Get started

Fastest path: zero install. Or self-host in one command.

⚡ Zero install

Use the free hosted instance

No download, no signup, no key of your own to manage. Point any OpenAI SDK's baseURL at the hosted mock and start testing right now:

baseURLhttps://api.llm-mock.dev/openai/v1

Prefer to self-host?

Prebuilt multi-arch image on GHCR, or run from source with Bun.

# Docker
docker run --rm -p 3000:3000 ghcr.io/axium-lab/llm-mock

# From source
git clone https://github.com/axium-lab/llm-mock.git
cd llm-mock
bun install
bun start

See the API reference for every endpoint, header and error shape.

Change one line, keep your code

The official openai SDK talks to llm-mock exactly as it talks to the real API. No mocking libraries, no request interception.

import OpenAI from "openai";

const client = new OpenAI({
  baseURL: "http://localhost:3000/openai/v1", // the only change in your app
  apiKey: "sk-mock-key-01",                   // any mock key, no real secret
});

const completion = await client.chat.completions.create({
  model: "gpt-4o",
  messages: [{ role: "user", content: "Hello!" }],
});

console.log(completion.choices[0].message.content); // "Echo: Hello!"

Built for real integration tests

Not a stub — a faithful contract, verified in CI against the official SDK.

OpenAI-compatible contract

Responses match the official API shapes, validated in CI with the official openai SDK as the client.

Streaming (SSE)

stream: true works on chat completions and the Responses API, with the real typed event stream.

Deterministic & stateless

Same request, same bytes. Snapshot-friendly, and identical on a laptop, in CI, or behind a load balancer.

Real error flows

Invalid keys, unknown models and validation errors return the exact OpenAI error envelope — test your error handling too.

Controllable replies

Need a specific answer? Send it in a request header. Nothing to register, nothing to clean up.

Zero setup

One Docker command, or bun install && bun start. Mock keys ship in the repo.

Provider support

Multi-provider by design: each provider mounts under its own URL prefix and implements its own API contract. Here's where each one stands today.

Provider	Prefix	Status
OpenAI	`/openai/v1`	✅ Supported
Anthropic	`/anthropic`	🔜 Planned
Gemini (AI Studio)	`/gemini`	🔜 Planned
Gemini Enterprise (Vertex AI)	—	🔜 Planned
Azure OpenAI	—	🔜 Planned