A Conversions API is a server-to-server connection through which an advertiser reports what happened after an ad was seen or clicked - a purchase, a lead, a sign-up, an in-store sale - directly from its own systems to an advertising platform. The event never passes through the visitor's browser. The name began as a Meta product label and, usually shortened to CAPI, now covers equivalent pipes at LinkedIn, Microsoft, OpenAI and Netflix, while other platforms use different names: the same channel is marketed as a conversions API by Meta and LinkedIn and an events API by TikTok. It exists because the browser tag that traditionally carried outcome data is increasingly blocked or stripped of identifiers, while automated bidding depends on that data to decide where money goes.

How a server event travels

The sequence starts with an ordinary action. A shopper completes checkout, and the advertiser's web server, commerce back end or customer relationship management (CRM) system records the order. Instead of waiting for JavaScript in the browser to report it, the advertiser's server, a server-side tag manager container or a partner connector builds a JSON payload and sends it by HTTP POST to the platform's endpoint, authenticated with an access token. The platform returns a status code, tries to match the event to a user, and feeds matched outcomes into reporting, attribution and optimisation.

Microsoft's version shows the plumbing plainly. Its documentation, last updated on August 4, 2026, specifies requests to capi.uet.microsoft.com on the path /v1/{tagId}/events, a Bearer token tied to a Universal Event Tracking (UET) tag, HTTP 200 on success and 400 or 401 on failure, and batches of up to 1,000 events. By default one invalid event causes the whole batch to be refused.

Meta links server events to a dataset ID and, according to its developer documentation, processes them like events from the Meta Pixel, the mobile software development kits (SDKs) or offline uploads. Connections take three broad forms: a direct build by the advertiser's engineers, a partner integration through a commerce platform or customer data platform, and, since April 2026, a platform-managed option. PPC Land's comparison of Meta's setup routes put a direct build at two to four weeks of developer time.

Advertisers, agencies and analytics engineers do the sending, often through vendors such as Stape, Tealium or Segment.

Fields, matching and deduplication

Each implementation revolves around a compact event object. In Meta's specification, event_name and event_time are required, with event_time expressed as a Unix timestamp in seconds. That timestamp can be up to seven days old; according to Meta, if any event in a request is older, the entire request fails and nothing is processed. A user_data object carries customer information, custom_data holds business values such as order value and currency, and event_source_url is mandatory for website events. An action_source field records where the event happened, and an opt_out flag limits an event to attribution only.

Matching depends on identifiers. Email addresses and phone numbers are normalised and hashed with SHA-256 before sending, with phone numbers first converted to the E.164 international format. Click identifiers fill the gaps. Microsoft advises storing the latest msclkid value per user for a suggested 90 days, since capturing it on the server path becomes the advertiser's responsibility rather than the tag's.

Deduplication is where many set-ups go wrong. Platforms recommend running the browser tag and the server connection together, so the same purchase can arrive twice. Meta resolves this through event_name and event_id: when browser and server events match within 48 hours, only the first counts, and if both land within five minutes of each other, the browser copy is preferred. OpenAI deduplicates on Pixel ID, event name and event_id, while Microsoft asks for a shared, stable eventId on both paths.

Consider an order placed at 14:02 that fires a browser Purchase event with the ID ord-58213, followed a minute later by a server event carrying the same ID. One purchase is recorded. Had the server sent 58213 without the prefix, two would be logged, and bidding would learn from inflated revenue.

Origin and evolution

Facebook's server pathway was first known as the Server-Side API. By autumn 2020 it was operating as the Conversions API: the first release of a widely used server-side Google Tag Manager template for sending Facebook Conversions API events, published by Simo Ahava, is dated October 2, 2020. The timing reflected pressure from Apple. According to Facebook's developer blog, Aggregated Event Measurement began rolling out alongside Apple's SKAdNetwork on April 27, 2021, following the iOS 14.5 tracking prompt.

Scope widened in stages. Meta's Graph API changelog states that the Conversions API gained full support for offline events in February 2023. The separate Offline Conversions API lost offline event support from Graph API v17.0 and was retired when v16.0 expired in May 2025, after an earlier third-quarter 2024 target slipped. Business messaging events followed in June 2024.

TikTok calls its pipe the Events API and in November 2023 consolidated web, app and offline signals into one endpoint. LinkedIn's Conversions API arrived in 2023, with submitted data retained for at most 180 days. Google never adopted the label. Its server route runs through the Google Ads API and, since December 9, 2025, the Data Manager API, which accepts up to 2,000 conversion events per request over REST or gRPC.

Fragmentation eventually produced a standard. The IAB Technology Laboratory (IAB Tech Lab) closed public comment on its Event and Conversion API specification on February 20, 2026, and ECAPI 1.0 was finalised with contributors including Meta, Google, Walmart, TikTok, Roku, NBCUniversal and Paramount.

Why it matters for marketers

Automated bidding allocates budget against the conversions it can observe. When events go missing, a channel can look weaker than it is. The Conversions API is therefore less a technical upgrade than a precondition for fair comparison between channels.

Most of the evidence is self-reported. A 26-page IAB guide published on October 30, 2025 said two-thirds of advertisers saw improved return on ad spend (ROAS) after implementation, based on a survey run between July 22 and August 19, 2025. The same research found 67% of implementations operating alongside pixels. Meta states that advertisers with a web Conversions API set-up averaged a 17.8% lower cost per result than those without - a platform figure, not an independent audit.

The technique has also left social media. Netflix announced its own Conversion API on March 4, 2026, naming Tinuiti as an early testing partner, and DIRECTV became the first multichannel video programming distributor to connect to LiveRamp's CAPI Hub in April 2026.

Limitations and disputes

Server delivery changes transport, not law. German courts have treated Meta's Business Tools, a category covering both the Pixel and the Conversions API, as collecting personal data without a valid legal basis. The Dresden Higher Regional Court ordered Meta on February 3, 2026, in case 4 U 292/25, to pay four users 1,500 euros each, and another Dresden senate confirmed a similar award on April 13, 2026. Microsoft's API treats consent as granted when the adStorageConsent field is omitted, leaving implementers to pass any denial. The ECAPI specification likewise standardises transmission rather than consent.

Blocking has not disappeared. Server routing was promoted partly as a way around ad blockers, yet filter lists began naming server-side tag manager subdomains in May 2026, and one AdGuard list carried more than 3,400 rules against first-party tracking hostnames. A consultant at a server-side tagging vendor, cited in PPC Land's server-side explainer, put the resulting data gap at 5% to 40% depending on audience - an estimate from a party that sells the infrastructure. Simo Ahava, co-founder at Simmer, wrote in July 2024 that using first-party set-ups to circumvent ad blockers is "ethically questionable", as recorded in PPC Land's Google tag gateway explainer.

Identity does not move entirely to the server. Microsoft still requires a client-side ID Sync pixel for audience building, and the same browser defences can cut it. Integration cost is another obstacle: according to IAB Tech Lab research cited by Innovid, 72% of publishers named technical integration as a major barrier to adoption. A successful response is not proof of success either, since Microsoft silently drops optional fields it cannot accept while still returning HTTP 200.

Not the same as

ECAPI. An IAB Tech Lab specification defining a shared event format. A Conversions API is a platform's own endpoint, which may or may not accept ECAPI payloads.

Pixel or site tag. Client-side JavaScript reporting from the browser. Platforms recommend running it alongside a Conversions API rather than replacing it.

Server-side tagging. An architecture, such as server-side Google Tag Manager, that moves tag execution onto an advertiser-controlled server. It is one possible sender of Conversions API calls, not the API itself.

Enhanced conversions. A Google feature that supplements tag-based conversions with hashed first-party data. It can use an API route, but PPC Land's enhanced conversions explainer separates it from the cross-platform server pattern.

Recent developments

Meta announced a one-click set-up on April 15, 2026, and the web-only, platform-managed connection appeared in Events Manager later that month. OpenAI launched a pixel and Conversions API for ChatGPT ads on May 5, 2026, granting early access to conversion-optimised campaigns to accounts configured by June 1. LiveRamp connected its CAPI Hub to ChatGPT on June 10. Microsoft's documentation describes a pilot provisioned per account, with no general availability date stated as of September 2026. Safari 27, expected in autumn 2026, blocks connections to Bing and LinkedIn tracking endpoints by IP address, according to a review of WebKit beta source code.

Timeline

  • October 2, 2020: First release of Simo Ahava's server-side Google Tag Manager template for Facebook Conversions API events
  • April 27, 2021: Facebook begins rolling out Aggregated Event Measurement and SKAdNetwork support after iOS 14.5
  • February 2023: Meta's Conversions API gains full support for offline events
  • May 23, 2023: Graph API v17.0 ends offline event support in the Offline Conversions API
  • 2023: LinkedIn introduces its Conversions API
  • November 2023: TikTok consolidates web, app and offline signals into one Events API endpoint
  • June 2024: Meta extends the Conversions API to business messaging events
  • May 2025: Meta retires the Offline Conversions API as Graph API v16.0 expires
  • October 30, 2025: IAB publishes its 26-page guide on Conversion APIs for connected TV
  • December 9, 2025: Google launches the Data Manager API
  • February 3, 2026: Dresden Higher Regional Court rules against Meta over Business Tools tracking
  • February 20, 2026: Public comment on the IAB Tech Lab ECAPI specification closes
  • March 4, 2026: Netflix announces a proprietary Conversion API
  • April 13, 2026: A second Dresden senate confirms damages against Meta
  • April 15, 2026: Meta announces one-click Conversions API set-up
  • April 16, 2026: DIRECTV connects to LiveRamp's CAPI Hub
  • May 3, 2026: ECAPI 1.0 is finalised
  • May 5, 2026: OpenAI launches a pixel and Conversions API for ChatGPT ads
  • May 2026: Ad blocker filter lists begin naming server-side tag manager subdomains
  • June 10, 2026: LiveRamp's CAPI Hub connects to ChatGPT ads
  • July 6, 2026: A WebKit source review finds Safari 27 blocking Bing and LinkedIn tracking endpoints by IP address
  • August 4, 2026: Microsoft's Conversions API documentation is last updated, describing a per-account pilot

Summary

Who. Advertisers, agencies and analytics engineers send the events, often through vendors or server-side tag managers. Meta, LinkedIn, TikTok, Microsoft, OpenAI, Netflix and others receive them through their own endpoints, while the IAB Tech Lab maintains ECAPI as a shared format.

What. A server-to-server interface that transmits conversion and other marketing events, with hashed identifiers and click IDs for matching, from an advertiser's systems to an advertising platform, normally alongside a browser pixel with event IDs used for deduplication.

When. Meta's version was operating under the Conversions API name by October 2020, absorbed offline events in 2023, and received a one-click set-up in April 2026. ECAPI 1.0 was finalised in May 2026, and Microsoft's pilot documentation was last updated on August 4, 2026.

Where. Between advertiser infrastructure - web servers, commerce back ends, CRM systems, server-side containers - and platform endpoints such as capi.uet.microsoft.com, across web, app, offline, messaging and connected TV use cases.

Why. Browser tags lose events to ad blockers, cookie limits, stripped click identifiers and network-level blocking, and automated bidding can only optimise against outcomes it sees. Server delivery restores much of that signal, though it leaves consent obligations, identity gaps and integration costs unresolved.