Use a Third-Party API with Claude Code: ANTHROPIC_BASE_URL and settings.json

LLM API Guide Editorial (SeedRouter team) · Published 2026-10-08 · Updated 2026-10-08 · 6 min

Disclosure: LLM API Guide is maintained by the SeedRouter team, and this guide uses SeedRouter as the example. It applies to any gateway that offers an Anthropic Messages-compatible API.

What you need

Value	SeedRouter example
Gateway root URL	`https://seedrouter.net` (no `/v1`)
API key	An `sk-...` key created in the seedrouter.net dashboard
Model ID	e.g. `claude-sonnet-5-5` or `claude-opus-5-5`; copy it from the dashboard catalog

Option 1: Try it with environment variables

macOS / Linux:

export SEEDROUTER_API_KEY="sk-your-key"
export ANTHROPIC_BASE_URL="https://seedrouter.net"
export ANTHROPIC_AUTH_TOKEN="$SEEDROUTER_API_KEY"
export ANTHROPIC_MODEL="claude-sonnet-5-5"

claude

Windows PowerShell:

$env:ANTHROPIC_BASE_URL = "https://seedrouter.net"
$env:ANTHROPIC_AUTH_TOKEN = "sk-your-key"
$env:ANTHROPIC_MODEL = "claude-sonnet-5-5"
claude

These variables only last for the current terminal window.

Option 2: Make it permanent in settings.json

Claude Code's user settings file is ~/.claude/settings.json (Windows: C:\Users\<you>\.claude\settings.json). Values in its env block are applied on every start:

{
  "env": {
    "ANTHROPIC_BASE_URL": "https://seedrouter.net",
    "ANTHROPIC_AUTH_TOKEN": "sk-your-key",
    "ANTHROPIC_MODEL": "claude-sonnet-5-5"
  }
}

If the file already has other settings, merge the env block in and mind the commas. The file now contains a secret, so keep it out of Git and out of support chats.

Choosing models

ANTHROPIC_MODEL sets the default model.
Type /model inside Claude Code to switch.
A gateway's model IDs can differ from official names, so copy them from the gateway's catalog.

Check that requests go through the gateway

Start claude and ask a simple question.
Type /status inside Claude Code to see the current connection details.
Open the gateway dashboard's usage logs; the request and its charge should appear there.

Common problems

Symptom	Cause	Fix
401	Wrong key, extra whitespace, or a key from another site	Copy the key from the dashboard again
404	`/v1` added to ANTHROPIC_BASE_URL, or a typo in the domain	Use only the gateway root URL
Auth conflict warning	Both an account sign-in and an environment key are present	Run `/logout`, restart, or remove the extra variable
Model unavailable	The model ID is not in your account's catalog	Check the dashboard model list
Long task drops midway	Network or proxy interruption, or an intermediary closing a long silent stream	Retry and check the gateway status page

Summary

Three variables do the work: ANTHROPIC_BASE_URL, ANTHROPIC_AUTH_TOKEN and ANTHROPIC_MODEL. Test them in your shell first, then move them into settings.json. On SeedRouter, Claude models are priced at 30% of official API pricing (see the live price table on the SeedRouter website), and new accounts get two free popular-model trials.

FAQ

Should ANTHROPIC_BASE_URL include /v1?

No. Claude Code appends /v1/messages itself. For SeedRouter use https://seedrouter.net.

What is the difference between ANTHROPIC_AUTH_TOKEN and ANTHROPIC_API_KEY?

AUTH_TOKEN is sent as an Authorization Bearer header and API_KEY as an x-api-key header. Use the one your gateway documents; SeedRouter's example uses ANTHROPIC_AUTH_TOKEN.

I signed in with a Claude account before. Will that conflict?

You may see an auth conflict warning. Run /logout inside Claude Code to drop the account sign-in, then start it again with the environment variables.