SSAI vs CSAI: choosing ad insertion for your CTV app
The short answer: on-demand libraries monetize well with client-side insertion (CSAI), live and FAST streams practically require server-side insertion (SSAI), and many publishers run both in one app. The longer answer — how each works, what it costs, and where each one breaks — is below.
CSAI: the player does the work
In client-side ad insertion, the video player on the device pauses content, calls the ad server, receives a VAST response, plays the ad and returns to content. Every measurement beacon fires from the device itself, which makes measurement precise and interactive formats — QR actions, remote clicks — natural.
The price is complexity. Your player must parse VAST, resolve wrapper chains, handle timeouts and errors, and manage smooth transitions in and out of ad breaks — on every device generation you support. An ad that stutters on a three-year-old TV is a viewer lost and an impression uncounted.
SSAI: the stream does the work
In server-side ad insertion, the work moves to the cloud. An SSAI service takes your content stream, requests personalized ads from the ad server per household, and stitches them into the stream itself. The device plays one continuous video — no client ad logic, no ad blocking, identical behavior across platforms.
This is why SSAI is the standard for FAST channels and live content: there is no natural pause in a live stream where a client could go fetch an ad. The trade-off is measurement. The player no longer knows where ads are, so beacons must fire from the server side or from a lightweight client SDK — and buyers actively test SSAI measurement for spoofing.
The measurement divide
CSAI measurement is straightforward: impression, start, quartile and complete events fire from the device that played the ad. Verification vendors trust it because the signals originate where the viewing happened.
SSAI measurement requires deliberate design. Server-side beacons must carry enough device and session identity to be verifiable, and reputable setups include client-side confirmation beacons. Get this wrong in either direction — too little identity looks like fraud, too much creates privacy exposure — and buyers will discount or reject your inventory. This is the single most technical decision in a CTV stack; treat it as one.
Cost and engineering effort
CSAI shifts cost into engineering: player work per platform, per device generation, plus ongoing maintenance as platforms update. SSAI shifts cost into infrastructure: per-hour stitching fees and stream processing, with simpler clients. Neither is universally cheaper — CSAI hurts more with many platforms and few viewers, SSAI hurts more with huge viewing hours.
Publisher-side integration effort for either route is days, not months. The long pole is always store certification and demand approval, not the ad code.
Match the method to the content
On-demand libraries — AVOD apps, catch-up catalogs — fit CSAI: breaks are known in advance, sessions are intentional, and precise client measurement is worth the player work. Linear and FAST streams fit SSAI: there is no client-side moment to insert an ad into a live feed, and one stitched stream serves every device identically.
The hybrid is common and sensible: one app, CSAI for the VOD section, SSAI for the linear channels. Your ad server sees both; the viewer notices nothing.
Where each approach breaks
CSAI fails on the device: deep VAST wrapper chains that time out before reaching a media file, transition glitches between ad and content, beacons lost on older hardware. SSAI fails in the cloud: misaligned SCTE-35 markers, decision chains too slow to fill a live break, measurement that cannot survive buyer verification.
Know your failure mode before launch. Test CSAI on the oldest device you support with a network proxy attached. Test SSAI against a verification vendor's methodology, not just your own dashboard.
A decision framework
Four questions settle it. Is your content live or on demand? Live points to SSAI. How many platforms do you support? Many platforms raise CSAI's player cost. How important are interactive formats? They favor client-side. And what does your demand require? Some buyers specify measurement expectations that effectively choose for you.
When in doubt, start where the industry landed: CSAI for VOD, SSAI for linear, one reporting layer across both.
How this works with Mobadly
Integration routes are confirmed during onboarding and depend on your platform and stream architecture — SDK, VAST tag, SSAI and server-to-server are the options we evaluate. The inventory review happens before commercial terms, which are agreed individually and confirmed in writing.
Register as a publisher, choose your inventory type, and describe your current stack — the review will tell you quickly which route fits and what your measurement needs to look like.