How to Use ANTHROPIC_BASE_URL to Cut API Costs

The ANTHROPIC_BASE_URL environment variable lets you route all Anthropic API calls through a local proxy, without changing any code. RelayPlane uses this to route simple tasks to cheaper models, saving you 40-60% on API costs.

What is ANTHROPIC_BASE_URL?

Every Anthropic SDK (Python, TypeScript, and tools like OpenClaw and Claude Code) checks for the ANTHROPIC_BASE_URL environment variable before making API calls. If set, all requests go to that URL instead of api.anthropic.com.

This means you can intercept every API call your agent makes, and make intelligent decisions about which model to actually use.

Quick Setup (60 seconds)

Step 1: Install the RelayPlane proxy

$npm install -g @relayplane/proxy

Step 2: Initialize the proxy (first-time setup)

$relayplane init

Sets up your config and API keys on first run.

Step 3: Start the proxy

$relayplane start

Starts on port 4100 by default. Runs locally on your machine.

Step 4: Set the environment variable

$export ANTHROPIC_BASE_URL=http://localhost:4100

Step 5: Use your tools normally

$openclaw

That's it. All requests now route through RelayPlane. Simple tasks go to Haiku. Complex tasks stay on Opus/Sonnet.

Tools That Support ANTHROPIC_BASE_URL

OpenClaw

Full support. Just set the env var and run.

Claude Code

Full support via ANTHROPIC_BASE_URL.

Cursor

Supports custom API endpoints in settings.

Aider

Respects ANTHROPIC_BASE_URL automatically.

Python SDK

anthropic.Anthropic() checks the env var.

TypeScript SDK

new Anthropic() checks the env var.

How This Saves You Money

When your agent sends a request for claude-4-opus, RelayPlane analyzes the request metadata (task type, token count, complexity) and decides whether a cheaper model like Haiku would work just as well.

# Typical agent session with RelayPlane:
[09:41:02] Reading file... → haiku ($0.001) was opus ($0.06)
[09:41:03] Listing directory... → haiku ($0.001) was opus ($0.06)
[09:41:04] Planning refactor... → sonnet ($0.02) was opus ($0.06)
[09:41:06] Complex architecture... → opus ($0.06) kept as-is
Session savings: $0.16 → $0.08 (50% saved)

Common Questions

Does this add latency?

Less than 5ms for local routing. The proxy runs on your machine, no network hop.

What if the cheap model fails?

RelayPlane automatically retries with a better model. Your agent never sees the failure.

Can I see what's being routed?

Run relayplane --audit to see every routing decision in real-time.

Does RelayPlane see my prompts?

No. The proxy runs locally on your machine. Your prompts go directly to Anthropic. Telemetry is on by default: only anonymous metadata (task type, token count) is shared. Disable: relayplane telemetry off

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