Claude Code CLI Third-Party API Quick Start: Environment Variable Setup

Claude Code CLI can be pointed to a third-party model endpoint through environment variables. With NixAPI configured, you can keep using the claude command while sending model requests to a NixAPI-compatible endpoint.

The three key variables are:

• ANTHROPIC_BASE_URL
• ANTHROPIC_API_KEY
• ANTHROPIC_MODEL

Important: for Claude Code CLI, set ANTHROPIC_BASE_URL to https://nixapi.com without /v1.

---

Prerequisites

Check Node.js first:

node -v

Node.js 18 or later is recommended. If it is missing, install it from the Node.js website.

Then create a NixAPI API Key from the NixAPI console. Copy model IDs from the NixAPI supported models page.

---

Step 1: Install Claude Code CLI

For macOS, Linux, or WSL:

curl -fsSL https://claude.ai/install.sh | bash

For Windows PowerShell:

irm https://claude.ai/install.ps1 | iex

Verify installation:

claude --version

If the command is not found, reopen the terminal and check the PATH instructions from the installer.

---

Step 2: Set Environment Variables

macOS / Linux / WSL

For the current terminal:

export ANTHROPIC_BASE_URL=https://nixapi.com
export ANTHROPIC_API_KEY=nix-your-api-key
export ANTHROPIC_MODEL=claude-sonnet-4-20250514

To make it persistent in zsh:

echo 'export ANTHROPIC_BASE_URL=https://nixapi.com' >> ~/.zshrc
echo 'export ANTHROPIC_API_KEY=nix-your-api-key' >> ~/.zshrc
echo 'export ANTHROPIC_MODEL=claude-sonnet-4-20250514' >> ~/.zshrc
source ~/.zshrc

Windows PowerShell

[Environment]::SetEnvironmentVariable("ANTHROPIC_BASE_URL", "https://nixapi.com", "User")
[Environment]::SetEnvironmentVariable("ANTHROPIC_API_KEY", "nix-your-api-key", "User")
[Environment]::SetEnvironmentVariable("ANTHROPIC_MODEL", "claude-sonnet-4-20250514", "User")

Close PowerShell and reopen it.

Do not add /v1 here. Claude Code CLI uses ANTHROPIC_BASE_URL and builds paths differently from the OpenAI SDK.

---

Step 3: Optional config.json

If you prefer a local config file, create:

• macOS / Linux: ~/.claude/config.json
• Windows: C:\Users\your-name\.claude\config.json

Example:

{
  "env": {
    "ANTHROPIC_BASE_URL": "https://nixapi.com",
    "ANTHROPIC_API_KEY": "nix-your-api-key",
    "ANTHROPIC_MODEL": "claude-sonnet-4-20250514"
  }
}

Save the file and reopen your terminal.

---

Step 4: Start Claude Code

Enter your project:

cd your-project
claude

If you previously logged in with an official account and the CLI still asks for official authentication, run:

/logout

Restart the CLI so it picks up the environment variables.

Test with:

Read the current project structure and give me a short code-quality checklist.

If you get a response, Claude Code CLI is using the configured endpoint.

---

Model Suggestions

Start with:

• claude-sonnet-4-20250514: daily coding, fixes, explanations
• claude-opus-4-7: deeper architecture analysis and long tasks
• gpt-5.4: general text and code tasks
• deepseek-v4: coding and reasoning comparison

Pricing varies by model. Check the NixAPI model pricing page before long-running tasks. NixAPI currently has a limited-time top-up exchange rate: RMB 0.80 = USD 1.00.

---

Troubleshooting

claude doctor shows network issues

Check whether the current terminal can read the variables:

echo $ANTHROPIC_BASE_URL
echo $ANTHROPIC_MODEL

In PowerShell:

echo $env:ANTHROPIC_BASE_URL
echo $env:ANTHROPIC_MODEL

If they are empty, the environment variables are not loaded.

API Key error

Check that ANTHROPIC_API_KEY contains a valid key from the NixAPI console and has no extra spaces.

Model not found

Copy the exact model ID from the NixAPI supported models page.

Why no /v1?

This is a Claude Code CLI-specific detail. Use:

https://nixapi.com

Do not use:

https://nixapi.com/v1

---

Summary

Claude Code CLI third-party API setup is driven by environment variables. With ANTHROPIC_BASE_URL=https://nixapi.com, ANTHROPIC_API_KEY, and ANTHROPIC_MODEL set correctly, the claude command can call models through NixAPI.