SSAI vs. CSAI: A Technical Breakdown for Publishers – How Your VAST Tags Behave Differently

We’re in the middle of a gold rush in the world of video advertising. But unlike the 1800s, today’s boom is digital—and it’s happening on your TV. Connected TV (CTV) and Over-The-Top (OTT) platforms have become the hottest destinations for advertisers, offering premium video inventory, measurable audiences, and high completion rates. For publishers and content platforms, the growth potential is massive—but it comes with technical complexities that can make or break monetization.

One of the most important technical decisions for publishers today revolves around how ads are delivered: Server-Side Ad Insertion (SSAI) vs. Client-Side Ad Insertion (CSAI). This choice affects everything from ad tracking, error rates, and verification standards (like OMID or SIMID), to viewer experience and fill rates.

In this article, we’ll explore:

• What SSAI and CSAI are
• Why SSAI has become popular in CTV
• How VAST behavior differs between the two
• The impact on error tracking, OMID/SIMID support, and verification
• Visual diagrams of ad request flows
• Key considerations for publishers choosing between them

What is SSAI?

Server-Side Ad Insertion (SSAI)—sometimes called “ad stitching”—is a process where the ad is stitched into the video stream on the server side. The viewer receives a continuous stream with no distinction between content and ad. This eliminates the need for a player to stop, load, and play the ad separately.

Pros of SSAI:

• Seamless user experience (no buffering during ad transitions)
• Avoids ad blockers (ads are part of the stream)
• Better control on CTV devices with limited client capabilities

Cons of SSAI:

• Limited support for client-side tracking and verification
• Complex to implement correctly
• Inconsistent support for interactive formats like SIMID

What is CSAI?

Client-Side Ad Insertion (CSAI) is the traditional method where the video player pauses content and fetches the ad through a VAST tag from the client (browser or app).

Pros of CSAI:

• Full support for VAST 4.2+, OMID, SIMID, and interactive formats
• More granular error and event tracking
• Easier integration with verification vendors and measurement SDKs

Cons of CSAI:

• More susceptible to ad blockers
• Risk of buffering or playback failure during ad loads
• Device and player compatibility issues, especially on CTV

VAST Behavior Differences: SSAI vs. CSAI

The Video Ad Serving Template (VAST) standard is central to video monetization. But its behavior changes significantly depending on whether SSAI or CSAI is used.

In CSAI:

• The client (video player) parses the VAST tag.
• Tracking URLs (impression, start, quartiles, complete) are called directly from the device.
• Error codes are reported in real-time.
• Verification (OMID/SIMID) is executed by the SDK on the client.

In SSAI:

• The server parses the VAST tag and stitches the ad into the stream.
• Tracking events are often proxied or fired server-side (less accurate).
• Error reporting may be delayed or missing entirely if the stream fails.
• Verification (OMID/SIMID) is either unsupported or simulated by SSAI vendors.

Visual Breakdown: SSAI vs CSAI Ad Request Flow

SSAI Ad Flow Diagram:

Player
--> SSAI Server
--> Ad Server (VAST Request)
--> Content + Ad Stream (stitched)
--> Fires Tracking Events (server-side or via beacon injection)

CSAI Ad Flow Diagram:

Player
--> Ad Server (VAST Request)
--> Receives VAST
--> Loads & Plays Ad
--> Fires Tracking Events (client-side)
--> Resumes Content

These diagrams illustrate a critical difference: who controls the VAST—and when.

VAST Error Tracking: Why Some Errors Are More Common in SSAI

Because SSAI systems pre-fetch and pre-stitch ads, they often obscure VAST error codes that are normally caught at runtime in CSAI setups. Common implications:

In CSAI:

• Error 402 (timeout) or 403 (no ads) is directly reported by the player.
• Error 405 (VAST media not playable) is detected on-device.
• Ad load and playback issues are transparent.

In SSAI:

• The server handles errors silently.
• Often only generic error codes are passed back, if any.
• Reporting may rely on synthetic tracking (less accurate).

OMID & SIMID: Compatibility Breakdown

CSAI

• OMID Support: Full support (SDK-enabled)
• SIMID Support: Full (interactive overlays)
• Third-Party Viewability: Broad support
• Error Codes: Full VAST error tracking

SSAI

• OMID Support: Limited / Emulated
• SIMID Support: Not supported (non-interactive)
• Third-Party Viewability: Vendor-specific / limited
• Error Codes: Limited / abstracted

OMID (Open Measurement Interface Definition) enables third-party viewability and verification in video environments. SIMID (Secure Interactive Media Interface Definition) supports interactive creatives—think skip buttons or overlays. These both require client-side JS SDK execution, which SSAI can't natively provide.

Some SSAI vendors inject tracking beacons or simulate OMID-like measurement, but these often lack IAB certification or full compatibility.

SSAI Vendors & Hybrid Approaches

Popular SSAI vendors include:

• Google DAI
• Amazon IVS
• Brightcove SSAI
• Yospace
• JW Player SSAI

Some publishers now adopt hybrid SSAI + CSAI models:

• SSAI for core ad delivery
• CSAI fallback for verification or interactivity
• Beacon injection to maintain partial client-side tracking

So Which One Should You Choose?

Use SSAI if:

• You prioritize seamless playback and ad blocker resistance
• Your platform is CTV-heavy and apps are limited in capabilities
• You can partner with vendors who handle tracking and verification workarounds

Use CSAI if:

• You rely on third-party verification (OMID, MOAT, IAS, etc.)
• You want to support interactive creatives or SIMID
• Your player environment (web, mobile) supports SDKs and error reporting

Final Thoughts: It's Not One Size Fits All

As CTV and OTT continue to grow, publishers must adapt technically—not just creatively. SSAI is not a silver bullet, and CSAI is not outdated. They each bring unique strengths and trade-offs that can impact ad revenue, measurement accuracy, and user experience.

If you’re a publisher in the middle of this gold rush, your most valuable asset isn’t just your content—it’s your technical infrastructure. Understand how your VAST tags behave in each setup, and choose wisely.