Onboarding Zod transform silently broken — web signups assigned wrong checkout currency

2026-03-16 | Rajiv | Resolved

TL;DR

During a refactor of the onboarding flow, an unobtrusive inconspicuous call to the handleSubmit function of the Vee-Validate form validation library was removed. This function was vital for triggering the Zod transformations on onboarding data, including mapping countries to currencies. This caused all web-signups to get CHF currencies, and get wrong prices presented in the store. This ultimately led to a lower conversion rate for 2 weeks.

Main Mistakes & Lessons

Next steps:

• Introduce postmortems as a standard practice
• Take integration of AI code reviews more seriously

Machine-generated structured context below.

Summary

PR #1539 ("fix: onboarding data integrity", merged March 3) removed handleSubmit from the portal onboarding form to implement per-slide validation. This inadvertently disabled the Zod .transform() that derives currency (and other fields) from user input. Since the profile table defaults currency to CHF, ~99% of web signups in DE/NL (and likely all non-Swiss markets) have been assigned the wrong checkout currency for 13 days.

Impact

• 4,248 confirmed DE/NL web signups received currency = CHF instead of EUR since March 3.
• These users were charged Swiss prices at Stripe checkout (e.g. 16 CHF/month instead of 12 EUR/month).
• Pricing experiment contaminated: 32 out of 186 conversions (17%) in the DE/NL experiment period were charged in CHF. Their PostHog revenue_in_eur property records the EUR list price, not what they actually paid.
• All Zod transforms broken, not just currency:
  • subject_of_studies normalization (economics → business) stopped running.
  • newsletter_opt_in override for Apple/Google OAuth users stopped running.
• Scope extends beyond DE/NL — any non-Swiss, non-USD web signup in this period got CHF.

Root Causes

The onboarding form used vee-validate's handleSubmit to submit the form. handleSubmit internally runs Zod validation and the .transform() chain, then passes the transformed output to the callback. The PR replaced handleSubmit with a manual function that reads the reactive values object directly. In vee-validate, values contains raw form input — not Zod-transformed output. The transform that computes currency = countryToCurrency(country) became dead code.

Because Supabase's .update() ignores undefined fields, the currency: undefined in the payload simply didn't touch the column, leaving the DB default (CHF) intact.

Trigger

PR #1539 merged March 3, 2026 at 10:58 CET.

Resolution

1. Code fix: PR #1590 — calls schema.parse(values) before updateProfile so the Zod transform runs. Merged 2026-03-16 09:28 UTC.
2. Currency mapping inverted: PR #1594 — countryToCurrency() now lists CHF countries (Switzerland only) and defaults to EUR, instead of listing EUR countries and defaulting to CHF. Merged 2026-03-16 14:37 UTC.

Detection

Discovered during a pricing experiment data integrity audit. Cross-referencing PostHog subscription:create events with Stripe transactions in Metabase revealed 57 DE/NL conversions charged in CHF instead of EUR. Tracing back to the profile currency field exposed the regression.

Action Items

Lessons Learned

What went well

• The pricing experiment audit caught this before experiment results were presented.
• The Zod transform logic itself was correct — it just wasn't being called.

What went wrong

• handleSubmit was removed without understanding it was the only thing triggering the Zod transform.
• CodeRabbit's auto-generated summary ("Currency now auto-selects based on chosen country") was misleading — it described the existing transform as if it were a new feature, masking that it was actually broken.
• This wasn't realistically preventable by a test — the failure was silent (no error, no crash). A test would only catch it if someone specifically asserted "currency must not be undefined in the update payload", which is an unlikely test to write. Stricter AI code review could have flagged that removing handleSubmit would break the transform chain.
• No monitoring on the currency distribution of new signups. A simple alert on "% of DE/NL signups with CHF" would have caught this on day one.

Where we got lucky

• The bug was discovered 13 days in, not months later.
• The affected users can be precisely identified via _created >= '2026-03-03' AND signup_via = 'web'.
• The data migration to correct currency is straightforward since country was saved correctly.

Timeline

Supporting information

• PR: https://github.com/studyflash-ai/studyflash/pull/1539
• Commit: 6a469a89a
• Affected file: apps/portal/app/pages/onboarding/index.vue
• DB default: profiles.currency column defaults to 'CHF'::text
• The countryToCurrency() function in packages/common/src/mappers.ts is correct — it was just never invoked.

Verification query

SELECT country, currency, count(*)
FROM profiles
WHERE country IN ('Germany', 'Netherlands')
  AND _created >= '2026-03-03'
  AND signup_via = 'web'
GROUP BY country, currency;
-- Before fix: ~99% CHF
-- After fix: should show 0 CHF rows