Using Insomnia Environment Variables and Pre‑request Scripts for Dynamic API Calls

Use Environment Variables for Dynamic Requests

When you need the same API call to hit different servers (dev, staging, prod) or to inject per‑run values such as timestamps or random IDs, Insomnia lets you define variables once and reuse them across all requests in a workspace.

How the substitution works

Insomnia scans the URL, headers, query string and body of a request before it is sent. Any text that matches the pattern {{variable_name}} is replaced with the value of variable_name from the currently selected environment. If the variable is missing, the placeholder stays unchanged, which can cause malformed requests.

Worked configuration

1. Create a new workspace (File → New Workspace) and save it as api-demo.workspace.json.
2. Open the Environments pane, click “+” and add an environment named test. Add a variable base_url with value https://httpbin.org.
3. Add another environment named prod with the same variable name but value https://api.example.com.
4. Create a new GET request. Set the URL to {{base_url}}/get. In the Headers tab add a custom header X-Request-ID: {{request_id}} (we will set this later).
5. Select the test environment from the dropdown at the top‑left. The request preview pane should show the resolved URL as https://httpbin.org/get.
6. To make the request ID change each time, open the Pre‑request Script tab and paste:

// Generate a short random string and store it in the environment\nconst randomId = Math.random().toString(36).substring(2, 9);\npm.environment.set('request_id', randomId);\n

1. Send the request twice. Insomnia will execute the pre‑request script before each call, so the X-Request-ID header will contain a different value each time.
2. Switch the environment dropdown to prod and resend. The same request now points to https://api.example.com/get without any further edits.

Limits and common mistakes

• Variable names are case‑sensitive, must start with a letter or underscore, and cannot contain spaces. Using an invalid name leaves the placeholder untouched.
• If a referenced variable is not defined in the active environment, Insomnia does not error; it sends the literal {{placeholder}} string, often resulting in 400/404 responses that are hard to trace.
• Environment files are stored as plain JSON inside the workspace. Storing secrets such as API keys or passwords in these files exposes them to anyone with access to the workspace. For sensitive data, consider using a local environment file (not committed to VCS) or an external secret manager.
• Pre‑request scripts run in a sandboxed Node‑like context; they cannot make external network calls or access the file system.