diff --git a/lib/api/schemas.ts b/lib/api/schemas.ts
index 12b2a0ed..571627b8 100644
--- a/lib/api/schemas.ts
+++ b/lib/api/schemas.ts
@@ -3,6 +3,7 @@ import { normaliseSwish, isValidSwish } from '@/lib/payments/swish'
 import { normalizeVatNumber } from '@/lib/vat/vat-number'
 import { isSaneDateString } from '@/lib/utils'
 import { countCalendarMonths } from '@/lib/bookkeeping/accruals/compute'
+import { DimensionsBagSchema } from '@/lib/bookkeeping/dimension-resolver'
 
 // ============================================================
 // Shared primitives
@@ -686,6 +687,13 @@ export const CreateJournalEntryLineSchema = z.object({
   amount_in_currency: z.number().optional(),
   exchange_rate: z.number().positive().optional(),
   tax_code: z.string().optional(),
+  // SIE dimension map {sie_dim_no: object_code}, e.g. {"1":"KS01","6":"P001"}.
+  // Single source of truth for the constraints lives in dimension-resolver so
+  // the staged pending-operations path validates identically. Wins per key
+  // over the cost_center/project aliases.
+  dimensions: DimensionsBagSchema.optional(),
+  // Deprecated aliases for dimensions['1'] / dimensions['6'] — kept forever
+  // for API/MCP compatibility.
   cost_center: z.string().optional(),
   project: z.string().optional(),
 })
diff --git a/lib/bookkeeping/__tests__/dimension-resolver.test.ts b/lib/bookkeeping/__tests__/dimension-resolver.test.ts
new file mode 100644
index 00000000..2d00a7c7
--- /dev/null
+++ b/lib/bookkeeping/__tests__/dimension-resolver.test.ts
@@ -0,0 +1,134 @@
+import { describe, it, expect } from 'vitest'
+import {
+  normalizeLineDimensions,
+  lineDimensionColumns,
+  coerceDimensionsBag,
+  DIM_COST_CENTER,
+  DIM_PROJECT,
+} from '@/lib/bookkeeping/dimension-resolver'
+
+describe('normalizeLineDimensions', () => {
+  it('returns empty map for a line with no dimension data', () => {
+    expect(normalizeLineDimensions({})).toEqual({})
+    expect(normalizeLineDimensions({ cost_center: null, project: null })).toEqual({})
+  })
+
+  it('maps the deprecated aliases to SIE keys 1 and 6', () => {
+    expect(normalizeLineDimensions({ cost_center: 'KS01', project: 'P001' })).toEqual({
+      '1': 'KS01',
+      '6': 'P001',
+    })
+  })
+
+  it('passes an explicit bag through', () => {
+    expect(normalizeLineDimensions({ dimensions: { '1': 'KS01', '7': 'ANST-4' } })).toEqual({
+      '1': 'KS01',
+      '7': 'ANST-4',
+    })
+  })
+
+  it('lets the explicit bag win over aliases per key', () => {
+    expect(
+      normalizeLineDimensions({
+        dimensions: { '1': 'KS-BAG' },
+        cost_center: 'KS-ALIAS',
+        project: 'P-ALIAS',
+      })
+    ).toEqual({ '1': 'KS-BAG', '6': 'P-ALIAS' })
+  })
+
+  it('treats an explicit empty string in the bag as clearing that dimension', () => {
+    expect(
+      normalizeLineDimensions({ dimensions: { '1': '' }, cost_center: 'KS-ALIAS' })
+    ).toEqual({})
+  })
+
+  it('trims whitespace and drops blank values', () => {
+    expect(
+      normalizeLineDimensions({ dimensions: { '6': '  P001  ' }, cost_center: '   ' })
+    ).toEqual({ '6': 'P001' })
+  })
+
+  it('drops non-numeric and zero/negative keys', () => {
+    expect(
+      normalizeLineDimensions({
+        dimensions: { projekt: 'X', '0': 'Y', '6': 'P001' } as Record<string, string>,
+      })
+    ).toEqual({ '6': 'P001' })
+  })
+
+  it("canonicalizes leading-zero keys ('01' -> '1') so mirrors are derived", () => {
+    const dims = normalizeLineDimensions({ dimensions: { '01': 'KS01', '06': 'P001' } })
+    expect(dims).toEqual({ '1': 'KS01', '6': 'P001' })
+    expect(lineDimensionColumns(dims)).toEqual({ cost_center: 'KS01', project: 'P001' })
+  })
+
+  it("clearing via a leading-zero key ('01': '') also clears the alias-filled '1'", () => {
+    expect(
+      normalizeLineDimensions({ dimensions: { '01': '' }, cost_center: 'KS-ALIAS' })
+    ).toEqual({})
+  })
+
+  it('reversal parity: empty bag + populated aliases equals alias-only input', () => {
+    // reverseEntry passes {dimensions: {}, cost_center, project} for legacy
+    // rows; storno passes the row directly — both must normalize identically.
+    expect(
+      normalizeLineDimensions({ dimensions: {}, cost_center: 'KS01', project: 'P001' })
+    ).toEqual(normalizeLineDimensions({ cost_center: 'KS01', project: 'P001' }))
+  })
+})
+
+describe('coerceDimensionsBag (boundary validator for staged payloads)', () => {
+  it('returns undefined for non-objects', () => {
+    expect(coerceDimensionsBag(undefined)).toBeUndefined()
+    expect(coerceDimensionsBag(null)).toBeUndefined()
+    expect(coerceDimensionsBag('P001')).toBeUndefined()
+    expect(coerceDimensionsBag(['6', 'P001'])).toBeUndefined()
+  })
+
+  it('accepts a valid bag and normalizes values', () => {
+    expect(coerceDimensionsBag({ '6': ' P001 ', '1': 'KS01' })).toEqual({
+      '6': 'P001',
+      '1': 'KS01',
+    })
+  })
+
+  it('rejects the WHOLE bag on any invalid entry — same as the API schema', () => {
+    // Numeric value (no silent coercion), invalid key, leading-zero key:
+    // exactly what CreateJournalEntryLineSchema would reject.
+    expect(coerceDimensionsBag({ '6': 42 })).toBeUndefined()
+    expect(coerceDimensionsBag({ '6': 42, '1': 'KS01' })).toBeUndefined()
+    expect(coerceDimensionsBag({ projekt: 'P001', '6': 'P001' })).toBeUndefined()
+    expect(coerceDimensionsBag({ '06': 'P001' })).toBeUndefined()
+  })
+
+  it('enforces the same length/charset constraints as the Zod line schema', () => {
+    expect(coerceDimensionsBag({ '6': 'x'.repeat(41) })).toBeUndefined()
+    expect(coerceDimensionsBag({ '6': 'P"1' })).toBeUndefined()
+    expect(coerceDimensionsBag({ '6': 'P{1}' })).toBeUndefined()
+    expect(coerceDimensionsBag({ '6': 'x'.repeat(40) })).toEqual({ '6': 'x'.repeat(40) })
+  })
+
+  it('returns undefined for an empty or whitespace-only bag', () => {
+    expect(coerceDimensionsBag({})).toBeUndefined()
+  })
+})
+
+describe('lineDimensionColumns', () => {
+  it('derives both mirrors from the map', () => {
+    expect(lineDimensionColumns({ [DIM_COST_CENTER]: 'KS01', [DIM_PROJECT]: 'P001' })).toEqual({
+      cost_center: 'KS01',
+      project: 'P001',
+    })
+  })
+
+  it('returns nulls for missing keys', () => {
+    expect(lineDimensionColumns({})).toEqual({ cost_center: null, project: null })
+    expect(lineDimensionColumns({ '7': 'ANST-4' })).toEqual({ cost_center: null, project: null })
+  })
+
+  it('round-trips with normalizeLineDimensions (mirror consistency)', () => {
+    const dims = normalizeLineDimensions({ cost_center: 'KS01', dimensions: { '6': 'P001' } })
+    expect(lineDimensionColumns(dims)).toEqual({ cost_center: 'KS01', project: 'P001' })
+  })
+})
diff --git a/lib/bookkeeping/dimension-resolver.ts b/lib/bookkeeping/dimension-resolver.ts
new file mode 100644
index 00000000..6a87c1cd
--- /dev/null
+++ b/lib/bookkeeping/dimension-resolver.ts
@@ -0,0 +1,108 @@
+/**
+ * Dimension resolver — the single place line dimensions are normalized and
+ * mirrored (dev_docs/dimensions_implementation_plan.md).
+ *
+ * Storage model: journal_entry_lines.dimensions is a JSONB map keyed by SIE
+ * dimension number ({"1":"KS01","6":"P001"}) and is the single source of
+ * truth. The legacy cost_center/project TEXT columns are deterministic mirrors
+ * of keys '1'/'6' during the dual-write window (they become GENERATED columns
+ * in a later migration). Every journal_entry_lines writer MUST derive the
+ * mirror columns via lineDimensionColumns() — never set them independently.
+ */
+
+import { z } from 'zod'
+
+/** SIE dimension numbers with first-class mirror columns. */
+export const DIM_COST_CENTER = '1'
+export const DIM_PROJECT = '6'
+
+export type LineDimensions = Record<string, string>
+
+/**
+ * THE schema for a dimensions bag ({sie_dim_no: object_code}) — the single
+ * source of truth for its constraints. The API layer
+ * (CreateJournalEntryLineSchema) and the staged pending-operations path
+ * (coerceDimensionsBag) both use this exact schema, so the two validation
+ * layers cannot drift. Keys are canonical SIE dimension numbers (no leading
+ * zeros); values must not contain characters that break SIE field framing.
+ */
+export const DimensionsBagSchema = z.record(
+  z.string().regex(/^[1-9]\d*$/, 'Dimensionsnyckel måste vara ett SIE-dimensionsnummer'),
+  z.string().min(1).max(40).regex(/^[^"{}]+$/, 'Dimensionskod får inte innehålla ", { eller }')
+)
+
+interface DimensionAliasInput {
+  dimensions?: LineDimensions | null
+  cost_center?: string | null
+  project?: string | null
+}
+
+/**
+ * Merge the explicit `dimensions` bag with the deprecated cost_center/project
+ * aliases into one canonical map. The explicit bag wins per key; aliases only
+ * fill keys the bag does not set. Empty/blank values and non-numeric keys are
+ * dropped so the stored map never carries junk entries.
+ */
+export function normalizeLineDimensions(line: DimensionAliasInput): LineDimensions {
+  const out: LineDimensions = {}
+
+  const costCenter = line.cost_center?.trim()
+  if (costCenter) out[DIM_COST_CENTER] = costCenter
+  const project = line.project?.trim()
+  if (project) out[DIM_PROJECT] = project
+
+  if (line.dimensions) {
+    for (const [key, value] of Object.entries(line.dimensions)) {
+      if (!/^\d+$/.test(key) || Number(key) < 1) continue
+      // Canonical numeric form: '01' and '1' must land on the same key, or
+      // lineDimensionColumns misses the mirror and reports split the value.
+      const dimNo = String(Number(key))
+      const trimmed = typeof value === 'string' ? value.trim() : ''
+      if (!trimmed) {
+        // Explicit empty string in the bag means "clear this dimension" — it
+        // must also override a non-empty alias, so remove any alias-filled key.
+        delete out[dimNo]
+        continue
+      }
+      out[dimNo] = trimmed
+    }
+  }
+
+  return out
+}
+
+/**
+ * Boundary validator for an untyped dimensions bag (staged pending-operation
+ * params, tool payloads). Delegates to DimensionsBagSchema — the exact schema
+ * the API layer uses — so the staged path cannot drift from API validation.
+ * Whole-bag semantics: a bag containing ANY invalid entry is rejected
+ * (returns undefined) rather than partially salvaged; staged payloads were
+ * already schema-validated at staging time, so an invalid entry here means
+ * drift or tampering — booking then proceeds without dimensions, which are
+ * never load-bearing for validity. Interior normalization
+ * (normalizeLineDimensions) stays permissive on charset by design — it must
+ * preserve legacy DB values verbatim on reversal/correction; this function is
+ * the input gate.
+ */
+export function coerceDimensionsBag(raw: unknown): LineDimensions | undefined {
+  if (raw === undefined || raw === null) return undefined
+  const parsed = DimensionsBagSchema.safeParse(raw)
+  if (!parsed.success) return undefined
+  const dims = normalizeLineDimensions({ dimensions: parsed.data })
+  return Object.keys(dims).length > 0 ? dims : undefined
+}
+
+/**
+ * Derive the legacy mirror columns from the canonical map. Pure function —
+ * divergence between `dimensions` and cost_center/project is impossible as
+ * long as every writer goes through this.
+ */
+export function lineDimensionColumns(dimensions: LineDimensions): {
+  cost_center: string | null
+  project: string | null
+} {
+  return {
+    cost_center: dimensions[DIM_COST_CENTER] ?? null,
+    project: dimensions[DIM_PROJECT] ?? null,
+  }
+}
diff --git a/lib/bookkeeping/engine.ts b/lib/bookkeeping/engine.ts
index 3a003934..ee397e2b 100644
--- a/lib/bookkeeping/engine.ts
+++ b/lib/bookkeeping/engine.ts
@@ -14,6 +14,7 @@ import {
   JournalEntryNotFoundError,
 } from '@/lib/bookkeeping/errors'
 import { resolveDefaultSeriesForSource } from '@/lib/bookkeeping/voucher-series-resolver'
+import { normalizeLineDimensions, lineDimensionColumns } from '@/lib/bookkeeping/dimension-resolver'
 import { backfillStandardBASAccounts } from '@/lib/bookkeeping/account-backfill'
 import { syncInvoiceStatusFromPaymentEntry, isPaymentSourceType } from '@/lib/bookkeeping/payment-sync'
 import { getActor } from '@/lib/bookkeeping/actor-context'
@@ -185,21 +186,26 @@ function buildLineInserts(
   lines: CreateJournalEntryLineInput[],
   accountIdMap: Map<string, string>
 ) {
-  return lines.map((line, index) => ({
-    journal_entry_id: entryId,
-    account_number: line.account_number,
-    account_id: accountIdMap.get(line.account_number) || null,
-    debit_amount: Math.round((line.debit_amount || 0) * 100) / 100,
-    credit_amount: Math.round((line.credit_amount || 0) * 100) / 100,
-    currency: line.currency || 'SEK',
-    amount_in_currency: line.amount_in_currency ? Math.round(line.amount_in_currency * 100) / 100 : null,
-    exchange_rate: line.exchange_rate || null,
-    line_description: line.line_description || null,
-    tax_code: line.tax_code || null,
-    cost_center: line.cost_center || null,
-    project: line.project || null,
-    sort_order: index,
-  }))
+  return lines.map((line, index) => {
+    // dimensions JSONB is the source of truth; cost_center/project are
+    // derived mirrors (dual-write window — see lib/bookkeeping/dimension-resolver.ts)
+    const dimensions = normalizeLineDimensions(line)
+    return {
+      journal_entry_id: entryId,
+      account_number: line.account_number,
+      account_id: accountIdMap.get(line.account_number) || null,
+      debit_amount: Math.round((line.debit_amount || 0) * 100) / 100,
+      credit_amount: Math.round((line.credit_amount || 0) * 100) / 100,
+      currency: line.currency || 'SEK',
+      amount_in_currency: line.amount_in_currency ? Math.round(line.amount_in_currency * 100) / 100 : null,
+      exchange_rate: line.exchange_rate || null,
+      line_description: line.line_description || null,
+      tax_code: line.tax_code || null,
+      dimensions,
+      ...lineDimensionColumns(dimensions),
+      sort_order: index,
+    }
+  })
 }
 
 /**
@@ -662,6 +668,7 @@ export async function reverseEntry(
       : undefined,
     exchange_rate: line.exchange_rate || undefined,
     tax_code: line.tax_code || undefined,
+    dimensions: line.dimensions || undefined,
     cost_center: line.cost_center || undefined,
     project: line.project || undefined,
   }))
diff --git a/lib/core/bookkeeping/storno-service.ts b/lib/core/bookkeeping/storno-service.ts
index ee145cef..fde72521 100644
--- a/lib/core/bookkeeping/storno-service.ts
+++ b/lib/core/bookkeeping/storno-service.ts
@@ -6,6 +6,7 @@ import type {
   JournalEntryLine,
 } from '@/types'
 import { validateBalance, getNextVoucherNumber } from '@/lib/bookkeeping/engine'
+import { normalizeLineDimensions, lineDimensionColumns } from '@/lib/bookkeeping/dimension-resolver'
 import { backfillStandardBASAccounts } from '@/lib/bookkeeping/account-backfill'
 import { resolvePeriodStatusForDate } from '@/lib/core/bookkeeping/period-service'
 import {
@@ -271,22 +272,25 @@ export async function correctEntry(
     throw new BookkeepingDatabaseError('create_reversal_entry', reversalError?.message)
   }
 
-  // Insert reversed lines (swap debit and credit)
-  const reversalLineInserts = originalLines.map((line, index) => ({
-    journal_entry_id: reversalEntry.id,
-    account_number: line.account_number,
-    account_id: line.account_id || null,
-    debit_amount: Math.round((Number(line.credit_amount) || 0) * 100) / 100,
-    credit_amount: Math.round((Number(line.debit_amount) || 0) * 100) / 100,
-    currency: line.currency || 'SEK',
-    amount_in_currency: line.amount_in_currency ? -Number(line.amount_in_currency) : null,
-    exchange_rate: line.exchange_rate || null,
-    line_description: `Storno: ${line.line_description || ''}`,
-    tax_code: line.tax_code || null,
-    cost_center: line.cost_center || null,
-    project: line.project || null,
-    sort_order: index,
-  }))
+  // Insert reversed lines (swap debit and credit, preserve dimensions)
+  const reversalLineInserts = originalLines.map((line, index) => {
+    const dimensions = normalizeLineDimensions(line)
+    return {
+      journal_entry_id: reversalEntry.id,
+      account_number: line.account_number,
+      account_id: line.account_id || null,
+      debit_amount: Math.round((Number(line.credit_amount) || 0) * 100) / 100,
+      credit_amount: Math.round((Number(line.debit_amount) || 0) * 100) / 100,
+      currency: line.currency || 'SEK',
+      amount_in_currency: line.amount_in_currency ? -Number(line.amount_in_currency) : null,
+      exchange_rate: line.exchange_rate || null,
+      line_description: `Storno: ${line.line_description || ''}`,
+      tax_code: line.tax_code || null,
+      dimensions,
+      ...lineDimensionColumns(dimensions),
+      sort_order: index,
+    }
+  })
 
   const { error: reversalLinesError } = await supabase
     .from('journal_entry_lines')
@@ -352,23 +356,26 @@ export async function correctEntry(
     correctedEntry = newEntry
 
     // Insert corrected lines
-    const correctedLineInserts = correctedLines.map((line, index) => ({
-      journal_entry_id: correctedEntry.id,
-      account_number: line.account_number,
-      account_id: accountIdMap.get(line.account_number) || null,
-      debit_amount: Math.round((line.debit_amount || 0) * 100) / 100,
-      credit_amount: Math.round((line.credit_amount || 0) * 100) / 100,
-      currency: line.currency || 'SEK',
-      amount_in_currency: line.amount_in_currency
-        ? Math.round(line.amount_in_currency * 100) / 100
-        : null,
-      exchange_rate: line.exchange_rate || null,
-      line_description: line.line_description || null,
-      tax_code: line.tax_code || null,
-      cost_center: line.cost_center || null,
-      project: line.project || null,
-      sort_order: index,
-    }))
+    const correctedLineInserts = correctedLines.map((line, index) => {
+      const dimensions = normalizeLineDimensions(line)
+      return {
+        journal_entry_id: correctedEntry.id,
+        account_number: line.account_number,
+        account_id: accountIdMap.get(line.account_number) || null,
+        debit_amount: Math.round((line.debit_amount || 0) * 100) / 100,
+        credit_amount: Math.round((line.credit_amount || 0) * 100) / 100,
+        currency: line.currency || 'SEK',
+        amount_in_currency: line.amount_in_currency
+          ? Math.round(line.amount_in_currency * 100) / 100
+          : null,
+        exchange_rate: line.exchange_rate || null,
+        line_description: line.line_description || null,
+        tax_code: line.tax_code || null,
+        dimensions,
+        ...lineDimensionColumns(dimensions),
+        sort_order: index,
+      }
+    })
 
     const { error: correctedLinesError } = await supabase
       .from('journal_entry_lines')
@@ -526,6 +533,7 @@ export async function recordateEntry(
         line.amount_in_currency != null ? Number(line.amount_in_currency) : undefined,
       exchange_rate: line.exchange_rate != null ? Number(line.exchange_rate) : undefined,
       tax_code: line.tax_code || undefined,
+      dimensions: line.dimensions || undefined,
       cost_center: line.cost_center || undefined,
       project: line.project || undefined,
     }))
diff --git a/lib/pending-operations/commit.ts b/lib/pending-operations/commit.ts
index f8843aa0..9b34c2f3 100644
--- a/lib/pending-operations/commit.ts
+++ b/lib/pending-operations/commit.ts
@@ -26,6 +26,7 @@ import {
   createCreditNoteJournalEntry,
 } from '@/lib/bookkeeping/invoice-entries'
 import { createJournalEntry, findFiscalPeriod, reverseEntry, validateBalance } from '@/lib/bookkeeping/engine'
+import { coerceDimensionsBag } from '@/lib/bookkeeping/dimension-resolver'
 import { cancelOrphanedPaymentEntry } from '@/lib/bookkeeping/cancel-orphaned-entry'
 import { runWithActor } from '@/lib/bookkeeping/actor-context-node'
 import type { CommitActor } from '@/lib/bookkeeping/actor-context'
@@ -2667,6 +2668,9 @@ function normalizeVoucherLines(raw: unknown): CreateJournalEntryLineInput[] {
       amount_in_currency: line.amount_in_currency !== undefined ? Number(line.amount_in_currency) : undefined,
       exchange_rate: line.exchange_rate !== undefined ? Number(line.exchange_rate) : undefined,
       tax_code: line.tax_code ? String(line.tax_code) : undefined,
+      // Boundary-validated with the same constraints as the Zod line schema —
+      // staged payloads must not bypass API-layer validation (SOC 2 PI1.1).
+      dimensions: coerceDimensionsBag(line.dimensions),
       cost_center: line.cost_center ? String(line.cost_center) : undefined,
       project: line.project ? String(line.project) : undefined,
     }
diff --git a/scripts/checks/no-new-antipatterns.mjs b/scripts/checks/no-new-antipatterns.mjs
index 6893e05f..7896d63d 100644
--- a/scripts/checks/no-new-antipatterns.mjs
+++ b/scripts/checks/no-new-antipatterns.mjs
@@ -16,6 +16,12 @@
  *   2. naive-ore-round — `Math.round(x * 100) / 100`, which is subtly wrong on
  *      exact-half values (see lib/money.ts `roundOre`). Tracked as a count.
  *      The canonical rounding modules are excluded.
+ *   3. direct-jel-insert — a file that inserts into `journal_entry_lines`
+ *      outside the sanctioned writers. During the dimensions dual-write window
+ *      every line writer must derive cost_center/project via
+ *      lineDimensionColumns() from the dimensions JSONB map
+ *      (lib/bookkeeping/dimension-resolver.ts) — a new direct insert site can
+ *      silently diverge the mirror columns. Tracked as a file-set.
  *
  * Usage:
  *   node scripts/checks/no-new-antipatterns.mjs            # check (CI)
@@ -75,6 +81,37 @@ function findRawRouteAuth() {
     .sort()
 }
 
+// Sanctioned journal_entry_lines insert sites. engine/storno write mirrors via
+// dimension-resolver; sie-import and sandbox seed write neither dims nor
+// mirrors (DB defaults keep them consistent).
+const JEL_INSERT_SANCTIONED = new Set([
+  'lib/bookkeeping/engine.ts',
+  'lib/core/bookkeeping/storno-service.ts',
+  'lib/import/sie-import.ts',
+  'app/api/sandbox/seed/route.ts',
+])
+// Matches an insert CHAINED on the lines table (`.from('journal_entry_lines').insert(`,
+// with optional whitespace/newlines in the chain) — select-only readers don't count.
+const JEL_INSERT_CHAIN_RE = /\.from\(\s*['"]journal_entry_lines['"]\s*\)\s*\.\s*(insert|upsert)\(/
+
+/** Files that insert into journal_entry_lines outside the sanctioned writers. */
+function findDirectJelInserts() {
+  const files = [
+    ...walk(path.join(ROOT, 'lib'), ['.ts', '.tsx']),
+    ...walk(path.join(ROOT, 'app'), ['.ts', '.tsx']),
+    ...walk(path.join(ROOT, 'extensions'), ['.ts', '.tsx']),
+  ]
+  return files
+    .filter((f) => {
+      const r = rel(f)
+      if (JEL_INSERT_SANCTIONED.has(r)) return false
+      if (r.includes('__tests__/') || r.endsWith('.test.ts')) return false
+      return JEL_INSERT_CHAIN_RE.test(fs.readFileSync(f, 'utf8'))
+    })
+    .map(rel)
+    .sort()
+}
+
 /** Count of naive Math.round(x*100)/100 occurrences (lines) across source. */
 function countNaiveRound() {
   const files = [
@@ -96,6 +133,7 @@ function countNaiveRound() {
 const current = {
   rawRouteAuth: findRawRouteAuth(),
   naiveOreRound: countNaiveRound(),
+  directJelInsert: findDirectJelInserts(),
 }
 
 const isUpdate = process.argv.includes('--update')
@@ -136,6 +174,22 @@ if (newAuthFiles.length) {
   console.error('  → wrap the route in withRouteContext (or call requireAuth) so MFA is enforced.')
 }
 
+// 1b. direct-jel-insert: allowlist lives in this file (JEL_INSERT_SANCTIONED),
+// no baseline — any unsanctioned insert site is a hard failure.
+if (current.directJelInsert.length) {
+  failed = true
+  console.error(
+    `\n✗ direct-jel-insert: ${current.directJelInsert.length} file(s) insert into journal_entry_lines ` +
+      `outside the sanctioned writers:`,
+  )
+  current.directJelInsert.forEach((f) => console.error(`    ${f}`))
+  console.error(
+    '  → route line writes through lib/bookkeeping/engine.ts, or derive cost_center/project via\n' +
+      '    lineDimensionColumns() (lib/bookkeeping/dimension-resolver.ts) and add the file to\n' +
+      '    JEL_INSERT_SANCTIONED in this script with a justification.',
+  )
+}
+
 // 2. naive-ore-round: count may not increase.
 if (current.naiveOreRound > baseline.naiveOreRound.count) {
   failed = true
@@ -160,5 +214,5 @@ if (failed) {
   process.exit(1)
 }
 console.log(
-  `\n✓ Antipattern guard passed (raw-route-auth: ${current.rawRouteAuth.length}, naive-ore-round: ${current.naiveOreRound}).`,
+  `\n✓ Antipattern guard passed (raw-route-auth: ${current.rawRouteAuth.length}, naive-ore-round: ${current.naiveOreRound}, direct-jel-insert: 0).`,
 )
diff --git a/supabase/migrations/20260702084500_dimensions_substrate.sql b/supabase/migrations/20260702084500_dimensions_substrate.sql
new file mode 100644
index 00000000..639d462b
--- /dev/null
+++ b/supabase/migrations/20260702084500_dimensions_substrate.sql
@@ -0,0 +1,402 @@
+-- Dimensions substrate (PR1 of the dimensions plan — dev_docs/dimensions_implementation_plan.md)
+--
+-- Adds the SIE-native dimension registry (dimensions = #DIM/#UNDERDIM,
+-- dimension_values = #OBJEKT) and the single-source-of-truth `dimensions jsonb`
+-- map on journal_entry_lines ({"1":"KS01","6":"P001"}, keyed by SIE dimension
+-- number). The legacy free-text journal_entry_lines.cost_center / .project
+-- columns become deterministic mirrors of keys '1'/'6', kept in sync by
+-- lib/bookkeeping/dimension-resolver.ts during the dual-write window (they are
+-- demoted to GENERATED columns in a later migration, per the plan).
+--
+-- Registry tables are born company_id-native with NO user_id column. This is a
+-- deliberate deviation from the legacy new-table template: it finishes the
+-- user_id -> company_id lift for this domain (dev_docs/dimensions_architecture.md
+-- §3.8). write_audit_log() reads user_id via to_jsonb() and tolerates its absence.
+--
+-- Corrective note: the comment in 20240101000011_alter_existing_tables.sql:66-68
+-- claiming cost_center_id/project_id UUID FK columns + indexes exist was never
+-- true — no such columns were ever created. This migration is the real
+-- referential structure for dimensions.
+
+-- =============================================================================
+-- 1. dimensions (= SIE #DIM / #UNDERDIM registry)
+-- =============================================================================
+CREATE TABLE public.dimensions (
+  id                uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  company_id        uuid NOT NULL REFERENCES public.companies(id) ON DELETE CASCADE,
+  -- Byrå/firm-shared taxonomy axis. Bare nullable column by design (SHAPE
+  -- decision: never NOT NULL); the FK to firms(id) is wired when firms lands.
+  firm_id           uuid NULL,
+  sie_dim_no        int  NOT NULL CHECK (sie_dim_no >= 1),
+  -- #UNDERDIM parent (e.g. kostnadsbärare 2 -> kostnadsställe 1)
+  parent_sie_dim_no int  NULL CHECK (parent_sie_dim_no IS NULL OR parent_sie_dim_no <> sie_dim_no),
+  name              text NOT NULL CHECK (char_length(name) BETWEEN 1 AND 80),
+  -- SIE asymmetry as data: dim 1 balances reset each fiscal year, dim 6 accumulates
+  resets_annually   boolean NOT NULL DEFAULT true,
+  -- Seeded dims 1 & 6: undeletable, number immutable (enforced by trigger below)
+  is_system         boolean NOT NULL DEFAULT false,
+  is_active         boolean NOT NULL DEFAULT true,
+  sort_order        int  NOT NULL DEFAULT 100,
+  created_at        timestamptz NOT NULL DEFAULT now(),
+  updated_at        timestamptz NOT NULL DEFAULT now(),
+
+  UNIQUE (company_id, sie_dim_no),
+  -- Composite key target so dimension_values can FK on (dimension_id, company_id)
+  -- and cross-company value rows are impossible by construction.
+  UNIQUE (id, company_id)
+);
+
+ALTER TABLE public.dimensions ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY "view own-company dimensions"
+  ON public.dimensions FOR SELECT USING (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "insert own-company dimensions"
+  ON public.dimensions FOR INSERT WITH CHECK (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "update own-company dimensions"
+  ON public.dimensions FOR UPDATE USING (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "delete own-company dimensions"
+  ON public.dimensions FOR DELETE USING (company_id IN (SELECT public.user_company_ids()));
+
+CREATE INDEX idx_dimensions_company_id ON public.dimensions (company_id);
+
+COMMENT ON COLUMN public.dimensions.resets_annually IS
+  'SIE4 balance semantics: true = flow-period dimension whose balances reset each fiscal year (dim 1 kostnadsställe — must NOT carry #IB/#OIB opening balances on export); false = accumulating dimension spanning years (dim 6 projekt). The SIE export/import path (PR2+) must honour this; it is data, not enforcement.';
+
+CREATE TRIGGER dimensions_updated_at
+  BEFORE UPDATE ON public.dimensions
+  FOR EACH ROW EXECUTE FUNCTION public.update_updated_at_column();
+
+CREATE TRIGGER audit_dimensions
+  AFTER INSERT OR UPDATE OR DELETE ON public.dimensions
+  FOR EACH ROW EXECUTE FUNCTION public.write_audit_log();
+
+-- =============================================================================
+-- 2. dimension_values (= SIE #OBJEKT)
+-- =============================================================================
+CREATE TABLE public.dimension_values (
+  id              uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  company_id      uuid NOT NULL REFERENCES public.companies(id) ON DELETE CASCADE,
+  dimension_id    uuid NOT NULL,
+  -- Object code as written into journal_entry_lines.dimensions and SIE #OBJEKT.
+  -- DB bound is deliberately loose (only chars that break SIE field framing are
+  -- forbidden) so legacy free-text survives the backfill; the strict Fortnox
+  -- format (^[A-Za-z0-9ÅÄÖåäö_+\-]{1,20}$) is enforced at the API layer for
+  -- user-created codes.
+  code            text NOT NULL CHECK (char_length(code) BETWEEN 1 AND 40 AND code !~ '["{}]'),
+  name            text NOT NULL CHECK (char_length(name) BETWEEN 1 AND 120),
+  parent_value_id uuid NULL REFERENCES public.dimension_values(id) ON DELETE SET NULL,
+  -- Inactivate, never delete, once referenced by a posted line (trigger below).
+  is_active       boolean NOT NULL DEFAULT true,
+  start_date      date NULL,
+  end_date        date NULL,
+  attributes      jsonb NOT NULL DEFAULT '{}'::jsonb CHECK (jsonb_typeof(attributes) = 'object'),
+  created_at      timestamptz NOT NULL DEFAULT now(),
+  updated_at      timestamptz NOT NULL DEFAULT now(),
+
+  UNIQUE (company_id, dimension_id, code),
+  -- Same-company integrity by construction (see dimensions UNIQUE (id, company_id))
+  FOREIGN KEY (dimension_id, company_id)
+    REFERENCES public.dimensions (id, company_id) ON DELETE CASCADE
+);
+
+ALTER TABLE public.dimension_values ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY "view own-company dimension_values"
+  ON public.dimension_values FOR SELECT USING (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "insert own-company dimension_values"
+  ON public.dimension_values FOR INSERT WITH CHECK (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "update own-company dimension_values"
+  ON public.dimension_values FOR UPDATE USING (company_id IN (SELECT public.user_company_ids()));
+CREATE POLICY "delete own-company dimension_values"
+  ON public.dimension_values FOR DELETE USING (company_id IN (SELECT public.user_company_ids()));
+
+CREATE INDEX idx_dimension_values_company_id ON public.dimension_values (company_id);
+CREATE INDEX idx_dimension_values_dimension_id ON public.dimension_values (dimension_id);
+
+CREATE TRIGGER dimension_values_updated_at
+  BEFORE UPDATE ON public.dimension_values
+  FOR EACH ROW EXECUTE FUNCTION public.update_updated_at_column();
+
+CREATE TRIGGER audit_dimension_values
+  AFTER INSERT OR UPDATE OR DELETE ON public.dimension_values
+  FOR EACH ROW EXECUTE FUNCTION public.write_audit_log();
+
+-- =============================================================================
+-- 3. Registry guard triggers
+-- =============================================================================
+
+-- 3a. dimensions: system dims are undeletable; sie_dim_no is immutable (it is
+-- the key used inside journal_entry_lines.dimensions — renumbering would
+-- silently orphan every tagged line); is_system cannot be flipped; a dimension
+-- whose number is referenced by any posted/reversed line cannot be deleted.
+CREATE OR REPLACE FUNCTION public.enforce_dimension_registry_guards()
+RETURNS trigger
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path TO 'public'
+AS $$
+BEGIN
+  IF TG_OP = 'UPDATE' THEN
+    IF NEW.sie_dim_no <> OLD.sie_dim_no THEN
+      RAISE EXCEPTION 'Dimensionsnumret kan inte ändras (rader är taggade med numret).';
+    END IF;
+    IF NEW.is_system <> OLD.is_system THEN
+      RAISE EXCEPTION 'is_system kan inte ändras.';
+    END IF;
+    RETURN NEW;
+  END IF;
+
+  -- DELETE
+  IF OLD.is_system THEN
+    RAISE EXCEPTION 'Systemdimensionen % (%) kan inte tas bort — avaktivera den istället.',
+      OLD.sie_dim_no, OLD.name;
+  END IF;
+  IF EXISTS (
+    SELECT 1
+    FROM public.journal_entries je
+    JOIN public.journal_entry_lines jel ON jel.journal_entry_id = je.id
+    WHERE je.company_id = OLD.company_id
+      AND je.status IN ('posted', 'reversed')
+      AND jel.dimensions ? OLD.sie_dim_no::text
+  ) THEN
+    RAISE EXCEPTION 'Dimensionen % (%) används på bokförda verifikat och kan inte tas bort — avaktivera den istället.',
+      OLD.sie_dim_no, OLD.name;
+  END IF;
+  RETURN OLD;
+END;
+$$;
+
+CREATE TRIGGER enforce_dimension_registry_guards
+  BEFORE UPDATE OR DELETE ON public.dimensions
+  FOR EACH ROW EXECUTE FUNCTION public.enforce_dimension_registry_guards();
+
+-- 3b. dimension_values: a code referenced by any posted/reversed line cannot be
+-- deleted (BFL 7-year philosophy / Fortnox "Avslutat") — inactivate instead.
+-- Fires on direct DELETE and on cascade from dimensions.
+CREATE OR REPLACE FUNCTION public.enforce_dimension_value_retention()
+RETURNS trigger
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path TO 'public'
+AS $$
+DECLARE
+  v_dim_no int;
+BEGIN
+  SELECT d.sie_dim_no INTO v_dim_no
+  FROM public.dimensions d
+  WHERE d.id = OLD.dimension_id;
+
+  IF v_dim_no IS NULL THEN
+    -- Parent dimension row already gone (same-statement cascade edge) — nothing
+    -- left to validate against.
+    RETURN OLD;
+  END IF;
+
+  IF EXISTS (
+    SELECT 1
+    FROM public.journal_entries je
+    JOIN public.journal_entry_lines jel ON jel.journal_entry_id = je.id
+    WHERE je.company_id = OLD.company_id
+      AND je.status IN ('posted', 'reversed')
+      AND jel.dimensions ->> v_dim_no::text = OLD.code
+  ) THEN
+    RAISE EXCEPTION 'Värdet "%" används på bokförda verifikat och kan inte tas bort — arkivera det istället.',
+      OLD.code;
+  END IF;
+  RETURN OLD;
+END;
+$$;
+
+CREATE TRIGGER enforce_dimension_value_retention
+  BEFORE DELETE ON public.dimension_values
+  FOR EACH ROW EXECUTE FUNCTION public.enforce_dimension_value_retention();
+
+-- =============================================================================
+-- 4. ensure_company_dimensions(company_id) — lazy get-or-create of system dims
+-- =============================================================================
+-- No eager per-company seeding: core stays zero-config for companies that never
+-- touch dimensions. Called by registry CRUD, the engine resolver and SIE import
+-- on first use. Idempotent.
+CREATE OR REPLACE FUNCTION public.ensure_company_dimensions(p_company_id uuid)
+RETURNS void
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path TO 'public'
+AS $$
+BEGIN
+  -- Tenant guard: authenticated callers must be members of the company.
+  -- Service-role callers (auth.uid() IS NULL) are trusted — server code scopes
+  -- by company_id (defense-in-depth pattern used across the codebase).
+  IF auth.uid() IS NOT NULL
+     AND p_company_id NOT IN (SELECT public.user_company_ids()) THEN
+    RAISE EXCEPTION 'not a member of company %', p_company_id;
+  END IF;
+
+  INSERT INTO public.dimensions (company_id, sie_dim_no, name, resets_annually, is_system, sort_order)
+  VALUES
+    (p_company_id, 1, 'Kostnadsställe', true,  true, 10),
+    (p_company_id, 6, 'Projekt',        false, true, 20)
+  ON CONFLICT (company_id, sie_dim_no) DO NOTHING;
+END;
+$$;
+
+REVOKE EXECUTE ON FUNCTION public.ensure_company_dimensions(uuid) FROM PUBLIC, anon;
+GRANT EXECUTE ON FUNCTION public.ensure_company_dimensions(uuid) TO authenticated, service_role;
+
+-- =============================================================================
+-- 5. journal_entry_lines.dimensions — the single source of truth line tag
+-- =============================================================================
+-- NOT NULL DEFAULT '{}' is metadata-only on PG11+ (no table rewrite).
+ALTER TABLE public.journal_entry_lines
+  ADD COLUMN dimensions jsonb NOT NULL DEFAULT '{}'::jsonb;
+ALTER TABLE public.journal_entry_lines
+  ADD CONSTRAINT jel_dimensions_is_object CHECK (jsonb_typeof(dimensions) = 'object');
+
+-- Containment queries (reports: dimensions @> '{"6":"P001"}') ride the GIN;
+-- hot dims 1/6 get partial expression indexes for equality/grouping paths.
+CREATE INDEX idx_jel_dimensions_gin
+  ON public.journal_entry_lines USING gin (dimensions jsonb_path_ops);
+CREATE INDEX idx_jel_dimensions_dim1
+  ON public.journal_entry_lines ((dimensions ->> '1')) WHERE dimensions ? '1';
+CREATE INDEX idx_jel_dimensions_dim6
+  ON public.journal_entry_lines ((dimensions ->> '6')) WHERE dimensions ? '6';
+
+COMMENT ON COLUMN public.journal_entry_lines.dimensions IS
+  'SIE dimension map {sie_dim_no: object_code}, e.g. {"1":"KS01","6":"P001"}. Single source of truth for line dimensions; cost_center/project mirror keys 1/6 during the dual-write window (see lib/bookkeeping/dimension-resolver.ts).';
+COMMENT ON COLUMN public.journal_entry_lines.cost_center IS
+  'Legacy mirror of dimensions->>''1'' (SIE #DIM 1 object code). Kept in sync by lineDimensionColumns(); will become a GENERATED column. NB: the 20240101000011 comment about cost_center_id/project_id FK columns was never true.';
+COMMENT ON COLUMN public.journal_entry_lines.project IS
+  'Legacy mirror of dimensions->>''6'' (SIE #DIM 6 object code). Kept in sync by lineDimensionColumns(); will become a GENERATED column.';
+
+-- =============================================================================
+-- 6. Backfill — representation copy of existing dimension data
+-- =============================================================================
+-- Copies the legacy TEXT columns into the JSONB map on already-posted lines.
+-- This is a pure representation change (same values, new column) with no
+-- accounting content touched — accounts, amounts, dates and descriptions are
+-- byte-identical. The line-immutability trigger blocks ALL updates to posted
+-- lines regardless of column, so it is disabled for exactly this statement
+-- (sanctioned precedent: 20260415000000_schema_sync.sql cleanup routine).
+--
+-- ⚠️ REVIEWER GUIDANCE BEFORE REUSING THIS PATTERN (BFL 5 kap 5§ / BFNAR
+-- 2013:2: corrections must preserve original + change visibly; overwriting
+-- verifikat content is forbidden). Disabling this trigger is defensible ONLY
+-- when ALL of the following hold, as they do here:
+--   1. The UPDATE writes exclusively to a column that carries NO verifikat
+--      content (here: the brand-new `dimensions` column, populated from values
+--      already stored on the same row — no information is created or lost).
+--   2. Accounts, amounts, dates, descriptions and linkage columns are
+--      untouched (this statement's SET clause names only `dimensions`).
+--   3. The change is reviewed under the Swedish-compliance CI workflow.
+-- A migration that needs to touch actual verifikat content must instead go
+-- through storno/rättelse (correctEntry) — never this pattern.
+--
+-- Concurrency: the DISABLE/UPDATE/ENABLE sequence runs inside ONE transaction.
+-- ALTER TABLE ... DISABLE TRIGGER takes ACCESS EXCLUSIVE on the table and
+-- holds it until COMMIT, so no concurrent writer can slip a line in while the
+-- trigger is off — the unguarded window the pattern would otherwise open
+-- during a live deployment does not exist. (If the migration runner already
+-- wraps the file in a transaction, the inner BEGIN degrades to a warning.)
+BEGIN;
+
+ALTER TABLE public.journal_entry_lines
+  DISABLE TRIGGER enforce_journal_entry_line_immutability;
+
+-- NULLIF: an empty-string legacy mirror must not mint a {"n": ""} entry —
+-- normalizeLineDimensions treats empty string as "cleared" and never stores it.
+UPDATE public.journal_entry_lines
+SET dimensions = jsonb_strip_nulls(
+  jsonb_build_object('1', NULLIF(cost_center, ''), '6', NULLIF(project, ''))
+)
+WHERE NULLIF(cost_center, '') IS NOT NULL OR NULLIF(project, '') IS NOT NULL;
+
+ALTER TABLE public.journal_entry_lines
+  ENABLE TRIGGER enforce_journal_entry_line_immutability;
+
+COMMIT;
+
+-- =============================================================================
+-- 7. Migrate the legacy registry tables into the new registry
+-- =============================================================================
+-- cost_centers/projects were company-scoped by 20260330130000 but are
+-- write-dead (no code path ever INSERTed; sie-export is the only reader and is
+-- switched to the new registry in PR2). Copy their rows, then seed system dims
+-- for any company that has legacy registry rows or tagged lines.
+
+-- 7a. System dims for every company with legacy registry rows or tagged lines.
+INSERT INTO public.dimensions (company_id, sie_dim_no, name, resets_annually, is_system, sort_order)
+SELECT DISTINCT src.company_id, 1, 'Kostnadsställe', true, true, 10
+FROM (
+  SELECT company_id FROM public.cost_centers
+  UNION
+  SELECT je.company_id
+  FROM public.journal_entry_lines jel
+  JOIN public.journal_entries je ON je.id = jel.journal_entry_id
+  WHERE jel.cost_center IS NOT NULL
+) src
+ON CONFLICT (company_id, sie_dim_no) DO NOTHING;
+
+INSERT INTO public.dimensions (company_id, sie_dim_no, name, resets_annually, is_system, sort_order)
+SELECT DISTINCT src.company_id, 6, 'Projekt', false, true, 20
+FROM (
+  SELECT company_id FROM public.projects
+  UNION
+  SELECT je.company_id
+  FROM public.journal_entry_lines jel
+  JOIN public.journal_entries je ON je.id = jel.journal_entry_id
+  WHERE jel.project IS NOT NULL
+) src
+ON CONFLICT (company_id, sie_dim_no) DO NOTHING;
+
+-- 7b. Copy legacy registry rows (sanitized to satisfy the code CHECK).
+INSERT INTO public.dimension_values (company_id, dimension_id, code, name, is_active)
+SELECT cc.company_id, d.id,
+       left(regexp_replace(cc.code, '["{}]', '', 'g'), 40),
+       left(cc.name, 120),
+       cc.is_active
+FROM public.cost_centers cc
+JOIN public.dimensions d ON d.company_id = cc.company_id AND d.sie_dim_no = 1
+WHERE char_length(regexp_replace(cc.code, '["{}]', '', 'g')) >= 1
+ON CONFLICT (company_id, dimension_id, code) DO NOTHING;
+
+INSERT INTO public.dimension_values (company_id, dimension_id, code, name, is_active, start_date, end_date)
+SELECT p.company_id, d.id,
+       left(regexp_replace(p.code, '["{}]', '', 'g'), 40),
+       left(p.name, 120),
+       p.is_active, p.start_date, p.end_date
+FROM public.projects p
+JOIN public.dimensions d ON d.company_id = p.company_id AND d.sie_dim_no = 6
+WHERE char_length(regexp_replace(p.code, '["{}]', '', 'g')) >= 1
+ON CONFLICT (company_id, dimension_id, code) DO NOTHING;
+
+-- 7c. Placeholder values for orphaned free-text codes on lines (referential
+-- validity holds retroactively without polluting pickers: is_active = false).
+INSERT INTO public.dimension_values (company_id, dimension_id, code, name, is_active)
+SELECT DISTINCT je.company_id, d.id,
+       left(regexp_replace(jel.cost_center, '["{}]', '', 'g'), 40),
+       jel.cost_center,
+       false
+FROM public.journal_entry_lines jel
+JOIN public.journal_entries je ON je.id = jel.journal_entry_id
+JOIN public.dimensions d ON d.company_id = je.company_id AND d.sie_dim_no = 1
+WHERE jel.cost_center IS NOT NULL
+  AND char_length(regexp_replace(jel.cost_center, '["{}]', '', 'g')) >= 1
+ON CONFLICT (company_id, dimension_id, code) DO NOTHING;
+
+INSERT INTO public.dimension_values (company_id, dimension_id, code, name, is_active)
+SELECT DISTINCT je.company_id, d.id,
+       left(regexp_replace(jel.project, '["{}]', '', 'g'), 40),
+       jel.project,
+       false
+FROM public.journal_entry_lines jel
+JOIN public.journal_entries je ON je.id = jel.journal_entry_id
+JOIN public.dimensions d ON d.company_id = je.company_id AND d.sie_dim_no = 6
+WHERE jel.project IS NOT NULL
+  AND char_length(regexp_replace(jel.project, '["{}]', '', 'g')) >= 1
+ON CONFLICT (company_id, dimension_id, code) DO NOTHING;
+
+-- The legacy cost_centers/projects tables are intentionally NOT dropped here:
+-- sie-export still reads them until PR2 switches it to the new registry. A
+-- follow-up cleanup migration drops both tables after that release.
+
+NOTIFY pgrst, 'reload schema';
diff --git a/tests/pg/dimensions-substrate.pg.test.ts b/tests/pg/dimensions-substrate.pg.test.ts
new file mode 100644
index 00000000..3d707b81
--- /dev/null
+++ b/tests/pg/dimensions-substrate.pg.test.ts
@@ -0,0 +1,295 @@
+import { describe, it, expect } from 'vitest'
+import { randomUUID } from 'node:crypto'
+import { getPool, withUserContext } from './setup'
+import { seedCompany, insertDraftJournalEntry } from './fixtures'
+
+// PR1 dimensions substrate (20260702084500_dimensions_substrate.sql):
+// registry tables + RLS, ensure_company_dimensions RPC, registry guard
+// triggers, jel.dimensions column + CHECK, and — the load-bearing property —
+// that the dimensions map on a POSTED line is frozen by the existing
+// line-immutability trigger with zero new triggers.
+
+async function seedWithDimensions() {
+  const seeded = await seedCompany()
+  await getPool().query(`SELECT public.ensure_company_dimensions($1)`, [seeded.companyId])
+  return seeded
+}
+
+async function getDimensionId(companyId: string, sieDimNo: number): Promise<string> {
+  const { rows } = await getPool().query(
+    `SELECT id FROM public.dimensions WHERE company_id = $1 AND sie_dim_no = $2`,
+    [companyId, sieDimNo],
+  )
+  return rows[0].id
+}
+
+async function insertValue(params: {
+  companyId: string
+  dimensionId: string
+  code: string
+  name?: string
+}): Promise<string> {
+  const id = randomUUID()
+  await getPool().query(
+    `INSERT INTO public.dimension_values (id, company_id, dimension_id, code, name)
+     VALUES ($1, $2, $3, $4, $5)`,
+    [id, params.companyId, params.dimensionId, params.code, params.name ?? params.code],
+  )
+  return id
+}
+
+// Insert a balanced line pair where the debit line carries a dimensions map.
+async function insertDimensionedLines(
+  journalEntryId: string,
+  dimensions: Record<string, string>,
+  amount = 1000,
+): Promise<string> {
+  const lineId = randomUUID()
+  await getPool().query(
+    `INSERT INTO public.journal_entry_lines
+       (id, journal_entry_id, account_number, debit_amount, credit_amount, dimensions, cost_center, project)
+     VALUES ($1, $2, '4010', $3, 0, $4, $5, $6),
+            (gen_random_uuid(), $2, '1930', 0, $3, '{}', NULL, NULL)`,
+    [
+      lineId,
+      journalEntryId,
+      amount,
+      JSON.stringify(dimensions),
+      dimensions['1'] ?? null,
+      dimensions['6'] ?? null,
+    ],
+  )
+  return lineId
+}
+
+async function commitEntry(companyId: string, journalEntryId: string): Promise<void> {
+  await getPool().query(
+    `SELECT voucher_number FROM public.commit_journal_entry($1::uuid, $2::uuid)`,
+    [companyId, journalEntryId],
+  )
+}
+
+describe('ensure_company_dimensions', () => {
+  it('creates system dims 1 and 6 idempotently', async () => {
+    const { companyId } = await seedCompany()
+    await getPool().query(`SELECT public.ensure_company_dimensions($1)`, [companyId])
+    await getPool().query(`SELECT public.ensure_company_dimensions($1)`, [companyId])
+
+    const { rows } = await getPool().query(
+      `SELECT sie_dim_no, name, resets_annually, is_system
+       FROM public.dimensions WHERE company_id = $1 ORDER BY sie_dim_no`,
+      [companyId],
+    )
+    expect(rows).toEqual([
+      { sie_dim_no: 1, name: 'Kostnadsställe', resets_annually: true, is_system: true },
+      { sie_dim_no: 6, name: 'Projekt', resets_annually: false, is_system: true },
+    ])
+  })
+
+  it('rejects an authenticated caller who is not a member of the company', async () => {
+    const { companyId } = await seedCompany()
+    const outsider = await seedCompany()
+
+    await expect(
+      withUserContext(outsider.userId, (client) =>
+        client.query(`SELECT public.ensure_company_dimensions($1)`, [companyId]),
+      ),
+    ).rejects.toThrow(/not a member/)
+  })
+
+  it('allows a member to call it through RLS context', async () => {
+    const { userId, companyId } = await seedCompany()
+    await withUserContext(userId, async (client) => {
+      await client.query(`SELECT public.ensure_company_dimensions($1)`, [companyId])
+      const { rows } = await client.query(
+        `SELECT sie_dim_no FROM public.dimensions WHERE company_id = $1 ORDER BY sie_dim_no`,
+        [companyId],
+      )
+      expect(rows.map((r) => r.sie_dim_no)).toEqual([1, 6])
+    })
+  })
+})
+
+describe('registry RLS', () => {
+  it('hides other companies dimensions and blocks cross-company inserts', async () => {
+    const a = await seedWithDimensions()
+    const b = await seedWithDimensions()
+
+    await withUserContext(a.userId, async (client) => {
+      const { rows } = await client.query(`SELECT company_id FROM public.dimensions`)
+      expect(rows.every((r) => r.company_id === a.companyId)).toBe(true)
+
+      const dimId = await getDimensionId(b.companyId, 6)
+      await expect(
+        client.query(
+          `INSERT INTO public.dimension_values (company_id, dimension_id, code, name)
+           VALUES ($1, $2, 'X', 'X')`,
+          [b.companyId, dimId],
+        ),
+      ).rejects.toThrow(/row-level security/)
+    })
+  })
+
+  it('lets a member manage values in their own company (incl. DELETE)', async () => {
+    const { userId, companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+
+    await withUserContext(userId, async (client) => {
+      await client.query(
+        `INSERT INTO public.dimension_values (company_id, dimension_id, code, name)
+         VALUES ($1, $2, 'P001', 'Projekt Alpha')`,
+        [companyId, dimId],
+      )
+      await client.query(
+        `UPDATE public.dimension_values SET is_active = false
+         WHERE company_id = $1 AND code = 'P001'`,
+        [companyId],
+      )
+      const del = await client.query(
+        `DELETE FROM public.dimension_values WHERE company_id = $1 AND code = 'P001'`,
+        [companyId],
+      )
+      expect(del.rowCount).toBe(1)
+    })
+  })
+})
+
+describe('registry guard triggers', () => {
+  it('blocks deleting a system dimension', async () => {
+    const { companyId } = await seedWithDimensions()
+    await expect(
+      getPool().query(
+        `DELETE FROM public.dimensions WHERE company_id = $1 AND sie_dim_no = 6`,
+        [companyId],
+      ),
+    ).rejects.toThrow(/kan inte tas bort/)
+  })
+
+  it('blocks renumbering a dimension', async () => {
+    const { companyId } = await seedWithDimensions()
+    await expect(
+      getPool().query(
+        `UPDATE public.dimensions SET sie_dim_no = 7 WHERE company_id = $1 AND sie_dim_no = 6`,
+        [companyId],
+      ),
+    ).rejects.toThrow(/kan inte ändras/)
+  })
+
+  it('code CHECK forbids SIE-framing-breaking characters', async () => {
+    const { companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    await expect(
+      insertValue({ companyId, dimensionId: dimId, code: 'P"1' }),
+    ).rejects.toThrow(/check/i)
+  })
+})
+
+describe('dimension_values retention', () => {
+  it('blocks deleting a value referenced by a posted line, allows unreferenced', async () => {
+    const { userId, companyId, fiscalPeriodId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    await insertValue({ companyId, dimensionId: dimId, code: 'P001' })
+    await insertValue({ companyId, dimensionId: dimId, code: 'P002' })
+
+    const entryId = await insertDraftJournalEntry({ userId, companyId, fiscalPeriodId })
+    await insertDimensionedLines(entryId, { '6': 'P001' })
+    await commitEntry(companyId, entryId)
+
+    await expect(
+      getPool().query(
+        `DELETE FROM public.dimension_values WHERE company_id = $1 AND code = 'P001'`,
+        [companyId],
+      ),
+    ).rejects.toThrow(/arkivera/)
+
+    const del = await getPool().query(
+      `DELETE FROM public.dimension_values WHERE company_id = $1 AND code = 'P002'`,
+      [companyId],
+    )
+    expect(del.rowCount).toBe(1)
+  })
+
+  it('blocks deleting a non-system dimension whose number is on posted lines (cascade path)', async () => {
+    const { userId, companyId, fiscalPeriodId } = await seedWithDimensions()
+    // Custom dim 20 with a tagged, posted line
+    await getPool().query(
+      `INSERT INTO public.dimensions (company_id, sie_dim_no, name) VALUES ($1, 20, 'Avdelning')`,
+      [companyId],
+    )
+    const entryId = await insertDraftJournalEntry({ userId, companyId, fiscalPeriodId })
+    await insertDimensionedLines(entryId, { '20': 'SYD' })
+    await commitEntry(companyId, entryId)
+
+    await expect(
+      getPool().query(
+        `DELETE FROM public.dimensions WHERE company_id = $1 AND sie_dim_no = 20`,
+        [companyId],
+      ),
+    ).rejects.toThrow(/kan inte tas bort/)
+  })
+})
+
+describe('journal_entry_lines.dimensions', () => {
+  it('rejects non-object values via CHECK', async () => {
+    const { userId, companyId, fiscalPeriodId } = await seedCompany()
+    const entryId = await insertDraftJournalEntry({ userId, companyId, fiscalPeriodId })
+    await expect(
+      getPool().query(
+        `INSERT INTO public.journal_entry_lines
+           (journal_entry_id, account_number, debit_amount, credit_amount, dimensions)
+         VALUES ($1, '1930', 100, 0, '["not","a","map"]')`,
+        [entryId],
+      ),
+    ).rejects.toThrow(/jel_dimensions_is_object/)
+  })
+
+  it('defaults to {} so dimension-less writers stay valid', async () => {
+    const { userId, companyId, fiscalPeriodId } = await seedCompany()
+    const entryId = await insertDraftJournalEntry({ userId, companyId, fiscalPeriodId })
+    await getPool().query(
+      `INSERT INTO public.journal_entry_lines
+         (journal_entry_id, account_number, debit_amount, credit_amount)
+       VALUES ($1, '1930', 100, 0), ($1, '3001', 0, 100)`,
+      [entryId],
+    )
+    const { rows } = await getPool().query(
+      `SELECT dimensions FROM public.journal_entry_lines WHERE journal_entry_id = $1`,
+      [entryId],
+    )
+    expect(rows.map((r) => r.dimensions)).toEqual([{}, {}])
+  })
+
+  it('is mutable on drafts but frozen on posted lines (inherits immutability)', async () => {
+    const { userId, companyId, fiscalPeriodId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    await insertValue({ companyId, dimensionId: dimId, code: 'P001' })
+
+    const entryId = await insertDraftJournalEntry({ userId, companyId, fiscalPeriodId })
+    const lineId = await insertDimensionedLines(entryId, { '6': 'P001' })
+
+    // Draft: retagging is allowed
+    await getPool().query(
+      `UPDATE public.journal_entry_lines SET dimensions = '{"6":"P002"}', project = 'P002' WHERE id = $1`,
+      [lineId],
+    )
+
+    await commitEntry(companyId, entryId)
+
+    // Posted: ANY update of the dimensions map is blocked by the existing trigger
+    await expect(
+      getPool().query(
+        `UPDATE public.journal_entry_lines SET dimensions = '{"6":"P999"}' WHERE id = $1`,
+        [lineId],
+      ),
+    ).rejects.toThrow(/Cannot UPDATE lines of a posted journal entry/)
+
+    // And the committed map survived intact
+    const { rows } = await getPool().query(
+      `SELECT dimensions, cost_center, project FROM public.journal_entry_lines WHERE id = $1`,
+      [lineId],
+    )
+    expect(rows[0].dimensions).toEqual({ '6': 'P002' })
+    expect(rows[0].project).toBe('P002')
+    expect(rows[0].cost_center).toBeNull()
+  })
+})
diff --git a/types/index.ts b/types/index.ts
index 35928a22..9fad5b40 100644
--- a/types/index.ts
+++ b/types/index.ts
@@ -1364,6 +1364,10 @@ export interface JournalEntryLine {
   exchange_rate: number | null
   line_description: string | null
   tax_code: string | null
+  // SIE dimension map {sie_dim_no: object_code}, e.g. {"1":"KS01","6":"P001"}.
+  // Source of truth; cost_center/project mirror keys '1'/'6'. Optional so
+  // pre-migration fixtures and partial selects stay type-valid.
+  dimensions?: Record<string, string>
   cost_center: string | null
   project: string | null
   sort_order: number
@@ -1705,7 +1709,12 @@ export interface CreateJournalEntryLineInput {
   amount_in_currency?: number
   exchange_rate?: number
   tax_code?: string
+  // SIE dimension map {sie_dim_no: object_code}. Wins per key over the
+  // deprecated cost_center/project aliases (normalizeLineDimensions).
+  dimensions?: Record<string, string>
+  /** @deprecated alias for dimensions['1'] — kept for API/MCP compatibility */
   cost_center?: string
+  /** @deprecated alias for dimensions['6'] — kept for API/MCP compatibility */
   project?: string
 }