The June 2026 Shopify Scripts Sunset: Why Custom Agency Apps Are a Trap
Every Shopify Plus merchant had to migrate off Ruby scripts by June 30, 2026. Agencies pitched $40K custom Shopify Function builds. Here's why porting your scripts to a bespoke Rust app is almost always the wrong move — and what to do instead.
If you ran a Shopify Plus store with Ruby scripts enforcing B2B pricing, hiding shipping rates, or guarding margin at checkout, Shopify turned them off for good on June 30, 2026.
That was the date Shopify pulled the plug on legacy checkout scripts. Every Plus merchant had to move to Shopify Functions, a completely different runtime that compiles your logic into WebAssembly and runs it inside Shopify's infrastructure. The performance promise is real — fast execution, enterprise-grade scale. The migration reality is where many brands made, or are still making, a very expensive mistake.
The mistake most merchants are making
Agencies noticed the deadline. If you talked to a Shopify SI in the run-up to it, you probably saw the pitch: we will rebuild your scripts as a custom Shopify Function. Forty thousand dollars. Six to eight weeks. Your checkout logic, ported one-for-one to Rust and compiled into WASM.
On the surface that sounds like exactly what you need. In practice you inherit a maintenance problem that is harder to fix than the one you are trying to leave behind.
Why WASM is nothing like Ruby
Ruby scripts were fragile but forgiving. A junior dev with admin access could write fifty lines of code, paste it into Shopify, and ship a new promo rule in an afternoon. Not ideal, but fast.
Shopify Functions are a different engineering discipline entirely. You write in Rust (or JavaScript, which also compiles to WebAssembly). You work inside strict memory and instruction limits. And Shopify enforces a hard execution budget per invocation, around 11 million instructions — go over it and the function fails. There is no graceful degradation, no "it ran a bit slow today." The logic just does not run.
That alone would be manageable if the code was stable and rarely changed. But checkout logic is never stable. It changes every time marketing runs a new promotion, every time finance updates a margin floor, every time a new wholesale tier gets added. Every one of those changes now means a Rust code review, a recompile, a redeploy, and a prayer that you did not introduce a memory leak.
The hidden costs of the custom agency app
Here is what actually happens in month three after the migration.
The CMO wants to change the margin floor for Black Friday. She used to be able to have someone edit a Ruby script in an hour. Now she files a Jira ticket with the agency. The ticket sits in their queue for a week. When it ships, the test coverage is thin and a bug slips through. By the time it is fixed, the promo is half over.
The ERP data is stale. Your custom app has hardcoded COGS assumptions because the agency could not figure out how to pull live cost data from NetSuite inside the function's execution limits. So when collagen prices go up 18% next quarter and nobody updates the function, you ship three weeks of orders against the old numbers. The margin leak only shows up in the month-end P&L.
The agency's backend has a memory leak. Their Rust code works fine for 99% of carts but crashes on a specific combination of line items that only shows up at scale. You hit it on Cyber Monday. Your checkout is down for forty minutes. Who is financially liable? The answer, buried in the MSA, is: you are.
The common thread is that the agency built a custom app, which means you now own a custom app. You own its bug fixes, its upgrades, its integration debt, and its single point of failure.
What to do instead
The smart move is not to port scripts one-for-one. It is to split the job and stop treating checkout logic as bespoke code at all.
Promotion logic belongs in managed tools: Shopify's native discount settings, or a no-code Functions builder that someone else maintains. Margin enforcement belongs somewhere it can actually see your costs. Managed margin layers for Shopify Plus — Agentis is one, there are others — do that work after the order is placed and before it ships, instead of inside checkout. Your finance and marketing teams configure the rules; nobody recompiles a binary to change a discount cap.
Three things make this model structurally better than a custom agency build:
Zero-code rule updates. A margin floor, a discount cap, a MAP threshold — configured, not coded. The person who owns the business logic (finance, marketing, ops) is the person who updates it. There is no ticket queue, no agency retainer, no waiting.
Live ERP cost. Because the margin check runs on the placed order rather than inside checkout, it is not squeezed into a function's execution budget. A post-order layer like Agentis reads your NetSuite cost data, adds freight, fees, and FX, and checks true net margin within 60 seconds of each order. No hardcoded COGS assumptions.
Checkout is never at risk. Nothing runs in the checkout path, so a bad rule cannot take checkout down. The worst case is an order that gets flagged or held before fulfillment for a person to review. That is not something an agency-built checkout app can promise.
The real question to ask your agency
If you are still mid-migration, you do not need to cancel the project. You need to ask a different question: is the thing they are building for you actually better than a managed alternative?
Specifically, ask:
- Can finance change a margin floor without a developer?
- Does it read live NetSuite COGS, or are costs hardcoded?
- What is the circuit breaker behavior on a rule failure?
- Who is liable if the function crashes checkout?
- What does the ongoing maintenance cost look like over 24 months?
If the answers are "no," "hardcoded," "checkout goes down," "you are," and "another fifteen thousand a year in retainers" — you are buying a Rust version of the exact problem you had with Ruby scripts. You are just paying more for it and making it harder to change.
The migration is a governance decision, not a construction project
June 30, 2026 was never just an IT migration. It was a financial governance decision with a date attached. The merchants who treated it as an IT project spent forty thousand dollars to rebuild brittle logic in a harder language. The better outcome is a setup where promotion rules are configured, not coded, and every order is checked against your margin policy before it ships.
Your checkout is already running a financial policy engine. The only question is whether that policy is managed by people who own the business outcome, or hidden inside a Rust binary that nobody but the agency can read.
Pick carefully. A custom binary is hard to unwind.
Want to see what order-level margin enforcement looks like on Shopify Plus? Start a free 7-day audit — no commitment, no rewrite. Or read more on why Shopify Scripts ended and what the Shopify scripts migration actually involves.