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# Simulations: test every path through your API tools

A simulation can now answer an API tool from the test case. You describe the records and rules for each test. You can then test every branch of a flow, with no test accounts to set up or reset. Use it to find Playbook gaps and to test fixes in the improvement loop. Every API tool stays **Live** by default. Set an API tool to **Mock** to use it.

## What changes

* **Mock in simulations**, on each API tool. A mocked API tool is answered from the new **External systems** field of the test case. Its API traffic profile, a summary of how your API usually answers, gives the answer its shape. No request reaches your system. Set several API tools at once with **Set simulation mode** on the **Tools** page.
* **External systems**, a new test case field. Describe the records your systems hold and the rules they follow for the test.
* **Block in simulations**, on each code tool, MCP tool, and HTTP Request block. These have no mock. Code tools and MCP tools run for real by default, and so does an HTTP Request block in a voice simulation. A blocked call sends nothing, and the run fails.
* A **Mocked** or **Live** badge on each API tool call in a simulation transcript.
* A Web Chat or Email simulation no longer sends your webhooks, and no simulation writes to Zendesk Messaging. A voice simulation still sends webhooks, because it is a real call.

Mock API tools work on Web Chat, Email, and Voice. See [Mock API tools](/docs/optimization/testing/simulations/mock-api-tools).