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examples: Add streaming ChatGPT proxy from FOSDEM 2025 (apple#723)
### Motivation We recently demoed [building a streaming ChatGPT proxy with Swift OpenAPI, at FOSDEM 2025][0]. This PR adds the resulting package for people to refer to as reference material. ### Modifications - Add package reflecting final state of the presentation. - Add a `.env.example` file. - Add covering README with link to presentation, abstract, and usage. - Add VS Code settings for Dev Containers. ### Result Reference project available for people watching the presentation. ### Test Plan CI will build the example. [0]: https://fosdem.org/2025/schedule/event/fosdem-2025-5230-live-coding-a-streaming-chatgpt-proxy-with-swift-openapi-from-scratch-/
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‎.licenseignore

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.gitignore
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.licenseignore
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.swiftformatignore
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.unacceptablelanguageignore
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.spi.yml
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.swift-format
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.github/*
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Examples/HelloWorldiOSClientAppExample/HelloWorldiOSClientApp/Assets.xcassets/*
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Examples/HelloWorldiOSClientAppExample/HelloWorldiOSClientApp/Preview*
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Examples/**/Generated*
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Examples/streaming-chatgpt-proxy/.env.example
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Examples/streaming-chatgpt-proxy/players.txt
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**/Makefile
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**/*.html
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.editorconfig

‎.swiftformatignore

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Sources/swift-openapi-generator/Documentation.docc
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Examples/**/Generated/*
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Examples/**/GeneratedSources/*
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Examples/streaming-chatgpt-proxy/**
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**Package.swift

‎.unacceptablelanguageignore

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Examples/streaming-chatgpt-proxy/Sources/ChatGPT/openapi.yaml

‎Examples/streaming-chatgpt-proxy/.devcontainer/devcontainer.json

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{
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"name": "Swift",
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"image": "swift:6.0",
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"features": {
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"ghcr.io/devcontainers/features/common-utils:2": {},
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"ghcr.io/devcontainers/features/git:1": {}
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},
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"runArgs": [
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"--cap-add=SYS_PTRACE",
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"--security-opt",
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"seccomp=unconfined"
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],
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"customizations": {
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"vscode": {
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"settings": {
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"lldb.library": "/usr/lib/liblldb.so"
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},
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"extensions": [
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"sswg.swift-lang",
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"42Crunch.vscode-openapi"
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]
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}
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},
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// Use 'forwardPorts' to make a list of ports inside the container available locally.
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// "forwardPorts": [],
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// Set `remoteUser` to `root` to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
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"remoteUser": "root"
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}

‎Examples/streaming-chatgpt-proxy/.env.example

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# Rename this file to just .env and paste your token below.
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OPENAI_TOKEN=PASTE_YOUR_TOKEN_HERE

‎Examples/streaming-chatgpt-proxy/.gitignore

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.DS_Store
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.build/
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.build-linux/
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.env
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/Packages
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/*.xcodeproj
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xcuserdata/
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DerivedData/
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.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
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.vscode
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/Package.resolved
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.ci/
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.docc-build/

‎Examples/streaming-chatgpt-proxy/.vscode/settings.json

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{
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"editor.lightbulb.enabled": "off",
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"explorer.decorations.badges": false,
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"explorer.decorations.colors": false,
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"files.exclude": {
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"**/.build": true,
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"**/.build-linux": true,
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"**/.gitignore": true,
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"**/.swiftpm": true,
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"**/.vscode": true,
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".env": true,
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"*.o": true,
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"*.d": true,
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"*.swiftdeps": true,
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"*.swiftdeps~": true,
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},
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"outline.problems.colors": false,
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"outline.problems.badges": false,
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"lldb.library": "/usr/lib/liblldb.so",
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"lldb.launch.expressions": "native",
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"swift.diagnosticsStyle": "default",
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"swift.disableAutoResolve": true,
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"swift.sourcekit-lsp.backgroundIndexing": "off",
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"swift.sourcekit-lsp.disable": true,
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"swift.buildPath": ".build-linux",
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"workbench.colorCustomizations": {
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"editorError.foreground": "#00000000",
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"editorWarning.foreground": "#00000000",
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},
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"workbench.startupEditor": "none"
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}

‎Examples/streaming-chatgpt-proxy/Package.swift

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// swift-tools-version: 5.9
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import PackageDescription
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let package = Package(
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name: "fosdem-2025-demo",
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platforms: [.macOS(.v14)],
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dependencies: [
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.package(url: "https://github.com/apple/swift-openapi-generator", from: "1.6.0"),
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.package(url: "https://github.com/apple/swift-openapi-runtime", from: "1.6.0"),
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.package(url: "https://github.com/apple/swift-openapi-urlsession", from: "1.0.2"),
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.package(url: "https://github.com/swift-server/swift-openapi-vapor", from: "1.0.1"),
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.package(url: "https://github.com/swift-server/swift-openapi-async-http-client", from: "1.1.0"),
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],
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targets: [
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.target(
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name: "ChatGPT",
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dependencies: [
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.product(name: "OpenAPIRuntime", package: "swift-openapi-runtime"),
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],
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plugins: [
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.plugin(name: "OpenAPIGenerator", package: "swift-openapi-generator"),
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]
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),
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.executableTarget(
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name: "ClientCLI",
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dependencies: [
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.product(name: "OpenAPIURLSession", package: "swift-openapi-urlsession"),
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],
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plugins: [
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.plugin(name: "OpenAPIGenerator", package: "swift-openapi-generator"),
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]
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),
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.executableTarget(
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name: "ProxyServer",
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dependencies: [
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"ChatGPT",
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.product(name: "OpenAPIAsyncHTTPClient", package: "swift-openapi-async-http-client"),
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.product(name: "OpenAPIVapor", package: "swift-openapi-vapor"),
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],
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plugins: [
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.plugin(name: "OpenAPIGenerator", package: "swift-openapi-generator"),
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]
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),
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]
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)

‎Examples/streaming-chatgpt-proxy/README.md

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# Streaming ChatGPT Proxy
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An example project using [Swift OpenAPI Generator](https://github.com/apple/swift-openapi-generator).
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> **Disclaimer:** This example is deliberately simplified and is intended for illustrative purposes only.
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## Overview
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A tailored API server, backed by ChatGPT, and client CLI, with end-to-end
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streaming.
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This package is the reference sources for the demo presented at [FOSDEM 2025:
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_Live coding a streaming ChatGPT proxy with Swift OpenAPI—from
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scratch!_][fosdem25-swift-openapi]
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> Join us as we build a ChatGPT client, from scratch, using Swift OpenAPI Generator. We’ll take advantage of Swift OpenAPI’s pluggable HTTP transports to reuse the same generated client to make upstream calls from a Linux server, providing end-to-end streaming, backed by async sequences, without buffering upstream responses.
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>
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> In this session you’ll learn how to:
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>
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> * Generate a type-safe ChatGPT macOS client and use URLSession OpenAPI transport.
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> * Stream LLM responses using Server Sent Events (SSE).
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> * Bootstrap a Linux proxy server using the Vapor OpenAPI transport.
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> * Use the same generated ChatGPT client within the proxy by switching to the AsyncHTTPClient transport.
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> * Efficiently transform responses from SSE to JSON Lines, maintaining end-to-end streaming.
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The example provides an API for a fictitious _ChantGPT_ service, which produces
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creative chants to sing at basketball games. 🙌 🏀 🙌
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## Usage
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The upstream calls to ChatGPT require an API token, which is configured using the `OPENAI_TOKEN` environment variable.
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Rename `.env.example` to `.env` and replace the placeholder with your token.
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Build and run the server using:
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```console
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% swift run ProxyServer
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2025-01-30T09:12:23+0000 notice codes.vapor.application : [Vapor] Server starting on http://127.0.0.1:8080
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...
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```
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Then, from another terminal, run the proxy client using:
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```console
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% swift run ClientCLI "That team with the Bull logo"
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Build of product 'ClientCLI' complete! (7.24s)
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🧑‍💼: That one with the bull logo
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---
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🤖: **"Charge Ahead, Chicago Bulls!"**
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(Verse 1)
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Red and black, we’re on the prowl,
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Chicago Bulls, hear us growl!
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From the Windy City, we take the lead,
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Charging forward with lightning speed!
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(Chorus)
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B-U-L-L-S, Bulls! Bulls! Bulls!
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We’re the team that never dulls!
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Hoops and hustle, heart and soul,
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Chicago Bulls, we’re on a roll!
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...
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```
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## Linux development with VS Code Dev Containers
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The package also contains configuration for developing with VS Code [Dev
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Containers][dev-containers].
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If you have the Dev Containers extension installed, use the `Dev Containers: Reopen in Container` command to switch to build and run for Linux.
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[fosdem25-swift-openapi]: https://fosdem.org/2025/schedule/event/fosdem-2025-5230-live-coding-a-streaming-chatgpt-proxy-with-swift-openapi-from-scratch-/
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[dev-containers]: https://code.visualstudio.com/docs/devcontainers/containers

‎Examples/streaming-chatgpt-proxy/Sources/ChatGPT/Middleware.swift

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//===----------------------------------------------------------------------===//
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//
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// This source file is part of the SwiftOpenAPIGenerator open source project
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//
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// Copyright (c) 2025 Apple Inc. and the SwiftOpenAPIGenerator project authors
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// Licensed under Apache License v2.0
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//
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// See LICENSE.txt for license information
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// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
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//
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// SPDX-License-Identifier: Apache-2.0
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//
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//===----------------------------------------------------------------------===//
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import Foundation
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import HTTPTypes
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import OpenAPIRuntime
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package struct HeaderFieldMiddleware: ClientMiddleware {
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var name: HTTPField.Name
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var value: String
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package static func authTokenFromEnvironment(_ environmentVariable: String) -> Self {
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guard let token = ProcessInfo.processInfo.environment[environmentVariable] else {
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fatalError("Please set \(environmentVariable) environment variable.")
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}
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return Self(name: .authorization, value: "Bearer \(token)")
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}
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package func intercept(
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_ request: HTTPRequest,
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body: HTTPBody?,
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baseURL: URL,
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operationID: String,
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next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
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) async throws -> (HTTPResponse, HTTPBody?) {
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var request = request
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request.headerFields[name] = value
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return try await next(request, body, baseURL)
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}
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}

‎Examples/streaming-chatgpt-proxy/Sources/ChatGPT/openapi-generator-config.yaml

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generate: [types, client]
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accessModifier: package
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filter:
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paths:
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- /chat/completions
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namingStrategy: idiomatic

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