Configure a model provider

This guide helps you connect Harn to a model provider. It covers the checks you can run before you put a provider in a program.

1. Check the local installation

Run the doctor command first:

harn doctor
harn doctor --check-providers

The first command reports the local Harn setup. The second also checks the configured provider paths. It does not print secret values.

2. Find a provider and model

List the models that Harn knows about:

harn models list
harn models list --provider anthropic

Inspect one model before you use it:

harn models info claude-sonnet-5
harn provider dispatch-explain anthropic claude-sonnet-5

The catalog is the source of truth for current aliases and capabilities. Do not copy an old model name from an example when the catalog gives you a newer one.

3. Set the provider credential

Set the credential in your shell or secret manager. Common cloud providers use these variables:

Provider	Environment variable
Anthropic	ANTHROPIC_API_KEY
OpenAI	OPENAI_API_KEY
Google Gemini	GEMINI_API_KEY
OpenRouter	OPENROUTER_API_KEY
Together AI	TOGETHER_AI_API_KEY
DeepInfra	DEEPINFRA_API_KEY
NVIDIA NIM	NVIDIA_API_KEY
Hugging Face	HUGGINGFACE_API_KEY

The exact provider configuration can change. Use the provider reference for API details, or run harn doctor --check-providers after setting the variable.

4. Test the connection

Use the model test command for a small smoke test:

harn models test claude-sonnet-5 --provider anthropic

The command sends a test request. It can use provider credits. Use a model that is available to your account.

5. Use the provider in a program

Keep the provider and model in the call options:

fn main(harness: Harness) {
  const response = harness.llm.call(
    "Reply with one short greeting.",
    nil,
    { provider: "anthropic", model: "claude-sonnet-5", max_tokens: 64 }
  )
  harness.stdio.println(response.text)
}

For a single project, put stable defaults in harn.toml when the provider reference says the setting is supported. Keep credentials out of that file.

Local providers

Start your local server first, then use the provider's name and model from the catalog:

harn models list --provider ollama
harn provider ready ollama --model <model>

For an OpenAI-compatible server, check the endpoint and model settings in the provider reference.

Use the mock provider in tests

The mock provider needs no credentials and returns deterministic responses. Use it for syntax checks, unit tests, and examples. A mock run proves that the program reaches Harn's model-call boundary; it does not prove that a cloud provider is configured or that a model produces useful answers.

See mock LLM responses for queued responses and error cases.