Perfect—let’s make the whole guide Swift‑native and wire it to gpt-5-codex as the default model. This version shows the Responses API tool‑calling loop, built‑in tools (web search), custom tools, structured outputs (text.format), and streaming.

Why gpt-5-codex? It’s a GPT‑5 variant optimized for agentic coding and is Responses‑only—built to drive coding agents (think Codex/CLI/IDE workflows). (OpenAI) Structured outputs + function/tool calling + previous_response_id are the core building blocks in Responses. (OpenAI Platform) Prompting tips for this model differ slightly from plain GPT‑5; the cookbook notes it’s Responses‑only and has a few behavior differences. (OpenAI Cookbook)

---

TL;DR setup

• Use gpt-5-codex with Responses API. Put your key in OPENAI_API_KEY. (OpenAI Platform)

• Tools come in two flavors:

  • Built‑ins like web_search (and others) you enable in the tools array; you can request sources with include. (OpenAI Platform)
  • Custom functions you define via JSON Schema; model returns function_call items with a call_id and arguments. You run the tool, then send back function_call_output with that call_id. (OpenAI Platform)

• Multi‑turn/state: chain requests with previous_response_id so the model can continue reasoning with prior items. (OpenAI Platform)

• Strict JSON: use Structured Outputs via text.format or strict function schemas. (OpenAI Platform)

• Streaming: enable SSE to receive deltas and tool‑call events. (OpenAI Platform)

---

1) Swift package bootstrap

Package.swift

// swift-tools-version: 6.0
import PackageDescription

let package = Package(
  name: "SwiftCodexAgent",
  platforms: [.macOS(.v13)],
  products: [.executable(name: "codex-agent", targets: ["CodexAgent"])],
  targets: [.executableTarget(name: "CodexAgent", path: "Sources")]
)

Environment

• export OPENAI_API_KEY=sk-...

---

2) Minimal Responses client (REST + SSE)

Sources/OpenAIResponsesClient.swift

import Foundation

struct SSEEvent { let event: String?; let data: String }

final class OpenAIResponsesClient {
    private let apiKey: String
    private let base = URL(string: "https://api.openai.com/v1/responses")!

    init(apiKey: String = ProcessInfo.processInfo.environment["OPENAI_API_KEY"] ?? "") {
        precondition(!apiKey.isEmpty, "Set OPENAI_API_KEY")
        self.apiKey = apiKey
    }

    // One-shot request
    func create(body: [String: Any]) async throws -> [String: Any] {
        var req = URLRequest(url: base)
        req.httpMethod = "POST"
        req.setValue("application/json", forHTTPHeaderField: "Content-Type")
        req.setValue("Bearer \(apiKey)", forHTTPHeaderField: "Authorization")
        req.httpBody = try JSONSerialization.data(withJSONObject: body, options: [])

        let (data, resp) = try await URLSession.shared.data(for: req)
        guard let http = resp as? HTTPURLResponse, (200..<300).contains(http.statusCode) else {
            throw NSError(domain: "OpenAI", code: (resp as? HTTPURLResponse)?.statusCode ?? -1,
                          userInfo: ["response": String(data: data, encoding: .utf8) ?? ""])
        }
        return (try JSONSerialization.jsonObject(with: data)) as? [String: Any] ?? [:]
    }

    // Streaming (SSE)
    func stream(body: [String: Any], onEvent: @escaping (SSEEvent) -> Void) async throws {
        var req = URLRequest(url: base)
        req.httpMethod = "POST"
        req.setValue("application/json", forHTTPHeaderField: "Content-Type")
        req.setValue("text/event-stream", forHTTPHeaderField: "Accept")
        req.setValue("Bearer \(apiKey)", forHTTPHeaderField: "Authorization")

        var b = body; b["stream"] = true
        req.httpBody = try JSONSerialization.data(withJSONObject: b, options: [])

        let (bytes, resp) = try await URLSession.shared.bytes(for: req)
        guard let http = resp as? HTTPURLResponse, (200..<300).contains(http.statusCode) else {
            throw NSError(domain: "OpenAI-SSE", code: (resp as? HTTPURLResponse)?.statusCode ?? -1)
        }

        var event: String?; var data = ""
        for try await line in bytes.lines {
            if line.hasPrefix("event:") {
                event = line.dropFirst("event:".count).trimmingCharacters(in: .whitespaces)
            } else if line.hasPrefix("data:") {
                let d = line.dropFirst("data:".count).trimmingCharacters(in: .whitespaces)
                data += d + "\n"
            } else if line.isEmpty, !data.isEmpty {
                onEvent(SSEEvent(event: event, data: data))
                event = nil; data = ""
            }
        }
    }
}

• Streaming uses SSE; you’ll see deltas and tool‑call events as they happen. (OpenAI Platform)

---

3) Tool protocol + registry (your custom functions)

Sources/Tools.swift

import Foundation

protocol Tool {
    var name: String { get }
    var about: String { get }
    var parameters: [String: Any] { get } // JSON Schema
    func call(with args: [String: Any]) async throws -> Any
}

final class ToolRegistry {
    private var tools: [String: Tool] = [:]
    func register(_ t: Tool) { tools[t.name] = t }
    func get(_ name: String) -> Tool? { tools[name] }

    /// Compose the "tools" array for Responses:
    /// includeBuiltIns e.g. ["web_search"] (platform-hosted)
    func spec(includeBuiltIns: [String] = []) -> [[String: Any]] {
        var out: [[String: Any]] = includeBuiltIns.map { ["type": $0] }
        for t in tools.values {
            out.append([
                "type": "function",
                "name": t.name,
                "description": t.about,
                "parameters": t.parameters,
                "strict": true // schema-enforced arguments
            ])
        }
        return out
    }
}

• Built‑ins (like web_search) are declared by type; custom tools are type: "function" with a JSON Schema and (optionally) strict: true. (OpenAI Platform)

---

4) A couple of custom tools for coding agents

Sources/CustomTools.swift

import Foundation

/// Very simple "save a file" tool to let the agent create code/test files.
struct SaveFileTool: Tool {
    let name = "save_file"
    let about = "Write a UTF-8 text file into a workspace directory."
    let workspace = FileManager.default.temporaryDirectory.appendingPathComponent("codex_workspace")

    var parameters: [String: Any] {
        ["type": "object",
         "properties": [
            "path": ["type": "string", "description": "Relative path under workspace, e.g. src/main.swift"],
            "content": ["type": "string", "description": "File content (UTF-8)"]
         ],
         "required": ["path", "content"],
         "additionalProperties": false]
    }

    func call(with args: [String: Any]) async throws -> Any {
        let rel = (args["path"] as? String) ?? "untitled.txt"
        let content = (args["content"] as? String) ?? ""
        try FileManager.default.createDirectory(at: workspace, withIntermediateDirectories: true)
        let dest = workspace.appendingPathComponent(rel)
        try FileManager.default.createDirectory(at: dest.deletingLastPathComponent(), withIntermediateDirectories: true)
        try content.data(using: .utf8)?.write(to: dest)
        return ["saved": dest.path]
    }
}

/// Dummy "run tests" tool: simulates a test run with results.
/// In a real agent, you'd invoke `swift test` (or your runner) in a sandbox.
struct RunTestsTool: Tool {
    let name = "run_tests"
    let about = "Execute unit tests in the workspace and return a summary."

    var parameters: [String: Any] {
        ["type": "object",
         "properties": [
           "command": ["type": "string", "description": "Ignored in demo; default swift test"]
         ],
         "additionalProperties": false]
    }

    func call(with args: [String: Any]) async throws -> Any {
        // Simulate a pass:
        return ["passed": true, "tests": 3, "failures": 0, "duration_ms": 1420]
    }
}

---

5) The agent loop (handles tool‑calls, batching outputs, continuation)

Sources/Agent.swift

import Foundation

struct FunctionCall { let callId: String; let name: String; let arguments: [String: Any] }

final class Agent {
    private let client: OpenAIResponsesClient
    private let model: String
    private let tools: ToolRegistry
    private let includeBuiltIns: [String]

    init(client: OpenAIResponsesClient, model: String, tools: ToolRegistry, includeBuiltIns: [String] = []) {
        self.client = client; self.model = model; self.tools = tools; self.includeBuiltIns = includeBuiltIns
    }

    @discardableResult
    func ask(_ prompt: String,
             forceToolChoice: String? = nil,
             structuredJSONSchema: [String: Any]? = nil,
             includeWebSources: Bool = false) async throws -> String {

        var previousId: String?
        var finalText = ""
        var toolsSpec = tools.spec(includeBuiltIns: includeBuiltIns)

        var toolChoice: Any = "auto"
        if let forced = forceToolChoice { toolChoice = ["type": "function", "name": forced] } // or "web_search"

        var textOpts: [String: Any]?
        if let schema = structuredJSONSchema {
            textOpts = ["format": [
                "type": "json_schema",
                "json_schema": [
                    "name": schema["name"] ?? "Result",
                    "schema": schema["schema"] ?? [:]
                ],
                "strict": true
            ]]
        }

        var body: [String: Any] = [
            "model": model,
            "input": prompt,           // can also be an array of items
            "tools": toolsSpec,
            "tool_choice": toolChoice
        ]
        if let textOpts { body["text"] = textOpts }
        if includeWebSources {
            // Ask Responses to include sources for web_search calls:
            body["include"] = ["web_search_call.action.sources"]
        }

        var resp = try await client.create(body: body)
        previousId = resp["id"] as? String
        finalText += extractText(resp)

        // Loop while there are function calls
        var safety = 0
        while true {
            let calls = functionCalls(resp)
            if calls.isEmpty { break }

            let outputs: [[String: Any]] = try await withThrowingTaskGroup(of: [String: Any].self) { group in
                for c in calls {
                    group.addTask {
                        guard let tool = self.tools.get(c.name) else {
                            return ["type": "function_call_output", "call_id": c.callId, "output": "{\"error\":\"unknown tool\"}"]
                        }
                        let result = try await tool.call(with: c.arguments)
                        let data = try JSONSerialization.data(withJSONObject: result, options: [])
                        let json = String(data: data, encoding: .utf8) ?? "\"\""
                        return ["type": "function_call_output", "call_id": c.callId, "output": json]
                    }
                }
                var outs: [[String: Any]] = []
                for try await o in group { outs.append(o) }
                return outs
            }

            var cont: [String: Any] = [
                "model": model,
                "previous_response_id": previousId as Any,
                "input": outputs,
                "tools": toolsSpec,
                "tool_choice": "auto"
            ]
            if let textOpts { cont["text"] = textOpts }
            if includeWebSources { cont["include"] = ["web_search_call.action.sources"] }

            resp = try await client.create(body: cont)
            previousId = resp["id"] as? String
            finalText += extractText(resp)

            safety += 1; if safety > 8 { break }
        }

        return finalText.trimmingCharacters(in: .whitespacesAndNewlines)
    }

    private func functionCalls(_ resp: [String: Any]) -> [FunctionCall] {
        guard let output = resp["output"] as? [[String: Any]] else { return [] }
        var out: [FunctionCall] = []
        for item in output {
            guard (item["type"] as? String) == "function_call" else { continue }
            let callId = (item["call_id"] as? String) ?? UUID().uuidString
            let name = (item["name"] as? String) ?? ""
            var args: [String: Any] = [:]
            if let argStr = item["arguments"] as? String,
               let data = argStr.data(using: .utf8),
               let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any] {
                args = json
            }
            out.append(FunctionCall(callId: callId, name: name, arguments: args))
        }
        return out
    }

    private func extractText(_ resp: [String: Any]) -> String {
        guard let output = resp["output"] as? [[String: Any]] else { return "" }
        var text = ""
        for item in output {
            guard let t = item["type"] as? String else { continue }
            if t == "output_text" { text += (item["text"] as? String) ?? "" }
            if t == "message",
               let segments = item["content"] as? [[String: Any]] {
                for seg in segments where (seg["type"] as? String) == "output_text" {
                    text += (seg["text"] as? String) ?? ""
                }
            }
        }
        return text
    }
}

• Tool calls arrive as function_call items; you must echo each call’s call_id when sending function_call_output in the next request, paired with previous_response_id. (OpenAI Platform)
• input can be a string or an array of items; the code keeps it simple with a string on turn 1, then an array of outputs on continuation. (OpenAI Platform)
• tool_choice lets you keep "auto" or force a specific tool for determinism. (OpenAI Platform)
• If you enable web_search, you can ask Responses to include citations/sources with include: ["web_search_call.action.sources"]. (OpenAI Platform)

---

6) Run the agent (with gpt-5-codex)

Sources/main.swift

import Foundation

@main
struct CodexAgentMain {
    static func main() async throws {
        let client = OpenAIResponsesClient()

        let registry = ToolRegistry()
        registry.register(SaveFileTool())
        registry.register(RunTestsTool())

        // Add built-ins if you want the model to browse while coding.
        let agent = Agent(
            client: client,
            model: "gpt-5-codex",
            tools: registry,
            includeBuiltIns: ["web_search"] // optional; remove if you want offline-only
        )

        // A coding-style instruction tailored for GPT-5-Codex
        // (Model-specific prompting guidance lives in the cookbook.)
        // Ask it to create a Fibonacci implementation + tests and persist to files, then run tests.
        let answer = try await agent.ask(
            """
            Create a Swift module with:
            - src/Fibonacci.swift exporting fibonacci(n: Int) -> Int
            - Tests/FibonacciTests.swift with 3 unit tests
            Save files using the save_file tool, then run tests with run_tests.
            If web results help, cite sources.
            """,
            includeWebSources: true
        )

        print("\n=== FINAL ===\n\(answer)\n")
    }
}

• gpt-5-codex is Responses‑only, tuned for coding agents (CLI/IDE/Codex). (OpenAI Platform)
• For prompt style specific to this model (e.g., more explicit developer instructions), see the GPT‑5‑Codex prompting guide. (OpenAI Cookbook)

---

7) Structured outputs (two ways you’ll actually use)

• Strict function args (we set "strict": true in tool specs). Great for deterministic tool invocation. (OpenAI Platform)
• Final JSON via text.format (recommended when the answer should be parseable JSON).

Example: ask for a machine‑readable test report.

let testReportSchema: [String: Any] = [
  "name": "TestReport",
  "schema": [
    "type": "object",
    "properties": [
      "passed": ["type": "boolean"],
      "tests": ["type": "integer"],
      "failures": ["type": "integer"],
      "duration_ms": ["type": "integer"]
    ],
    "required": ["passed","tests","failures"]
  ]
]

let structured = try await agent.ask(
  "Return the latest test run summary as JSON only.",
  structuredJSONSchema: testReportSchema
)
print(structured)

• In Responses, structured text output is configured under text.format with type: "json_schema" (this supersedes older response_format patterns). (OpenAI Platform)

---

8) Streaming (optional, handy for IDE/CLI UX)

let body: [String: Any] = [
  "model": "gpt-5-codex",
  "input": "Draft a minimal README for the Fibonacci module.",
  "tools": [["type": "web_search"]],
  "tool_choice": "auto",
  "include": ["web_search_call.action.sources"]
]

try await OpenAIResponsesClient().stream(body: body) { ev in
    if let e = ev.event { print("EVENT:", e) }
    print(ev.data) // JSON envelope chunks (text deltas, tool calls, etc.)
}

• SSE streaming delivers token/tool events as they happen—perfect for “live coding” UI. (OpenAI Platform)

---

9) Patterns you’ll reuse (and pitfalls)

• Multi‑tool turn: model may emit multiple function_call items—execute them, then send one batch of function_call_output items in input + the previous_response_id. (OpenAI Platform)
• Force determinism: set tool_choice to a specific function or built‑in when needed (e.g., always run_tests before answering). (OpenAI Platform)
• State: previous_response_id chains turns; instructions from older turns aren’t auto‑reapplied unless you pass them again or depend on the model’s reasoning items from that ID. (OpenAI Platform)
• Web search: if enabled, you can pull back sources via include for auditability. (OpenAI Platform)
• Prompting for gpt-5-codex: favor concise developer directives and explicit tool usage steps; cookbook notes it’s not a drop‑in for GPT‑5 and that some UI params differ. (OpenAI Cookbook)

---

10) Minimal request shapes (cheat sheet)

• Basic

{ "model": "gpt-5-codex", "input": "hello" }

• Built‑in web search

{
  "model": "gpt-5-codex",
  "input": "What Swift version ships with Xcode 16.1? Cite sources.",
  "tools": [{ "type": "web_search" }],
  "tool_choice": "auto",
  "include": ["web_search_call.action.sources"]
}

(OpenAI Platform)

• Custom function definition

{
  "type": "function",
  "name": "save_file",
  "description": "Write a UTF-8 text file.",
  "parameters": {
    "type": "object",
    "properties": { "path": { "type": "string" }, "content": { "type": "string" } },
    "required": ["path","content"],
    "additionalProperties": false
  },
  "strict": true
}

(OpenAI Platform)

• Continue after tool calls

{
  "model": "gpt-5-codex",
  "previous_response_id": "<id from prior call>",
  "input": [
    { "type": "function_call_output", "call_id": "<call_id_1>", "output": "{\"saved\":\"/tmp/.../Fibonacci.swift\"}" },
    { "type": "function_call_output", "call_id": "<call_id_2>", "output": "{\"passed\":true,\"tests\":3}" }
  ]
}

(OpenAI Platform)

• Structured JSON answer

{
  "model": "gpt-5-codex",
  "input": "Return a JSON test summary only.",
  "text": {
    "format": {
      "type": "json_schema",
      "json_schema": { "name": "TestReport", "schema": { "type": "object", "properties": { "passed": { "type": "boolean" } }, "required": ["passed"] } },
      "strict": true
    }
  }
}

(OpenAI Platform)

---

References (official)

• Responses API reference (request shape, parameters, streaming) (OpenAI Platform)
• Function/Tool calling flow (including function_call_output + call_id) (OpenAI Platform)
• Conversation state / previous_response_id (OpenAI Platform)
• Built‑in web_search (tools guide + sources via include) (OpenAI Platform)
• Structured Outputs (text.format + JSON Schema) & migration notes (OpenAI Platform)
• gpt-5-codex model (what it is, availability) + system‑card addendum + launch post (OpenAI Platform)

---

Want me to spin this into a tiny SPM repo scaffold (swift run codex-agent) with a workspace folder and a real swift test runner tool so you can drive red‑green‑refactor from the model?