microsoft/autogen · error · Exception

Missing ANTHROPIC_API_KEY environment variable.

Error message

Missing ANTHROPIC_API_KEY environment variable.

What it means

Guard exception in the 'create an Anthropic agent' documentation snippet: ANTHROPIC_API_KEY is null, so the code throws instead of building an AnthropicClient with an empty credential. Identical pattern to the other Anthropic samples; the message differs only in wording ('Missing' vs 'Please set').

Source

Thrown at dotnet/samples/AgentChat/AutoGen.Anthropic.Sample/Create_Anthropic_Agent.cs:15

// Copyright (c) Microsoft Corporation. All rights reserved.
// Create_Anthropic_Agent.cs

using AutoGen.Anthropic.Extensions;
using AutoGen.Anthropic.Utils;
using AutoGen.Core;

namespace AutoGen.Anthropic.Sample;

public static class Create_Anthropic_Agent
{
    public static async Task RunAsync()
    {
        #region create_anthropic_agent
        var apiKey = Environment.GetEnvironmentVariable("ANTHROPIC_API_KEY") ?? throw new Exception("Missing ANTHROPIC_API_KEY environment variable.");
        var anthropicClient = new AnthropicClient(new HttpClient(), AnthropicConstants.Endpoint, apiKey);
        var agent = new AnthropicClientAgent(anthropicClient, "assistant", AnthropicConstants.Claude3Haiku);
        #endregion

        #region register_middleware
        var agentWithConnector = agent
            .RegisterMessageConnector()
            .RegisterPrintMessage();
        #endregion register_middleware

        await agentWithConnector.SendAsync(new TextMessage(Role.Assistant, "Hello", from: "user"));
    }
}

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Export ANTHROPIC_API_KEY before dotnet run.
2. For CI, add it as a masked secret variable in the pipeline.
3. Use launchSettings.json or .env loading so IDE runs see it too.
4. Verify the key with a minimal curl to the Anthropic API if the error persists after setting it.

static string RequireEnv(string name) =>
    Environment.GetEnvironmentVariable(name)
    ?? throw new InvalidOperationException($"Missing {name} environment variable.");

var apiKey = RequireEnv("ANTHROPIC_API_KEY");

When it happens

Trigger: Executing Create_Anthropic_Agent.RunAsync without ANTHROPIC_API_KEY defined for the process.

Common situations: Fresh clone run without setting secrets; shell/IDE environment mismatch; CI job lacking the secret variable.

Related errors

• Please set ANTHROPIC_API_KEY environment variable.
• Missing ANTHROPIC_API_KEY environment variable.
• Please set OPENAI_API_KEY environment variable.
• Please set OPENAI_API_KEY environment variable.
• AZURE_OPENAI_API_KEY is not set

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/717e912f441a2cc0. Report an issue: GitHub.