# Execution Mode

> Choose where tests run: Shift-Left Engine on server for CI/CD and schedules, or Shift-Left Agent locally for development and private APIs.

Source: https://totalshiftleft.ai/help-center/product-documentation/execution-mode

## Overview

Execution Mode decides where test cases are executed for an endpoint.

Use this selection before running tests so behavior matches your environment and workflow.

## Shift-Left Engine

- Runs tests on the server.
- Best for CI/CD pipelines and scheduled runs.
- Status is typically shown as **Always Available**.

Choose this mode when you want centralized execution and automation-ready behavior.

## Shift-Left Agent

- Runs tests locally on your machine.
- Best for development workflows and private/internal APIs.
- Status can appear as **Not Running** when the local agent process is not active.

Choose this mode when local network access or developer-side execution is required.

## If Shift-Left Agent is not available

When the UI shows **Shift-Left Agent Not Available**, use this flow:

1. Download Shift-Left Agent.
2. Install Shift-Left Agent.
3. Start Shift-Left Agent.

After startup, refresh status in Studio and select Agent mode again.

## Start and stop Agent

- **Start Agent**: launch the local agent service/app so Studio can execute local tests.
- **Stop Agent**: stop the local service when local execution is no longer needed.

If status remains unavailable after start, verify installation health and local connectivity.

## Related articles

- [Generate endpoint test cases](/help-center/product-documentation/ai-generated-tests)
- [Test case](/help-center/product-documentation/test-case)
- [Run tests and review results](/help-center/product-documentation/test-run)
- [Test execution](/help-center/product-documentation/test-execution)