Some validations and enrichments can't be done inside WeTransform alone. Is this VAT number real? Does this customer ID exist in your database? What is the current price of this SKU in your ERP? These checks require calling an external system, in real time, during the transformation.
That's what Power Functions are for.
⚡ What is a Power Function?
A Power Function is a rule action that calls an external API during the transformation process. WeTransform sends the relevant data to the external system, reads the response, and writes the result directly into your file, visible on the Finalize screen like any other rule output. You choose when to trigger it, on which column, and under which conditions.
🏭 Real-world examples
🔍 VAT validation — for each row, send the VAT number to a business registry API. If invalid, flag the row as an error before submission.
💶 Price lookup — fetch the current price of each SKU from your ERP and write it into the output, so the file always carries live data.
📍 Address lookup — validate a delivery address against a mapping service. Correct misspellings or flag unrecognized addresses automatically.
🪪 Customer ID check — verify that a customer_id in the incoming file actually exists in your CRM before accepting the file.
🔀 Two ways to connect your system
🟢 Fetch mode — no development needed. You describe the HTTP request once (URL, headers, with placeholders like {sku} for the row's values), and WeTransform calls your API once per row and reads the result straight from the response. If the response is JSON, you pick the exact value with a path like price.amount, using a Test button that runs the request on sample data. If your API already exists, there is nothing to build. Rows with identical values share a single call, and results are cached.
🔵 Callback mode — full control, for high volume. WeTransform notifies your backend that a batch is ready; your backend fetches the rows, applies its own logic, and pushes the results back. Requests are HMAC-signed. This mode requires development and suits advanced, high-volume integrations.
Start with Fetch mode. Move to Callback mode only if you need custom batching or heavy processing on your side.
⚙️ How to use a Power Function in a rule
Once configured in Integrations → Power Functions, the function becomes available natively for your users. Open the Rules editor, create a new rule, and in the THEN section select "Power functions" in the action dropdown. You'll see the list of available functions, both native ones and any custom ones set up in your account.
🔑 Some functions require an API key. Native functions like "Generate content with AI" require an OpenAI API key, and "Send an email" requires a Sendgrid key. Custom Power Functions in Fetch mode use the API key of the system you're calling, passed through the {token} placeholder. These are configured in your account settings.
✅ What happens after the function runs?
While the function runs, the affected columns show a "pending" status on the Finalize screen, and the results appear as they come in. If the function flags a row, it appears as a regular error in Finalize, with the same correction options as any other error.
Every execution is also traceable: it appears in the steps overview with its impacted rows, and selecting the affected cells and clicking Explain shows why the function wrote or flagged each value. See How to understand every change made to your file.
👨💻 For your developers: the full technical reference for both modes, including a live demo endpoint to try Fetch mode, is at api.wetransform.com/en/documentation/power-functions.
👉 What to do next