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Cursor API Key Setup — APIMaster Third-Party Key Guide

Use APIMaster.ai API keys in Cursor for DeepSeek, GPT, and Claude. Covers Settings custom models, GPT/Claude pitfalls, and the official Codex / Claude Code extension workflow.

Cursor is an AI-powered code editor. APIMaster.ai provides OpenAI-compatible and Claude Messages APIs. You can connect via Cursor’s built-in third-party key settings or via official extensions.

Start with Get API Key. Keep keys on your machine only—do not share in chats or screenshots.

Which approach to use

Goal Recommended
Models not built into Cursor (e.g. DeepSeek) Cursor Settings → Models + custom model id
GPT-5.4 / GPT-5.5 billed via APIMaster Codex extension + ~/.codex config
Claude Sonnet / Opus billed via APIMaster Claude Code extension + ~/.claude config
General coding agent Cline (OpenAI Compatible) also works

Prerequisites

  1. Cursor installed (Windows / macOS / Linux).
  2. Cursor’s third-party API key feature requires Cursor Pro.
  3. APIMaster API Key from the console.
  4. Target model id from the marketplace.

Method 1: Cursor Settings (good for DeepSeek, etc.)

Works for model ids Cursor does not ship natively (e.g. deepseek-v4-pro).

Step 1: API Key and Base URL

  1. Open Cursor Settings → Models (or API Keys).

  2. Enable OpenAI API Key and paste your APIMaster key.

  3. Enable Override OpenAI Base URL:

    https://apimaster.ai/v1

    Include the /v1 suffix.

Cursor Settings: API key and override base URL

Step 2: Add a custom model

  1. Click Add model.
  2. Enter a marketplace model id (e.g. deepseek-v4-pro).
  3. Enable it and select it in the chat model picker.

Add and enable deepseek-v4-pro

Step 3: Test

Send a message in Cursor chat. If the footer shows deepseek-v4-pro and you get a reply, traffic is going through APIMaster.

Method 1 limits: built-in GPT & Claude

With the same Settings third-party key, GPT and Claude names often behave differently:

1. Cannot add duplicate built-in names

Adding gpt-5.4, gpt-5.5, or some Claude ids may fail—Cursor already reserves those slots for official Cursor routes.

Fix: Use Method 2 (Codex / Claude Code extensions) for GPT and Claude on APIMaster.

2. Built-in GPT errors after configuring APIMaster

Selecting Cursor’s built-in GPT-5.4 / GPT-5.5 while Override Base URL points to APIMaster can error—the built-in route conflicts with your custom base URL.

Fix: Use the Codex side panel, not built-in GPT models.

3. Built-in Claude uses Cursor Pro quota

Built-in Claude/Sonnet in the main chat usually bills Cursor Pro, not your APIMaster key.

Fix: Use the Claude Code extension panel.

Method 2: Codex & Claude Code extensions (GPT & Claude)

Install official extensions and point them at APIMaster via the same config files as CLI.

Install

  • Codex – OpenAI's coding agent (openai)
  • Claude Code for VS Code (Anthropic)

Install Codex from marketplace

Install Claude Code from marketplace

Configure APIMaster

Extension Guide Files Base URL
Codex (GPT) Codex setup ~/.codex/config.toml, auth.json https://apimaster.ai/v1
Claude Code Claude Code setup ~/.claude/settings.json https://apimaster.ai (no /v1)

See each guide for full file contents and backup instructions.

Use in Cursor

Open the CODEX or CLAUDE CODE activity bar icon and chat there—billing uses APIMaster, separate from Cursor’s built-in models.

Codex panel in Cursor

Claude Code panel in Cursor

Troubleshooting

Issue Fix
No API key / override option Requires Cursor Pro
401 Check key; no extra spaces
404 model Match marketplace id exactly
Cannot add gpt-5.4 Use Codex extension
Claude still uses Pro quota Use Claude Code panel
Codex hits OpenAI Full-file replace ~/.codex configs per guide

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