Using the Response Body Transform middleware with Tyk Classic APIs

Last updated: 3 minutes read.

The response body transform middleware provides a way to modify the payload of API responses before they are returned to the client.

This middleware is configured in the Tyk Classic API Definition at the endpoint level. You can do this via the Tyk Dashboard API or in the API Designer.

If you’re using the newer Tyk OAS APIs, then check out the Tyk OAS page.

Configuring the middleware in the Tyk Classic API Definition

To enable the middleware you must add a new transform_response object to the extended_paths section of your API definition.

The transform_response object has the following configuration:

  • path: the path to match on
  • method: this method to match on
  • template_data: details of the Go template to be applied for the transformation of the response body

The Go template is described in the template_data object by the following fields:

  • input_type: the format of input data the parser should expect (either xml or json)
  • enable_session: set this to true to make session metadata available to the transform template
  • template_mode: instructs the middleware to look for the template either in a file or in a base64 encoded blob; the actual file location (or base64 encoded template) is provided in template_source
  • template_source: if template_mode is set to file, this will be the path to the text file containing the template; if template_mode is set to blob, this will be a base64 encoded representation of your template

For example:

    "extended_paths": {
        "transform_response": [
                "path": "/anything",
                "method": "POST",
                "template_data": {
                    "template_mode": "file",
                    "template_source": "./templates/transform_test.tmpl",
                    "input_type": "json",
                    "enable_session": true 

In this example, the Response Body Transform middleware is directed to use the template located in the file at location ./templates/transform_test.tmpl. The input (pre-transformation) response payload will be json format and session metadata will be available for use in the transformation.


Tyk will load and evaluate the template file when the Gateway starts up. If you modify the template, you will need to restart Tyk in order for the changes to take effect.

Prior to Tyk 5.3, there was an additional step to enable response body transformation. You would need to add the following to the Tyk Classic API definition:

        {"name": "response_body_transform"}

If using the Endpoint Designer in the Tyk Dashboard, this would be added automatically.

We removed the need to configure the response_processors element in Tyk 5.3.0.

Configuring the middleware in the API Designer

You can use the API Designer in the Tyk Dashboard to configure the response body transform middleware for your Tyk Classic API by following these steps.

Step 1: Add an endpoint for the path and select the plugin

From the Endpoint Designer add an endpoint that matches the path for which you want to perform the transformation. Select the Body Transforms plugin.

Endpoint designer

Step 2: Configure the middleware

Ensure that you have selected the RESPONSE tab, then select your input type, and then add the template you would like to use to the Template input box.

Setting the body response transform

Step 3: Test the Transform

If you have sample input data, you can use the Input box to add it, and then test it using the Test button. You will see the effect of the template on the sample input in the Output box.

Testing the body transform function

Step 4: Save the API

Use the save or create buttons to save the changes and activate the Response Body Transform middleware.