What is a conversions API?
By the HonestTag team ยท Published October 24, 2026
A conversions API is an ad platform's server-to-server way in for conversions such as purchases. Instead of a pixel in the shopper's browser reporting the sale, a server reports it. Most platforms now have one, and most recommend running it alongside their pixel. The rule that matters: when both report the same sale, both copies need the duplicate key that platform names, plus any other condition it names, such as a time window, or the platform may count the sale twice.
Why platforms built them
The platforms give their own reasons:
- TikTok: "Connectivity issues and browser inconsistency can impact conversions reported via Pixel" (TikTok Business Help).
- Reddit offers its API "for more accurate tracking, especially in cookie-restricted environments" (Reddit Ads API).
- OpenAI: "Measure conversions without depending solely on browser event delivery" (OpenAI Ads).
Install HonestTag free and run the Mirror plan beside your current tracking. It sends no purchases or conversions to your ad platforms, so it cannot double-count a sale or change your current setup, and it shows which platform claims hold up against the new customers in your real orders. Free at any store size.
What each platform calls it
| Platform | Its name | Duplicate key | Window it states |
|---|---|---|---|
| Meta | Conversions API | event_id and event_name | 48 hours |
| TikTok | Events API | event and event_id | After 5 minutes, within 48 hours of the first event |
| Conversions API | event_id | None stated | |
| Snapchat | Conversions API (CAPI) | event_id and timestamp | 48 hours |
| Conversions API | conversion_id | None stated | |
| Microsoft Advertising | Conversions API (CAPI) | tagId, eventId and eventName | None stated |
| OpenAI (ChatGPT ads) | Conversions API (CAPI) | Pixel ID, event_name and id | None stated |
Google Ads says that when two conversions for the same conversion action have the same transaction ID, it "will know the second conversion is a duplicate," and that the ID "should be consistent across both the tag and server-to-server uploads" (Google Ads Help). Google Ads conversion tracking covers it.
Pixel, API, or both?
Both, say most platforms. Meta recommends the Conversions API "alongside their Meta Pixel," TikTok recommends, for website connections, the Events API "with your existing Pixel integration," Snapchat recommends its API to web advertisers "alongside a Pixel integration," and OpenAI says "Use both to combine browser activity with confirmed backend conversions."
If TikTok shows no purchases at all, my TikTok pixel shows no purchases walks through the causes.
Each platform sets its own conditions for treating two copies as one sale, and the table above lists them. Meta, for example, discards the later copy only when the event name and event ID match, both went to the same pixel and they arrive within 48 hours. Two tools that each send a purchase with their own IDs can leave the platform with two purchases. Duplicate purchase events goes deeper.
Check what each platform is claiming. The free Mirror plan puts what Google Ads, Meta, Microsoft Advertising and TikTok claim beside their first-click new customers, and paid plans send verified orders to what you connect.
What a conversions API does not change
- Consent still applies. Shopify says server pixels "are still subject to your store's customer privacy and consent settings" (Shopify Help Center). Microsoft says "By default, all events are processed with the consent state set to granted," and a sender can include its adStorageConsent parameter; with the value D (Denied), events "will not be used for any advertising purposes, including conversion attribution and retargeting" (Microsoft Learn).
- Old events have a limit. Meta returns an error for the entire request when any event in it is more than 7 days old (Meta for Developers). Microsoft says "eventTime must be within the last 7 days" (Microsoft Learn).
- Other platforms are unaffected. A server event sent to one platform does not stop another platform from claiming the same order.
What HonestTag does
- The free Mirror plan sends no purchases to any platform. It shows what Google Ads, Meta, Microsoft Advertising and TikTok claim beside the new customers whose first recorded ad click or ad-tagged link was for that platform, counted from your own orders.
- On a paid plan, verified orders go to the destinations you connect, within your plan's simultaneous-delivery limit, through each platform's own API: Meta's Conversions API, TikTok's Events API, the Pinterest, Reddit and OpenAI Conversions APIs, Google's Data Manager API with the order ID as the transaction ID, and Microsoft's API as an offline conversion for each goal.
The server-side delivery docs list what each platform receives.
Frequently asked questions
Is a conversions API the same as server-side tracking?
Close. A conversions API is the platform's door for server-sent events. Server-side tracking is the practice of sending events from a server, through that door or another.
Do I still need the pixel if I use a conversions API?
Most platforms recommend both for a website: Meta, TikTok (for website connections), Snapchat (for web advertisers), Reddit and OpenAI each say so. Meta tells you to share the same events through both, because the Conversions API can capture website events that the pixel might miss due to network issues or page loading errors.
Will the pixel and the conversions API count a sale twice?
Not when both copies meet that platform's conditions: its duplicate key, plus any window or other rule it states. Meta, for example, discards the later copy when the event name and event ID match, both go to the same pixel and they arrive within 48 hours. Without a match, the platform may count the sale twice.
Does a conversions API get around a shopper's consent choice?
Not for Shopify server pixels: Shopify says they are still subject to your store's customer privacy and consent settings, and events might be restricted when a customer hasn't consented to tracking. Microsoft says that by default all events are processed with the consent state set to granted, and a sender can include its adStorageConsent parameter; events marked Denied will not be used for any advertising purposes, including conversion attribution and retargeting.
Check what each platform is claiming
The free Mirror plan puts what Google Ads, Meta, Microsoft Advertising and TikTok claim beside their first-click new customers, and paid plans send verified orders to what you connect.
- No risk to your setup. The Mirror plan sends no purchases or conversions to your ad platforms, so it cannot double-count a sale or change your current setup.
- Claims against real orders. It shows what Google Ads, Meta, Microsoft Advertising and TikTok claim beside the new customers whose first recorded ad click or ad-tagged link was that platform, counted from your own orders.
- Free at any store size. The Mirror plan stays free whatever your sales.
Related reading: the Meta Conversions API, and server-side tracking.