feat: add --reasoning CLI flag (#314)

This PR adds a new CLI flag: `--reasoning`, which allows users to
customize the reasoning effort level (`low`, `medium`, or `high`) used
by OpenAI's `o` models.
By introducing the `--reasoning` flag, users gain more flexibility when
working with the models. It enables optimization for either speed or
depth of reasoning, depending on specific use cases.
This PR resolves #107

- **Flag**: `--reasoning`
- **Accepted Values**: `low`, `medium`, `high`
- **Default Behavior**: If not specified, the model uses the default
reasoning level.

## Example Usage

```bash
codex --reasoning=low "Write a simple function to calculate factorial"

---------

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

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

@@ -676,7 +676,7 @@ export class AgentLoop {
           try {
             let reasoning: Reasoning | undefined;
             if (this.model.startsWith("o")) {
-              reasoning = { effort: "high" };
+              reasoning = { effort: this.config.reasoningEffort ?? "high" };
               if (this.model === "o3" || this.model === "o4-mini") {
                 reasoning.summary = "auto";
               }

codex-cli/src/utils/config.ts

@@ -7,6 +7,7 @@
 // compiled `dist/` output used by the published CLI.
 
 import type { FullAutoErrorMode } from "./auto-approval-mode.js";
+import type { ReasoningEffort } from "openai/resources.mjs";
 
 import { AutoApprovalMode } from "./auto-approval-mode.js";
 import { log } from "./logger/log.js";
@@ -62,6 +63,8 @@ export const OPENAI_TIMEOUT_MS =
   parseInt(process.env["OPENAI_TIMEOUT_MS"] || "0", 10) || undefined;
 export const OPENAI_BASE_URL = process.env["OPENAI_BASE_URL"] || "";
 export let OPENAI_API_KEY = process.env["OPENAI_API_KEY"] || "";
+
+export const DEFAULT_REASONING_EFFORT = "high";
 export const OPENAI_ORGANIZATION = process.env["OPENAI_ORGANIZATION"] || "";
 export const OPENAI_PROJECT = process.env["OPENAI_PROJECT"] || "";
 
@@ -142,6 +145,9 @@ export type StoredConfig = {
     saveHistory?: boolean;
     sensitivePatterns?: Array<string>;
   };
+  /** User-defined safe commands */
+  safeCommands?: Array<string>;
+  reasoningEffort?: ReasoningEffort;
 };
 
 // Minimal config written on first run.  An *empty* model string ensures that
@@ -165,6 +171,7 @@ export type AppConfig = {
   approvalMode?: AutoApprovalMode;
   fullAutoErrorMode?: FullAutoErrorMode;
   memory?: MemoryConfig;
+  reasoningEffort?: ReasoningEffort;
   /** Whether to enable desktop notifications for responses */
   notify?: boolean;
 
@@ -366,6 +373,7 @@ export const loadConfig = (
     notify: storedConfig.notify === true,
     approvalMode: storedConfig.approvalMode,
     disableResponseStorage: storedConfig.disableResponseStorage ?? false,
+    reasoningEffort: storedConfig.reasoningEffort,
   };
 
   // -----------------------------------------------------------------------
@@ -480,6 +488,7 @@ export const saveConfig = (
     provider: config.provider,
     providers: config.providers,
     approvalMode: config.approvalMode,
+    reasoningEffort: config.reasoningEffort,
   };
 
   // Add history settings if they exist