Add custom tool support to Response API

[jamesrochabrun]

Summary

Adds support for custom tools that return plain text instead of JSON in the Response API.

Changes

• Tool.swift: Add CustomTool struct and .custom case to Tool enum
• OutputItem.swift: Add CustomToolCall struct and .customToolCall case for response handling
• ToolChoiceMode.swift: Add CustomToolChoice struct and .customTool case for forced tool selection

Benefits

Custom tools enable models to return plain text responses without JSON wrapping, which is useful for:

• Code execution tools (e.g., print("hello world") instead of {"code": "print(\"hello world\")"))
• Mathematical expressions
• Structured text outputs
• Avoiding unnecessary JSON overhead

Example Usage

// Define a custom tool
let customTool = Tool.custom(.init(
    name: "code_exec",
    description: "Executes arbitrary Python code."
))

// Use in Response API
let parameters = ModelResponseParameter(
    input: .string("Use the code_exec tool to print hello world."),
    model: .custom("gpt-5"),
    tools: [customTool]
)

// Response will include CustomToolCall with plain text input
// e.g., input: "print(\"hello world\")"

Implementation Notes

• Only implements explicitly documented OpenAI API features
• Custom tool outputs for multi-turn conversations are not included as they are not documented
• All changes follow existing patterns for FunctionTool and FunctionToolCall
• Build completed successfully with no errors

Files Changed

• Sources/OpenAI/Public/Shared/Tool.swift (+33 lines)
• Sources/OpenAI/Public/ResponseModels/Response/OutputItem.swift (+30 lines)
• Sources/OpenAI/Public/Shared/ToolChoiceMode.swift (+27 lines)

Total: 3 files changed, 90 insertions

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