diff --git a/DECISIONS.md b/DECISIONS.md
index 514107fa..5b2223c9 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -768,3 +768,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-03] Tax depreciation (issue #324) is elected per fiscal period on a pooled snapshot chain, not per asset: fiscal_periods carries method, rule, opening, base, deduction and closing values, with DB guards enforcing method continuity and opening equal to the previous closing; assets keep depreciation_method linear for book depreciation, and company_settings.tax_depreciation_method was dropped because a second company-level method meant an admin/member RLS mismatch and a non-atomic second write; the annual snapshot chain is authoritative.
 [2026-08-03] kompletteringsregel_20 with a positive basis and zero acquisition cohorts is refused rather than computed: reducing over an empty cohort set would claim a full write-off the cohort evidence does not support (IL 18 kap. 17 §), so such periods require manual review instead of an automatic deduction.
 [2026-08-04] Balance-sheet synthetic result = complement of classes 1-2 (class 0/9/null rows included), not classes 3-8: a resultatavslut posted to 2099 without zeroing class 3-8 then self-cancels inside the residual instead of double-counting equity; mirrors balansrapport's residual definition (#1333)
+[2026-08-04] v1 settings PATCH writes directly (no staging), following the v1 customers precedent: REST callers are already scope-gated, staging is an MCP segregation-of-duties concept.
diff --git a/app/api/v1/companies/[companyId]/settings/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/settings/__tests__/route.test.ts
new file mode 100644
index 00000000..54f4b73d
--- /dev/null
+++ b/app/api/v1/companies/[companyId]/settings/__tests__/route.test.ts
@@ -0,0 +1,453 @@
+/**
+ * Integration tests for PATCH /api/v1/companies/:companyId/settings.
+ *
+ * Modeled on the v1 customers route tests: mocked API-key auth + a flexible
+ * Supabase proxy mock; no real network or database access.
+ */
+import { beforeAll, beforeEach, describe, expect, it, vi } from 'vitest'
+
+beforeAll(() => {
+  // Belt-and-braces: ensure we never reach a real DB from this test suite.
+  if (process.env.NODE_ENV !== 'test') {
+    throw new Error(
+      `settings route tests require NODE_ENV=test (got ${process.env.NODE_ENV ?? 'undefined'})`,
+    )
+  }
+  process.env.NEXT_PUBLIC_SUPABASE_URL ||= 'http://localhost:54321'
+  process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY ||= 'test-anon-key'
+})
+
+vi.mock('@/lib/auth/api-keys', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/auth/api-keys')>('@/lib/auth/api-keys')
+  return {
+    ...actual,
+    validateApiKey: vi.fn(),
+    createServiceClientNoCookies: vi.fn(),
+  }
+})
+
+vi.mock('@supabase/supabase-js', async () => {
+  const actual = await vi.importActual<typeof import('@supabase/supabase-js')>('@supabase/supabase-js')
+  return { ...actual, createClient: vi.fn().mockReturnValue({}) }
+})
+
+import { validateApiKey, createServiceClientNoCookies } from '@/lib/auth/api-keys'
+import { PATCH as updateSettings } from '../route'
+
+const mockValidate = validateApiKey as ReturnType<typeof vi.fn>
+const mockServiceClient = createServiceClientNoCookies as ReturnType<typeof vi.fn>
+
+function makeFlexibleSupabase(byTable: Record<string, { data?: unknown; error?: unknown }>) {
+  // Records update payloads and .select() projection strings so tests can
+  // assert what the route writes and which columns it fetches back.
+  const captured: {
+    update: unknown[]
+    selects: Record<string, string[]>
+  } = { update: [], selects: {} }
+  const buildChain = (table: string): unknown => {
+    const handler: ProxyHandler<object> = {
+      get(_target, prop) {
+        if (prop === 'then') {
+          return (resolve: (v: unknown) => void) =>
+            resolve(byTable[table] ?? { data: null, error: null })
+        }
+        return (...args: unknown[]) => {
+          if (prop === 'update') captured.update.push(args[0])
+          if (prop === 'select' && typeof args[0] === 'string') {
+            ;(captured.selects[table] ??= []).push(args[0])
+          }
+          return buildChain(table)
+        }
+      },
+    }
+    return new Proxy({}, handler)
+  }
+  return { from: vi.fn((table: string) => buildChain(table)), captured }
+}
+
+const COMPANY_ID = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa'
+const USER_ID = 'user-1'
+
+function companyParams(companyId: string) {
+  return { params: Promise.resolve({ companyId }) }
+}
+
+function makePatchRequest(url: string, body: unknown, extraHeaders: Record<string, string> = {}): Request {
+  return new Request(url, {
+    method: 'PATCH',
+    headers: {
+      Authorization: 'Bearer test-fixture-not-a-real-key',
+      'Content-Type': 'application/json',
+      'Idempotency-Key': 'abcd1234-4444-4abc-8def-1234567890ab',
+      ...extraHeaders,
+    },
+    body: JSON.stringify(body),
+  })
+}
+
+function withWriteScope() {
+  mockValidate.mockResolvedValue({
+    userId: USER_ID,
+    companyId: COMPANY_ID,
+    apiKeyId: 'ak_1',
+    apiKeyName: 'CI key',
+    scopes: ['companies:write'],
+    mode: 'live',
+  })
+}
+
+const SAMPLE_SETTINGS = {
+  bank_name: 'Testbanken',
+  clearing_number: null,
+  account_number: null,
+  // '991-2346' passes the Bankgiro Luhn check (see lib/bankgiro Luhn tests).
+  bankgiro: '991-2346',
+  plusgiro: null,
+  swish: null,
+  iban: null,
+  bic: null,
+  default_our_reference: 'Anna Andersson',
+  email: 'faktura@acme.test',
+  phone: null,
+  website: null,
+  invoice_email_texts: null,
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  withWriteScope()
+})
+
+describe('PATCH /api/v1/companies/:companyId/settings', () => {
+  it('returns 401 UNAUTHORIZED for an invalid API key', async () => {
+    mockValidate.mockResolvedValue({ error: 'Invalid API key', status: 401 })
+    mockServiceClient.mockReturnValue(makeFlexibleSupabase({}))
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        bank_name: 'X',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(401)
+    const body = await res.json()
+    expect(body.error.code).toBe('UNAUTHORIZED')
+  })
+
+  it('rejects keys without the companies:write scope', async () => {
+    mockValidate.mockResolvedValue({
+      userId: USER_ID,
+      companyId: COMPANY_ID,
+      apiKeyId: 'ak_1',
+      apiKeyName: 'CI key',
+      scopes: ['companies:read'],
+      mode: 'live',
+    })
+    mockServiceClient.mockReturnValue(makeFlexibleSupabase({}))
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        bank_name: 'X',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(403)
+    const body = await res.json()
+    expect(body.error.code).toBe('INSUFFICIENT_SCOPE')
+    expect(body.error.details.required_scope).toBe('companies:write')
+  })
+
+  it('returns 404 when the caller is not a member of the company in the URL', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: null, error: null },
+      }),
+    )
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        bank_name: 'X',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(404)
+    const body = await res.json()
+    expect(body.error.code).toBe('NOT_FOUND')
+  })
+
+  it('returns 404 when the company has no settings row', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        company_settings: { data: null, error: null },
+      }),
+    )
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        bank_name: 'X',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(404)
+    const body = await res.json()
+    expect(body.error.code).toBe('NOT_FOUND')
+    expect(body.error.details).toEqual({ resource: 'company_settings' })
+  })
+
+  it('rejects requests without an Idempotency-Key header', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      }),
+    )
+
+    const req = new Request(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+      method: 'PATCH',
+      headers: {
+        Authorization: 'Bearer test-fixture-not-a-real-key',
+        'Content-Type': 'application/json',
+      },
+      body: JSON.stringify({ bank_name: 'X' }),
+    })
+
+    const res = await updateSettings(req, companyParams(COMPANY_ID))
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+  })
+
+  it('returns 400 for a body that is not valid JSON', async () => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const req = new Request(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+      method: 'PATCH',
+      headers: {
+        Authorization: 'Bearer test-fixture-not-a-real-key',
+        'Content-Type': 'application/json',
+        'Idempotency-Key': 'abcd1234-4444-4abc-8def-1234567890ab',
+      },
+      body: '{"bank_name": not-json',
+    })
+
+    const res = await updateSettings(req, companyParams(COMPANY_ID))
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+    expect(body.error.details).toEqual({ field: 'body', message: 'Body is not valid JSON.' })
+    expect(supabaseMock.captured.update).toHaveLength(0)
+  })
+
+  it.each([
+    ['a bare array', [{ bank_name: 'X' }]],
+    ['a bare string', 'bank_name=X'],
+    ['a bare number', 42],
+    ['null', null],
+  ])('returns 400 for a JSON body that is not an object (%s)', async (_label, jsonBody) => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, jsonBody),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+    expect(body.error.details).toEqual({ field: 'body', message: 'Body must be a JSON object.' })
+    expect(supabaseMock.captured.update).toHaveLength(0)
+  })
+
+  it('rejects an empty body (at least one field required)', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      }),
+    )
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {}),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+  })
+
+  it('rejects unknown fields, including the internal column name default_our_reference', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      }),
+    )
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        default_our_reference: 'Sneaky',
+        vat_registered: true,
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+    const fields = body.error.details.issues.map((i: { field: string }) => i.field)
+    expect(fields).toContain('default_our_reference')
+    expect(fields).toContain('vat_registered')
+  })
+
+  it('returns 400 for a bankgiro that fails the Luhn check', async () => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        // Right shape (regex passes) but wrong check digit: 991-2346 is valid.
+        bankgiro: '991-2345',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+    const issue = body.error.details.issues.find((i: { field: string }) => i.field === 'bankgiro')
+    expect(issue).toBeTruthy()
+    expect(issue.message).toBe('Invalid Bankgiro number')
+    // Nothing was written.
+    expect(supabaseMock.captured.update).toHaveLength(0)
+  })
+
+  it('returns 400 for an unknown invoice email placeholder', async () => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        invoice_email_texts: { sv: { body: 'Hej! Se faktura {faktura_nr}.' } },
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('VALIDATION_ERROR')
+    const issue = body.error.details.issues.find(
+      (i: { field: string }) => i.field === 'invoice_email_texts.sv.body',
+    )
+    expect(issue).toBeTruthy()
+    expect(issue.message).toContain('{faktura_nr}')
+    expect(supabaseMock.captured.update).toHaveLength(0)
+  })
+
+  it('updates settings and maps contact_person onto default_our_reference', async () => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      company_settings: { data: SAMPLE_SETTINGS, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        contact_person: 'Anna Andersson',
+        bankgiro: '991-2346',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    // The response uses the public field name, mapped from the DB column.
+    expect(body.data.company_id).toBe(COMPANY_ID)
+    expect(body.data.contact_person).toBe('Anna Andersson')
+    expect(body.data.bankgiro).toBe('991-2346')
+    // The update payload carries the DB column name, not contact_person.
+    const updatePayload = supabaseMock.captured.update[0] as Record<string, unknown>
+    expect(updatePayload.default_our_reference).toBe('Anna Andersson')
+    expect(updatePayload.bankgiro).toBe('991-2346')
+    expect(updatePayload.contact_person).toBeUndefined()
+    // The response projection reads the column back.
+    expect(supabaseMock.captured.selects['company_settings']?.[0]).toContain('default_our_reference')
+  })
+
+  it('keeps unsupplied fields undefined (never null) in the update payload', async () => {
+    // The route builds a literal 13-column update payload where unsupplied
+    // fields are undefined; supabase-js JSON serialization drops them, so
+    // the stored values survive a partial PATCH. A future `?? null` on that
+    // payload would silently CLEAR every column the caller did not send
+    // (null is a real write that empties the column). This test pins the
+    // undefined-not-null contract and fails on any such regression.
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      company_settings: { data: { ...SAMPLE_SETTINGS, bank_name: 'Nya Banken' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/settings`, {
+        bank_name: 'Nya Banken',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(200)
+    expect(supabaseMock.captured.update).toHaveLength(1)
+    const updatePayload = supabaseMock.captured.update[0] as Record<string, unknown>
+    expect(updatePayload.bank_name).toBe('Nya Banken')
+
+    const unsuppliedKeys = Object.keys(updatePayload).filter((key) => key !== 'bank_name')
+    // Guard the guard: the literal payload declares every column, so the
+    // unsupplied set must be non-empty for the loop below to prove anything.
+    expect(unsuppliedKeys.length).toBeGreaterThan(0)
+    for (const key of unsuppliedKeys) {
+      expect(
+        updatePayload[key],
+        `unsupplied column "${key}" must be undefined in the update payload, never null`,
+      ).toBeUndefined()
+    }
+    // What actually reaches PostgREST after JSON serialization: only the
+    // supplied column remains.
+    expect(JSON.parse(JSON.stringify(updatePayload))).toEqual({ bank_name: 'Nya Banken' })
+  })
+
+  it('dry-run merges the proposed changes with the current row and writes nothing', async () => {
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      company_settings: { data: SAMPLE_SETTINGS, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateSettings(
+      makePatchRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/settings?dry_run=true`,
+        { contact_person: 'Bo Berg' },
+      ),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(200)
+    expect(res.headers.get('X-Dry-Run')).toBe('true')
+    const body = await res.json()
+    expect(body.data.dry_run).toBe(true)
+    expect(body.data.preview.contact_person).toBe('Bo Berg')
+    // Unchanged fields from the current record are preserved.
+    expect(body.data.preview.bank_name).toBe('Testbanken')
+    expect(supabaseMock.captured.update).toHaveLength(0)
+  })
+})
diff --git a/app/api/v1/companies/[companyId]/settings/route.ts b/app/api/v1/companies/[companyId]/settings/route.ts
new file mode 100644
index 00000000..072d5912
--- /dev/null
+++ b/app/api/v1/companies/[companyId]/settings/route.ts
@@ -0,0 +1,332 @@
+/**
+ * /api/v1/companies/{companyId}/settings: company-settings writes.
+ *
+ * PATCH: partial update of invoice payment details (bank account, Bankgiro,
+ *        Plusgiro, Swish, IBAN/BIC), company contact details shown on
+ *        invoices (email, phone, website, contact_person), and the custom
+ *        invoice email texts. Idempotent (mandatory Idempotency-Key).
+ *        Dry-runnable.
+ *
+ * The field set is deliberately identical to the MCP staging tool
+ * gnubok_update_company_settings and validation is the SAME shared schema
+ * (UpdateCompanySettingsParamsSchema): Luhn-checked Bankgiro/Plusgiro and a
+ * fixed placeholder whitelist for the invoice email texts. Do not widen this
+ * surface toward the internal /api/settings PUT: that route accepts tax and
+ * legal profile fields and regenerates tax deadlines as a side effect.
+ *
+ * The write is direct (no staged operation), following the v1 customers
+ * precedent: REST callers are already gated by the companies:write scope.
+ *
+ * No GET here yet: a read endpoint is a possible follow-up (the MCP tool
+ * gnubok_get_company_settings covers reads today).
+ */
+
+import { z } from 'zod'
+import { ok } from '@/lib/api/v1/response'
+import { dryRunPreview } from '@/lib/api/v1/dry-run'
+import { registerEndpoint, dataEnvelope } from '@/lib/api/v1/registry'
+import { withApiV1 } from '@/lib/api/v1/with-api-v1'
+import { v1ErrorResponse, v1ErrorResponseFromCode } from '@/lib/api/v1/errors'
+import { InvoiceEmailTextsSchema, UpdateSettingsSchema } from '@/lib/api/schemas'
+import { UpdateCompanySettingsParamsSchema } from '@/lib/pending-operations/schemas/company-settings'
+
+// Flat body keys copied into the update payload verbatim. Mirrors the MCP
+// tool gnubok_update_company_settings field for field; contact_person is
+// handled separately because it aliases the default_our_reference column.
+const FLAT_BODY_KEYS = [
+  'bank_name',
+  'clearing_number',
+  'account_number',
+  'bankgiro',
+  'plusgiro',
+  'swish',
+  'iban',
+  'bic',
+  'email',
+  'phone',
+  'website',
+  'invoice_email_texts',
+] as const
+
+const KNOWN_BODY_KEYS: ReadonlySet<string> = new Set([...FLAT_BODY_KEYS, 'contact_person'])
+
+interface SettingsRow {
+  bank_name: string | null
+  clearing_number: string | null
+  account_number: string | null
+  bankgiro: string | null
+  plusgiro: string | null
+  swish: string | null
+  iban: string | null
+  bic: string | null
+  default_our_reference: string | null
+  email: string | null
+  phone: string | null
+  website: string | null
+  invoice_email_texts: unknown
+}
+
+const CompanySettingsResource = z.object({
+  company_id: z.string().uuid(),
+  bank_name: z.string().nullable(),
+  clearing_number: z.string().nullable(),
+  account_number: z.string().nullable(),
+  bankgiro: z.string().nullable(),
+  plusgiro: z.string().nullable(),
+  swish: z.string().nullable(),
+  iban: z.string().nullable(),
+  bic: z.string().nullable(),
+  contact_person: z.string().nullable(),
+  email: z.string().nullable(),
+  phone: z.string().nullable(),
+  website: z.string().nullable(),
+  invoice_email_texts: InvoiceEmailTextsSchema.nullable(),
+})
+
+// Documentation body schema (OpenAPI + agent tool docs). Field shapes are
+// reused from UpdateSettingsSchema, exactly like the shared changes schema
+// composes them; contact_person exposes the default_our_reference column
+// under its public name. Runtime validation goes through the shared
+// UpdateCompanySettingsParamsSchema in the handler so the REST endpoint and
+// the MCP tool can never drift apart on the Swedish-domain rules.
+const V1PatchCompanySettingsSchema = z
+  .object({
+    bank_name: UpdateSettingsSchema.shape.bank_name,
+    clearing_number: UpdateSettingsSchema.shape.clearing_number,
+    account_number: UpdateSettingsSchema.shape.account_number,
+    bankgiro: UpdateSettingsSchema.shape.bankgiro,
+    plusgiro: UpdateSettingsSchema.shape.plusgiro,
+    swish: UpdateSettingsSchema.shape.swish,
+    iban: UpdateSettingsSchema.shape.iban,
+    bic: UpdateSettingsSchema.shape.bic,
+    contact_person: UpdateSettingsSchema.shape.default_our_reference,
+    email: UpdateSettingsSchema.shape.email,
+    phone: UpdateSettingsSchema.shape.phone,
+    website: UpdateSettingsSchema.shape.website,
+    invoice_email_texts: UpdateSettingsSchema.shape.invoice_email_texts,
+  })
+  .strict()
+
+function toSettingsResource(companyId: string, row: SettingsRow) {
+  return {
+    company_id: companyId,
+    bank_name: row.bank_name ?? null,
+    clearing_number: row.clearing_number ?? null,
+    account_number: row.account_number ?? null,
+    bankgiro: row.bankgiro ?? null,
+    plusgiro: row.plusgiro ?? null,
+    swish: row.swish ?? null,
+    iban: row.iban ?? null,
+    bic: row.bic ?? null,
+    contact_person: row.default_our_reference ?? null,
+    email: row.email ?? null,
+    phone: row.phone ?? null,
+    website: row.website ?? null,
+    invoice_email_texts: row.invoice_email_texts ?? null,
+  }
+}
+
+/**
+ * Map a Zod issue path from the shared `{ changes: {...} }` wrapper back to
+ * the public body field names: strip the `changes` prefix and rename
+ * `default_our_reference` (the DB column) to `contact_person` (the only name
+ * this endpoint accepts in the request body).
+ */
+function formatIssueField(path: ReadonlyArray<PropertyKey>): string {
+  const rest = path[0] === 'changes' ? path.slice(1) : [...path]
+  if (rest.length === 0) return 'body'
+  return rest
+    .map((segment, index) =>
+      index === 0 && segment === 'default_our_reference' ? 'contact_person' : String(segment),
+    )
+    .join('.')
+}
+
+registerEndpoint({
+  operation: 'companies.settings.update',
+  method: 'PATCH',
+  path: '/api/v1/companies/:companyId/settings',
+  summary: 'Partially update company settings.',
+  description:
+    'Patches the company payment details (bank account, Bankgiro, Plusgiro, Swish, IBAN/BIC), the contact details shown on invoices (contact_person, email, phone, website), and the custom invoice email texts. All fields optional; at least one must be supplied. Idempotent (mandatory Idempotency-Key). Dry-runnable. The same validation as the MCP staging tool applies: Bankgiro/Plusgiro numbers are Luhn-checked and invoice email texts only accept a fixed placeholder set.',
+  useWhen:
+    'You need to change the payment or contact details that appear on invoices, or override the invoice email texts, directly over REST instead of the staged MCP flow.',
+  doNotUseFor:
+    'Legal or tax profile changes (org number, VAT registration, fiscal year, accounting method): those are not exposed on the public API. Reading settings (no GET endpoint yet; use the MCP tool gnubok_get_company_settings).',
+  pitfalls: [
+    'Idempotency-Key is mandatory; calls without it return 400.',
+    'contact_person is stored as default_our_reference: the default "Our reference" value on new invoices.',
+    'bankgiro and plusgiro must carry a valid Luhn check digit; null or empty string clears them.',
+    'invoice_email_texts only accepts the placeholders {fakturanummer} {kundnamn} {förnamn} {företag} {förfallodatum} {belopp}; any other {token} is rejected. Null clears every override.',
+  ],
+  example: {
+    request: { bankgiro: '991-2346', contact_person: 'Anna Andersson' },
+    response: {
+      data: {
+        company_id: 'aaaa1111-2222-4333-8444-555566667777',
+        bank_name: 'Testbanken',
+        clearing_number: null,
+        account_number: null,
+        bankgiro: '991-2346',
+        plusgiro: null,
+        swish: null,
+        iban: null,
+        bic: null,
+        contact_person: 'Anna Andersson',
+        email: 'faktura@acme.example',
+        phone: null,
+        website: null,
+        invoice_email_texts: null,
+      },
+      meta: { request_id: 'req_...', api_version: '2026-05-12' },
+    },
+  },
+  scope: 'companies:write',
+  // Matches lib/pending-operations/risk-tiers.ts (update_company_settings):
+  // payment settings control where customers send money on future invoices.
+  risk: 'medium',
+  idempotent: true,
+  reversible: true,
+  dryRunSupported: true,
+  request: { body: V1PatchCompanySettingsSchema },
+  response: { success: dataEnvelope(CompanySettingsResource) },
+})
+
+export const PATCH = withApiV1<{ params: Promise<{ companyId: string }> }>(
+  'companies.settings.update',
+  async (request, ctx) => {
+    let rawBody: unknown
+    try {
+      rawBody = await request.json()
+    } catch {
+      return v1ErrorResponseFromCode('VALIDATION_ERROR', ctx.log, {
+        requestId: ctx.requestId,
+        details: { field: 'body', message: 'Body is not valid JSON.' },
+      })
+    }
+
+    if (rawBody === null || typeof rawBody !== 'object' || Array.isArray(rawBody)) {
+      return v1ErrorResponseFromCode('VALIDATION_ERROR', ctx.log, {
+        requestId: ctx.requestId,
+        details: { field: 'body', message: 'Body must be a JSON object.' },
+      })
+    }
+    const body = rawBody as Record<string, unknown>
+
+    // Reject unknown fields under their public names before the alias
+    // mapping, so the caller is told about `contact_person`, never about the
+    // internal column name.
+    const unknownKeys = Object.keys(body).filter((key) => !KNOWN_BODY_KEYS.has(key))
+    if (unknownKeys.length > 0) {
+      return v1ErrorResponseFromCode('VALIDATION_ERROR', ctx.log, {
+        requestId: ctx.requestId,
+        details: {
+          issues: unknownKeys.map((key) => ({ field: key, message: 'Unknown field.' })),
+        },
+      })
+    }
+
+    // Build the changes payload exactly like the MCP tool: copy the flat keys
+    // verbatim and alias the public contact_person field onto the
+    // default_our_reference column.
+    const rawChanges: Record<string, unknown> = {}
+    for (const key of FLAT_BODY_KEYS) {
+      if (body[key] !== undefined) rawChanges[key] = body[key]
+    }
+    if (body.contact_person !== undefined) {
+      rawChanges.default_our_reference = body.contact_person
+    }
+
+    // Shared Swedish-domain validation (same schema as the MCP staging tool):
+    // Luhn-checked bankgiro/plusgiro, placeholder whitelist on the invoice
+    // email texts, and the at-least-one-field rule.
+    const parsed = UpdateCompanySettingsParamsSchema.safeParse({ changes: rawChanges })
+    if (!parsed.success) {
+      return v1ErrorResponseFromCode('VALIDATION_ERROR', ctx.log, {
+        requestId: ctx.requestId,
+        details: {
+          issues: parsed.error.issues.map((issue) => ({
+            field: formatIssueField(issue.path),
+            message: issue.message,
+          })),
+        },
+      })
+    }
+    const changes = parsed.data.changes
+
+    // Dry-run: fetch the current row, merge the proposed changes, return the
+    // merged preview. No DB write.
+    if (ctx.dryRun) {
+      // Literal projection (not a shared const): the schema guard
+      // (tests/schema/no-phantom-columns.test.ts) can only verify columns in
+      // inline literals. Same column set as the MCP tool; excludes tax/legal
+      // profile columns on purpose (see the module doc).
+      const { data: current, error: fetchErr } = await ctx.supabase
+        .from('company_settings')
+        .select('bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic, default_our_reference, email, phone, website, invoice_email_texts')
+        .eq('company_id', ctx.companyId!)
+        .maybeSingle()
+
+      if (fetchErr) {
+        return v1ErrorResponse(fetchErr, ctx.log, { requestId: ctx.requestId })
+      }
+      if (!current) {
+        ctx.log.warn('companies.settings.update dry-run: settings row not found', {
+          companyId: ctx.companyId,
+        })
+        return v1ErrorResponseFromCode('NOT_FOUND', ctx.log, {
+          requestId: ctx.requestId,
+          details: { resource: 'company_settings' },
+        })
+      }
+
+      return dryRunPreview(
+        toSettingsResource(ctx.companyId!, { ...(current as unknown as SettingsRow), ...changes }),
+        { requestId: ctx.requestId, log: ctx.log },
+      )
+    }
+
+    // Literal payload (not the parsed object): the schema guard can then
+    // statically verify every column name. Fields the caller did not supply
+    // are `undefined` here and are dropped by supabase-js JSON serialization,
+    // so only supplied fields are written; explicit null still clears.
+    const { data, error } = await ctx.supabase
+      .from('company_settings')
+      .update({
+        bank_name: changes.bank_name,
+        clearing_number: changes.clearing_number,
+        account_number: changes.account_number,
+        bankgiro: changes.bankgiro,
+        plusgiro: changes.plusgiro,
+        swish: changes.swish,
+        iban: changes.iban,
+        bic: changes.bic,
+        default_our_reference: changes.default_our_reference,
+        email: changes.email,
+        phone: changes.phone,
+        website: changes.website,
+        invoice_email_texts: changes.invoice_email_texts,
+      })
+      .eq('company_id', ctx.companyId!)
+      .select('bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic, default_our_reference, email, phone, website, invoice_email_texts')
+      .maybeSingle()
+
+    if (error) {
+      return v1ErrorResponse(error, ctx.log, { requestId: ctx.requestId })
+    }
+    if (!data) {
+      ctx.log.warn('companies.settings.update: settings row not found', {
+        companyId: ctx.companyId,
+      })
+      return v1ErrorResponseFromCode('NOT_FOUND', ctx.log, {
+        requestId: ctx.requestId,
+        details: { resource: 'company_settings' },
+      })
+    }
+
+    return ok(toSettingsResource(ctx.companyId!, data as unknown as SettingsRow), {
+      requestId: ctx.requestId,
+    })
+  },
+  { requireIdempotencyKey: true },
+)
diff --git a/lib/api/v1/__tests__/__snapshots__/spec-snapshot.test.ts.snap b/lib/api/v1/__tests__/__snapshots__/spec-snapshot.test.ts.snap
index b77da9aa..e24c53d2 100644
--- a/lib/api/v1/__tests__/__snapshots__/spec-snapshot.test.ts.snap
+++ b/lib/api/v1/__tests__/__snapshots__/spec-snapshot.test.ts.snap
@@ -1,6 +1,6 @@
 // Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
 
-exports[`v1 spec snapshot > matches the recorded endpoint count > endpoint-count 1`] = `123`;
+exports[`v1 spec snapshot > matches the recorded endpoint count > endpoint-count 1`] = `124`;
 
 exports[`v1 spec snapshot > matches the recorded endpoint key set > endpoint-keys 1`] = `
 [
@@ -69,6 +69,7 @@ exports[`v1 spec snapshot > matches the recorded endpoint key set > endpoint-key
   "PATCH /api/v1/companies/:companyId/invoices/:id",
   "PATCH /api/v1/companies/:companyId/salary-runs/:id",
   "PATCH /api/v1/companies/:companyId/salary-runs/:id/lines/:lineId",
+  "PATCH /api/v1/companies/:companyId/settings",
   "PATCH /api/v1/companies/:companyId/supplier-invoices/:id",
   "PATCH /api/v1/companies/:companyId/suppliers/:id",
   "PATCH /api/v1/companies/:companyId/webhooks/:id",
@@ -134,6 +135,7 @@ exports[`v1 spec snapshot > matches the recorded scope catalogue > endpoint-scop
 [
   "bookkeeping:write",
   "companies:read",
+  "companies:write",
   "compliance:read",
   "customers:read",
   "customers:write",
diff --git a/lib/api/v1/load-routes.ts b/lib/api/v1/load-routes.ts
index 411a9437..99bf0ef9 100644
--- a/lib/api/v1/load-routes.ts
+++ b/lib/api/v1/load-routes.ts
@@ -161,4 +161,7 @@ import '@/app/api/v1/companies/[companyId]/dimensions/[id]/values/[valueId]/rout
 // #895: articles read (artikelregister) for invoice line linkage.
 import '@/app/api/v1/companies/[companyId]/articles/route'
 
+// #1348: company-settings write (PATCH, MCP-tool-identical field set).
+import '@/app/api/v1/companies/[companyId]/settings/route'
+
 export {}
diff --git a/lib/auth/api-keys.ts b/lib/auth/api-keys.ts
index fbdf214b..56ffe25c 100644
--- a/lib/auth/api-keys.ts
+++ b/lib/auth/api-keys.ts
@@ -23,7 +23,7 @@ export const API_KEY_SCOPES = {
   'payroll:write':      { label: 'Löner: skriv',        description: 'Skapa lönekörning, beräkna, generera AGI (3 verktyg)' },
   // v1 REST API: added Phase 1
   'companies:read':     { label: 'Företag: läs',        description: 'Lista och visa företagsprofiler som API-nyckeln har tillgång till' },
-  'companies:write':    { label: 'Företag: skriv',      description: 'Uppdatera företagsinställningar via stagade verktyg' },
+  'companies:write':    { label: 'Företag: skriv',      description: 'Uppdatera företagsinställningar via stagade verktyg eller REST-endpointen PATCH /api/v1/companies/{companyId}/settings' },
   'events:read':        { label: 'Händelser: läs',      description: 'Polla händelseloggen (event_log) som webhook-fallback' },
   'webhooks:manage':    { label: 'Webhooks: hantera',   description: 'Skapa, lista, uppdatera och radera webhook-prenumerationer' },
   'operations:read':    { label: 'Operationer: läs',    description: 'Hämta status för långkörande operationer (importer, bokslut, omvärdering)' },
diff --git a/lib/auth/scopes.ts b/lib/auth/scopes.ts
index 68beeb65..924b37df 100644
--- a/lib/auth/scopes.ts
+++ b/lib/auth/scopes.ts
@@ -39,6 +39,9 @@ export const V1_ENDPOINT_SCOPES: Record<string, ApiKeyScope> = {
   // Companies
   'GET /api/v1/companies': 'companies:read',
   'GET /api/v1/companies/:companyId': 'companies:read',
+  // Issue #1348: company-settings write (same field set as the MCP tool
+  // gnubok_update_company_settings; direct write, no staging).
+  'PATCH /api/v1/companies/:companyId/settings': 'companies:write',
 
   // Operations (async long-running tasks)
   'GET /api/v1/operations/:id': 'operations:read',