Setting Up Claude Code with a Custom Provider

Claude Code is a handy coding agent that runs right in your terminal. By default it uses Anthropic models through Anthropic’s own API, but you can point it at a custom provider like 9router for more model options, often at a lower cost. I covered 9router in detail in this guide, and the whole setup here is just a few lines in one JSON file.

Quick setup

In a hurry? Replace the credentials below and copy this prompt to your coding agent in the terminal:

Configure Claude Code custom provider in `settings.json`.

Detect the OS and use:
* Windows: `%USERPROFILE%\.claude\settings.json`
* Linux/WSL/macOS: `~/.claude/settings.json`

Create or update the file, preserving existing settings. Add/replace `env` with:

{
  "ANTHROPIC_API_KEY": "<API_KEY>",
  "ANTHROPIC_BASE_URL": "<BASE_URL>",
  "ANTHROPIC_DEFAULT_OPUS_MODEL": "<MODEL>",
  "ANTHROPIC_DEFAULT_SONNET_MODEL": "<MODEL>",
  "ANTHROPIC_DEFAULT_HAIKU_MODEL": "<MODEL>"
}

Ask for missing API key, base URL, or model. Keep valid JSON and do not expose the full API key.

Step 1: Install Claude Code

If you do not have Claude Code yet, install it first.

macOS and Linux: open your terminal and run:

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

Windows (PowerShell): run:

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

Alternative on macOS with Homebrew:

brew install --cask claude-code

Step 2: Configure the Custom Provider

Claude Code reads its settings from a settings.json file. The location depends on your OS:

• Windows: C:\Users\<YourUsername>\.claude\settings.json (resolves from %USERPROFILE%\.claude\settings.json)
• Linux / WSL: ~/.claude/settings.json (resolves to /home/<username>/.claude/settings.json)
• macOS: ~/.claude/settings.json (resolves to /Users/<username>/.claude/settings.json)

Open the file and add your custom provider configuration inside the env block. For example, with 9router it looks like this:

{
  "env": {
    "ANTHROPIC_API_KEY": "YOUR_CUSTOM_PROVIDER_API_KEY",
    "ANTHROPIC_BASE_URL": "http://localhost:20218/v1",
    "ANTHROPIC_DEFAULT_OPUS_MODEL": "provider-name/model-name",
    "ANTHROPIC_DEFAULT_SONNET_MODEL": "provider-name/model-name",
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "provider-name/model-name"
  }
}

The env block sets environment variables for Claude Code. ANTHROPIC_BASE_URL points to your provider’s endpoint, and the model variables pick which model each Claude Code tier uses.

Fun fact: the hardest part of this step is usually remembering where you put the comma 🫪

Step 3: Verify the Configuration

Save the file and start Claude Code from your terminal:

claude

You will be asked to confirm that you want to use your custom API key. Select yes:

Then check the header: it should show your selected custom model. Try sending a message to confirm everything responds correctly.

And you are good to go. Claude Code now runs with your custom provider.