diff --git a/components/onboarding/journey/OnboardingJourney.tsx b/components/onboarding/journey/OnboardingJourney.tsx
index ea88094f..cb3f7a4c 100644
--- a/components/onboarding/journey/OnboardingJourney.tsx
+++ b/components/onboarding/journey/OnboardingJourney.tsx
@@ -514,7 +514,9 @@ export default function OnboardingJourney({
           ? parseStartMonthDay(state.ticLookup?.fiscalYear?.startMonthDay)
           : null
         const firstYearSuggested = state.lookupRan
-          ? deriveFirstYearDefaults(state.ticLookup?.registrationDate).isFirstFiscalYear
+          ? deriveFirstYearDefaults(state.ticLookup?.registrationDate, Date.now(), {
+              noClosedPeriod: state.ticLookup?.fiscalYear == null,
+            }).isFirstFiscalYear
           : false
         const confirmMode = startMonth !== null
         const title = confirmMode ? t('journey_fy_confirm_title') : t('journey_fy_ask_title')
diff --git a/extensions/general/mcp-server/__tests__/lookup-company.test.ts b/extensions/general/mcp-server/__tests__/lookup-company.test.ts
index 6a3586b3..e898a163 100644
--- a/extensions/general/mcp-server/__tests__/lookup-company.test.ts
+++ b/extensions/general/mcp-server/__tests__/lookup-company.test.ts
@@ -153,6 +153,19 @@ describe('gnubok_lookup_company', () => {
     expect('fiscal_year_start_month' in suggested).toBe(false)
   })
 
+  it('treats no-closed-period as first year up to 18 months after registration (extended first year)', async () => {
+    // The Arcim case: registered 13 months ago, no annual report filed, first
+    // räkenskapsår runs to 31 Dec under the BFL 3 kap 3 § 18-month cap.
+    mocks.lookupCompanyByOrgNumber.mockResolvedValue(
+      found({ fiscalYear: null, registrationDate: Date.now() - 396 * 24 * 60 * 60 * 1000 })
+    )
+    const result = await run('5560000001')
+
+    const ask = result.still_to_ask as string[]
+    expect(ask.some((q) => q.includes('first_fiscal_year'))).toBe(true)
+    expect(ask.some((q) => q.includes('calendar year or broken year'))).toBe(false)
+  })
+
   it('returns not_found with the full question list when no company matches', async () => {
     mocks.lookupCompanyByOrgNumber.mockResolvedValue(null)
     const result = await run('5560000001')
diff --git a/extensions/general/mcp-server/__tests__/payload-size.bench.test.ts b/extensions/general/mcp-server/__tests__/payload-size.bench.test.ts
index bb7994c7..7f639562 100644
--- a/extensions/general/mcp-server/__tests__/payload-size.bench.test.ts
+++ b/extensions/general/mcp-server/__tests__/payload-size.bench.test.ts
@@ -226,9 +226,13 @@ describe('tools/list payload size guard', () => {
     //     tools; the onboarding skill's first instruction is to call it, and
     //     a search-only tool is uncallable on Claude.ai. Descriptions were
     //     trimmed first; the tool costs ~265 tokens against ~0 headroom.
+    //   * 61.5K to 62K with gnubok_sie_preflight (migration-first onboarding):
+    //     the scan-before-import step the skill instructs for shared SIE
+    //     files, so default-catalog for the same Claude.ai reason; ~390
+    //     tokens (schema carries the mappings-passthrough contract).
     // Long-term answer to growth is leaning harder on gnubok_search_tools: if this
     // fires again, prefer trimming descriptions or making a tool opt-in via search
     // before bumping further.
-    expect(approxTokens).toBeLessThan(61_500)
+    expect(approxTokens).toBeLessThan(62_000)
   })
 })
diff --git a/extensions/general/mcp-server/__tests__/sie-preflight.test.ts b/extensions/general/mcp-server/__tests__/sie-preflight.test.ts
new file mode 100644
index 00000000..81c4e158
--- /dev/null
+++ b/extensions/general/mcp-server/__tests__/sie-preflight.test.ts
@@ -0,0 +1,187 @@
+/**
+ * gnubok_sie_preflight: the read-only "does this file look correct" scan
+ * that runs before gnubok_import_sie. Uses the real SIE parser on inline
+ * fixture files; only the Supabase lookups (company orgnr, duplicate
+ * imports, stored mappings) are mocked.
+ */
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { tools } from '../server'
+
+const tool = tools.find((t) => t.name === 'gnubok_sie_preflight')!
+const COMPANY_ID = '11111111-1111-4111-8111-111111111111'
+
+// Current-year fiscal year: a past year would trigger the (correct)
+// "omföring av årets resultat saknas" warning and turn verdict ok into
+// ok_with_warnings.
+const YEAR = new Date().getFullYear()
+const VALID_SIE = [
+  '#FLAGGA 0',
+  '#FORMAT PC8',
+  '#SIETYP 4',
+  '#FNAMN "Testbolaget AB"',
+  '#ORGNR 556000-0001',
+  `#RAR 0 ${YEAR}0101 ${YEAR}1231`,
+  '#KONTO 1930 "Bank"',
+  '#KONTO 3001 "Försäljning"',
+  `#VER A 1 ${YEAR}0115 "Faktura 1"`,
+  '{',
+  '#TRANS 1930 {} 1000.00',
+  '#TRANS 3001 {} -1000.00',
+  '}',
+].join('\n')
+
+const UNBALANCED_SIE = VALID_SIE.replace('#TRANS 3001 {} -1000.00', '#TRANS 3001 {} -900.00')
+
+type MockConfig = {
+  companyOrg?: string | null
+  duplicateFile?: { id: string; imported_at: string } | null
+  duplicatePeriod?: Record<string, unknown> | null
+}
+
+function mockSupabase(config: MockConfig = {}) {
+  const makeChain = (table: string) => {
+    const called: string[] = []
+    const chain: Record<string | symbol, unknown> = new Proxy(
+      {},
+      {
+        get(_t, prop: string | symbol) {
+          if (prop === 'then') {
+            return (resolve: (v: unknown) => void) => {
+              if (table === 'companies') {
+                resolve({
+                  data: {
+                    org_number: 'companyOrg' in config ? config.companyOrg : '5560000001',
+                  },
+                  error: null,
+                })
+              } else if (table === 'sie_imports') {
+                if (called.includes('single')) {
+                  resolve({ data: config.duplicateFile ?? null, error: null })
+                } else {
+                  resolve({
+                    data: config.duplicatePeriod ? [config.duplicatePeriod] : [],
+                    error: null,
+                  })
+                }
+              } else {
+                resolve({ data: [], error: null })
+              }
+            }
+          }
+          return (..._args: unknown[]) => {
+            called.push(String(prop))
+            return chain
+          }
+        },
+      }
+    )
+    return chain
+  }
+  return { from: (table: string) => makeChain(table) }
+}
+
+async function run(args: Record<string, unknown>, config?: MockConfig) {
+  return (await tool.execute(
+    { filename: 'export.se', ...args },
+    COMPANY_ID,
+    'user-1',
+    mockSupabase(config) as never
+  )) as Record<string, unknown>
+}
+
+describe('gnubok_sie_preflight', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('is a read-only tool that stages nothing', () => {
+    expect(tool).toBeDefined()
+    expect(tool.annotations.readOnlyHint).toBe(true)
+    expect(tool.annotations.destructiveHint).toBe(false)
+  })
+
+  it('passes a clean matching file with verdict ok and passthrough mappings', async () => {
+    const result = await run({ file_content: VALID_SIE })
+
+    expect(result.verdict).toBe('ok')
+    const file = result.file as Record<string, unknown>
+    expect(file.company_name).toBe('Testbolaget AB')
+    expect(file.voucher_count).toBe(1)
+    expect((result.org_number_match as Record<string, unknown>).match).toBe(true)
+    expect(result.duplicate).toBeNull()
+    // Mappings are shaped for direct passthrough to gnubok_import_sie.
+    const mappings = result.mappings as Array<Record<string, unknown>>
+    expect(mappings.length).toBeGreaterThan(0)
+    expect(mappings[0]).toHaveProperty('sourceAccount')
+    expect(mappings[0]).toHaveProperty('targetAccount')
+  })
+
+  it('flags an org-number mismatch as another company\'s bookkeeping', async () => {
+    const result = await run({ file_content: VALID_SIE }, { companyOrg: '5599999999' })
+
+    expect(result.verdict).toBe('ok_with_warnings')
+    expect((result.org_number_match as Record<string, unknown>).match).toBe(false)
+    expect(result.instructions).toContain('STOP')
+  })
+
+  it('reports unverified instead of ok when the company has no org number', async () => {
+    const result = await run({ file_content: VALID_SIE }, { companyOrg: null })
+    const match = result.org_number_match as Record<string, unknown>
+    expect(match.verified).toBe(false)
+    expect(match.match).toBeNull()
+  })
+
+  it('returns verdict invalid with the balance error for an unbalanced voucher', async () => {
+    const result = await run({ file_content: UNBALANCED_SIE })
+
+    expect(result.verdict).toBe('invalid')
+    const validation = result.validation as { errors: string[] }
+    expect(validation.errors.length).toBeGreaterThan(0)
+    expect(result.instructions).toContain('do not import')
+  })
+
+  it('returns verdict duplicate when the same file hash is already imported', async () => {
+    const result = await run(
+      { file_content: VALID_SIE },
+      { duplicateFile: { id: 'imp-1', imported_at: '2026-08-01T00:00:00Z' } }
+    )
+
+    expect(result.verdict).toBe('duplicate')
+    expect((result.duplicate as Record<string, unknown>).kind).toBe('file')
+  })
+
+  it('returns verdict duplicate when the fiscal year overlaps a completed import', async () => {
+    const result = await run(
+      { file_content: VALID_SIE },
+      {
+        duplicatePeriod: {
+          id: 'imp-2',
+          fiscal_year_start: '2025-01-01',
+          fiscal_year_end: '2025-12-31',
+          imported_at: '2026-08-01T00:00:00Z',
+        },
+      }
+    )
+
+    expect(result.verdict).toBe('duplicate')
+    expect((result.duplicate as Record<string, unknown>).kind).toBe('period')
+  })
+
+  it('decodes CP437 bytes from file_content_base64 so åäö survive', async () => {
+    // 'Försäljning' and 'Testbolaget' with CP437 bytes: ö=0x94, ä=0x84.
+    const cp437 = Buffer.from(
+      VALID_SIE.replace(/ö/g, '\x94').replace(/ä/g, '\x84').replace(/å/g, '\x86'),
+      'latin1'
+    )
+    const result = await run({ file_content_base64: cp437.toString('base64') })
+
+    expect(result.verdict).toBe('ok')
+    const mappings = result.mappings as Array<{ sourceName?: string }>
+    expect(mappings.some((m) => m.sourceName === 'Försäljning')).toBe(true)
+  })
+
+  it('rejects a call with neither content field', async () => {
+    await expect(run({})).rejects.toMatchObject({ code: 'VALIDATION_ERROR' })
+  })
+})
diff --git a/extensions/general/mcp-server/server.ts b/extensions/general/mcp-server/server.ts
index abdc1c18..44f54f50 100644
--- a/extensions/general/mcp-server/server.ts
+++ b/extensions/general/mcp-server/server.ts
@@ -1444,6 +1444,26 @@ export function isDefaultCatalogTool(tool: { catalogVisibility?: 'default' | 'se
   return tool.catalogVisibility !== 'search'
 }
 
+/**
+ * Resolve SIE file content from tool args: plain text (the model read the
+ * attachment) or base64 (exact bytes, e.g. from a code-execution sandbox).
+ * The base64 path runs the same encoding detection as the HTTP upload route,
+ * so CP437 exports keep their åäö instead of arriving pre-mangled through a
+ * host's UTF-8 read. Returns null when neither field is usable.
+ */
+async function decodeSieToolContent(args: Record<string, unknown>): Promise<string | null> {
+  if (typeof args.file_content === 'string' && args.file_content.length > 0) {
+    return args.file_content
+  }
+  if (typeof args.file_content_base64 === 'string' && args.file_content_base64.length > 0) {
+    const { detectEncoding, decodeBuffer } = await import('@/lib/import/sie-parser')
+    const buffer = Buffer.from(args.file_content_base64, 'base64')
+    const arrayBuffer = buffer.buffer.slice(buffer.byteOffset, buffer.byteOffset + buffer.byteLength)
+    return decodeBuffer(arrayBuffer, detectEncoding(arrayBuffer))
+  }
+  return null
+}
+
 /**
  * Absolute origin for links the user opens in a browser. A deployment
  * without NEXT_PUBLIC_APP_URL falls back to localhost in getCanonicalBaseUrl,
@@ -3109,14 +3129,22 @@ export const tools: McpTool[] = [
       stillToAsk.push('accounting_method (accrual = faktureringsmetoden, cash = kontantmetoden; never guess)')
 
       const startMonth = parseStartMonthDay(lookup.fiscalYear?.startMonthDay)
-      const firstYear = deriveFirstYearDefaults(lookup.registrationDate)
+      // No closed fiscal period in the registry = no annual report filed yet
+      // = still in the first räkenskapsår, which BFL 3 kap 3 § lets run up to
+      // 18 months. The 12-month window alone misses extended first years.
+      const firstYear = deriveFirstYearDefaults(lookup.registrationDate, Date.now(), {
+        noClosedPeriod: lookup.fiscalYear == null,
+      })
+      const registrationIso = lookup.registrationDate
+        ? new Date(lookup.registrationDate).toISOString().slice(0, 10)
+        : null
       if (startMonth !== null) {
         stillToAsk.push(
           `fiscal year: registry shows ${lookup.fiscalYear?.startMonthDay} to ${lookup.fiscalYear?.endMonthDay}; ask "stämmer detta?" instead of an open question`
         )
       } else if (firstYear.isFirstFiscalYear) {
         stillToAsk.push(
-          `fiscal year: company registered recently; suggest a first fiscal year starting ${firstYear.firstYearStart} (first_fiscal_year start/end; an enskild firma's first year must end 31 December)`
+          `fiscal year: no closed period in the registry, so this is the FIRST räkenskapsår; suggest first_fiscal_year start ${registrationIso ?? firstYear.firstYearStart} (registration date) and end 31 December (max 18 months from start; an enskild firma's first year must end 31 December), ask only "stämmer det?"`
         )
       } else {
         stillToAsk.push('fiscal year: calendar year or broken year (no registry data)')
@@ -3325,7 +3353,7 @@ export const tools: McpTool[] = [
         instructions:
           active.length > 0
             ? 'At least one bank is connected and syncing. To add another bank, give the user the connect_url.'
-            : 'On claude.ai/Claude Desktop a connect card with an open-in-browser button is rendered with this result; on other clients give the user the connect_url as a link. They must be logged in to Accounted there, pick their bank and approve with BankID; consent lasts up to 180 days and the first transactions arrive within a minute. Tell them to come back here when done, then continue with gnubok_list_uncategorized_transactions.',
+            : 'On claude.ai/Claude Desktop a connect card with an open-in-browser button is rendered with this result; on other clients give the user the connect_url as a link. They must be logged in to Accounted there, pick their bank, approve with BankID (consent up to 180 days), then CONFIRM WHICH ACCOUNTS to sync in the dialog that opens; the first transactions arrive within a minute of that save. Banks cap PSD2 history (often ~90 days): older history comes via SIE import, not the bank. When the user is back, call this tool again to verify status=active, then continue straight to gnubok_list_uncategorized_transactions without asking.',
       }
     },
   },
@@ -16119,15 +16147,179 @@ export const tools: McpTool[] = [
     },
   },
 
+  {
+    name: 'gnubok_sie_preflight',
+    title: 'SIE Preflight Scan',
+    description:
+      'Scan a SIE file BEFORE import: parse, validate (balances, IB, encoding), duplicate check, orgnr match against the company, suggested account mappings. Read-only, stages nothing. Call FIRST when the user shares a SIE file; pass the returned mappings to gnubok_import_sie.',
+    inputSchema: {
+      type: 'object',
+      additionalProperties: false,
+      properties: {
+        file_content: { type: 'string', description: 'Full SIE file contents as text' },
+        file_content_base64: { type: 'string', description: 'Exact file bytes base64-encoded (preferred when available: preserves CP437 åäö)' },
+        filename: { type: 'string' },
+      },
+      required: ['filename'],
+    },
+    outputSchema: {
+      type: 'object',
+      properties: {
+        verdict: { type: 'string', enum: ['ok', 'ok_with_warnings', 'invalid', 'duplicate'] },
+        file: { type: 'object' },
+        validation: { type: 'object' },
+        org_number_match: { type: ['object', 'null'] },
+        duplicate: { type: ['object', 'null'] },
+        mappings: { type: 'array', items: { type: 'object' } },
+        mapping_stats: { type: 'object' },
+        instructions: { type: 'string' },
+      },
+      required: ['verdict', 'file', 'validation', 'org_number_match', 'duplicate', 'mappings', 'mapping_stats', 'instructions'],
+    },
+    annotations: {
+      readOnlyHint: true,
+      destructiveHint: false,
+      idempotentHint: true,
+      openWorldHint: false,
+    },
+    async execute(args, companyId, _userId, supabase) {
+      const content = await decodeSieToolContent(args)
+      if (!content) {
+        throw Object.assign(
+          new Error('Provide the SIE file as file_content (text) or file_content_base64 (exact bytes).'),
+          { code: 'VALIDATION_ERROR' }
+        )
+      }
+
+      const { parseSIEFile, validateSIEFile } = await import('@/lib/import/sie-parser')
+      const { suggestMappings, getMappingStats, isSystemAccount } = await import('@/lib/import/account-mapper')
+      const { scanSieForCp1252Artifacts, formatSieArtifactWarning } = await import('@/lib/import/sie-artifact-scan')
+      const { generateImportPreview, checkDuplicateImport, checkDuplicatePeriodImport } = await import('@/lib/import/sie-import')
+      const { BAS_REFERENCE } = await import('@/lib/bookkeeping/bas-data')
+
+      let parsed
+      try {
+        parsed = parseSIEFile(content)
+      } catch (e) {
+        throw Object.assign(
+          new Error(`SIE-filen kunde inte tolkas: ${e instanceof Error ? e.message : 'okänt fel'}`),
+          { code: 'VALIDATION_ERROR' }
+        )
+      }
+
+      // Mojibake tripwire (warn, never block): a host that read CP437 bytes
+      // as UTF-8/Latin-1 mangles åäö in names. Numbers and structure survive,
+      // so the import still balances; the user decides whether names matter.
+      const artifactScan = scanSieForCp1252Artifacts(parsed)
+      const encodingWarnings: string[] = []
+      if (artifactScan.flagged) {
+        encodingWarnings.push(
+          `${formatSieArtifactWarning(artifactScan)} Tip: re-share the file as file_content_base64 to preserve the original bytes.`
+        )
+      }
+
+      const validation = validateSIEFile(parsed)
+
+      // Wrong-company tripwire: importing another company's bookkeeping is
+      // the worst silent failure this flow can have. Digits-only comparison;
+      // missing on either side reports unverified instead of ok.
+      const { data: companyRow } = await supabase
+        .from('companies')
+        .select('org_number')
+        .eq('id', companyId)
+        .maybeSingle()
+      const companyOrg = ((companyRow as { org_number?: string | null } | null)?.org_number ?? '').replace(/\D/g, '')
+      const fileOrg = (parsed.header.orgNumber ?? '').replace(/\D/g, '')
+      const orgMatch =
+        companyOrg && fileOrg
+          ? { verified: true, match: companyOrg === fileOrg, company_org_number: companyOrg, file_org_number: fileOrg }
+          : { verified: false, match: null, company_org_number: companyOrg || null, file_org_number: fileOrg || null }
+
+      const duplicateFile = await checkDuplicateImport(supabase, companyId, content)
+      let duplicatePeriod = null
+      if (!duplicateFile && parsed.stats.fiscalYearStart && parsed.stats.fiscalYearEnd) {
+        duplicatePeriod = await checkDuplicatePeriodImport(
+          supabase,
+          companyId,
+          parsed.stats.fiscalYearStart,
+          parsed.stats.fiscalYearEnd
+        )
+      }
+      const duplicate = duplicateFile
+        ? { kind: 'file', import_id: duplicateFile.id, imported_at: duplicateFile.imported_at }
+        : duplicatePeriod
+          ? {
+              kind: 'period',
+              import_id: duplicatePeriod.id,
+              fiscal_year_start: duplicatePeriod.fiscal_year_start,
+              fiscal_year_end: duplicatePeriod.fiscal_year_end,
+              imported_at: duplicatePeriod.imported_at,
+            }
+          : null
+
+      const bookkeepingAccounts = parsed.accounts.filter((a) => !isSystemAccount(a.number))
+      const { data: storedMappings } = await supabase
+        .from('sie_account_mappings')
+        .select('*')
+        .eq('company_id', companyId)
+      const mappings = suggestMappings(
+        bookkeepingAccounts,
+        BAS_REFERENCE,
+        (storedMappings as import('@/lib/import/types').SIEAccountMappingRecord[]) || undefined
+      )
+      const mappingStats = getMappingStats(mappings)
+      const preview = generateImportPreview(parsed, mappings)
+
+      const allWarnings = [...validation.warnings, ...encodingWarnings]
+      const verdict = !validation.valid
+        ? 'invalid'
+        : duplicate
+          ? 'duplicate'
+          : allWarnings.length > 0 || orgMatch.match === false
+            ? 'ok_with_warnings'
+            : 'ok'
+
+      return {
+        verdict,
+        file: {
+          filename: args.filename,
+          company_name: parsed.header.companyName,
+          org_number: parsed.header.orgNumber,
+          sie_type: parsed.header.sieType,
+          source_program: parsed.header.program ?? null,
+          fiscal_year: { start: parsed.stats.fiscalYearStart, end: parsed.stats.fiscalYearEnd },
+          account_count: bookkeepingAccounts.length,
+          voucher_count: parsed.stats.totalVouchers,
+          transaction_line_count: parsed.stats.totalTransactionLines,
+          opening_balance_total: preview.openingBalanceTotal ?? null,
+        },
+        validation: { valid: validation.valid, errors: validation.errors, warnings: allWarnings },
+        org_number_match: orgMatch,
+        duplicate,
+        mappings,
+        mapping_stats: mappingStats,
+        instructions:
+          verdict === 'invalid'
+            ? 'The file has blocking errors: report them to the user and do not import. A fresh export from the source system usually fixes them.'
+            : verdict === 'duplicate'
+              ? 'This file or fiscal year is already imported. Report it; use gnubok_undo_sie_import first if the user wants to replace it.'
+              : orgMatch.match === false
+                ? 'STOP: the file belongs to a different organisation than this company. Confirm with the user before any import.'
+                : 'Summarize the scan for the user (source system, fiscal year, voucher count, balance status, any warnings). On their go-ahead call gnubok_import_sie with this same file and the returned mappings; the import stages for approval.',
+      }
+    },
+  },
+
   {
     name: 'gnubok_import_sie',
     title: 'Import SIE File',
-    description: 'Stage SIE-file import (types 1-4, CP437/UTF-8/Latin-1). On commit creates fiscal period, opening balances, and journal entries. High-risk, always staged.',
+    description: 'Stage SIE-file import (types 1-4, CP437/UTF-8/Latin-1). On commit creates fiscal period, opening balances, and journal entries. High-risk, always staged. Run gnubok_sie_preflight first for the scan and the mappings.',
     inputSchema: {
       type: 'object',
       additionalProperties: false,
       properties: {
         file_content: { type: 'string', description: 'Full SIE file contents' },
+        file_content_base64: { type: 'string', description: 'Exact file bytes base64-encoded; alternative to file_content (preserves CP437 åäö)' },
         filename: { type: 'string', description: 'Original filename' },
         mappings: {
           type: 'array',
@@ -16141,17 +16333,19 @@ export const tools: McpTool[] = [
         opening_balance_series: { type: 'string', description: 'Series for the IB voucher; default avoids series used by the file' },
         update_account_names: { type: 'boolean', description: 'Use #KONTO names from the file for created and existing accounts (default true). Set false to keep BAS default names.' },
       },
-      required: ['file_content', 'filename', 'mappings'],
+      required: ['filename', 'mappings'],
     },
     outputSchema: STAGED_OPERATION_SCHEMA,
     annotations: { readOnlyHint: false, destructiveHint: false, idempotentHint: false, openWorldHint: false },
     async execute(args, companyId, userId, supabase, actor) {
-      const fileContent = args.file_content as string
+      // Decoded once here; the staged params carry the decoded text so
+      // commitImportSie re-parses exactly what was previewed.
+      const fileContent = await decodeSieToolContent(args)
       const filename = args.filename as string
       const mappings = args.mappings as unknown[] | undefined
 
       if (!fileContent || !filename || !Array.isArray(mappings)) {
-        throw new Error('file_content, filename, and mappings are required')
+        throw new Error('file_content (or file_content_base64), filename, and mappings are required')
       }
 
       // Parse + validate at stage time so the approver sees real content (which
diff --git a/extensions/general/mcp-server/skills/onboarding.ts b/extensions/general/mcp-server/skills/onboarding.ts
index cde6159e..ad258963 100644
--- a/extensions/general/mcp-server/skills/onboarding.ts
+++ b/extensions/general/mcp-server/skills/onboarding.ts
@@ -8,6 +8,14 @@ legally need a human with BankID: connecting (creating the account),
 approving the bank consent, and authorising Skatteverket. Everything else
 happens here.
 
+**Momentum rule: one confirmation, then keep going.** The only stop in this
+flow is the create-company preview ("stämmer detta? ja"). Never end a turn
+with "säg till när du vill fortsätta": after the confirm, call the connect
+tools immediately; when the user reports a connection done, verify it and
+continue straight into import or categorization. Staged writes still go
+through their normal approval, but that approval IS the conversation, not an
+extra pause around it.
+
 ## When to use
 
 - "Sätt upp bokföring för mitt AB / min enskilda firma"
@@ -24,105 +32,103 @@ inget konto skapar du det där (BankID eller e-post), det tar en minut." The
 call is retried automatically once connected. Do not send the user to the web
 app to sign up first.
 
-## Step 1: ask for the organisationsnummer, then look it up
+## Step 1: TWO opening questions, then look up
 
-Ask for ONE thing first: the **organisationsnummer** (10 digits; an enskild
-firma's org number is the owner's personnummer, fine to use here). Then call
-\`gnubok_lookup_company\` with it. This mirrors the in-app wizard: the public
-registry answers most of the questions, so the user confirms facts instead of
-filling in a form.
+Open with exactly two questions, together:
 
-The result carries three parts; use them exactly as intended:
+1. **Organisationsnummer?** (10 digits; an enskild firma's org number is the
+   owner's personnummer, fine to use here)
+2. **Har du bokfört i ett annat system tidigare?** (Fortnox, Visma, Bokio,
+   Björn Lundén, Briox, Wint, annat system, eller helt nytt bolag)
 
-- \`company\`: the registry facts. Present them as a SHORT summary for the
-  user to confirm: "Jag hittade Example AB, Storgatan 1 i Stockholm,
-  godkänd för F-skatt och momsregistrerad. Stämmer det?" Do NOT re-ask
-  what the registry already answered.
-- \`suggested_create_company_input\`: prefilled arguments for
-  \`gnubok_create_company\`. Merge the user's remaining answers into it.
-- \`still_to_ask\`: the questions the registry could not answer. Ask exactly
-  these and nothing more.
+Then call \`gnubok_lookup_company\` with the org number. The registry answers
+most of the form; present the facts as a SHORT summary to confirm ("Jag
+hittade Example AB, Storgatan 1 i Stockholm, godkänd för F-skatt och
+momsregistrerad. Stämmer det?") and ask ONLY what \`still_to_ask\` lists.
+Never re-ask what the registry answered.
 
 Rules baked into that split (same as the web onboarding):
 
 - **F-skatt** from the registry is a fact, both true and false.
 - **VAT** is a fact ONLY when positively registered. "No VAT registration
   found" is a question, never an assumption (ML 17 kap 24 §).
-- **Moms period** (\`monthly\`/\`quarterly\`/\`yearly\`) and **accounting
-  method** (\`accrual\`/\`cash\`) are ALWAYS the user's answer. Rules of
-  thumb if they are unsure: turnover under 1 MSEK may report VAT yearly,
-  under 40 MSEK quarterly, above that monthly (Skatteverket's registration
-  decision states the actual period); cash method is only allowed under
-  3 MSEK turnover and is common for small enskild firma, AB with invoices
-  usually run accrual.
-- **Enskild firma name**: the verksamhetsnamn is freely choosable; suggest
-  the registered name but let the user pick. An AB's registered name is a
-  fact.
-- **Fiscal year**: when the registry shows one, confirm it ("Ert
-  räkenskapsår är 1 januari till 31 december, stämmer det?") instead of
-  asking openly. For a company registered within the last 12 months,
-  suggest a first fiscal year from the registration date and pass
-  \`first_fiscal_year\` (BFL 3 kap.: it may be shorter than 12 months or up
-  to 18 months). Enskild firma is always calendar-year and its first year
-  always ends 31 December.
+- **Moms period** and **accounting method** are ALWAYS the user's answer.
+  Rules of thumb if unsure: under 1 MSEK turnover may report VAT yearly,
+  under 40 MSEK quarterly, above monthly; cash method only under 3 MSEK.
+- **Enskild firma name**: verksamhetsnamnet is freely choosable; suggest the
+  registered name but let the user pick. An AB's registered name is a fact.
+- **Fiscal year**: registry data becomes a confirm question, never an open
+  one. No closed period in the registry = FIRST räkenskapsår: suggest
+  registration date to 31 December (up to 18 months, BFL 3 kap 3 §).
 
-If the lookup returns \`not_found\` or \`unavailable\`, fall back to asking
-each question in \`still_to_ask\` (the full list) and continue; the flow is
-the same, just without prefill. A brand-new registration can take days to
-appear in the registry.
+\`not_found\`/\`unavailable\`: fall back to asking the \`still_to_ask\` list and
+continue. Only \`aktiebolag\` and \`enskild_firma\` are supported today.
 
-Only \`aktiebolag\` and \`enskild_firma\` are supported today; HB/KB/förening
-are not. A company marked CEASED in the registry: surface the warning, but
-the user may continue (they know their company best).
+## Step 2: preview, ONE confirm, create, keep moving
 
-## Step 2: preview, confirm, create
+Call \`gnubok_create_company\` WITHOUT \`confirm\`. Read the preview back in
+plain Swedish (form, orgnr, fiscal period dates, VAT + period, method).
+After the user's "ja": call it again with \`confirm: true\`, and in the SAME
+turn continue with step 3 or 4. Creation sets up chart, settings, first
+fiscal period, tax deadlines, and starts the 30-day trial; the connection
+uses the new company automatically.
 
-Call \`gnubok_create_company\` WITHOUT \`confirm\` first. Read the preview back
-to the user in plain Swedish: company form, org number, the fiscal period
-dates, VAT setup (registered + period), method. Only after an explicit "ja"
-call it again with the same arguments and \`confirm: true\`.
+## Step 3 (existing bookkeeping): import it FIRST
 
-What creation does in one step: company + owner membership, BAS chart of
-accounts for the company form, settings, the first fiscal period, and the
-automatic tax deadlines (moms, F-skatt, AGI, inkomstdeklaration). The 30-day
-trial with bank sync, Skatteverket, AI and e-mail starts immediately. This
-connection uses the new company automatically from the next call; no
-re-authentication.
+When the user had a previous system, history comes before the bank: it is
+the fastest path to a ledger that shows real value, and bank history rarely
+reaches far enough back anyway.
 
-## Step 3: connect the bank
+1. Tell them where to export: **Fortnox** Register → Exportera → SIE 4,
+   **Visma eEkonomi** Bokföring → Export SIE, **Bokio** Inställningar →
+   Exportera data → SIE, **Björn Lundén / Briox / Wint** under Export.
+   Every Swedish system exports SIE4 (.se/.sie); ask them to attach the
+   file here in the chat.
+2. When the file arrives, call \`gnubok_sie_preflight\` with its content
+   (\`file_content\` as read, or \`file_content_base64\` when exact bytes are
+   available: that preserves åäö in CP437 exports). Summarize the scan:
+   source system, fiscal years, verifikat count, balance status, org-number
+   match, warnings. This is the "does it look correct" moment: surface
+   problems BEFORE anything is written.
+3. On their go-ahead: \`gnubok_import_sie\` with the same file and the
+   preflight's \`mappings\`. It stages for approval; after commit verify with
+   \`gnubok_get_trial_balance\`.
+4. Multiple fiscal years = multiple files: import oldest first so IB/UB
+   chains. If the file is very large for chat, the web wizard at
+   \`/import?mode=sie\` is the fallback; Fortnox users can also run the full
+   API migration (invoices, customers, documents) at
+   \`/import?mode=migration&provider=fortnox\`.
 
-Call \`gnubok_connect_bank\`. It reports existing connections and returns a
-\`connect_url\`; on claude.ai/Claude Desktop a connect card with an
-open-in-browser button renders automatically. The user opens the link in a
-browser where they are logged in to Accounted, picks the bank and approves
-with BankID (PSD2 consent, up to 180 days). Transactions start syncing
-within a minute. If they prefer not to connect a bank, they can import bank
-statements as files in the web app instead; do not block on this step.
+## Step 4: connect bank and Skatteverket (together, no pause)
 
-## Step 4: connect Skatteverket (optional but recommended)
+Call \`gnubok_connect_bank\` AND \`gnubok_connect_skatteverket\` in the same
+turn; on claude.ai/Desktop both render connect cards with buttons.
 
-Call \`gnubok_connect_skatteverket\`. Same pattern: the user opens the
-connect link (card button on claude.ai/Desktop), identifies with BankID as
-firmatecknare at Skatteverket, and lands back in Accounted. This enables
-skattekonto sync and filing of momsdeklaration and arbetsgivardeklaration
-from here. Filing is never mandatory through Accounted: every declaration
-can be downloaded and filed manually.
+- Bank: BankID + PSD2 consent, then an **account selection dialog** in the
+  browser: transactions start syncing when the user saves it. Banks cap
+  PSD2 history (often ~90 days); older history is the SIE import's job.
+- Skatteverket: BankID as firmatecknare; enables skattekonto sync and
+  moms/AGI filing. Optional, never block on it.
 
-## Step 5: first bookkeeping
+When the user says they are done (or comes back), re-call
+\`gnubok_connect_bank\` to verify \`connected\`, then go DIRECTLY to step 5.
 
-Once transactions arrive: \`gnubok_list_uncategorized_transactions\` and the
-categorize flow (\`gnubok_suggest_categories\`, \`gnubok_categorize_transaction\`,
-approval). For a company with history in another system, offer the SIE
-import (\`gnubok_import_sie\` in the search catalog) before categorizing.
+## Step 5: first bookkeeping, immediately
+
+Call \`gnubok_list_uncategorized_transactions\` as soon as the bank is
+connected: do not ask whether to proceed. Walk the categorize flow
+(\`gnubok_suggest_categories\`, \`gnubok_categorize_transaction\`, approval).
+If nothing has synced yet, say so and check again on the user's next
+message instead of making them ask.
 
 ## Tools
 
 - \`gnubok_lookup_company\`: registry facts + prefill from the orgnr; call first
 - \`gnubok_create_company\`: preview (no confirm) then create (confirm=true)
-- \`gnubok_list_companies\`: see which companies this connection can reach
-- \`gnubok_connect_bank\`: status + connect link for PSD2 bank consent
-- \`gnubok_connect_skatteverket\`: status + connect link for Skatteverket
-- \`gnubok_get_agent_briefing\`: the company's settings and state once created
+- \`gnubok_sie_preflight\`: scan a shared SIE file, nothing written
+- \`gnubok_import_sie\`: staged import; use the preflight's mappings
+- \`gnubok_connect_bank\` / \`gnubok_connect_skatteverket\`: status + connect links
+- \`gnubok_list_companies\`, \`gnubok_get_agent_briefing\`: state checks
 - \`gnubok_list_uncategorized_transactions\`: the first real bookkeeping step
 
 ## Pitfalls
@@ -131,6 +137,8 @@ import (\`gnubok_import_sie\` in the search catalog) before categorizing.
   not work around it by claiming the company is not VAT-registered.
 - Do not create a company twice on a retry: check \`gnubok_list_companies\`
   if a create call was interrupted.
+- A preflight org-number mismatch means the file is another company's
+  bookkeeping: stop and confirm, never import across companies.
 - Bookkeeping duty starts when the company exists in Accounted with a fiscal
   period. Never create a company "to try things out" for a real
   organisation; use the sandbox in the web app for demos.
@@ -140,8 +148,8 @@ export const onboardingSkill: Skill = {
   slug: 'onboarding',
   name: 'Onboarding: New Company Setup',
   summary:
-    'Set up a company from the conversation: ask for the orgnr, prefill facts with gnubok_lookup_company, preview and create with gnubok_create_company, then the bank and Skatteverket connect links.',
-  tags: ['onboarding', 'setup', 'company', 'bank', 'skatteverket', 'agent-first'],
+    'Set up a company in chat: orgnr + previous system first, prefill via gnubok_lookup_company, one confirm to create, SIE history via preflight + staged import, then bank/Skatteverket cards.',
+  tags: ['onboarding', 'setup', 'company', 'bank', 'skatteverket', 'sie', 'migration', 'agent-first'],
   body,
   tier: 'workflow',
 }
diff --git a/lib/company/__tests__/first-year-defaults.test.ts b/lib/company/__tests__/first-year-defaults.test.ts
index c5e6ad1a..0162a0e8 100644
--- a/lib/company/__tests__/first-year-defaults.test.ts
+++ b/lib/company/__tests__/first-year-defaults.test.ts
@@ -46,6 +46,18 @@ describe('deriveFirstYearDefaults', () => {
     expect(deriveFirstYearDefaults(registered, NOW).isFirstFiscalYear).toBe(false)
   })
 
+  it('extends the window to 18 months when the registry shows no closed period', () => {
+    // Extended first räkenskapsår (BFL 3 kap 3 §): registered 13 months ago
+    // with no annual report filed is still the first year.
+    const registered = NOW - 13 * MONTH_MS
+    expect(
+      deriveFirstYearDefaults(registered, NOW, { noClosedPeriod: true }).isFirstFiscalYear
+    ).toBe(true)
+    expect(
+      deriveFirstYearDefaults(NOW - 19 * MONTH_MS, NOW, { noClosedPeriod: true }).isFirstFiscalYear
+    ).toBe(false)
+  })
+
   it('seeds first_year_start as the 1st of the UTC registration month', () => {
     const registered = new Date('2026-03-14T10:00:00Z').getTime()
     const result = deriveFirstYearDefaults(registered, NOW)
diff --git a/lib/company/first-year-defaults.ts b/lib/company/first-year-defaults.ts
index 019527ab..12f2a722 100644
--- a/lib/company/first-year-defaults.ts
+++ b/lib/company/first-year-defaults.ts
@@ -22,16 +22,21 @@ export function parseStartMonthDay(value: string | null | undefined): number | n
 /**
  * Derive the first-year defaults from TIC's `registrationDate`.
  * A company is treated as "first year" when registered less than 12 months
- * ago: comfortably inside BFL's 18-month cap on a first räkenskapsår, which
- * has no minimum length. Returns both the toggle state and a seeded
- * `first_year_start` (always the 1st of the registration month, the format
- * the date inputs expect).
+ * ago, or less than 18 months ago when the registry shows NO closed fiscal
+ * period (`noClosedPeriod`): a company that has never filed an annual
+ * report is still in its first räkenskapsår, and BFL 3 kap 3 § allows that
+ * first year to run up to 18 months (no minimum). The 12-month floor alone
+ * missed exactly the extended-first-year companies the signal exists for
+ * (Arcim, registered 13 months before onboarding, first year to 31 Dec).
+ * Returns both the toggle state and a seeded `first_year_start` (always the
+ * 1st of the registration month, the format the date inputs expect).
  *
  * `now` exists for tests; production callers omit it.
  */
 export function deriveFirstYearDefaults(
   registrationDate: number | null | undefined,
   now: number = Date.now(),
+  opts?: { noClosedPeriod?: boolean },
 ): {
   isFirstFiscalYear: boolean
   firstYearStart: string | undefined
@@ -43,8 +48,9 @@ export function deriveFirstYearDefaults(
   if (Number.isNaN(regDate.getTime())) {
     return { isFirstFiscalYear: false, firstYearStart: undefined }
   }
+  const windowMonths = opts?.noClosedPeriod ? 18 : 12
   const monthsAgo = (now - regDate.getTime()) / (1000 * 60 * 60 * 24 * 30.44)
-  if (monthsAgo >= 12) return { isFirstFiscalYear: false, firstYearStart: undefined }
+  if (monthsAgo >= windowMonths) return { isFirstFiscalYear: false, firstYearStart: undefined }
   const year = regDate.getUTCFullYear()
   const month = String(regDate.getUTCMonth() + 1).padStart(2, '0')
   return { isFirstFiscalYear: true, firstYearStart: `${year}-${month}-01` }