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# Simulations: see what your External systems text leaves out

A simulated run now tells you when your test case left out a value that an API tool set to **Mock** returns. Before, the AI Agent acted on an empty value, and the run looked like an AI Agent fault.

**Simulation analysis** lists **What External systems leaves out** below your text:

* An output that came back empty, when your text names neither the output nor its API tool.
* A record written as JSON that has no key that an output of the API tool reads.

Each item says what to add to the **External systems** text.

## Who this affects

Every test case that uses an API tool set to **Mock**. The list is available for 24 hours after the run.

See [Mock API tools](/docs/optimization/testing/simulations/mock-api-tools#what-the-simulation-flags).