diff --git a/.github/workflows/test-pg-real.yml b/.github/workflows/test-pg-real.yml
index 7e9b78aa..4a2fb211 100644
--- a/.github/workflows/test-pg-real.yml
+++ b/.github/workflows/test-pg-real.yml
@@ -33,6 +33,108 @@ jobs:
           PG_GATE_BASE: origin/${{ github.base_ref }}
         run: node scripts/check-pg-test-coverage.mjs
 
+  # Proves that an EXISTING database survives the PR's migrations, which the
+  # pg-real job below cannot: it applies every migration to an empty database,
+  # so a NOT NULL, a CHECK, a unique index or a backfill passes trivially
+  # against zero rows and can still fail (or silently corrupt) on production.
+  #
+  # Shape: schema at the merge-base -> seed real rows -> apply ONLY the new
+  # migrations -> assert the rows are intact. A PR that adds no migration skips
+  # straight past the apply step and costs one cheap no-op run.
+  pg-upgrade:
+    runs-on: ubuntu-latest
+
+    services:
+      postgres:
+        image: supabase/postgres:15.8.1.060
+        env:
+          POSTGRES_PASSWORD: postgres
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U postgres"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 20
+
+    env:
+      DATABASE_URL: postgresql://postgres:postgres@localhost:5432/postgres
+      PGPASSWORD: postgres
+
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          # Full history so the merge-base with the PR base branch exists.
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Install psql client
+        run: sudo apt-get update && sudo apt-get install -y --no-install-recommends postgresql-client
+
+      - name: Identify the migrations this PR adds
+        id: newmig
+        env:
+          BASE_REF: origin/${{ github.base_ref }}
+        run: |
+          set -euo pipefail
+          MERGE_BASE=$(git merge-base "$BASE_REF" HEAD)
+          echo "merge_base=$MERGE_BASE" >> "$GITHUB_OUTPUT"
+          echo "Merge base: $MERGE_BASE"
+
+          # Migrations present at HEAD but not at the merge base. Uses the git
+          # tree, not the filesystem, so a rebase or a merge commit cannot make
+          # an already-shipped migration look new.
+          git ls-tree -r --name-only HEAD -- supabase/migrations \
+            | grep '\.sql$' | sort > /tmp/head-migrations.txt
+          git ls-tree -r --name-only "$MERGE_BASE" -- supabase/migrations \
+            | grep '\.sql$' | sort > /tmp/base-migrations.txt
+          comm -23 /tmp/head-migrations.txt /tmp/base-migrations.txt > /tmp/new-migrations.txt
+
+          COUNT=$(wc -l < /tmp/new-migrations.txt | tr -d ' ')
+          echo "count=$COUNT" >> "$GITHUB_OUTPUT"
+          echo "New migrations ($COUNT):"
+          cat /tmp/new-migrations.txt
+
+      - name: Bootstrap storage schema
+        if: steps.newmig.outputs.count != '0'
+        run: psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -q -f tests/pg/bootstrap.sql
+
+      - name: Apply the schema as it stands at the merge base
+        if: steps.newmig.outputs.count != '0'
+        env:
+          MERGE_BASE: ${{ steps.newmig.outputs.merge_base }}
+        run: |
+          set -euo pipefail
+          # Read each migration out of the merge-base tree rather than the
+          # working tree: a PR that EDITS a shipped migration (forbidden, but
+          # this job must not be the thing that hides it) still gets the
+          # original applied here, so the edit shows up as a failure below.
+          while read -r f; do
+            echo "Applying (base) $f"
+            git show "$MERGE_BASE:$f" | psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -q
+          done < /tmp/base-migrations.txt
+
+      - name: Seed a real company with posted verifikat
+        if: steps.newmig.outputs.count != '0'
+        run: psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -q -f tests/pg/upgrade/seed.sql
+
+      - name: Apply ONLY the new migrations, against existing data
+        if: steps.newmig.outputs.count != '0'
+        run: |
+          set -euo pipefail
+          while read -r f; do
+            echo "Applying (new) $f"
+            psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -q -f "$f"
+          done < /tmp/new-migrations.txt
+
+      - name: Assert the existing data survived
+        if: steps.newmig.outputs.count != '0'
+        run: psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -f tests/pg/upgrade/assert.sql
+
+      - name: No migrations in this PR
+        if: steps.newmig.outputs.count == '0'
+        run: echo "This PR adds no migration; nothing to upgrade-test."
+
   pg-real:
     runs-on: ubuntu-latest
 
diff --git a/DECISIONS.md b/DECISIONS.md
index ee20fe31..afc6b919 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -743,3 +743,7 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-02] Out-of-order SIE IB resync requires exact date adjacency: the nearest later fiscal period can sit beyond a missing middle year, and replacing its authoritative IB with a non-adjacent UB would make that later period temporarily wrong until the gap was imported.
 [2026-08-03] Issue #1360 keeps EUR annual reports fail-closed at general eligibility rather than only digital filing: the ledger and annual-report model are SEK-denominated, so allowing a EUR profile to lock a paper version would mislabel SEK amounts; full EUR support requires a company accounting-currency model across the ledger, report builders, and iXBRL.
 [2026-08-03] Issue #1267 localizes the latest-posted-voucher label in the web views while PDF and spreadsheet exports remain Swedish: translating one export label would create mixed-language files, so full export localization stays a separate surface-wide change.
+
+[2026-08-03] Shared format contracts centralised in lib/invariants/ (org number, BAS account number, ISO date, fiscal year), each with its rationale recorded next to the rule. Trigger: four Skatteverket/Bolagsverket-bound export paths (KU10, AGI, SRU redovisare, iXBRL preflight) each had their own idea of a valid organisationsnummer, so a company stored with a space or in 12-digit form could file AGI all year and fail at the arsredovisning deadline. normalizeOrgNumber moved from lib/company-lookup/ and isSaneDateString from lib/utils.ts; both old paths re-export. The iXBRL check-digit verdict is warn, not error: we do not block a statutory filing on a Luhn assumption unverified against a primary source. KU10 12-digit passthrough pinned by test, not changed (open domain question). ROT/RUT brf_org_number left alone: different documented contract. Ratchet guard 8 holds the remaining 114 inline copies.
+
+[2026-08-03] CI gained a pg-upgrade job: apply the merge-base schema, seed real rows, apply ONLY the PR migrations, assert the data survived. Rationale: pg-real applies all 548 migrations to an EMPTY database, so a NOT NULL / CHECK / unique index / backfill passes against zero rows and can still break prod. Proven locally against supabase/postgres:15.8.1.060 with three bad migrations: a CHECK violating an ore-level row and a NOT NULL on a populated column both exit 0 on empty and exit 3 on seeded. Base migrations are read from the merge-base git tree, not the working tree, so a PR that edits a shipped migration still surfaces here.
diff --git a/lib/api/schemas.ts b/lib/api/schemas.ts
index cbf8d7d8..8ee3c8c4 100644
--- a/lib/api/schemas.ts
+++ b/lib/api/schemas.ts
@@ -1,7 +1,13 @@
 import { z } from 'zod'
 import { normaliseSwish, isValidSwish } from '@/lib/payments/swish'
 import { normalizeVatNumber } from '@/lib/vat/vat-number'
-import { isSaneDateString } from '@/lib/utils'
+import {
+  accountNumberSchema,
+  isoDateSchema,
+  saneIsoDateSchema,
+  fiscalYearSchema,
+} from '@/lib/invariants/zod'
+import { ISO_DATE_RE, ISO_DATE_MESSAGE_SV } from '@/lib/invariants/iso-date'
 import { countCalendarMonths } from '@/lib/bookkeeping/accruals/compute'
 import { DimensionsBagSchema } from '@/lib/bookkeeping/dimension-resolver'
 import { validateEmployeeBankAccount } from '@/lib/salary/payment/bank-account'
@@ -17,8 +23,13 @@ import type { AuditAction } from '@/types'
 /** UUID v4 string */
 const uuid = z.string().uuid()
 
-/** ISO date string (YYYY-MM-DD) */
-const isoDate = z.string().regex(/^\d{4}-\d{2}-\d{2}$/, 'Expected YYYY-MM-DD date format')
+/**
+ * ISO date string (YYYY-MM-DD).
+ *
+ * Shape only. From `lib/invariants/iso-date.ts` so that every schema below, the
+ * v1 routes, and the MCP surface reject a malformed date the same way.
+ */
+const isoDate = isoDateSchema
 
 /**
  * ISO date that must also be a real, in-range calendar date: not just the
@@ -26,12 +37,10 @@ const isoDate = z.string().regex(/^\d{4}-\d{2}-\d{2}$/, 'Expected YYYY-MM-DD dat
  * transaction form) so a 6-digit year or impossible date can't slip through
  * for user-entered dates. Use this over `isoDate` for free-text date input.
  */
-const saneIsoDate = z
-  .string()
-  .refine(isSaneDateString, 'Invalid or out-of-range date (expected YYYY-MM-DD, year 1900-2100)')
+const saneIsoDate = saneIsoDateSchema
 
 /** BAS account number: always a string of 4 digits */
-const accountNumber = z.string().regex(/^\d{4}$/, 'Account number must be exactly 4 digits')
+const accountNumber = accountNumberSchema
 
 /** Non-negative monetary amount (>= 0) */
 const nonNegativeAmount = z.number().nonnegative()
@@ -1751,7 +1760,7 @@ export const UpdateSettingsSchema = z.object({
   pays_salaries: z.boolean().optional(),
   sector_slug: z.string().nullable().optional(),
   // Bookkeeping lock
-  bookkeeping_locked_through: z.string().regex(/^\d{4}-\d{2}-\d{2}$/, 'Ogiltigt datumformat (YYYY-MM-DD)').nullable().optional(),
+  bookkeeping_locked_through: z.string().regex(ISO_DATE_RE, ISO_DATE_MESSAGE_SV).nullable().optional(),
   auto_lock_period_days: z.number().int().positive().nullable().optional(),
   // Voucher series
   default_voucher_series: z.string().regex(/^[A-Z]$/, 'Verifikationsserie måste vara en bokstav A-Z').optional(),
@@ -1989,10 +1998,7 @@ export const BankLinkSchema = z.object({
   // Settlement account being reconciled. The voucher must have a line on this
   // account and the transaction must belong to it. Defaults to '1930' in the
   // route for back-compat.
-  account_number: z
-    .string()
-    .regex(/^[0-9]{4}$/, 'Kontonummer måste vara 4 siffror')
-    .optional(),
+  account_number: accountNumber.optional(),
 })
 
 export const BankUnlinkSchema = z.object({
@@ -2014,10 +2020,7 @@ export const RunReconciliationSchema = z.object({
   date_to: isoDate.optional(),
   // BAS settlement account to reconcile against (e.g. '1930', '1932'). Defaults
   // to '1930' server-side so existing clients stay correct.
-  account_number: z
-    .string()
-    .regex(/^[0-9]{4}$/, 'Kontonummer måste vara 4 siffror')
-    .optional(),
+  account_number: accountNumber.optional(),
   dry_run: z.boolean().optional(),
   // Pairs the user ticked in the dry-run preview. When present on an apply
   // (dry_run false), only these pairs are committed: intersected server-side
@@ -2745,7 +2748,7 @@ const openingBalancesShape = {
   ytd_net: z.number().min(0).default(0),
   vacation_paid_days_remaining: z.number().min(0).max(40).default(0),
   vacation_saved_days_by_year: z
-    .record(z.string().regex(/^\d{4}$/, 'Nyckel måste vara ett fyrsiffrigt år'), z.number().min(0).max(40))
+    .record(fiscalYearSchema, z.number().min(0).max(40))
     .default({}),
   opening_semester_liability: z.number().min(0).default(0),
   opening_semester_liability_avgifter: z.number().min(0).default(0),
diff --git a/lib/bokslut/ixbrl/__tests__/fixtures.ts b/lib/bokslut/ixbrl/__tests__/fixtures.ts
index 1ff09bea..7e9606de 100644
--- a/lib/bokslut/ixbrl/__tests__/fixtures.ts
+++ b/lib/bokslut/ixbrl/__tests__/fixtures.ts
@@ -135,7 +135,7 @@ export const PREVIOUS: TrialBalancePair = {
 export function makeInput(): IxbrlArsredovisningInput {
   const mapping = mapTrialBalancesToK2(CURRENT, PREVIOUS)
   return {
-    company: { name: 'Testbolaget AB', orgNumber: '556999-9999', city: 'Sundsvall' },
+    company: { name: 'Testbolaget AB', orgNumber: '556999-9997', city: 'Sundsvall' },
     period: { start: '2025-01-01', end: '2025-12-31' },
     previousPeriod: { start: '2024-01-01', end: '2024-12-31' },
     isFirstFiscalYear: false,
diff --git a/lib/bokslut/ixbrl/validate/rules.ts b/lib/bokslut/ixbrl/validate/rules.ts
index b5cb96fb..c8980dec 100644
--- a/lib/bokslut/ixbrl/validate/rules.ts
+++ b/lib/bokslut/ixbrl/validate/rules.ts
@@ -14,6 +14,7 @@
  */
 
 import type { IxbrlArsredovisningInput } from '../types'
+import { isOrgNumberShaped, hasInvalidOrgNumberCheckDigit } from '@/lib/invariants/org-number'
 
 export interface PreflightIssue {
   /** Bolagsverket kontrollera code where one exists, else our own ACC-xxx. */
@@ -53,10 +54,35 @@ const RULES: Rule[] = [
     input.company.name.trim().length === 0
       ? issue('1020', 'error', 'Företagsnamnet saknas i årsredovisningen.')
       : null,
-  (input) =>
-    /^\d{6}-?\d{4}$/.test(input.company.orgNumber.trim())
-      ? null
-      : issue('1035', 'error', `Organisationsnumret "${input.company.orgNumber}" är inte giltigt (förväntat format NNNNNN-NNNN).`),
+  // Shared rule (lib/invariants/org-number.ts), so this validator agrees with
+  // the AGI, KU10 and SRU export paths about what a valid org number is. The
+  // previous local regex rejected the 12-digit form outright and skipped the
+  // check digit, so a company stored in 12-digit form could file AGI all year
+  // and then fail here with a message that did not say why.
+  (input) => {
+    const orgNumber = input.company.orgNumber.trim()
+    if (!isOrgNumberShaped(orgNumber)) {
+      return issue(
+        '1035',
+        'error',
+        `Organisationsnumret "${input.company.orgNumber}" är inte giltigt (förväntat format NNNNNN-NNNN).`,
+      )
+    }
+    // Warn, not error, deliberately. A wrong check digit is almost certainly a
+    // typo and Bolagsverket will reject it, so the user should see it. But
+    // whether every org number Bolagsverket accepts satisfies Luhn is a Swedish
+    // domain question we have not verified against a primary source, and an
+    // 'error' here blocks Skicka in. We do not block a statutory filing on an
+    // unverified assumption: surface it and let the user judge.
+    if (hasInvalidOrgNumberCheckDigit(orgNumber)) {
+      return issue(
+        '1035',
+        'warn',
+        `Organisationsnumret "${input.company.orgNumber}" har fel kontrollsiffra. Kontrollera sista siffran.`,
+      )
+    }
+    return null
+  },
   (input) =>
     input.forvaltningsberattelse.allmantOmVerksamheten.trim().length === 0
       ? issue('1051', 'error', 'Förvaltningsberättelsen saknas (Allmänt om verksamheten är tom).')
diff --git a/lib/company-lookup/normalize-org-number.ts b/lib/company-lookup/normalize-org-number.ts
index d6d7e464..48c88a17 100644
--- a/lib/company-lookup/normalize-org-number.ts
+++ b/lib/company-lookup/normalize-org-number.ts
@@ -1,33 +1,9 @@
-import { luhnValidate } from '@/lib/bankgiro/luhn'
-
 /**
- * Normalize an org number to Accounted's canonical 10-digit storage form.
+ * Org-number normalization for the company-lookup boundary.
  *
- * Accepts hyphen/space-formatted input in either of the two shapes Swedish
- * users commonly type:
- *  - 10 digits (5560125790 or 8001011231): stored as-is
- *  - 12 digits (198001011231): century prefix stripped
- *
- * Returns null for any other length, non-digit content, or invalid Luhn
- * check digit (the structural rule Bolagsverket and personnummer share).
- * Storing a structurally invalid org number would later be caught by
- * Skatteverket SRU and any receiving SIE4 system: refusing at the boundary
- * keeps Accounted's bookkeeping from accumulating under an unusable identifier.
- *
- * 10-digit storage matches the rest of the codebase: see
- * `lib/skatteverket/format.ts`, which converts 10→12 at export time by
- * prefixing with '16' (AB) or '19'/'20' (EF personnummer).
+ * The rule moved to `lib/invariants/org-number.ts`, where it sits with the
+ * other shared format contracts and with the Skatteverket-bound converters that
+ * have to agree with it. Re-exported here because this is where the lookup,
+ * TIC-refresh, VAT-number and årsredovisning callers import it from.
  */
-export function normalizeOrgNumber(raw: string | null | undefined): string | null {
-  if (!raw) return null
-  const cleaned = raw.replace(/[\s-]/g, '')
-  let canonical: string
-  if (/^\d{10}$/.test(cleaned)) {
-    canonical = cleaned
-  } else if (/^\d{12}$/.test(cleaned)) {
-    canonical = cleaned.substring(2)
-  } else {
-    return null
-  }
-  return luhnValidate(canonical) ? canonical : null
-}
+export { normalizeOrgNumber } from '@/lib/invariants/org-number'
diff --git a/lib/invariants/README.md b/lib/invariants/README.md
new file mode 100644
index 00000000..d3ce4646
--- /dev/null
+++ b/lib/invariants/README.md
@@ -0,0 +1,51 @@
+# `lib/invariants`
+
+Shared **product contracts**: the formats and bounds that more than one consumer
+must agree on, with the reason for each rule recorded next to it.
+
+## What belongs here
+
+A rule belongs here only when consumers **outside the owning module** need the
+identical invariant to validate input or to generate compatible output.
+
+Our consumers are the web app, the public `/api/v1` surface, the MCP server, the
+SIE importer and exporter, and the Skatteverket-bound generators (AGI, KU10,
+SRU, iXBRL). When two of those disagree about what a valid value looks like, the
+disagreement is invisible until a customer's filing fails.
+
+## What does not belong here
+
+- **Rules specific to one module.** They stay in that module.
+- **General helpers.** `lib/utils.ts`, `lib/money.ts`.
+- **Database questions.** "Is this account in the company's chart?" is
+  `lib/bookkeeping/account-validation.ts`, not a format rule.
+- **Business rules.** "Is this fiscal year open?" is a period question.
+
+## Naming
+
+The name must make the owning concept explicit. `isAccountNumber`, not
+`isValidNumber`. Two rules in here share a byte-identical regex (`account-number`
+and `fiscal-year` are both `/^\d{4}$/`) and mean entirely different things: the
+names are the only thing keeping a call site honest, so they carry the weight.
+
+## Layout
+
+| File | Owns |
+|---|---|
+| `org-number.ts` | Swedish organisationsnummer / personnummer: canonical 10-digit form, Luhn, Skatteverket 12-digit conversion |
+| `account-number.ts` | BAS account number format (4 digits, always a string) |
+| `iso-date.ts` | `YYYY-MM-DD` shape, plus a real-calendar-date check |
+| `fiscal-year.ts` | Four-digit räkenskapsår key |
+| `zod.ts` | Zod primitives built from the rules above, for API schemas |
+
+`zod.ts` is separate so that consumers which do not use Zod (the MCP server,
+report generators) can import a rule without pulling the dependency in.
+
+## Adding a rule
+
+1. Write the rule and the **reason** in the docblock. A rule without a recorded
+   reason gets re-litigated or worked around within a quarter.
+2. Export a pure validator (primitives in, boolean or `null` out).
+3. Add the Zod primitive to `zod.ts` if an API surface needs it.
+4. Replace the hand-rolled copies. `npm run check:guards` tracks how many remain
+   and fails if the count goes up.
diff --git a/lib/invariants/__tests__/formats.test.ts b/lib/invariants/__tests__/formats.test.ts
new file mode 100644
index 00000000..8d71703a
--- /dev/null
+++ b/lib/invariants/__tests__/formats.test.ts
@@ -0,0 +1,94 @@
+import { describe, it, expect } from 'vitest'
+import { isAccountNumber, accountClass, ACCOUNT_NUMBER_RE } from '@/lib/invariants/account-number'
+import { isIsoDateShaped, isSaneDateString, ISO_DATE_RE } from '@/lib/invariants/iso-date'
+import { isFiscalYear, FISCAL_YEAR_RE } from '@/lib/invariants/fiscal-year'
+import { isSaneDateString as isSaneDateStringFromUtils } from '@/lib/utils'
+
+describe('account number', () => {
+  it('accepts exactly four digits, as a string', () => {
+    expect(isAccountNumber('1930')).toBe(true)
+    expect(isAccountNumber('0000')).toBe(true)
+  })
+
+  it('rejects anything else', () => {
+    expect(isAccountNumber('193')).toBe(false)
+    expect(isAccountNumber('19300')).toBe(false)
+    expect(isAccountNumber('19a0')).toBe(false)
+    expect(isAccountNumber(' 1930')).toBe(false)
+    expect(isAccountNumber('')).toBe(false)
+    expect(isAccountNumber(null)).toBe(false)
+    expect(isAccountNumber(undefined)).toBe(false)
+  })
+
+  it('reads the BAS account class off the leading digit', () => {
+    expect(accountClass('1930')).toBe(1)
+    expect(accountClass('2440')).toBe(2)
+    expect(accountClass('3001')).toBe(3)
+    expect(accountClass('8910')).toBe(8)
+    expect(accountClass('nope')).toBeNull()
+  })
+})
+
+describe('iso date', () => {
+  it('shape check accepts the right shape, including impossible dates', () => {
+    expect(isIsoDateShaped('2026-01-31')).toBe(true)
+    // Shape only: this is the documented difference from isSaneDateString.
+    expect(isIsoDateShaped('2026-02-31')).toBe(true)
+    expect(isIsoDateShaped('2026-1-31')).toBe(false)
+    expect(isIsoDateShaped('31/01/2026')).toBe(false)
+    expect(isIsoDateShaped(null)).toBe(false)
+  })
+
+  it('sane check rejects impossible and out-of-range dates', () => {
+    expect(isSaneDateString('2026-01-31')).toBe(true)
+    expect(isSaneDateString('2026-02-31')).toBe(false)
+    expect(isSaneDateString('2024-13-40')).toBe(false)
+    // The native <input type="date"> 6-digit-year corruption.
+    expect(isSaneDateString('202403-02-05')).toBe(false)
+    expect(isSaneDateString('1899-12-31')).toBe(false)
+    expect(isSaneDateString('2101-01-01')).toBe(false)
+  })
+
+  it('is the same function utils re-exports, not a second copy', () => {
+    expect(isSaneDateStringFromUtils).toBe(isSaneDateString)
+  })
+})
+
+describe('fiscal year', () => {
+  it('accepts a four-digit year in range', () => {
+    expect(isFiscalYear('2026')).toBe(true)
+    expect(isFiscalYear(2026)).toBe(true)
+    expect(isFiscalYear('1900')).toBe(true)
+  })
+
+  it('rejects out-of-range and wrong shapes', () => {
+    expect(isFiscalYear('1899')).toBe(false)
+    expect(isFiscalYear('2201')).toBe(false)
+    expect(isFiscalYear('26')).toBe(false)
+    expect(isFiscalYear('')).toBe(false)
+    expect(isFiscalYear(null)).toBe(false)
+  })
+})
+
+describe('the deliberate regex collision', () => {
+  /**
+   * `account-number` and `fiscal-year` share a byte-identical regex and mean
+   * entirely different things. This test exists so that anyone tempted to
+   * "deduplicate" them has to read why they are separate first.
+   */
+  it('account number and fiscal year share a pattern but not a rule', () => {
+    expect(ACCOUNT_NUMBER_RE.source).toBe(FISCAL_YEAR_RE.source)
+
+    // '1930' is a real bank account and an absurd fiscal year in our range,
+    // '2026' is a real fiscal year and a valid BAS account number. Only the
+    // named rules can tell a caller which one it is holding.
+    expect(isAccountNumber('1930')).toBe(true)
+    expect(isFiscalYear('1930')).toBe(true)
+    expect(isFiscalYear('0000')).toBe(false)
+    expect(isAccountNumber('0000')).toBe(true)
+  })
+
+  it('the iso-date pattern is not the four-digit pattern', () => {
+    expect(ISO_DATE_RE.source).not.toBe(ACCOUNT_NUMBER_RE.source)
+  })
+})
diff --git a/lib/invariants/__tests__/org-number-cross-path.test.ts b/lib/invariants/__tests__/org-number-cross-path.test.ts
new file mode 100644
index 00000000..b98e444a
--- /dev/null
+++ b/lib/invariants/__tests__/org-number-cross-path.test.ts
@@ -0,0 +1,194 @@
+import { describe, it, expect } from 'vitest'
+import { formatRedovisare } from '@/lib/skatteverket/format'
+import { generateKU10Xml } from '@/lib/salary/ku/ku10-generator'
+import { generateAGIXml } from '@/lib/salary/agi/xml-generator'
+import { runPreflightChecks } from '@/lib/bokslut/ixbrl/validate/rules'
+import type { IxbrlArsredovisningInput } from '@/lib/bokslut/ixbrl/types'
+
+/**
+ * The four Skatteverket- and Bolagsverket-bound export paths must agree about
+ * what a valid org number is.
+ *
+ * Before `lib/invariants/org-number.ts` they did not: the SRU converter stripped
+ * hyphens only and threw on a space, KU10 stripped the first hyphen only, AGI
+ * stripped every non-digit (so stray letters passed), and the årsredovisning
+ * validator rejected the 12-digit form and skipped the check digit. A company
+ * stored in one of the awkward forms could file AGI for a year and then fail at
+ * the årsredovisning deadline with a message that did not say why.
+ *
+ * This test is the guard on that agreement. If a future change makes one path
+ * accept an input the others reject, it fails here rather than at a customer's
+ * deadline.
+ */
+
+const AB_10 = '5560125790'
+
+/** The input forms a Swedish user or a provider API actually produces. */
+const EQUIVALENT_FORMS = ['5560125790', '556012-5790', '556012 5790', '165560125790']
+
+/** The subset that is already 10 digits, ignoring separators. */
+const TEN_DIGIT_FORMS = ['5560125790', '556012-5790', '556012 5790']
+
+function ku10CompanyFixture(orgNumber: string) {
+  return {
+    orgNumber,
+    companyName: 'Testbolaget AB',
+    year: 2025,
+    contactName: 'Test Testsson',
+    contactPhone: '0700000000',
+    contactEmail: 'test@example.com',
+  }
+}
+
+function agiCompanyFixture(orgNumber: string) {
+  return {
+    orgNumber,
+    companyName: 'Testbolaget AB',
+    periodYear: 2025,
+    periodMonth: 3,
+    contactName: 'Test Testsson',
+    contactPhone: '0700000000',
+    contactEmail: 'test@example.com',
+  }
+}
+
+function ixbrlInputFixture(orgNumber: string): IxbrlArsredovisningInput {
+  // Only the org-number rule (code 1035) is asserted below. The remaining
+  // fields exist so the other preflight rules can run without throwing; their
+  // verdicts are filtered out.
+  return {
+    company: { name: 'Testbolaget AB', orgNumber },
+    period: { start: '2025-01-01', end: '2025-12-31' },
+    isFirstFiscalYear: false,
+    forvaltningsberattelse: {
+      allmantOmVerksamheten: 'Bolaget bedriver konsultverksamhet.',
+      resultatdisposition: { balanseratResultat: 0, aretsResultat: 0, summa: 0 },
+    },
+    faststallelseintyg: {
+      arsstammaDatum: '2026-05-15',
+      genereratDatum: '2026-05-20',
+      resultatdispositionDecision: 'enligt_forslag',
+      resultatdispositionOutcome: 'balanseras_i_ny_rakning',
+      signerFirstName: 'Test',
+      signerLastName: 'Testsson',
+    },
+    underskrifter: {
+      dateringsdatum: '2026-05-10',
+      signers: [{ firstName: 'Test', lastName: 'Testsson', role: 'Styrelseledamot' }],
+    },
+    totals: {
+      aretsResultat: { current: 0, previous: 0 },
+      egetKapitalSkulder: { current: 0, previous: 0 },
+      tillgangar: { current: 0, previous: 0 },
+    },
+    rr: [],
+    br: [],
+    warnings: [],
+  } as unknown as IxbrlArsredovisningInput
+}
+
+/** Does the årsredovisning preflight raise the org-number issue (code 1035)? */
+function ixbrlRejects(orgNumber: string): boolean {
+  const result = runPreflightChecks(ixbrlInputFixture(orgNumber))
+  return result.issues.some((i) => i.code === '1035')
+}
+
+/** Does the SRU redovisare conversion throw? */
+function redovisareRejects(orgNumber: string): boolean {
+  try {
+    formatRedovisare(orgNumber, 'aktiebolag')
+    return false
+  } catch {
+    return true
+  }
+}
+
+/** Does the AGI generator refuse to build for this org number? */
+function agiRejects(orgNumber: string): boolean {
+  try {
+    generateAGIXml(
+      agiCompanyFixture(orgNumber),
+      [],
+      { totalTax: 0, totalAvgifterBasis: 0, totalAvgifterAmount: 0, totalSjuklonekostnad: 0 } as never,
+    )
+    return false
+  } catch {
+    return true
+  }
+}
+
+describe('org number: the four export paths agree', () => {
+  it.each(EQUIVALENT_FORMS)('accepts %s everywhere', (form) => {
+    expect(redovisareRejects(form), 'SRU redovisare conversion').toBe(false)
+    expect(ixbrlRejects(form), 'årsredovisning preflight').toBe(false)
+    expect(agiRejects(form), 'AGI generator').toBe(false)
+    expect(() => generateKU10Xml(ku10CompanyFixture(form), []), 'KU10 generator').not.toThrow()
+  })
+
+  it('produces the same redovisare identity from every equivalent form', () => {
+    const identities = EQUIVALENT_FORMS.map((f) => formatRedovisare(f, 'aktiebolag'))
+    expect(new Set(identities).size, `got ${JSON.stringify(identities)}`).toBe(1)
+    expect(identities[0]).toBe('165560125790')
+  })
+
+  it('emits a separator-free identity into the KU10 file', () => {
+    for (const form of TEN_DIGIT_FORMS) {
+      const xml = generateKU10Xml(ku10CompanyFixture(form), [])
+      const match = xml.match(/<Organisationsnummer>([^<]*)<\/Organisationsnummer>/)
+      // Guard against a vacuous assertion: the element must actually be there.
+      expect(match, `input form ${form}: no <Organisationsnummer> in output`).not.toBeNull()
+      expect(match?.[1], `input form ${form}`).toBe(AB_10)
+    }
+  })
+
+  /**
+   * OPEN QUESTION, deliberately pinned rather than changed.
+   *
+   * KU10 strips separators but does not fold the 12-digit form down to the
+   * canonical 10 digits, so a company stored as `165560125790` files with 12
+   * digits. Whether Skatteverket's KU10 schema wants 10 or 12 here is a Swedish
+   * domain question that the `swedish-payroll` skill does not cover, and
+   * CLAUDE.md forbids answering it from training data.
+   *
+   * This is pre-existing behaviour (the old `replace('-', '')` did the same);
+   * this test pins it so the answer, when we get it, is a deliberate change with
+   * a failing test to update rather than a silent drift.
+   */
+  it('PINNED: KU10 passes a 12-digit stored org number through unfolded', () => {
+    const xml = generateKU10Xml(ku10CompanyFixture('165560125790'), [])
+    const match = xml.match(/<Organisationsnummer>([^<]*)<\/Organisationsnummer>/)
+    expect(match?.[1]).toBe('165560125790')
+  })
+
+  it.each([
+    ['5560125790x', 'stray characters'],
+    ['55601', 'too short'],
+  ])('rejects %s (%s) on every path that validates', (bad) => {
+    expect(redovisareRejects(bad), 'SRU redovisare conversion').toBe(true)
+    expect(ixbrlRejects(bad), 'årsredovisning preflight').toBe(true)
+    expect(agiRejects(bad), 'AGI generator').toBe(true)
+  })
+
+  it('surfaces a bad check digit without blocking the filing', () => {
+    const badCheckDigit = '5560125791'
+    const result = runPreflightChecks(ixbrlInputFixture(badCheckDigit))
+    const orgIssues = result.issues.filter((i) => i.code === '1035')
+
+    expect(orgIssues).toHaveLength(1)
+    // Warn, not error: a wrong check digit is surfaced to the user, but we do
+    // not block Skicka in on a domain assumption we have not verified against a
+    // primary source. See the rationale in validate/rules.ts.
+    expect(orgIssues[0].severity).toBe('warn')
+    // The org-number verdict must not be among the errors that block Skicka in.
+    // (`result.ok` is not asserted here: this fixture is minimal and trips other
+    // unrelated rules. The org-number rule's own severity is the contract.)
+    expect(result.errors.some((i) => i.code === '1035')).toBe(false)
+
+    // Export-time conversion stays permissive by design: see org-number.ts.
+    expect(redovisareRejects(badCheckDigit), 'SRU redovisare conversion').toBe(false)
+  })
+
+  it('the canonical form is what the display helper round-trips to', () => {
+    expect(formatRedovisare(AB_10, 'aktiebolag')).toBe('165560125790')
+  })
+})
diff --git a/lib/invariants/__tests__/org-number.test.ts b/lib/invariants/__tests__/org-number.test.ts
new file mode 100644
index 00000000..ba5799a6
--- /dev/null
+++ b/lib/invariants/__tests__/org-number.test.ts
@@ -0,0 +1,105 @@
+import { describe, it, expect } from 'vitest'
+import {
+  normalizeOrgNumber,
+  isValidOrgNumber,
+  isOrgNumberShaped,
+  hasInvalidOrgNumberCheckDigit,
+  stripOrgNumberFormatting,
+  formatOrgNumberDisplay,
+  toRedovisare12,
+} from '@/lib/invariants/org-number'
+
+// Real-shaped numbers with correct Luhn check digits.
+const AB_10 = '5560125790'
+const EF_10 = '8001011231'
+
+describe('stripOrgNumberFormatting', () => {
+  it('removes hyphens and spaces, nothing else', () => {
+    expect(stripOrgNumberFormatting('556012-5790')).toBe(AB_10)
+    expect(stripOrgNumberFormatting('556012 5790')).toBe(AB_10)
+    expect(stripOrgNumberFormatting(' 556012 - 5790 ')).toBe(AB_10)
+    // Letters are preserved so the caller's shape check can reject them,
+    // instead of a digit-strip silently making garbage look valid.
+    expect(stripOrgNumberFormatting('5560125790x')).toBe('5560125790x')
+  })
+})
+
+describe('normalizeOrgNumber', () => {
+  it('accepts the forms users actually type', () => {
+    expect(normalizeOrgNumber(AB_10)).toBe(AB_10)
+    expect(normalizeOrgNumber('556012-5790')).toBe(AB_10)
+    expect(normalizeOrgNumber('556012 5790')).toBe(AB_10)
+  })
+
+  it('strips the century prefix from the 12-digit form', () => {
+    expect(normalizeOrgNumber('165560125790')).toBe(AB_10)
+    expect(normalizeOrgNumber('198001011231')).toBe(EF_10)
+    expect(normalizeOrgNumber('19800101-1231')).toBe(EF_10)
+  })
+
+  it('rejects wrong length, non-digits and a bad check digit', () => {
+    expect(normalizeOrgNumber('55601257')).toBeNull()
+    expect(normalizeOrgNumber('5560125790x')).toBeNull()
+    expect(normalizeOrgNumber('5560125791')).toBeNull() // check digit off by one
+    expect(normalizeOrgNumber('')).toBeNull()
+    expect(normalizeOrgNumber(null)).toBeNull()
+    expect(normalizeOrgNumber(undefined)).toBeNull()
+  })
+})
+
+describe('shape versus check digit', () => {
+  it('separates "wrong format" from "wrong last digit"', () => {
+    expect(isOrgNumberShaped('5560125791')).toBe(true)
+    expect(isValidOrgNumber('5560125791')).toBe(false)
+    expect(hasInvalidOrgNumberCheckDigit('5560125791')).toBe(true)
+
+    // Wrong shape is not a check-digit problem.
+    expect(hasInvalidOrgNumberCheckDigit('55601')).toBe(false)
+    // A valid number is neither.
+    expect(hasInvalidOrgNumberCheckDigit(AB_10)).toBe(false)
+  })
+})
+
+describe('formatOrgNumberDisplay', () => {
+  it('renders NNNNNN-NNNN from any accepted input form', () => {
+    expect(formatOrgNumberDisplay(AB_10)).toBe('556012-5790')
+    expect(formatOrgNumberDisplay('556012 5790')).toBe('556012-5790')
+    expect(formatOrgNumberDisplay('165560125790')).toBe('556012-5790')
+  })
+
+  it('passes through anything that is not org-number shaped', () => {
+    expect(formatOrgNumberDisplay('nonsense')).toBe('nonsense')
+    expect(formatOrgNumberDisplay('')).toBe('')
+  })
+})
+
+describe('toRedovisare12', () => {
+  it('prefixes 16 for aktiebolag', () => {
+    expect(toRedovisare12(AB_10, 'aktiebolag')).toBe('165560125790')
+    expect(toRedovisare12('556012-5790', 'aktiebolag')).toBe('165560125790')
+  })
+
+  it('prefixes the century for enskild firma', () => {
+    // 80 is above the current two-digit year, so it belongs to the 1900s.
+    expect(toRedovisare12(EF_10, 'enskild_firma')).toBe('198001011231')
+  })
+
+  it('passes an already 12-digit value through untouched', () => {
+    expect(toRedovisare12('165560125790', 'aktiebolag')).toBe('165560125790')
+  })
+
+  it('accepts spaces, which the previous hyphen-only strip did not', () => {
+    expect(toRedovisare12('556012 5790', 'aktiebolag')).toBe('165560125790')
+  })
+
+  it('throws on a length it cannot interpret', () => {
+    expect(() => toRedovisare12('55601', 'aktiebolag')).toThrow(/Ogiltigt organisationsnummer/)
+  })
+
+  it('stays permissive about the check digit', () => {
+    // Export-time conversion must not start rejecting numbers that are already
+    // stored and filing: a failed export at a deadline is worse than letting
+    // Skatteverket reject it with its own message. See the module docblock.
+    expect(toRedovisare12('5560125791', 'aktiebolag')).toBe('165560125791')
+  })
+})
diff --git a/lib/invariants/account-number.ts b/lib/invariants/account-number.ts
new file mode 100644
index 00000000..7185b459
--- /dev/null
+++ b/lib/invariants/account-number.ts
@@ -0,0 +1,46 @@
+/**
+ * BAS account number format.
+ *
+ * ## The rule
+ *
+ * Exactly four digits, carried as a **string**. `'1930'`, never `1930`.
+ *
+ * ## Why a string
+ *
+ * Account numbers are identifiers, not quantities. Arithmetic on one is always
+ * a bug, and a numeric type invites it (`1930 + 1` is not an account). The
+ * string form also keeps leading digits meaningful: the first digit is the BAS
+ * account class (1 assets, 2 equity and liabilities, 3 revenue, 4-7 costs,
+ * 8 financial), which reporting code reads positionally.
+ *
+ * ## Why the shared constant, not a literal
+ *
+ * `/^\d{4}$/` appeared at 20 sites, sometimes as `/^[0-9]{4}$/`, with error
+ * messages that differed per site. Worse, the identical literal also means
+ * "a four-digit year" elsewhere in the codebase (see `fiscal-year.ts`), so the
+ * regex alone does not say what is being validated. Naming the rule makes the
+ * two impossible to confuse at a call site.
+ *
+ * Format only. Whether an account *exists and is active* in a company's chart
+ * is a database question: see `lib/bookkeeping/account-validation.ts`.
+ */
+
+/** Exactly four digits. */
+export const ACCOUNT_NUMBER_RE = /^\d{4}$/
+
+/** Canonical validation error message (Swedish: user-facing surface). */
+export const ACCOUNT_NUMBER_MESSAGE = 'Kontonummer måste vara 4 siffror'
+
+/** True when the input is a syntactically valid BAS account number. */
+export function isAccountNumber(raw: string | null | undefined): boolean {
+  return typeof raw === 'string' && ACCOUNT_NUMBER_RE.test(raw)
+}
+
+/**
+ * The BAS account class (leading digit) of an account number, or null when the
+ * input is not a valid account number.
+ */
+export function accountClass(raw: string | null | undefined): number | null {
+  if (!isAccountNumber(raw)) return null
+  return parseInt((raw as string).charAt(0), 10)
+}
diff --git a/lib/invariants/fiscal-year.ts b/lib/invariants/fiscal-year.ts
new file mode 100644
index 00000000..7a71eba5
--- /dev/null
+++ b/lib/invariants/fiscal-year.ts
@@ -0,0 +1,37 @@
+/**
+ * Four-digit calendar year, as used for räkenskapsår keys.
+ *
+ * ## Why this exists as its own name
+ *
+ * The regex is byte-identical to {@link ACCOUNT_NUMBER_RE} in
+ * `account-number.ts`. That collision is the reason both are named: a reader
+ * (or a codemod) scanning for `/^\d{4}$/` cannot tell a BAS account from a
+ * year, and the two have opposite consequences when confused. `lib/api/schemas.ts`
+ * already carried both meanings under the same literal.
+ *
+ * The bound is deliberately loose. Accounted holds historical räkenskapsår from
+ * SIE imports going back decades and must accept future years for planning, so
+ * this validates the shape and a sane range, not a business rule. Whether a
+ * given year has an open fiscal period is a database question.
+ */
+
+/** Exactly four digits. Identical to the account-number shape by coincidence, not by meaning. */
+export const FISCAL_YEAR_RE = /^\d{4}$/
+
+/** Canonical validation error message (Swedish: user-facing surface). */
+export const FISCAL_YEAR_MESSAGE = 'Nyckel måste vara ett fyrsiffrigt år'
+
+/** Lower bound: earlier years are certainly a typo, not an imported räkenskapsår. */
+export const FISCAL_YEAR_MIN = 1900
+
+/** Upper bound: generous enough for forward planning, tight enough to catch a mistyped account number. */
+export const FISCAL_YEAR_MAX = 2200
+
+/** True when the input is four digits inside the accepted range. */
+export function isFiscalYear(raw: string | number | null | undefined): boolean {
+  if (raw === null || raw === undefined) return false
+  const s = typeof raw === 'number' ? String(raw) : raw
+  if (!FISCAL_YEAR_RE.test(s)) return false
+  const n = parseInt(s, 10)
+  return n >= FISCAL_YEAR_MIN && n <= FISCAL_YEAR_MAX
+}
diff --git a/lib/invariants/index.ts b/lib/invariants/index.ts
new file mode 100644
index 00000000..67807d78
--- /dev/null
+++ b/lib/invariants/index.ts
@@ -0,0 +1,45 @@
+/**
+ * Shared product contracts: the formats and bounds more than one consumer must
+ * agree on. See `README.md` in this directory for what belongs here.
+ *
+ * Zod primitives are deliberately NOT re-exported from this barrel: import them
+ * from `@/lib/invariants/zod` so that non-Zod consumers keep a clean module
+ * graph.
+ */
+
+export {
+  ACCOUNT_NUMBER_RE,
+  ACCOUNT_NUMBER_MESSAGE,
+  isAccountNumber,
+  accountClass,
+} from './account-number'
+
+export {
+  ISO_DATE_RE,
+  ISO_DATE_MESSAGE,
+  ISO_DATE_MESSAGE_SV,
+  SANE_DATE_MESSAGE,
+  SANE_DATE_MIN_YEAR,
+  SANE_DATE_MAX_YEAR,
+  isIsoDateShaped,
+  isSaneDateString,
+} from './iso-date'
+
+export {
+  FISCAL_YEAR_RE,
+  FISCAL_YEAR_MESSAGE,
+  FISCAL_YEAR_MIN,
+  FISCAL_YEAR_MAX,
+  isFiscalYear,
+} from './fiscal-year'
+
+export {
+  ORG_NUMBER_LENGTH,
+  stripOrgNumberFormatting,
+  isOrgNumberShaped,
+  normalizeOrgNumber,
+  isValidOrgNumber,
+  hasInvalidOrgNumberCheckDigit,
+  formatOrgNumberDisplay,
+  toRedovisare12,
+} from './org-number'
diff --git a/lib/invariants/iso-date.ts b/lib/invariants/iso-date.ts
new file mode 100644
index 00000000..19a19e91
--- /dev/null
+++ b/lib/invariants/iso-date.ts
@@ -0,0 +1,69 @@
+import { parseISO, isValid } from 'date-fns'
+
+/**
+ * ISO calendar date format (`YYYY-MM-DD`).
+ *
+ * ## The rule
+ *
+ * Accounting dates are exchanged and stored as `YYYY-MM-DD`, never as a locale
+ * format and never as a timestamp. `lib/utils.ts` `formatDate()` renders this
+ * same shape for display, so what a user reads matches what the API accepts.
+ *
+ * ## Why it is centralised
+ *
+ * The literal `/^\d{4}-\d{2}-\d{2}$/` appeared at 68 sites outside tests, with
+ * at least three different error messages for the same failure ("Expected
+ * YYYY-MM-DD", "Expected YYYY-MM-DD date format", "Ogiltigt datumformat
+ * (YYYY-MM-DD)"). An API surface that rejects the same input three different
+ * ways is three surfaces to a client.
+ *
+ * ## Two rules, not one
+ *
+ * - {@link isIsoDateShaped} is a **shape** check. `2026-02-31` passes it.
+ * - {@link isSaneDateString} additionally requires the date to exist and to sit
+ *   in a plausible year range. Use it for anything a human typed.
+ *
+ * `isSaneDateString` moved here from `lib/utils.ts`, where it was already
+ * documented as "the ONE authoritative date rule shared by the client form and
+ * the server-side CreateTransactionSchema". `lib/utils.ts` re-exports it so
+ * existing imports keep working; this module is now its home because the same
+ * rule is needed by consumers that must not import UI helpers.
+ */
+
+/** `YYYY-MM-DD` shape. Does not check that the date exists. */
+export const ISO_DATE_RE = /^\d{4}-\d{2}-\d{2}$/
+
+/** Canonical validation error message. */
+export const ISO_DATE_MESSAGE = 'Expected YYYY-MM-DD'
+
+/** Swedish-facing variant, for surfaces that render errors to end users. */
+export const ISO_DATE_MESSAGE_SV = 'Ogiltigt datumformat (YYYY-MM-DD)'
+
+/** Message for the stricter {@link isSaneDateString} rule. */
+export const SANE_DATE_MESSAGE = 'Invalid or out-of-range date (expected YYYY-MM-DD, year 1900-2100)'
+
+/** Earliest year accepted for a user-entered date. */
+export const SANE_DATE_MIN_YEAR = 1900
+
+/** Latest year accepted for a user-entered date. */
+export const SANE_DATE_MAX_YEAR = 2100
+
+/** True when the input has `YYYY-MM-DD` shape. */
+export function isIsoDateShaped(raw: string | null | undefined): boolean {
+  return typeof raw === 'string' && ISO_DATE_RE.test(raw)
+}
+
+/**
+ * Strict ISO date validation for user-entered dates: correct shape, a date that
+ * actually exists, and a plausible year.
+ *
+ * Rejects impossible dates (`2024-13-40`, `2026-02-31`) and absurd years. The
+ * year bound exists because a mistyped or misparsed date most often lands far
+ * outside it, and an accounting date in year 0202 silently creates a fiscal
+ * period nobody can close.
+ */
+export function isSaneDateString(s: string): boolean {
+  if (!ISO_DATE_RE.test(s)) return false
+  const d = parseISO(s)
+  return isValid(d) && d.getFullYear() >= SANE_DATE_MIN_YEAR && d.getFullYear() <= SANE_DATE_MAX_YEAR
+}
diff --git a/lib/invariants/org-number.ts b/lib/invariants/org-number.ts
new file mode 100644
index 00000000..f2da068a
--- /dev/null
+++ b/lib/invariants/org-number.ts
@@ -0,0 +1,147 @@
+import { luhnValidate } from '@/lib/bankgiro/luhn'
+
+/**
+ * Swedish organisationsnummer / personnummer: the one place that decides what
+ * "a valid org number" means.
+ *
+ * ## The rule
+ *
+ * - **Canonical storage form is 10 digits, no separators** (`5560125790`).
+ * - Input may arrive as 10 or 12 digits, with spaces or hyphens, because that
+ *   is what users type and what provider APIs return. Both forms normalize to
+ *   the same 10 digits; the century prefix is dropped.
+ * - The last digit is a Luhn (mod-10) check digit, the structural rule
+ *   Bolagsverket and personnummer share.
+ *
+ * ## Why this module exists
+ *
+ * Before it, seven call sites each had their own idea of the rule, and four of
+ * them fed Skatteverket-bound output that must agree:
+ *
+ * | Site | Old rule | Failure |
+ * |---|---|---|
+ * | `lib/skatteverket/format.ts` | strip `-` only | threw on any input containing a space |
+ * | `lib/salary/ku/ku10-generator.ts` | `replace('-', '')` | first hyphen only, no space handling |
+ * | `lib/salary/agi/xml-generator.ts` | strip non-digits | no check-digit validation |
+ * | `lib/bokslut/ixbrl/validate/rules.ts` | `/^\d{6}-?\d{4}$/` | rejected the 12-digit form outright |
+ *
+ * A company stored with a space or in 12-digit form could file AGI all year and
+ * then fail on the årsredovisning, with no way for the user to tell why. The
+ * rules only stay in agreement if there is exactly one of them.
+ *
+ * ## Deliberate asymmetry: normalize everywhere, Luhn only at the boundary
+ *
+ * `normalizeOrgNumber` (Luhn-checked) guards data coming *in*. The export-time
+ * converter `toRedovisare12` is structural only: it must not start rejecting
+ * numbers that are already stored and have been filing successfully, because a
+ * failed export at a deadline is worse than a number Skatteverket will reject
+ * with its own message. Tighten the intake, not the outflow.
+ */
+
+/** Digits-only canonical storage length. */
+export const ORG_NUMBER_LENGTH = 10
+
+/**
+ * Strip the separators Swedish users and provider APIs put in org numbers.
+ * Does not validate: use {@link isOrgNumberShaped} or {@link normalizeOrgNumber}.
+ */
+export function stripOrgNumberFormatting(raw: string): string {
+  return raw.replace(/[\s-]/g, '')
+}
+
+/**
+ * True when the input is structurally an org number (10 or 12 digits after
+ * separators are stripped), regardless of check digit.
+ */
+export function isOrgNumberShaped(raw: string | null | undefined): boolean {
+  if (!raw) return false
+  const cleaned = stripOrgNumberFormatting(raw)
+  return /^\d{10}$/.test(cleaned) || /^\d{12}$/.test(cleaned)
+}
+
+/**
+ * Normalize an org number to Accounted's canonical 10-digit storage form.
+ *
+ * Accepts hyphen/space-formatted input in either of the two shapes Swedish
+ * users commonly type:
+ *  - 10 digits (5560125790 or 8001011231): stored as-is
+ *  - 12 digits (198001011231): century prefix stripped
+ *
+ * Returns null for any other length, non-digit content, or invalid Luhn check
+ * digit. Storing a structurally invalid org number would later be caught by
+ * Skatteverket SRU and any receiving SIE4 system: refusing at the boundary
+ * keeps Accounted's bookkeeping from accumulating under an unusable identifier.
+ */
+export function normalizeOrgNumber(raw: string | null | undefined): string | null {
+  if (!raw) return null
+  const cleaned = stripOrgNumberFormatting(raw)
+  let canonical: string
+  if (/^\d{10}$/.test(cleaned)) {
+    canonical = cleaned
+  } else if (/^\d{12}$/.test(cleaned)) {
+    canonical = cleaned.substring(2)
+  } else {
+    return null
+  }
+  return luhnValidate(canonical) ? canonical : null
+}
+
+/** True when {@link normalizeOrgNumber} accepts the input. */
+export function isValidOrgNumber(raw: string | null | undefined): boolean {
+  return normalizeOrgNumber(raw) !== null
+}
+
+/**
+ * True when the input is shaped like an org number but its check digit is
+ * wrong. Lets a validator tell the user *which* problem they have instead of
+ * one undifferentiated "ogiltigt organisationsnummer".
+ */
+export function hasInvalidOrgNumberCheckDigit(raw: string | null | undefined): boolean {
+  return isOrgNumberShaped(raw) && !isValidOrgNumber(raw)
+}
+
+/**
+ * Format a canonical org number for display: `NNNNNN-NNNN`.
+ * Returns the input unchanged when it is not org-number shaped.
+ */
+export function formatOrgNumberDisplay(raw: string | null | undefined): string {
+  if (!raw) return ''
+  const cleaned = stripOrgNumberFormatting(raw)
+  const ten = /^\d{12}$/.test(cleaned) ? cleaned.substring(2) : cleaned
+  if (!/^\d{10}$/.test(ten)) return raw
+  return `${ten.substring(0, 6)}-${ten.substring(6)}`
+}
+
+/**
+ * Convert an org number to Skatteverket's 12-digit "redovisare" format.
+ *
+ * - Organisationsnummer (aktiebolag): prefix `16` (5020000013 -> 165020000013)
+ * - Personnummer (enskild firma): prefix `19` or `20` by century
+ * - Input already in 12-digit form passes through untouched
+ *
+ * Structural only, no check-digit validation: see the module docblock for why
+ * the export path stays permissive.
+ *
+ * @throws when the input is not 10 or 12 digits after separators are stripped.
+ */
+export function toRedovisare12(
+  orgNumber: string,
+  entityType: 'enskild_firma' | 'aktiebolag',
+): string {
+  const clean = stripOrgNumberFormatting(orgNumber)
+
+  if (/^\d{12}$/.test(clean)) return clean
+
+  if (!/^\d{10}$/.test(clean)) {
+    throw new Error(`Ogiltigt organisationsnummer: ${orgNumber} (förväntar 10 eller 12 siffror)`)
+  }
+
+  if (entityType === 'aktiebolag') return `16${clean}`
+
+  // Enskild firma: personnummer. A two-digit year above the current one must
+  // belong to the previous century (someone born in 98 is 1998, not 2098).
+  const yearDigits = parseInt(clean.substring(0, 2), 10)
+  const currentTwoDigitYear = new Date().getFullYear() % 100
+  const prefix = yearDigits > currentTwoDigitYear ? '19' : '20'
+  return `${prefix}${clean}`
+}
diff --git a/lib/invariants/zod.ts b/lib/invariants/zod.ts
new file mode 100644
index 00000000..3ad64714
--- /dev/null
+++ b/lib/invariants/zod.ts
@@ -0,0 +1,48 @@
+import { z } from 'zod'
+import { ACCOUNT_NUMBER_RE, ACCOUNT_NUMBER_MESSAGE } from './account-number'
+import { ISO_DATE_RE, ISO_DATE_MESSAGE, SANE_DATE_MESSAGE, isSaneDateString } from './iso-date'
+import { FISCAL_YEAR_RE, FISCAL_YEAR_MESSAGE } from './fiscal-year'
+import { isValidOrgNumber, normalizeOrgNumber } from './org-number'
+
+/**
+ * Zod primitives built from the shared rules.
+ *
+ * Kept in a separate file so consumers that do not use Zod (the MCP server,
+ * report generators, SIE import) can import a rule without pulling the
+ * dependency into their module graph.
+ *
+ * `lib/api/schemas.ts` builds its own local aliases on top of these, so the
+ * ~100 schemas there inherit any correction made in one place.
+ */
+
+/** BAS account number: always a string of exactly 4 digits. */
+export const accountNumberSchema = z.string().regex(ACCOUNT_NUMBER_RE, ACCOUNT_NUMBER_MESSAGE)
+
+/** ISO date shape (`YYYY-MM-DD`). Does not check the date exists. */
+export const isoDateSchema = z.string().regex(ISO_DATE_RE, ISO_DATE_MESSAGE)
+
+/**
+ * ISO date that must also be a real, in-range calendar date. Use this over
+ * {@link isoDateSchema} for anything a human typed.
+ */
+export const saneIsoDateSchema = z.string().refine(isSaneDateString, SANE_DATE_MESSAGE)
+
+/** Four-digit räkenskapsår key. */
+export const fiscalYearSchema = z.string().regex(FISCAL_YEAR_RE, FISCAL_YEAR_MESSAGE)
+
+/**
+ * Swedish org number in any accepted input form (10 or 12 digits, spaces or
+ * hyphens), validated including its Luhn check digit.
+ *
+ * Does **not** transform: schemas that persist the value should call
+ * `normalizeOrgNumber` explicitly at the write site so the canonical form is
+ * visible in the calling code rather than hidden in a parser.
+ */
+export const orgNumberSchema = z
+  .string()
+  .refine(isValidOrgNumber, 'Ogiltigt organisationsnummer (10 eller 12 siffror, giltig kontrollsiffra)')
+
+/** Org number that is normalized to the canonical 10-digit storage form on parse. */
+export const normalizedOrgNumberSchema = orgNumberSchema.transform(
+  (v) => normalizeOrgNumber(v) as string,
+)
diff --git a/lib/salary/agi/xml-generator.ts b/lib/salary/agi/xml-generator.ts
index 9bd3a4f3..9b55abb6 100644
--- a/lib/salary/agi/xml-generator.ts
+++ b/lib/salary/agi/xml-generator.ts
@@ -1,4 +1,5 @@
 import { decryptPersonnummer } from '../personnummer'
+import { isOrgNumberShaped } from '@/lib/invariants/org-number'
 
 /**
  * AGI XML generator: Arbetsgivardeklaration på individnivå.
@@ -197,11 +198,14 @@ export class AGIIncompleteDataError extends Error {
 
 function assertRequiredCompanyData(company: AGICompanyData): void {
   const missing: string[] = []
-  const orgNumberDigits = (company.orgNumber || '').replace(/\D/g, '')
   // Skatteverket's IDENTITET type requires either 10 digits (AB orgnr, we prefix
-  // with "16") or 12 digits (personnummer for enskild firma). Any other length
-  // is a data-entry error that we cannot silently fix.
-  if (orgNumberDigits.length !== 10 && orgNumberDigits.length !== 12) missing.push('organisationsnummer')
+  // with "16") or 12 digits (personnummer for enskild firma). Any other shape is
+  // a data-entry error that we cannot silently fix.
+  //
+  // Shared rule (lib/invariants/org-number.ts), not a local digit-strip: the
+  // previous `replace(/\D/g, '')` also swallowed letters, so a field holding
+  // stray text still measured 10 digits and passed.
+  if (!isOrgNumberShaped(company.orgNumber)) missing.push('organisationsnummer')
   if (!company.contactName.trim()) missing.push('kontaktperson (namn)')
   if (!company.contactPhone.trim()) missing.push('telefon')
   if (!company.contactEmail.trim()) missing.push('e-post')
diff --git a/lib/salary/ku/ku10-generator.ts b/lib/salary/ku/ku10-generator.ts
index 73f78906..e91805bf 100644
--- a/lib/salary/ku/ku10-generator.ts
+++ b/lib/salary/ku/ku10-generator.ts
@@ -1,5 +1,6 @@
 import { decryptPersonnummer } from '../personnummer'
 import { getBranding } from '@/lib/branding/service'
+import { stripOrgNumberFormatting } from '@/lib/invariants/org-number'
 
 /**
  * KU10 (Kontrolluppgift): Annual employee income statement.
@@ -49,7 +50,10 @@ export function generateKU10Xml(
   employees: KU10EmployeeData[]
 ): string {
   const lines: string[] = []
-  const orgNr = company.orgNumber.replace('-', '')
+  // Shared rule (lib/invariants/org-number.ts). The previous
+  // `replace('-', '')` removed only the FIRST hyphen and left spaces intact, so
+  // an org number entered as "556012 5790" reached Skatteverket with a space in it.
+  const orgNr = stripOrgNumberFormatting(company.orgNumber)
 
   lines.push('<?xml version="1.0" encoding="UTF-8"?>')
   lines.push('<Skatteverket xmlns="http://xmls.skatteverket.se/se/skatteverket/ai/instans/infoForBeskworksgivku/1.0"')
diff --git a/lib/skatteverket/format.ts b/lib/skatteverket/format.ts
index 9dbe6260..84c4534c 100644
--- a/lib/skatteverket/format.ts
+++ b/lib/skatteverket/format.ts
@@ -1,4 +1,5 @@
 import type { VatPeriodType } from '@/types'
+import { toRedovisare12 } from '@/lib/invariants/org-number'
 
 /**
  * Convert a Accounted org_number to Skatteverket's 12-digit "redovisare" format.
@@ -6,27 +7,18 @@ import type { VatPeriodType } from '@/types'
  * Rules:
  * - Organisationsnummer (10 digits, e.g. 5020000013): prefix with "16" → 165020000013
  * - Personnummer (10 digits, e.g. 8501011234): prefix with "19" or "20" based on century
- * - Strip any hyphens before processing
+ * - Separators (hyphens and spaces) are stripped before processing
+ *
+ * Delegates to `lib/invariants/org-number.ts` so that this converter, the AGI
+ * and KU10 generators, and the årsredovisning validator cannot drift apart.
+ * The previous local implementation stripped hyphens only and threw on any
+ * input containing a space.
  */
 export function formatRedovisare(
   orgNumber: string,
   entityType: 'enskild_firma' | 'aktiebolag'
 ): string {
-  const clean = orgNumber.replace(/-/g, '')
-
-  if (clean.length === 12) return clean
-
-  if (clean.length !== 10) {
-    throw new Error(`Ogiltigt organisationsnummer: ${orgNumber} (förväntar 10 eller 12 siffror)`)
-  }
-
-  if (entityType === 'aktiebolag') return `16${clean}`
-
-  // Enskild firma: personnummer
-  const yearDigits = parseInt(clean.substring(0, 2), 10)
-  const currentTwoDigitYear = new Date().getFullYear() % 100
-  const prefix = yearDigits > currentTwoDigitYear ? '19' : '20'
-  return `${prefix}${clean}`
+  return toRedovisare12(orgNumber, entityType)
 }
 
 /**
diff --git a/lib/utils.ts b/lib/utils.ts
index 34ec3e31..a55cd7aa 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -72,15 +72,15 @@ export function formatDate(date: Date | string): string {
  *
  * The shape check (4-digit year) is what stops the native <input type="date">
  * 6-digit-year corruption ('202403-02-05'); the parse + range check also
- * rejects impossible dates (2024-13-40) and absurd years. Exported as the ONE
- * authoritative date rule shared by the client form and the server-side
+ * rejects impossible dates (2024-13-40) and absurd years. The ONE authoritative
+ * date rule shared by the client form and the server-side
  * CreateTransactionSchema, so the two validation layers can never drift.
+ *
+ * Implementation lives in `lib/invariants/iso-date.ts` alongside the other
+ * shared format contracts; re-exported here because this is where callers have
+ * always imported it from.
  */
-export function isSaneDateString(s: string): boolean {
-  if (!/^\d{4}-\d{2}-\d{2}$/.test(s)) return false
-  const d = parseISO(s)
-  return isValid(d) && d.getFullYear() >= 1900 && d.getFullYear() <= 2100
-}
+export { isSaneDateString } from '@/lib/invariants/iso-date'
 
 /**
  * Date + time for audit / metadata displays: `2026-05-11 14:30`. ISO-ordered
diff --git a/scripts/checks/antipatterns-baseline.json b/scripts/checks/antipatterns-baseline.json
index 097b81cd..3b1a43f1 100644
--- a/scripts/checks/antipatterns-baseline.json
+++ b/scripts/checks/antipatterns-baseline.json
@@ -9,6 +9,9 @@
   "naiveOreRound": {
     "count": 641
   },
+  "handRolledInvariants": {
+    "count": 114
+  },
   "ledgerScanningReports": {
     "count": 4,
     "files": [
diff --git a/scripts/checks/no-new-antipatterns.mjs b/scripts/checks/no-new-antipatterns.mjs
index 630dd962..f550a310 100644
--- a/scripts/checks/no-new-antipatterns.mjs
+++ b/scripts/checks/no-new-antipatterns.mjs
@@ -51,6 +51,13 @@
  *      extensionRegistry.get('<id>') so a disabled extension never exposes a
  *      live surface (allowlisted file-set, may only shrink). Implementation
  *      and rationale in extension-route-guards.mjs.
+ *   8. hand-rolled-invariant: a shared format rule (BAS account number, ISO
+ *      date, four-digit fiscal year) spelled out inline instead of imported
+ *      from lib/invariants/. The BAS account rule was written out at 20 sites
+ *      and the ISO date rule at 68, with error messages that differed per site;
+ *      the four Skatteverket-bound org-number paths disagreed outright about
+ *      what "valid" meant, which is the kind of drift a customer only discovers
+ *      when a filing fails at the deadline. Tracked as a count.
  *
  * Usage:
  *   node scripts/checks/no-new-antipatterns.mjs            # check (CI)
@@ -83,6 +90,25 @@ const RAW_AUTH_RE = /\.auth\.getUser\(/
 const GUARD_RE = /requireAuth\(|withRouteContext[<(]/
 const NAIVE_ROUND_RE = /Math\.round\([^\n]*\*\s*100\s*\)\s*\/\s*100/
 
+// 8. hand-rolled-invariant. Shared format contracts live in lib/invariants/
+// (account number, ISO date, four-digit fiscal year, org number). Before that
+// module the BAS account rule was written out at 20 sites and the ISO date rule
+// at 68, with error messages that differed per site, and the four
+// Skatteverket-bound org-number paths did not agree on what "valid" meant.
+//
+// Only the two unambiguous regex families are counted. An org-number
+// digit-strip is too varied in shape to match reliably by regex; the
+// cross-path test in lib/invariants/__tests__/org-number-cross-path.test.ts is
+// the guard on that one instead.
+const HAND_ROLLED_INVARIANT_RES = [
+  // /^\d{4}$/ or /^[0-9]{4}$/  → accountNumberSchema or fiscalYearSchema
+  /\/\^(?:\\d|\[0-9\])\{4\}\$\//,
+  // /^\d{4}-\d{2}-\d{2}$/      → isoDateSchema or ISO_DATE_RE
+  /\/\^(?:\\d|\[0-9\])\{4\}-(?:\\d|\[0-9\])\{2\}-(?:\\d|\[0-9\])\{2\}\$\//,
+]
+// The sanctioned home of these rules: must not count against itself.
+const INVARIANT_EXEMPT_PREFIX = 'lib/invariants/'
+
 function walk(dir, exts, out = []) {
   let entries
   try {
@@ -231,6 +257,31 @@ function countNaiveRound() {
   return count
 }
 
+/**
+ * Occurrences of a shared format rule written out by hand instead of imported
+ * from lib/invariants/. Counted, not file-setted: the campaign lowers the
+ * number file by file and the count may only go down.
+ */
+function countHandRolledInvariants() {
+  const files = [
+    ...walk(path.join(ROOT, 'lib'), ['.ts', '.tsx']),
+    ...walk(path.join(ROOT, 'app'), ['.ts', '.tsx']),
+    ...walk(path.join(ROOT, 'components'), ['.ts', '.tsx']),
+    ...walk(path.join(ROOT, 'extensions'), ['.ts', '.tsx']),
+  ]
+  let count = 0
+  for (const f of files) {
+    const relPath = rel(f)
+    if (relPath.startsWith(INVARIANT_EXEMPT_PREFIX)) continue
+    // Tests legitimately spell out the pattern they are asserting about.
+    if (relPath.includes('__tests__/') || relPath.endsWith('.test.ts')) continue
+    for (const line of fs.readFileSync(f, 'utf8').split('\n')) {
+      if (HAND_ROLLED_INVARIANT_RES.some((re) => re.test(line))) count++
+    }
+  }
+  return count
+}
+
 // Dependencies pinned to an EXACT version on purpose, because a bump broke prod
 // and must not silently return via `npm update`, a dependabot bump, or a manual
 // install. Any drift (in package.json OR the lockfile) fails CI. See DECISIONS.md.
@@ -563,6 +614,7 @@ function findRawUserErrors() {
 const current = {
   rawRouteAuth: findRawRouteAuth(),
   naiveOreRound: countNaiveRound(),
+  handRolledInvariants: countHandRolledInvariants(),
   ledgerScanningReports: findLedgerScanningReports(),
   directJelInsert: findDirectJelInserts(),
   pinnedDepViolations: findPinnedDepViolations(),
@@ -579,6 +631,7 @@ if (isUpdate) {
       'Ratchet baseline for scripts/checks/no-new-antipatterns.mjs. These counts may only decrease. Re-run with --update after a migration lowers them. Goal: both reach 0 (A1 route-auth campaign, D1 rounding codemod).',
     rawRouteAuth: { count: current.rawRouteAuth.length, files: current.rawRouteAuth },
     naiveOreRound: { count: current.naiveOreRound },
+    handRolledInvariants: { count: current.handRolledInvariants },
     ledgerScanningReports: {
       count: current.ledgerScanningReports.length,
       files: current.ledgerScanningReports,
@@ -675,6 +728,20 @@ if (current.sekLabelledAmounts.length) {
   )
 }
 
+// 1e2. hand-rolled-invariant: counted, may only go down.
+if (current.handRolledInvariants > (baseline.handRolledInvariants?.count ?? Infinity)) {
+  failed = true
+  console.error(
+    `\n✗ hand-rolled-invariant: ${current.handRolledInvariants} inline copies of a shared format rule ` +
+      `(baseline ${baseline.handRolledInvariants?.count}):`,
+  )
+  console.error(
+    '  → import the rule instead: accountNumberSchema / isoDateSchema / saneIsoDateSchema /\n' +
+      '    fiscalYearSchema from @/lib/invariants/zod, or the ACCOUNT_NUMBER_RE / ISO_DATE_RE\n' +
+      '    constants from @/lib/invariants. See lib/invariants/README.md.',
+  )
+}
+
 // 1f. cross-extension-import: a physical extension route may only import its
 // own extension. No baseline: the count is 0 today, any hit is a hard failure.
 if (current.extensionRoutes.crossImports.length) {
@@ -772,5 +839,5 @@ if (failed) {
   process.exit(1)
 }
 console.log(
-  `\n✓ Antipattern guard passed (raw-route-auth: ${current.rawRouteAuth.length}, naive-ore-round: ${current.naiveOreRound}, ledger-scanning-report: ${current.ledgerScanningReports.length}, direct-jel-insert: 0, pinned-dep: 0, raw-user-error: 0, sek-labelled-amount: 0, cross-extension-import: 0, ungated-extension-route: ${current.extensionRoutes.ungated.length}/${UNGATED_EXTENSION_ROUTES.size} allowlisted).`,
+  `\n✓ Antipattern guard passed (raw-route-auth: ${current.rawRouteAuth.length}, naive-ore-round: ${current.naiveOreRound}, hand-rolled-invariant: ${current.handRolledInvariants}, ledger-scanning-report: ${current.ledgerScanningReports.length}, direct-jel-insert: 0, pinned-dep: 0, raw-user-error: 0, sek-labelled-amount: 0, cross-extension-import: 0, ungated-extension-route: ${current.extensionRoutes.ungated.length}/${UNGATED_EXTENSION_ROUTES.size} allowlisted).`,
 )
diff --git a/tests/pg/upgrade/assert.sql b/tests/pg/upgrade/assert.sql
new file mode 100644
index 00000000..7eafcf99
--- /dev/null
+++ b/tests/pg/upgrade/assert.sql
@@ -0,0 +1,85 @@
+-- Upgrade-path assertions: run AFTER the pull request's migrations have been
+-- applied on top of the seeded database.
+--
+-- Every check raises an exception on failure, so `psql -v ON_ERROR_STOP=1`
+-- fails the job. Keep the assertions about invariants that must hold for ANY
+-- migration, not about the specifics of one change.
+
+DO $$
+DECLARE
+  v_entries       INT;
+  v_lines         INT;
+  v_debits        NUMERIC;
+  v_credits       NUMERIC;
+  v_ore           NUMERIC;
+  v_company       INT;
+  v_period        INT;
+  v_max_voucher   INT;
+BEGIN
+  -- 1. The company, its membership and its fiscal period survived.
+  SELECT count(*) INTO v_company
+    FROM public.companies WHERE id = '22222222-2222-2222-2222-222222222222';
+  IF v_company <> 1 THEN
+    RAISE EXCEPTION 'upgrade: seeded company disappeared (found %)', v_company;
+  END IF;
+
+  SELECT count(*) INTO v_period
+    FROM public.fiscal_periods WHERE id = '33333333-3333-3333-3333-333333333333';
+  IF v_period <> 1 THEN
+    RAISE EXCEPTION 'upgrade: seeded fiscal period disappeared (found %)', v_period;
+  END IF;
+
+  -- 2. All three posted verifikat survived, still posted. A migration must
+  --    never silently drop or unpost a posted entry (BFL 5 kap).
+  SELECT count(*) INTO v_entries
+    FROM public.journal_entries
+    WHERE company_id = '22222222-2222-2222-2222-222222222222'
+      AND status = 'posted';
+  IF v_entries <> 3 THEN
+    RAISE EXCEPTION 'upgrade: expected 3 posted entries after migration, found %', v_entries;
+  END IF;
+
+  -- 3. Every line survived.
+  SELECT count(*) INTO v_lines
+    FROM public.journal_entry_lines l
+    JOIN public.journal_entries e ON e.id = l.journal_entry_id
+    WHERE e.company_id = '22222222-2222-2222-2222-222222222222';
+  IF v_lines <> 7 THEN
+    RAISE EXCEPTION 'upgrade: expected 7 journal entry lines after migration, found %', v_lines;
+  END IF;
+
+  -- 4. The ledger still balances. This is the assertion that catches a
+  --    migration which retypes, rescales or rounds a money column.
+  SELECT COALESCE(sum(l.debit_amount), 0), COALESCE(sum(l.credit_amount), 0)
+    INTO v_debits, v_credits
+    FROM public.journal_entry_lines l
+    JOIN public.journal_entries e ON e.id = l.journal_entry_id
+    WHERE e.company_id = '22222222-2222-2222-2222-222222222222';
+  IF v_debits <> v_credits THEN
+    RAISE EXCEPTION 'upgrade: ledger no longer balances after migration (debits %, credits %)',
+      v_debits, v_credits;
+  END IF;
+  IF v_debits <> 20623.45 THEN
+    RAISE EXCEPTION 'upgrade: total debits changed from 20623.45 to %', v_debits;
+  END IF;
+
+  -- 5. The öre survived exactly. Money is NUMERIC and must not drift.
+  SELECT l.debit_amount INTO v_ore
+    FROM public.journal_entry_lines l
+    WHERE l.journal_entry_id = '44444444-4444-4444-4444-444444444403'
+      AND l.account_number = '6570';
+  IF v_ore IS DISTINCT FROM 123.45 THEN
+    RAISE EXCEPTION 'upgrade: öre-level amount drifted from 123.45 to %', v_ore;
+  END IF;
+
+  -- 6. Voucher numbers are intact and still sequential from 1.
+  SELECT max(voucher_number) INTO v_max_voucher
+    FROM public.journal_entries
+    WHERE company_id = '22222222-2222-2222-2222-222222222222';
+  IF v_max_voucher <> 3 THEN
+    RAISE EXCEPTION 'upgrade: highest voucher number changed to %, expected 3', v_max_voucher;
+  END IF;
+
+  RAISE NOTICE 'upgrade-path assertions passed: % entries, % lines, balanced at %',
+    v_entries, v_lines, v_debits;
+END $$;
diff --git a/tests/pg/upgrade/seed.sql b/tests/pg/upgrade/seed.sql
new file mode 100644
index 00000000..b3b481a9
--- /dev/null
+++ b/tests/pg/upgrade/seed.sql
@@ -0,0 +1,108 @@
+-- Upgrade-path fixture: a small but REAL company, seeded against the schema as
+-- it stood BEFORE the pull request's migrations.
+--
+-- Why this exists
+-- ---------------
+-- The `pg-real` job applies all migrations to an EMPTY database. Empty means
+-- zero rows, so a migration that adds a NOT NULL, adds a CHECK, creates a
+-- unique index or backfills passes trivially in CI and can still fail (or
+-- silently corrupt) on production, where the rows exist. This fixture gives the
+-- new migrations something to break.
+--
+-- Deliberately narrow: only long-stable core tables, because this file has to
+-- execute against an OLDER schema than the one in the working tree. Anything
+-- newer than the merge-base does not exist yet. Adding a column reference here
+-- to a recently-added column will make the job fail on every PR, not just the
+-- bad ones. If a new table needs upgrade coverage, add it here only once it has
+-- been in main long enough that the merge-base always has it.
+--
+-- CI FIXTURE ONLY. NEVER a template for real data.
+-- ------------------------------------------------
+-- The inserts below write `journal_entries` with status='posted' and their
+-- lines directly, bypassing lib/bookkeeping/engine.ts and the atomic
+-- commit_journal_entry RPC. That is legitimate here and ONLY here: this runs
+-- against a throwaway CI database that is destroyed with the job, so there is
+-- no verifikationsnummer sequence to keep gapless and no retention obligation
+-- (BFL 5 kap 6-7 §). Seeding this way is the only way to hand the migrations
+-- pre-existing posted rows to break.
+--
+-- Do not copy this pattern into a seed script, a migration, a repair script, or
+-- anything that touches a real database. Every production journal write goes
+-- through the engine (CLAUDE.md Hard Rule 2). If you need posted entries in a
+-- real database, use the engine.
+--
+-- Fixed UUIDs so assert.sql can find the rows without threading state.
+
+BEGIN;
+
+INSERT INTO auth.users (id, email, instance_id)
+VALUES (
+  '11111111-1111-1111-1111-111111111111',
+  'pg-upgrade@test.invalid',
+  '00000000-0000-0000-0000-000000000000'::uuid
+);
+
+INSERT INTO public.companies (id, name, entity_type, created_by)
+VALUES (
+  '22222222-2222-2222-2222-222222222222',
+  'Uppgraderingsbolaget AB',
+  'aktiebolag',
+  '11111111-1111-1111-1111-111111111111'
+);
+
+INSERT INTO public.company_members (company_id, user_id, role)
+VALUES (
+  '22222222-2222-2222-2222-222222222222',
+  '11111111-1111-1111-1111-111111111111',
+  'owner'
+);
+
+INSERT INTO public.fiscal_periods
+  (id, user_id, company_id, name, period_start, period_end, is_closed)
+VALUES (
+  '33333333-3333-3333-3333-333333333333',
+  '11111111-1111-1111-1111-111111111111',
+  '22222222-2222-2222-2222-222222222222',
+  '2026',
+  '2026-01-01',
+  '2026-12-31',
+  FALSE
+);
+
+-- Three posted verifikat with balanced lines. Posted (not draft) is the point:
+-- these are the rows a careless migration corrupts, and the ones the
+-- immutability triggers protect.
+INSERT INTO public.journal_entries
+  (id, user_id, company_id, fiscal_period_id, voucher_number, voucher_series,
+   entry_date, description, source_type, status)
+VALUES
+  ('44444444-4444-4444-4444-444444444401',
+   '11111111-1111-1111-1111-111111111111',
+   '22222222-2222-2222-2222-222222222222',
+   '33333333-3333-3333-3333-333333333333',
+   1, 'A', '2026-03-01', 'Försäljning', 'manual', 'posted'),
+  ('44444444-4444-4444-4444-444444444402',
+   '11111111-1111-1111-1111-111111111111',
+   '22222222-2222-2222-2222-222222222222',
+   '33333333-3333-3333-3333-333333333333',
+   2, 'A', '2026-03-15', 'Lokalhyra', 'manual', 'posted'),
+  ('44444444-4444-4444-4444-444444444403',
+   '11111111-1111-1111-1111-111111111111',
+   '22222222-2222-2222-2222-222222222222',
+   '33333333-3333-3333-3333-333333333333',
+   3, 'A', '2026-04-01', 'Bankavgift', 'manual', 'posted');
+
+INSERT INTO public.journal_entry_lines
+  (journal_entry_id, account_number, debit_amount, credit_amount)
+VALUES
+  ('44444444-4444-4444-4444-444444444401', '1930', 12500.00, 0),
+  ('44444444-4444-4444-4444-444444444401', '3001', 0, 10000.00),
+  ('44444444-4444-4444-4444-444444444401', '2611', 0, 2500.00),
+  ('44444444-4444-4444-4444-444444444402', '5010', 8000.00, 0),
+  ('44444444-4444-4444-4444-444444444402', '1930', 0, 8000.00),
+  -- An öre-level amount, so a migration that rounds or retypes the money
+  -- columns shows up in the assertion instead of passing on round numbers.
+  ('44444444-4444-4444-4444-444444444403', '6570', 123.45, 0),
+  ('44444444-4444-4444-4444-444444444403', '1930', 0, 123.45);
+
+COMMIT;