Anthropic API Key

agent1.ts:

import {createAgent} from 'langchain';
import 'dotenv/config'const agent = createAgent({model: 'claude-sonnet-4-5-20250929'}
);const response = await agent.invoke({messages: [{role: 'user', content: 'what is sum of 2+2'}]});
console.log(response);

 

.env:

ANTHROPIC_API_KEY='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'

 

In the agent1.ts file I didn't use ANTHROPIC_API_KEY, how did it know the API key?

 

Ah! This is a subtle but important behavior in Node.js/TypeScript with environment variables. Let me break it down carefully.

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1. How environment variables work

When you do:

import 'dotenv/config';

• The dotenv package automatically loads variables from your .env file into process.env.

• process.env is a global object in Node.js that any module can access.

• So after import 'dotenv/config', any code in your project can access environment variables like this:

console.log(process.env.ANTHROPIC_API_KEY);

---

2. Why your code “knows” the API key

• createAgent (from langchain) internally reads the environment variable ANTHROPIC_API_KEY from process.env) when you create the agent.

• You didn’t have to pass it explicitly, because langchain looks for it automatically.

So even though your code doesn’t mention the API key, langchain uses:

process.env.ANTHROPIC_API_KEY

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✅ Key takeaway

ANTHROPIC_API_KEY is implicitly passed through the environment, not as a function argument. This is a common pattern for API keys in Node.js libraries — it avoids hardcoding secrets in code.

 

If I change ANTHROPIC_API_KEY to another name, will the script still work?

 

ChatGPT said:

No — if you rename ANTHROPIC_API_KEY in your .env file, your script will stop working unless you also explicitly tell langchain (or whatever library you’re using) where to find the API key.

Here’s why:

1. langchain looks for a specific environment variable called ANTHROPIC_API_KEY.

2. If it doesn’t find it in process.env, it has no key to authenticate requests to Anthropic’s API.

3. So changing the name in .env breaks the implicit loading.

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How to fix it if you rename the key

You have two main options:

Option 1: Keep using ANTHROPIC_API_KEY in .env (simplest).

Option 2: Pass the API key explicitly in your code:

import { createAgent } from 'langchain';
import 'dotenv/config';const agent = createAgent({model: 'claude-sonnet-4-5-20250929',anthropicApiKey: process.env.MY_CUSTOM_KEY // use your renamed key
});const response = await agent.invoke({messages: [{ role: 'user', content: 'what is sum of 2+2' }]
});console.log(response);

• Here, you renamed your .env variable to MY_CUSTOM_KEY.

• Then you read it with process.env.MY_CUSTOM_KEY and pass it explicitly.

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💡 Rule of thumb: Most Node.js libraries that use API keys via environment variables are hardcoded to look for a specific name. Renaming requires either keeping the expected name or passing the value manually.