opencode-openai-codex-auth/docs/getting-started.md

Getting Started

Complete installation and setup guide for the OpenCode OpenAI Codex Auth Plugin.

⚠️ Before You Begin

This plugin is for personal development use only. It uses OpenAI's official OAuth authentication for individual coding assistance with your ChatGPT Plus/Pro subscription.

Not intended for: Commercial services, API resale, multi-user applications, or any use that violates OpenAI's Terms of Use.

For production applications, use the OpenAI Platform API.

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Prerequisites

• OpenCode installed (installation guide)
• ChatGPT Plus or Pro subscription (required for Codex access)
• Node.js 18+ (for OpenCode)

Installation

Step 1: Add Plugin to Config

OpenCode automatically installs plugins - no npm install needed!

Choose your configuration style:

⚠️ REQUIRED: Full Configuration (Only Supported Setup)

IMPORTANT: You MUST use the full configuration. This is the ONLY officially supported setup for GPT 5.1 models.

Why the full config is required:

• GPT 5 models can be temperamental and need proper configuration
• Minimal configs are NOT supported and will fail unpredictably
• OpenCode features require proper model metadata
• This configuration has been tested and verified to work

Add this to ~/.config/opencode/opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-openai-codex-auth"],
  "provider": {
    "openai": {
      "options": {
        "reasoningEffort": "medium",
        "reasoningSummary": "auto",
        "textVerbosity": "medium",
        "include": ["reasoning.encrypted_content"],
        "store": false
      },
      "models": {
        "gpt-5.1-codex-low": {
          "name": "GPT 5.1 Codex Low (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "low",
            "reasoningSummary": "auto",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-codex-medium": {
          "name": "GPT 5.1 Codex Medium (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "medium",
            "reasoningSummary": "auto",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-codex-high": {
          "name": "GPT 5.1 Codex High (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "high",
            "reasoningSummary": "detailed",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-codex-mini-medium": {
          "name": "GPT 5.1 Codex Mini Medium (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "medium",
            "reasoningSummary": "auto",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-codex-mini-high": {
          "name": "GPT 5.1 Codex Mini High (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "high",
            "reasoningSummary": "detailed",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-low": {
          "name": "GPT 5.1 Low (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "low",
            "reasoningSummary": "auto",
            "textVerbosity": "low",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-medium": {
          "name": "GPT 5.1 Medium (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "medium",
            "reasoningSummary": "auto",
            "textVerbosity": "medium",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        },
        "gpt-5.1-high": {
          "name": "GPT 5.1 High (OAuth)",
          "limit": {
            "context": 272000,
            "output": 128000
          },
          "options": {
            "reasoningEffort": "high",
            "reasoningSummary": "detailed",
            "textVerbosity": "high",
            "include": ["reasoning.encrypted_content"],
            "store": false
          }
        }
      }
    }
  }
}

What you get:

• ✅ GPT 5.1 Codex (Low/Medium/High reasoning)
• ✅ GPT 5.1 Codex Mini (Medium/High reasoning)
• ✅ GPT 5.1 (Low/Medium/High reasoning)
• ✅ 272k context + 128k output window for every preset
• ✅ All visible in OpenCode model selector
• ✅ Optimal settings for each reasoning level

Note

: All gpt-5.1-codex-mini* presets use 272k context / 128k output limits.

Prompt caching is enabled out of the box: when OpenCode sends its session identifier as prompt_cache_key, the plugin forwards it untouched so multi-turn runs reuse prior work. The CODEX_MODE bridge prompt bundled with the plugin is kept in sync with the latest Codex CLI release, so the OpenCode UI and Codex share the same tool contract. If you hit your ChatGPT subscription limits, the plugin returns a friendly Codex-style message with the 5-hour and weekly usage windows so you know when capacity resets.

⚠️ CRITICAL: This full configuration is REQUIRED. OpenCode's context auto-compaction and usage sidebar only work with this full configuration. GPT 5 models are temperamental and need proper setup - minimal configurations are NOT supported.

❌ Minimal Configuration (NOT SUPPORTED - DO NOT USE)

DO NOT use minimal configurations - they will NOT work reliably with GPT 5:

// ❌ DO NOT USE THIS
{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-openai-codex-auth"],
  "model": "openai/gpt-5-codex"
}

Why this doesn't work:

• GPT 5 models need proper configuration to work reliably
• Missing model metadata breaks OpenCode features
• Cannot guarantee stable operation

Step 2: Authenticate

opencode auth login

1. Select "OpenAI"
2. Choose "ChatGPT Plus/Pro (Codex Subscription)"
3. Browser opens automatically for OAuth flow
4. Log in with your ChatGPT account
5. Done! Token saved to ~/.opencode/auth/openai.json

⚠️ Important: If you have the official Codex CLI running, stop it first (both use port 1455 for OAuth callback).

Step 3: Test It

# Quick test
opencode run "write hello world to test.txt" --model=openai/gpt-5.1-codex-medium

# Or start interactive session
opencode

You'll see all 8 GPT 5.1 variants (5.1, 5.1 Codex, and 5.1 Codex Mini presets) in the model selector!

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Configuration Locations

OpenCode checks multiple config files in order:

1. Project config: ./.opencode.json (current directory)
2. Parent configs: Walks up directory tree
3. Global config: ~/.config/opencode/opencode.json

Recommendation: Use global config for plugin, project config for model/agent overrides.

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⚠️ Updating the Plugin (Important!)

OpenCode does NOT automatically update plugins.

When a new version is released, you must manually update:

# Step 1: Clear plugin cache
(cd ~ && sed -i.bak '/"opencode-openai-codex-auth"/d' .cache/opencode/package.json && rm -rf .cache/opencode/node_modules/opencode-openai-codex-auth)

# Step 2: Restart OpenCode - it will reinstall the latest version
opencode

When to update:

• New features released
• Bug fixes available
• Security updates

Check for updates: Releases Page

Pro tip: Subscribe to release notifications on GitHub to get notified of updates.

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Local Development Setup

For plugin development or testing unreleased changes:

{
  "plugin": ["file:///absolute/path/to/opencode-openai-codex-auth/dist"]
}

Note: Must point to dist/ folder (built output), not root.

Build the plugin:

cd opencode-openai-codex-auth
npm install
npm run build

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Verifying Installation

Check Plugin is Loaded

opencode --version
# Should not show any plugin errors

Check Authentication

cat ~/.opencode/auth/openai.json
# Should show OAuth credentials (if authenticated)

Test API Access

# Enable logging to verify requests
ENABLE_PLUGIN_REQUEST_LOGGING=1 opencode run "test" --model=openai/gpt-5-codex

# Check logs
ls ~/.opencode/logs/codex-plugin/
# Should show request logs

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Next Steps

• Configuration Guide - Advanced config options
• Troubleshooting - Common issues and solutions
• Developer Docs - Technical deep dive

Back to: Documentation Home