diff --git a/src/api.ts b/src/api.ts
index c4b643d..de10b20 100644
--- a/src/api.ts
+++ b/src/api.ts
@@ -10,6 +10,8 @@ import {
 import { InlineAISettings } from "./settings";
 import { App, MarkdownView, Notice } from "obsidian";
 import { EditorView } from "@codemirror/view";
+import { callCodexApi } from "./codex-client";
+import { getValidCodexToken, CodexTokens } from "./codex-auth";
 import { setGeneratedResponseEffect } from "./modules/AIExtension";
 import { parseCommand } from "./modules/commands/parser";
 import { MessageQueue } from "./modules/messageHistory/queue";
@@ -163,6 +165,10 @@ export class ChatApiManager {
 						},
 					});
 
+				case "codex":
+					// Handled directly in callApi — no LangChain client needed
+					return null;
+
 				default:
 					new Notice(`⚠️ Unsupported provider: ${settings.provider}`);
 					return null;
@@ -184,6 +190,10 @@ export class ChatApiManager {
 		systemMessage: string,
 		message: string,
 	): Promise<string> {
+		if (this.settings.provider === "codex") {
+			return this.callCodexProvider(systemMessage, message);
+		}
+
 		if (!this.chatClient) {
 			new Notice(
 				"⚠️ Chat client is not initialized. Please check your settings.",
@@ -209,6 +219,40 @@ export class ChatApiManager {
 		}
 	}
 
+	private async callCodexProvider(systemMessage: string, message: string): Promise<string> {
+		const s = this.settings;
+		if (!s.codexAccess || !s.codexRefresh || !s.codexAccountId) {
+			new Notice("⚠️ Codex: not signed in — open Settings → InlineAI and click 'Sign in with ChatGPT'");
+			return "⚠️ Codex not authenticated.";
+		}
+
+		try {
+			const tokens: CodexTokens = {
+				access: s.codexAccess,
+				refresh: s.codexRefresh,
+				expires: s.codexExpires ?? 0,
+				accountId: s.codexAccountId,
+			};
+
+			const accessToken = await getValidCodexToken(tokens, async (refreshed) => {
+				this.settings.codexAccess = refreshed.access;
+				this.settings.codexRefresh = refreshed.refresh;
+				this.settings.codexExpires = refreshed.expires;
+			});
+
+			if (!accessToken) {
+				new Notice("⚠️ Codex: session expired — please sign in again");
+				return "⚠️ Codex session expired.";
+			}
+
+			return await callCodexApi(systemMessage, message, accessToken, s.codexAccountId, s.model);
+		} catch (error: any) {
+			console.error("Codex error:", error);
+			new Notice(`❌ Codex: ${error.message}`);
+			return "⚠️ Codex request failed.";
+		}
+	}
+
 	/**
 	 * Handles user input and updates the editor with the response.
 	 * @param systemPrompt - The system prompt to send to the chat API.
diff --git a/src/codex-auth.ts b/src/codex-auth.ts
new file mode 100644
index 0000000..be18945
--- /dev/null
+++ b/src/codex-auth.ts
@@ -0,0 +1,201 @@
+import * as http from "http";
+import { Notice } from "obsidian";
+
+const CLIENT_ID = "app_EMoamEEZ73f0CkXaXp7hrann";
+const AUTHORIZE_URL = "https://auth.openai.com/oauth/authorize";
+const TOKEN_URL = "https://auth.openai.com/oauth/token";
+const REDIRECT_URI = "http://localhost:1455/auth/callback";
+const SCOPE = "openid profile email offline_access";
+const CALLBACK_PORT = 1455;
+
+export interface CodexTokens {
+	access: string;
+	refresh: string;
+	expires: number;
+	accountId: string;
+}
+
+async function generatePKCE(): Promise<{ verifier: string; challenge: string }> {
+	const array = new Uint8Array(32);
+	crypto.getRandomValues(array);
+	const verifier = btoa(String.fromCharCode(...array))
+		.replace(/\+/g, "-")
+		.replace(/\//g, "_")
+		.replace(/=/g, "");
+
+	const encoder = new TextEncoder();
+	const data = encoder.encode(verifier);
+	const digest = await crypto.subtle.digest("SHA-256", data);
+	const challenge = btoa(String.fromCharCode(...new Uint8Array(digest)))
+		.replace(/\+/g, "-")
+		.replace(/\//g, "_")
+		.replace(/=/g, "");
+
+	return { verifier, challenge };
+}
+
+function randomState(): string {
+	const array = new Uint8Array(16);
+	crypto.getRandomValues(array);
+	return Array.from(array, (b) => b.toString(16).padStart(2, "0")).join("");
+}
+
+function decodeJWT(token: string): Record<string, any> | null {
+	try {
+		const parts = token.split(".");
+		if (parts.length !== 3) return null;
+		return JSON.parse(atob(parts[1].replace(/-/g, "+").replace(/_/g, "/")));
+	} catch {
+		return null;
+	}
+}
+
+function extractAccountId(accessToken: string): string | null {
+	const payload = decodeJWT(accessToken);
+	if (!payload) return null;
+	const auth = payload["https://api.openai.com/auth"];
+	return auth?.user_id ?? auth?.account_id ?? null;
+}
+
+async function exchangeCode(code: string, verifier: string): Promise<CodexTokens | null> {
+	const res = await fetch(TOKEN_URL, {
+		method: "POST",
+		headers: { "Content-Type": "application/x-www-form-urlencoded" },
+		body: new URLSearchParams({
+			grant_type: "authorization_code",
+			client_id: CLIENT_ID,
+			code,
+			code_verifier: verifier,
+			redirect_uri: REDIRECT_URI,
+		}),
+	});
+
+	if (!res.ok) return null;
+
+	const json = await res.json() as any;
+	if (!json.access_token || !json.refresh_token) return null;
+
+	const accountId = extractAccountId(json.access_token);
+	if (!accountId) return null;
+
+	return {
+		access: json.access_token,
+		refresh: json.refresh_token,
+		expires: Date.now() + json.expires_in * 1000,
+		accountId,
+	};
+}
+
+export async function refreshCodexToken(tokens: CodexTokens): Promise<CodexTokens | null> {
+	const res = await fetch(TOKEN_URL, {
+		method: "POST",
+		headers: { "Content-Type": "application/x-www-form-urlencoded" },
+		body: new URLSearchParams({
+			grant_type: "refresh_token",
+			refresh_token: tokens.refresh,
+			client_id: CLIENT_ID,
+		}),
+	});
+
+	if (!res.ok) return null;
+
+	const json = await res.json() as any;
+	if (!json.access_token || !json.refresh_token) return null;
+
+	return {
+		access: json.access_token,
+		refresh: json.refresh_token,
+		expires: Date.now() + json.expires_in * 1000,
+		accountId: tokens.accountId,
+	};
+}
+
+export async function getValidCodexToken(
+	tokens: CodexTokens,
+	onRefresh: (t: CodexTokens) => Promise<void>,
+): Promise<string | null> {
+	if (tokens.expires > Date.now() + 60_000) return tokens.access;
+
+	const refreshed = await refreshCodexToken(tokens);
+	if (!refreshed) return null;
+
+	await onRefresh(refreshed);
+	return refreshed.access;
+}
+
+export async function startCodexOAuthFlow(): Promise<CodexTokens | null> {
+	const { verifier, challenge } = await generatePKCE();
+	const state = randomState();
+
+	const url = new URL(AUTHORIZE_URL);
+	url.searchParams.set("response_type", "code");
+	url.searchParams.set("client_id", CLIENT_ID);
+	url.searchParams.set("redirect_uri", REDIRECT_URI);
+	url.searchParams.set("scope", SCOPE);
+	url.searchParams.set("code_challenge", challenge);
+	url.searchParams.set("code_challenge_method", "S256");
+	url.searchParams.set("state", state);
+	url.searchParams.set("id_token_add_organizations", "true");
+	url.searchParams.set("codex_cli_simplified_flow", "true");
+	url.searchParams.set("originator", "codex_cli_rs");
+
+	return new Promise((resolve) => {
+		let resolved = false;
+		let server: http.Server | null = null;
+
+		const done = (tokens: CodexTokens | null) => {
+			if (resolved) return;
+			resolved = true;
+			server?.close();
+			resolve(tokens);
+		};
+
+		server = http.createServer(async (req, res) => {
+			if (!req.url?.startsWith("/auth/callback")) {
+				res.writeHead(404);
+				res.end();
+				return;
+			}
+
+			const params = new URL(req.url, "http://localhost").searchParams;
+			const code = params.get("code");
+			const returnedState = params.get("state");
+
+			if (!code || returnedState !== state) {
+				res.writeHead(400);
+				res.end("Invalid callback");
+				done(null);
+				return;
+			}
+
+			res.writeHead(200, { "Content-Type": "text/html" });
+			res.end("<html><body><h2>Signed in! You can close this tab.</h2></body></html>");
+
+			const tokens = await exchangeCode(code, verifier);
+			if (!tokens) {
+				new Notice("❌ Codex: failed to exchange auth code for tokens");
+			}
+			done(tokens);
+		});
+
+		server.on("error", (e: any) => {
+			if (e.code === "EADDRINUSE") {
+				new Notice("❌ Codex: port 1455 in use — close other Codex sessions first");
+			}
+			done(null);
+		});
+
+		server.listen(CALLBACK_PORT, "127.0.0.1", () => {
+			window.open(url.toString());
+			new Notice("🔐 Codex: browser opened — complete sign-in to continue");
+		});
+
+		// Timeout after 5 minutes
+		setTimeout(() => {
+			if (!resolved) {
+				new Notice("⚠️ Codex: sign-in timed out");
+				done(null);
+			}
+		}, 5 * 60 * 1000);
+	});
+}
diff --git a/src/codex-client.ts b/src/codex-client.ts
new file mode 100644
index 0000000..94e830e
--- /dev/null
+++ b/src/codex-client.ts
@@ -0,0 +1,127 @@
+const CODEX_API_URL = "https://chatgpt.com/backend-api/codex/responses";
+
+interface ResponsesInput {
+	type: "message";
+	role: "developer" | "user" | "assistant";
+	content: Array<{ type: "input_text"; text: string }>;
+}
+
+interface ResponsesBody {
+	model: string;
+	input: ResponsesInput[];
+	instructions: string;
+	store: false;
+	stream: true;
+	reasoning: { effort: string; summary: string };
+	text: { verbosity: string };
+	include: string[];
+}
+
+function normalizeModel(model: string): string {
+	const m = model.toLowerCase().trim();
+	if (m.includes("gpt-5.2-codex") || m.includes("gpt 5.2 codex")) return "gpt-5.2-codex";
+	if (m.includes("gpt-5.1-codex-max") || m.includes("codex-max")) return "gpt-5.1-codex-max";
+	if (m.includes("codex-mini-latest") || m.includes("codex-mini")) return "codex-mini-latest";
+	if (m.includes("gpt-5.1-codex") || m.includes("codex")) return "gpt-5.1-codex";
+	if (m.includes("gpt-5.2")) return "gpt-5.2";
+	if (m.includes("gpt-5.1")) return "gpt-5.1";
+	return "gpt-5.1-codex";
+}
+
+function parseSseText(sseBody: string): string {
+	const lines = sseBody.split("\n");
+	const parts: string[] = [];
+
+	for (const line of lines) {
+		if (!line.startsWith("data: ")) continue;
+		const data = line.slice(6).trim();
+		if (data === "[DONE]") break;
+
+		try {
+			const json = JSON.parse(data) as any;
+			// Responses API emits output[].content[].text deltas
+			for (const output of json.output ?? []) {
+				for (const content of output.content ?? []) {
+					if (content.type === "output_text" && content.text) {
+						parts.push(content.text);
+					}
+				}
+			}
+			// Delta format
+			const delta = json.delta;
+			if (delta?.type === "output_text" && delta.text) {
+				parts.push(delta.text);
+			}
+			// Snapshot format (non-streaming final)
+			if (json.type === "response.completed") {
+				const output = json.response?.output ?? [];
+				for (const item of output) {
+					for (const c of item.content ?? []) {
+						if (c.type === "output_text" && c.text) {
+							parts.push(c.text);
+						}
+					}
+				}
+			}
+		} catch {
+			// ignore malformed lines
+		}
+	}
+
+	return parts.join("");
+}
+
+export async function callCodexApi(
+	systemMessage: string,
+	userMessage: string,
+	accessToken: string,
+	accountId: string,
+	model: string,
+): Promise<string> {
+	const normalizedModel = normalizeModel(model);
+
+	const body: ResponsesBody = {
+		model: normalizedModel,
+		input: [
+			{
+				type: "message",
+				role: "developer",
+				content: [{ type: "input_text", text: systemMessage }],
+			},
+			{
+				type: "message",
+				role: "user",
+				content: [{ type: "input_text", text: userMessage }],
+			},
+		],
+		instructions: "",
+		store: false,
+		stream: true,
+		reasoning: { effort: "medium", summary: "auto" },
+		text: { verbosity: "medium" },
+		include: ["reasoning.encrypted_content"],
+	};
+
+	const res = await fetch(CODEX_API_URL, {
+		method: "POST",
+		headers: {
+			"Content-Type": "application/json",
+			"Authorization": `Bearer ${accessToken}`,
+			"chatgpt-account-id": accountId,
+			"OpenAI-Beta": "responses=experimental",
+			"originator": "codex_cli_rs",
+			"accept": "text/event-stream",
+		},
+		body: JSON.stringify(body),
+	});
+
+	if (!res.ok) {
+		const text = await res.text().catch(() => "");
+		throw new Error(`Codex API ${res.status}: ${text.slice(0, 200)}`);
+	}
+
+	const rawText = await res.text();
+	const result = parseSseText(rawText);
+	if (!result) throw new Error("Codex returned empty response");
+	return result;
+}
diff --git a/src/settings.ts b/src/settings.ts
index 6f32431..06f7beb 100644
--- a/src/settings.ts
+++ b/src/settings.ts
@@ -1,11 +1,12 @@
-import { App, PluginSettingTab, Setting } from "obsidian";
+import { App, PluginSettingTab, Setting, Notice } from "obsidian";
 import MyPlugin from "./main";
 import { cursorPrompt, selectionPrompt } from "./default_prompts";
 import { SlashCommand } from "./modules/commands/source";
+import { startCodexOAuthFlow } from "./codex-auth";
 
 // Interface for the settings
 export interface InlineAISettings {
-	provider: "openai" | "ollama" | "custom" | "gemini" | "azure";
+	provider: "openai" | "ollama" | "custom" | "gemini" | "azure" | "codex";
 	model: string;
 	apiKey?: string;
 	customURL?: string;
@@ -16,6 +17,11 @@ export interface InlineAISettings {
 	customCommands: SlashCommand[];
 	commandPrefix: string;
 	messageHistory: boolean;
+	// Codex subscription OAuth tokens
+	codexAccess?: string;
+	codexRefresh?: string;
+	codexExpires?: number;
+	codexAccountId?: string;
 }
 
 // Default settings values
@@ -55,7 +61,7 @@ export class InlineAISettingsTab extends PluginSettingTab {
 		new Setting(containerEl)
 			.setName("Provider")
 			.setDesc(
-				"Choose between OpenAI, Ollama, Azure OpenAI, Gemini, or a custom OpenAI-compatible endpoint.",
+				"Choose between OpenAI, Ollama, Azure OpenAI, Gemini, a custom OpenAI-compatible endpoint, or Codex (ChatGPT subscription).",
 			)
 			.addDropdown((dropdown) =>
 				dropdown
@@ -64,6 +70,7 @@ export class InlineAISettingsTab extends PluginSettingTab {
 					.addOption("azure", "Azure OpenAI")
 					.addOption("gemini", "Gemini")
 					.addOption("custom", "Custom/OpenAI-compatible")
+					.addOption("codex", "Codex (ChatGPT subscription)")
 					.setValue(this.plugin.settings.provider)
 					.onChange(async (value) => {
 						this.plugin.settings.provider = value as
@@ -71,12 +78,55 @@ export class InlineAISettingsTab extends PluginSettingTab {
 							| "ollama"
 							| "azure"
 							| "custom"
-							| "gemini";
+							| "gemini"
+							| "codex";
 						await this.saveSettings();
-						this.display(); // Refresh UI to show/hide API key field
+						this.display();
 					}),
 			);
 
+		// Codex subscription auth section
+		if (this.plugin.settings.provider === "codex") {
+			const isSignedIn = !!(
+				this.plugin.settings.codexAccess &&
+				this.plugin.settings.codexAccountId
+			);
+
+			new Setting(containerEl)
+				.setName("ChatGPT account")
+				.setDesc(
+					isSignedIn
+						? `Signed in (account: ${this.plugin.settings.codexAccountId})`
+						: "Not signed in — click to authenticate with your ChatGPT Plus/Pro subscription.",
+				)
+				.addButton((btn) => {
+					btn.setButtonText(isSignedIn ? "Sign out" : "Sign in with ChatGPT")
+						.setCta()
+						.onClick(async () => {
+							if (isSignedIn) {
+								this.plugin.settings.codexAccess = undefined;
+								this.plugin.settings.codexRefresh = undefined;
+								this.plugin.settings.codexExpires = undefined;
+								this.plugin.settings.codexAccountId = undefined;
+								await this.saveSettings();
+								this.display();
+							} else {
+								new Notice("Opening browser for ChatGPT sign-in…");
+								const tokens = await startCodexOAuthFlow();
+								if (tokens) {
+									this.plugin.settings.codexAccess = tokens.access;
+									this.plugin.settings.codexRefresh = tokens.refresh;
+									this.plugin.settings.codexExpires = tokens.expires;
+									this.plugin.settings.codexAccountId = tokens.accountId;
+									await this.saveSettings();
+									new Notice("✅ Codex: signed in successfully");
+									this.display();
+								}
+							}
+						});
+				});
+		}
+
 		// Model setting
 		new Setting(containerEl)
 			.setName("Model")