ai: OpenAI and Codex forced tool calls (#6588)

* feat(ai): support forced OpenAI Codex tool calls

* feat(ai): support OpenAI Responses tool choice

packages/ai/test/openai-codex-stream.test.ts

@@ -2,6 +2,7 @@ import { mkdtempSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { zstdDecompressSync } from "node:zlib";
+import { Type } from "typebox";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import {
 	closeOpenAICodexWebSocketSessions,
@@ -700,6 +701,61 @@ describe("openai-codex streaming", () => {
 		expect(requestedReasoning).toEqual({ effort: "xhigh", summary: "auto" });
 	});
 
+	it("forwards required tool choice", async () => {
+		const token = mockToken();
+		const encoder = new TextEncoder();
+		const sse = buildSSEPayload({ status: "completed" });
+		let requestedToolChoice: unknown;
+
+		vi.stubGlobal(
+			"fetch",
+			vi.fn(async (_input: string | URL, init?: RequestInit) => {
+				requestedToolChoice = decodeCodexRequestBody(init?.body)?.tool_choice;
+				return new Response(
+					new ReadableStream<Uint8Array>({
+						start(controller) {
+							controller.enqueue(encoder.encode(sse));
+							controller.close();
+						},
+					}),
+					{ status: 200, headers: { "content-type": "text/event-stream" } },
+				);
+			}),
+		);
+
+		const model: Model<"openai-codex-responses"> = {
+			id: "gpt-5.5",
+			name: "GPT-5.5",
+			api: "openai-codex-responses",
+			provider: "openai-codex",
+			baseUrl: "https://chatgpt.com/backend-api",
+			reasoning: true,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 400000,
+			maxTokens: 128000,
+		};
+
+		await streamOpenAICodexResponses(
+			model,
+			{
+				messages: [
+					{ role: "user", content: "Do not call ping. Respond with text instead.", timestamp: Date.now() },
+				],
+				tools: [
+					{
+						name: "ping",
+						description: "Ping",
+						parameters: Type.Object({ value: Type.String() }),
+					},
+				],
+			},
+			{ apiKey: token, transport: "sse", toolChoice: "required" },
+		).result();
+
+		expect(requestedToolChoice).toBe("required");
+	});
+
 	it.each(["gpt-5.3-codex", "gpt-5.4", "gpt-5.5"])("clamps %s minimal reasoning effort to low", async (modelId) => {
 		const tempDir = mkdtempSync(join(tmpdir(), "pi-codex-stream-"));
 		process.env.PI_CODING_AGENT_DIR = tempDir;