stripe subscription proration edge cases upgrade downgrade

Upgrade now, downgrade later -- and mean it

Upgrades and downgrades need opposite proration behavior on purpose: charge the difference now on upgrade, never mid-cycle-refund on downgrade (D10).

This is one of the things SaaS Billing + Tax Core already handles. Charge per-seat subscriptions: upgrades, downgrades, failed cards, invoices and sales tax.

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Is this you?

Plan changes need opposite proration behavior depending on direction, and getting it backwards either short-changes the business (an upgrade that defers its price difference to next invoice, so the customer used the better plan for free this cycle) or short-changes the customer (a downgrade that mid-cycle refunds instead of letting them keep what they already paid for).

Why this one is easy to get wrong

Stripe's proration_behavior parameter takes the same handful of values for both directions, so nothing in the API forces a developer to think about upgrade and downgrade as needing different settings -- it's easy to pick one proration_behavior value and apply it uniformly, and both directions will "work" in the sense of not erroring.

What you get instead

D10 pins upgrades to proration_behavior: 'always_invoice' (charge the difference immediately) with payment_behavior: 'error_if_incomplete' (a failed charge must not silently apply the upgrade anyway), and pins downgrades to a two-phase subscription schedule with proration_behavior: 'none' -- the customer keeps what they already paid for, no mid-cycle credit notes or refunds.

Source: ARCHITECTURE.md D10 — checkable in the pack you receive

How you actually use this

You don’t install a library or wire up an SDK. Your own coding agent builds the code in your project, and you keep it — no runtime dependency on us.

Step 1
Download and unzip
You get a folder: the docs that tell an agent what to build, a starting skeleton, and the test suite that decides when it's done.
Step 2
Open it in Claude Code or Cursor
Point your coding agent at the folder. Nothing to install, no account with us, no API key.
Step 3
Paste one prompt
The pack contains the exact prompt. Paste it as your first message and leave it alone — it works through the build itself, choosing a cheaper or stronger model per task.
Step 4
Run ./verify.sh
One command. It prints a pass or fail for every check. Green means the build is done — the same script we ran to produce the receipt on this page.

Typical build: about 18 minutes of your agent working, mostly unattended. Then you integrate the working module into your app the way you would any code you’d written yourself.

Why you can believe this

3 of 3 runs passed

We ran this pack from an empty folder 3 times and published exactly what happened — every check, the model, the token cost, the wall time. Not a testimonial, and not our opinion: the same verify.sh you run yourself. Read the full receipt →

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