feat: support multiple providers via Responses-Completion transformation (#247)

https://github.com/user-attachments/assets/9ecb51be-fa65-4e99-8512-abb898dda569

Implemented it as a transformation between Responses API and Completion
API so that it supports existing providers that implement the Completion
API and minimizes the changes needed to the codex repo.

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>
Co-authored-by: Fouad Matin <169186268+fouad-openai@users.noreply.github.com>
Co-authored-by: Fouad Matin <fouad@openai.com>

codex-cli/src/utils/agent/agent-loop.ts

@@ -1,16 +1,19 @@
 import type { ReviewDecision } from "./review.js";
 import type { ApplyPatchCommand, ApprovalPolicy } from "../../approvals.js";
 import type { AppConfig } from "../config.js";
+import type { ResponseEvent } from "../responses.js";
 import type {
   ResponseFunctionToolCall,
   ResponseInputItem,
   ResponseItem,
+  ResponseCreateParams,
 } from "openai/resources/responses/responses.mjs";
 import type { Reasoning } from "openai/resources.mjs";
 
 import { log } from "./log.js";
-import { OPENAI_BASE_URL, OPENAI_TIMEOUT_MS } from "../config.js";
+import { OPENAI_TIMEOUT_MS, getApiKey, getBaseUrl } from "../config.js";
 import { parseToolCallArguments } from "../parsers.js";
+import { responsesCreateViaChatCompletions } from "../responses.js";
 import {
   ORIGIN,
   CLI_VERSION,
@@ -39,6 +42,7 @@ const alreadyProcessedResponses = new Set();
 
 type AgentLoopParams = {
   model: string;
+  provider?: string;
   config?: AppConfig;
   instructions?: string;
   approvalPolicy: ApprovalPolicy;
@@ -58,6 +62,7 @@ type AgentLoopParams = {
 
 export class AgentLoop {
   private model: string;
+  private provider: string;
   private instructions?: string;
   private approvalPolicy: ApprovalPolicy;
   private config: AppConfig;
@@ -198,6 +203,7 @@ export class AgentLoop {
   // private cumulativeThinkingMs = 0;
   constructor({
     model,
+    provider = "openai",
     instructions,
     approvalPolicy,
     // `config` used to be required.  Some unit‑tests (and potentially other
@@ -214,6 +220,7 @@ export class AgentLoop {
     additionalWritableRoots,
   }: AgentLoopParams & { config?: AppConfig }) {
     this.model = model;
+    this.provider = provider;
     this.instructions = instructions;
     this.approvalPolicy = approvalPolicy;
 
@@ -236,7 +243,9 @@ export class AgentLoop {
     this.sessionId = getSessionId() || randomUUID().replaceAll("-", "");
     // Configure OpenAI client with optional timeout (ms) from environment
     const timeoutMs = OPENAI_TIMEOUT_MS;
-    const apiKey = this.config.apiKey ?? process.env["OPENAI_API_KEY"] ?? "";
+    const apiKey = getApiKey(this.provider);
+    const baseURL = getBaseUrl(this.provider);
+
     this.oai = new OpenAI({
       // The OpenAI JS SDK only requires `apiKey` when making requests against
       // the official API.  When running unit‑tests we stub out all network
@@ -245,7 +254,7 @@ export class AgentLoop {
       // errors inside the SDK (it validates that `apiKey` is a non‑empty
       // string when the field is present).
       ...(apiKey ? { apiKey } : {}),
-      baseURL: OPENAI_BASE_URL,
+      baseURL,
       defaultHeaders: {
         originator: ORIGIN,
         version: CLI_VERSION,
@@ -492,11 +501,23 @@ export class AgentLoop {
             const mergedInstructions = [prefix, this.instructions]
               .filter(Boolean)
               .join("\n");
+
+            const responseCall =
+              !this.config.provider ||
+              this.config.provider?.toLowerCase() === "openai"
+                ? (params: ResponseCreateParams) =>
+                    this.oai.responses.create(params)
+                : (params: ResponseCreateParams) =>
+                    responsesCreateViaChatCompletions(
+                      this.oai,
+                      params as ResponseCreateParams & { stream: true },
+                    );
             log(
               `instructions (length ${mergedInstructions.length}): ${mergedInstructions}`,
             );
+
             // eslint-disable-next-line no-await-in-loop
-            stream = await this.oai.responses.create({
+            stream = await responseCall({
               model: this.model,
               instructions: mergedInstructions,
               previous_response_id: lastResponseId || undefined,
@@ -720,7 +741,7 @@ export class AgentLoop {
 
         try {
           // eslint-disable-next-line no-await-in-loop
-          for await (const event of stream) {
+          for await (const event of stream as AsyncIterable<ResponseEvent>) {
             log(`AgentLoop.run(): response event ${event.type}`);
 
             // process and surface each item (no‑op until we can depend on streaming events)