Postman Simulator

Postman Simulator runs your service against simulated versions of the APIs it calls, so you can build and test without running every service. A simulation is a .sim.yaml file in your repository. It lists the mocks to run and how requests are routed to them, and it runs the same way in the Postman app, the CLI, CI, and through a coding agent. A mock stands in for one API, as code. It can keep state across requests, so when your service retries a payment, the payments mock stores a second charge. Simulations cover HTTP APIs and don't replace a final check against real services.

Install the Postman CLI with npm install -g postman-cli and run postman init in your repository. Then ask your agent: "Mock the APIs this service calls, run the service against them in a simulation, then make one of them fail and show me how the service handled it."

Prefer to work in the Postman app? The Learning Center covers the same steps: Simulate API conditions with Postman Simulator, Create a simulation, and Simulator in the CLI.

No. Developers create, edit, and run mocks and simulations in the Postman app or with the Postman CLI. Coding agents use the same files and commands, so you can start without an agent and add one later. Claude Code, OpenAI Codex, Cursor, Gemini CLI, Factory, and OpenCode work through the Postman plugin and skills.

You choose. A simulation serves mocks for the APIs you pick. In proxy mode, set the origin in each mock's config.yaml and trust the Simulator's CA certificate, and your service keeps its real URLs: calls to a mocked host go to its mock, and other calls go to the real services. The service you are building is not part of the simulation.

Requests that pass through the proxy reach real services and can change real data, so point them at test environments.

Yes. Each mock has named scenarios. Add an error such as a 503, a delay, or a rate limit to any mock and run again. You can override the conditions for a single run without changing the saved scenario, and the request log shows how your service handled each call.

Mocks come from your OpenAPI spec or collection, or from the team that owns the API and publishes them. Each simulation's context file records the sources and what has and has not been verified. Simulations let you test more conditions earlier and do not replace a final check against real services.

Yes. The .sim.yaml file lives in your repository. Start it with postman simulation run ./postman/simulations/checkout/checkout.sim.yaml, then run your tests against its address. The same command works in CI. Mocks are available on all plans. To let other developers, CI, and agents share a simulation instead of running their own, deploy it to a URL on your team's domain. Deploying needs a Solo, Team, or Enterprise plan (Postman pricing). The Postman Mock Servers page covers deployed URLs.

Postman mocks are JavaScript files in your repo that keep state and switch named scenarios, and the team that owns an API publishes one mock that every consumer reuses. Keep using staging for the services that are stable, and simulate the ones that are unstable, missing, or owned by another team.

Moving from another mocking tool: static routes and examples carry over through an OpenAPI spec, and your agent can rewrite stateful behavior as code. Review the result before you rely on it.