diff --git a/__tests__/classes-logic.test.ts b/__tests__/classes-logic.test.ts
new file mode 100644
index 0000000..cde6353
--- /dev/null
+++ b/__tests__/classes-logic.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect } from 'vitest'
+import {
+  effectiveCapacity,
+  computeSignupStatus,
+  sessionTiming,
+  canSignUp,
+  pickPromotion,
+} from '@/lib/classes-logic'
+
+describe('effectiveCapacity', () => {
+  it('session override wins, else class, else unlimited', () => {
+    expect(effectiveCapacity(5, 10)).toBe(5)
+    expect(effectiveCapacity(null, 10)).toBe(10)
+    expect(effectiveCapacity(undefined, undefined)).toBeNull()
+    expect(effectiveCapacity(0 as unknown as number, 10)).toBe(0)
+  })
+})
+
+describe('computeSignupStatus', () => {
+  it('unlimited capacity always registers', () => {
+    expect(computeSignupStatus(null, 999)).toBe('registered')
+  })
+  it('registers under capacity, waitlists at/over', () => {
+    expect(computeSignupStatus(3, 2)).toBe('registered')
+    expect(computeSignupStatus(3, 3)).toBe('waitlisted')
+    expect(computeSignupStatus(3, 4)).toBe('waitlisted')
+  })
+})
+
+describe('sessionTiming', () => {
+  const now = '2026-05-16T12:00:00.000Z'
+  it('classifies relative to now', () => {
+    expect(sessionTiming('2026-05-17T00:00:00.000Z', null, now)).toBe('upcoming')
+    expect(sessionTiming('2026-05-15T00:00:00.000Z', '2026-05-15T01:00:00.000Z', now)).toBe('past')
+    expect(sessionTiming('2026-05-16T11:00:00.000Z', '2026-05-16T13:00:00.000Z', now)).toBe('in_progress')
+  })
+  it('with no end, a started session is past once now exceeds start', () => {
+    expect(sessionTiming('2026-05-16T11:59:59.000Z', null, now)).toBe('past')
+  })
+})
+
+describe('canSignUp', () => {
+  const now = '2026-05-16T12:00:00.000Z'
+  it('rejects cancelled/completed/past', () => {
+    expect(canSignUp({ sessionStatus: 'cancelled', startsAt: '2026-06-01T00:00:00Z', now }).ok).toBe(false)
+    expect(canSignUp({ sessionStatus: 'completed', startsAt: '2026-06-01T00:00:00Z', now }).ok).toBe(false)
+    expect(canSignUp({ sessionStatus: 'scheduled', startsAt: '2026-05-01T00:00:00Z', endsAt: '2026-05-01T01:00:00Z', now }).ok).toBe(false)
+  })
+  it('allows an upcoming scheduled session', () => {
+    expect(canSignUp({ sessionStatus: 'scheduled', startsAt: '2026-06-01T00:00:00Z', now }).ok).toBe(true)
+  })
+})
+
+describe('pickPromotion', () => {
+  it('promotes the earliest waitlisted when a seat is free', () => {
+    const s = [
+      { id: 'r1', status: 'registered', signed_up_at: '2026-01-01T00:00:00Z' },
+      { id: 'w2', status: 'waitlisted', signed_up_at: '2026-01-03T00:00:00Z' },
+      { id: 'w1', status: 'waitlisted', signed_up_at: '2026-01-02T00:00:00Z' },
+    ]
+    expect(pickPromotion(s, 3)).toBe('w1')
+  })
+  it('returns null when still at capacity', () => {
+    const s = [
+      { id: 'r1', status: 'registered', signed_up_at: '2026-01-01T00:00:00Z' },
+      { id: 'r2', status: 'registered', signed_up_at: '2026-01-01T00:00:00Z' },
+      { id: 'w1', status: 'waitlisted', signed_up_at: '2026-01-02T00:00:00Z' },
+    ]
+    expect(pickPromotion(s, 2)).toBeNull()
+  })
+  it('returns null when nobody is waiting', () => {
+    expect(pickPromotion([{ id: 'r1', status: 'registered', signed_up_at: 'x' }], 5)).toBeNull()
+  })
+})
diff --git a/lib/classes-logic.ts b/lib/classes-logic.ts
new file mode 100644
index 0000000..6b028b5
--- /dev/null
+++ b/lib/classes-logic.ts
@@ -0,0 +1,99 @@
+// Pure, dependency-free decision logic for the classes feature. No Supabase,
+// no React, no Next. Unit-tested directly so capacity/waitlist/eligibility
+// rules cannot silently drift.
+
+export type SignupOutcome = 'registered' | 'waitlisted'
+export type SessionTiming = 'upcoming' | 'in_progress' | 'past'
+
+// A session-level capacity overrides the class default; null at both levels
+// means unlimited.
+export function effectiveCapacity(
+  sessionCapacity: number | null | undefined,
+  classCapacity: number | null | undefined,
+): number | null {
+  if (sessionCapacity != null) return sessionCapacity
+  if (classCapacity != null) return classCapacity
+  return null
+}
+
+// Where a new signup lands given the effective capacity and how many are
+// already registered (not counting waitlisted/cancelled).
+export function computeSignupStatus(
+  capacity: number | null,
+  currentRegistered: number,
+): SignupOutcome {
+  if (capacity == null) return 'registered'
+  return currentRegistered < capacity ? 'registered' : 'waitlisted'
+}
+
+function ms(value: string | null | undefined): number | null {
+  if (!value) return null
+  const t = new Date(value).getTime()
+  return Number.isNaN(t) ? null : t
+}
+
+export function sessionTiming(
+  startsAt: string,
+  endsAt: string | null | undefined,
+  now?: Date | string,
+): SessionTiming {
+  const t = now == null ? Date.now() : new Date(now).getTime()
+  const start = ms(startsAt)
+  if (start == null) return 'upcoming'
+  const end = ms(endsAt) ?? start
+  if (t < start) return 'upcoming'
+  if (t > end) return 'past'
+  return 'in_progress'
+}
+
+export type SignupEligibility = { ok: true } | { ok: false; reason: string }
+
+// Whether a member may newly sign up for a session.
+export function canSignUp(input: {
+  sessionStatus: string
+  startsAt: string
+  endsAt?: string | null
+  now?: Date | string
+}): SignupEligibility {
+  if (input.sessionStatus === 'cancelled') return { ok: false, reason: 'This session was cancelled.' }
+  if (input.sessionStatus === 'completed') return { ok: false, reason: 'This session is already completed.' }
+  if (sessionTiming(input.startsAt, input.endsAt, input.now) === 'past') {
+    return { ok: false, reason: 'This session has already ended.' }
+  }
+  return { ok: true }
+}
+
+// When a registered member cancels and a seat frees, the earliest-signed-up
+// waitlisted member is promoted. Returns that signup id, or null if there is
+// no free seat or no one waiting.
+export function pickPromotion(
+  signups: Array<{ id: string; status: string; signed_up_at: string }>,
+  capacity: number | null,
+): string | null {
+  if (capacity == null) {
+    // Unlimited: anyone waitlisted (shouldn't normally happen) can move up;
+    // promote the earliest to be safe.
+    const waiting = signups
+      .filter(s => s.status === 'waitlisted')
+      .sort((a, b) => a.signed_up_at.localeCompare(b.signed_up_at))
+    return waiting[0]?.id ?? null
+  }
+  const registered = signups.filter(s => s.status === 'registered').length
+  if (registered >= capacity) return null
+  const waiting = signups
+    .filter(s => s.status === 'waitlisted')
+    .sort((a, b) => a.signed_up_at.localeCompare(b.signed_up_at))
+  return waiting[0]?.id ?? null
+}
+
+export const SESSION_STATUS_LABEL: Record<string, string> = {
+  scheduled: 'Scheduled',
+  cancelled: 'Cancelled',
+  completed: 'Completed',
+}
+
+export const SIGNUP_STATUS_LABEL: Record<string, string> = {
+  registered: 'Registered',
+  waitlisted: 'Waitlisted',
+  cancelled: 'Cancelled',
+}
diff --git a/lib/validations.ts b/lib/validations.ts
index 918664d..9b57461 100644
--- a/lib/validations.ts
+++ b/lib/validations.ts
@@ -741,6 +741,94 @@ export const listMemberCertificationsSchema = z.object({
   memberId: z.string().uuid('Invalid member ID'),
 })
 
+// ─── Classes ─────────────────────────────────────────────────────────────────
+
+// Generic manual payment link only (no live payment integration). Empty
+// string -> null so a cleared field is stored as "no link".
+const paymentLink = z
+  .string()
+  .trim()
+  .max(2000)
+  .optional()
+  .nullable()
+  .transform(v => (v && v !== '' ? v : null))
+  .refine(
+    v => v === null || /^https?:\/\/.+/i.test(v),
+    'Payment link must be an http(s) URL',
+  )
+
+const classCapacity = z
+  .number()
+  .int('Capacity must be a whole number')
+  .positive('Capacity must be at least 1')
+  .max(100000)
+  .optional()
+  .nullable()
+
+const dateTimeString = z.string().min(1, 'A date and time is required').max(40)
+
+export const createClassSchema = z.object({
+  title: z.string().min(1, 'Title is required').max(200),
+  description: z.string().max(5000).optional().nullable(),
+  payment_link: paymentLink,
+  capacity: classCapacity,
+  grants_certification_id: z.string().uuid('Invalid certification ID').optional().nullable(),
+})
+
+export const updateClassSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+  title: z.string().min(1).max(200).optional(),
+  description: z.string().max(5000).optional().nullable(),
+  payment_link: paymentLink,
+  capacity: classCapacity,
+  grants_certification_id: z.string().uuid('Invalid certification ID').optional().nullable(),
+  is_active: z.boolean().optional(),
+})
+
+export const classIdSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+})
+
+export const createSessionSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+  starts_at: dateTimeString,
+  ends_at: z.string().max(40).optional().nullable().transform(v => (v && v !== '' ? v : null)),
+  location: z.string().max(300).optional().nullable(),
+  capacity: classCapacity,
+  notes: z.string().max(5000).optional().nullable(),
+})
+
+export const updateSessionSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+  starts_at: dateTimeString.optional(),
+  ends_at: z.string().max(40).optional().nullable().transform(v => (v && v !== '' ? v : null)),
+  location: z.string().max(300).optional().nullable(),
+  capacity: classCapacity,
+  notes: z.string().max(5000).optional().nullable(),
+  status: z.enum(['scheduled', 'cancelled', 'completed']).optional(),
+})
+
+export const sessionIdSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const signUpForClassSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const cancelSignupSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const markAttendanceSchema = z.object({
+  signupId: z.string().uuid('Invalid signup ID'),
+  attended: z.boolean(),
+})
+
+export const listSessionSignupsSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
 // ─── Generic ID schemas ──────────────────────────────────────────────────────
 
 export const uuidSchema = z.string().uuid('Invalid ID format')