diff --git a/apps/hq/src/api.ts b/apps/hq/src/api.ts
index 6bb5a12..19569c4 100644
--- a/apps/hq/src/api.ts
+++ b/apps/hq/src/api.ts
@@ -36,7 +36,8 @@ import {
   dismissReminder, getReminder, listQueue, listReminders, setSetting,
   type ReminderStatus,
 } from './repos-reminders'
-import { sendReminder, type SendReminderDeps } from './send-reminder'
+import { sendReminder, reminderContext, type SendReminderDeps } from './send-reminder'
+import { reminderEmail } from './reminder-templates'
 import { dashboardView } from './repos-dashboard'
 import {
   costRanking, listAwsUsage, previousMonth, upsertAwsUsage, type UpsertAwsUsageInput,
@@ -507,6 +508,21 @@ export function apiRouter(db: DB, gmailDeps?: Partial<GmailDeps>): Router {
     const status = typeof req.query['status'] === 'string' ? req.query['status'] as ReminderStatus : undefined
     res.json({ ok: true, reminders: status !== undefined ? listReminders(db, { status }) : listQueue(db) })
   })
+  r.get('/reminders/:id/preview', requireAuth, (req, res) => {
+    const id = String(req.params['id'] ?? '')
+    const reminder = getReminder(db, id)
+    if (reminder === null) { res.status(404).json({ ok: false, error: 'Reminder not found' }); return }
+    try {
+      if (reminder.ruleKind === 'follow_up' || reminder.ruleKind === 'email_bounced') {
+        throw new Error(`A ${reminder.ruleKind} reminder is an internal item, not a sendable email`)
+      }
+      const { ctx } = reminderContext(db, reminder, companySettings()['company.name'] ?? '')
+      const mail = reminderEmail(reminder.ruleKind, ctx)
+      res.json({ ok: true, subject: mail.subject, body: mail.bodyText })
+    } catch (err) {
+      res.status(400).json({ ok: false, error: err instanceof Error ? err.message : String(err) })
+    }
+  })
   r.post('/reminders/:id/send', requireAuth, (req, res) => {
     const id = String(req.params['id'] ?? '')
     if (getReminder(db, id) === null) { res.status(404).json({ ok: false, error: 'Reminder not found' }); return }
diff --git a/apps/hq/src/send-reminder.ts b/apps/hq/src/send-reminder.ts
index 2f09360..cb5753d 100644
--- a/apps/hq/src/send-reminder.ts
+++ b/apps/hq/src/send-reminder.ts
@@ -1,11 +1,11 @@
 import type { DB } from './db'
 import type { GmailDeps, SendResult } from './gmail'
 import { sendReminderEmail } from './gmail'
-import { getClient } from './repos-clients'
+import { getClient, type Client } from './repos-clients'
 import { getClientModule } from './repos-modules'
-import { getDocument } from './repos-documents'
+import { getDocument, type Doc } from './repos-documents'
 import { getAmc } from './repos-amc'
-import { getReminder, setReminderStatus } from './repos-reminders'
+import { getReminder, setReminderStatus, type Reminder } from './repos-reminders'
 import { reminderEmail, type ReminderContext } from './reminder-templates'
 import { documentHtml } from './templates'
 
@@ -19,6 +19,31 @@ export interface SendReminderDeps {
   now?: () => string
 }
 
+/** The per-rule email context + the resolved client/doc, shared by the send path
+ *  and GET /reminders/:id/preview so the previewed email equals the sent one. */
+export interface ReminderRender { ctx: ReminderContext; doc: Doc | null; client: Client }
+
+export function reminderContext(db: DB, reminder: Reminder, companyName: string): ReminderRender {
+  const client = getClient(db, reminder.clientId)
+  if (client === null) throw new Error('Client not found')
+  const ctx: ReminderContext = { clientName: client.name, companyName }
+  let doc: Doc | null = null
+  if (reminder.docId !== null) {
+    doc = getDocument(db, reminder.docId)
+    if (doc === null) throw new Error('Reminder document not found')
+    ctx.docNo = doc.docNo ?? undefined
+    ctx.amountPaise = doc.payablePaise
+    ctx.period = reminder.duePeriod
+  } else if (reminder.ruleKind === 'renewal_due') {
+    const cm = getClientModule(db, reminder.subjectId)
+    if (cm !== null && cm.nextRenewal !== null) ctx.dueDate = cm.nextRenewal
+  } else if (reminder.ruleKind === 'amc_expiring') {
+    const amc = getAmc(db, reminder.subjectId)
+    if (amc !== null) { ctx.coverage = amc.coverage; ctx.dueDate = amc.periodTo }
+  }
+  return { ctx, doc, client }
+}
+
 export async function sendReminder(
   db: DB, deps: SendReminderDeps, reminderId: string, userId: string,
 ): Promise<SendResult> {
@@ -27,30 +52,16 @@ export async function sendReminder(
   if (reminder.ruleKind === 'follow_up' || reminder.ruleKind === 'email_bounced') {
     throw new Error(`A ${reminder.ruleKind} reminder is an internal item, not a sendable email`)
   }
-  const client = getClient(db, reminder.clientId)
-  if (client === null) throw new Error('Client not found')
   const company = deps.company()
   const companyName = company['company.name'] ?? ''
+  const { ctx, doc, client } = reminderContext(db, reminder, companyName)
 
-  // Build the per-rule context.
-  const ctx: ReminderContext = { clientName: client.name, companyName }
   let attachment: { filename: string; data: Buffer } | undefined
   let documentId: string | undefined
-  if (reminder.docId !== null) {
-    const doc = getDocument(db, reminder.docId)
-    if (doc === null) throw new Error('Reminder document not found')
+  if (doc !== null) {
     documentId = doc.id
-    ctx.docNo = doc.docNo ?? undefined
-    ctx.amountPaise = doc.payablePaise
-    ctx.period = reminder.duePeriod
     const pdf = await deps.renderPdf(documentHtml(doc, client, company))
     attachment = { filename: `${(doc.docNo ?? 'draft').replaceAll('/', '-')}.pdf`, data: pdf }
-  } else if (reminder.ruleKind === 'renewal_due') {
-    const cm = getClientModule(db, reminder.subjectId)
-    if (cm !== null && cm.nextRenewal !== null) ctx.dueDate = cm.nextRenewal
-  } else if (reminder.ruleKind === 'amc_expiring') {
-    const amc = getAmc(db, reminder.subjectId)
-    if (amc !== null) { ctx.coverage = amc.coverage; ctx.dueDate = amc.periodTo }
   }
 
   const to = client.contacts.find((c) => c.email !== undefined && c.email !== '')?.email
diff --git a/apps/hq/test/reminder-preview.test.ts b/apps/hq/test/reminder-preview.test.ts
new file mode 100644
index 0000000..13ad555
--- /dev/null
+++ b/apps/hq/test/reminder-preview.test.ts
@@ -0,0 +1,51 @@
+// apps/hq/test/reminder-preview.test.ts
+import { describe, it, expect, afterAll } from 'vitest'
+import express from 'express'
+import { openDb } from '../src/db'
+import { seedIfEmpty } from '../src/seed'
+import { createStaff } from '../src/auth'
+import { createClient } from '../src/repos-clients'
+import { createModule, setPrice } from '../src/repos-modules'
+import { createDraft, issueDocument } from '../src/repos-documents'
+import { upsertReminder } from '../src/repos-reminders'
+import { apiRouter } from '../src/api'
+
+function appWith() {
+  const db = openDb(':memory:'); seedIfEmpty(db) // company.name='Tecnostac'
+  createStaff(db, { email: 'owner@test.in', displayName: 'Owner', role: 'owner', password: 'owner-password' })
+  const c = createClient(db, 'u1', { name: 'Acme', code: 'ACME', stateCode: '32', contacts: [{ name: 'Ravi', email: 'ravi@acme.in' }] })
+  const m = createModule(db, 'u1', { code: 'POS', name: 'POS' })
+  setPrice(db, 'u1', { moduleId: m.id, kind: 'yearly', pricePaise: 10_000_00, effectiveFrom: '2026-04-01' })
+  const inv = issueDocument(db, 'u1', createDraft(db, 'u1', { docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }] }).id)
+  const overdue = upsertReminder(db, { ruleKind: 'invoice_overdue', subjectId: inv.id, duePeriod: '2026-07', clientId: c.id, docId: inv.id })
+  const internal = upsertReminder(db, { ruleKind: 'follow_up', subjectId: 'i1', duePeriod: '2026-07-08', clientId: c.id })
+  const app = express(); app.use(express.json()); app.locals['db'] = db; app.use('/api', apiRouter(db))
+  const server = app.listen(0)
+  const base = `http://localhost:${(server.address() as { port: number }).port}/api`
+  return { server, base, inv, overdueId: overdue.id, internalId: internal.id }
+}
+
+describe('GET /reminders/:id/preview', () => {
+  const ctx = appWith()
+  afterAll(() => ctx.server.close())
+  const login = async () =>
+    (await (await fetch(`${ctx.base}/auth/login`, { method: 'POST', headers: { 'content-type': 'application/json' }, body: JSON.stringify({ email: 'owner@test.in', password: 'owner-password' }) })).json() as any).token
+  const get = async (token: string | null, id: string) => {
+    const res = await fetch(`${ctx.base}/reminders/${id}/preview`, { headers: token ? { authorization: `Bearer ${token}` } : {} })
+    return { status: res.status, json: await res.json() as any }
+  }
+  it('requires auth', async () => { expect((await get(null, ctx.overdueId)).status).toBe(401) })
+  it('previews the overdue email — subject + body match what would be sent', async () => {
+    const token = await login()
+    const r = await get(token, ctx.overdueId)
+    expect(r.status).toBe(200)
+    expect(r.json.subject).toContain(ctx.inv.docNo)  // 'Payment reminder — Invoice INV/26-27-0001 (Tecnostac)'
+    expect(r.json.subject).toContain('Tecnostac')
+    expect(r.json.body).toContain('Acme')            // clientName
+  })
+  it('404s an unknown reminder and 400s an internal follow_up', async () => {
+    const token = await login()
+    expect((await get(token, 'nope')).status).toBe(404)
+    expect((await get(token, ctx.internalId)).status).toBe(400)
+  })
+})