details

valueNotFound(OpenAI.Components.Schemas.OutputItem, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "item", intValue: nil)], debugDescription: "The anyOf structure did not decode into any child schema."))

Cannot initialize _TypePayload from invalid String value mcp_call

Root Cause

The server sends OutputItem with type "mcp_call" (with underscore), but the OutputItem decoder in ResponseOutputItemDoneEvent fails to decode this properly.

Steps to Reproduce

1. Configure an MCP tool using Tool.mcpTool()
2. Send a query using CreateModelResponseQuery with the MCP tool
3. When the model invokes the MCP tool, the stream processing fails with a DecodingError

Expected Behavior

The library should properly decode MCP tool call events without throwing errors.

Code Example

let mcpTool = Tool.mcpTool(
    Components.Schemas.MCPTool(
        _type: .mcp,
        serverLabel: "playwright-mcp",
        serverUrl: "https://example.com/mcp",
        headers: Components.Schemas.MCPTool.HeadersPayload(additionalProperties: ["Authorization": "Bearer token"]),
        allowedTools: .case1(["browser_navigate"]),
        requireApproval: .case2(.never)
    )
)

let query = CreateModelResponseQuery(
    input: .inputItemList(inputItems),
    model: "gpt-5-2025-08-07",
    stream: true,
    tools: [mcpTool]
)

let stream: AsyncThrowingStream<ResponseStreamEvent, Error> = client.responses.createResponseStreaming(query: query)

// This fails when MCP tool is called
for try await event in stream {
    // Processing fails with DecodingError
}

Environment

• Library Version: Latest main branch
• Swift Version: 5.x
• Platform: iOS
• Xcode Version: Latest

Workaround

Currently catching and ignoring the DecodingError allows the stream to continue, but this is not ideal as it may hide other legitimate errors.