diff --git a/server/utils/slack.ts b/server/utils/slack.ts
index f1fd674..94ac690 100644
--- a/server/utils/slack.ts
+++ b/server/utils/slack.ts
@@ -104,7 +104,7 @@ export class SlackService {
   /**
    * Find user in workspace by email
    */
-  private async findUserByEmail(email: string): Promise<string | null> {
+  async findUserByEmail(email: string): Promise<string | null> {
     try {
       const response = await this.client.users.lookupByEmail({ email });
       return response.user?.id || null;
diff --git a/server/utils/slackAccess.js b/server/utils/slackAccess.js
new file mode 100644
index 0000000..885ae12
--- /dev/null
+++ b/server/utils/slackAccess.js
@@ -0,0 +1,61 @@
+// Spec: docs/specs/wave-based-slack-onboarding.md
+//
+// Auto-detect existing Slack workspace membership for a Ghost Guild member
+// and flag them as `slackInvited` without sending a duplicate invite.
+//
+// Contract:
+// - Caller awaits this helper, but it never throws — all errors swallowed.
+// - Internal lookup races against a 3s timeout (resolves null on hang).
+// - No-op if Slack service isn't configured, lookup misses, or member is
+//   already flagged.
+
+import Member from '../models/member.js'
+import { getSlackService } from './slack.ts'
+
+const LOOKUP_TIMEOUT_MS = 3000
+
+export async function autoFlagPreExistingSlackAccess(member) {
+  try {
+    if (!member || !member._id || !member.email) return
+    if (member.slackInvited === true) return
+
+    const slackService = getSlackService()
+    if (!slackService) return
+
+    const timeoutPromise = new Promise((resolve) => {
+      setTimeout(() => resolve(null), LOOKUP_TIMEOUT_MS)
+    })
+
+    let userId = null
+    try {
+      userId = await Promise.race([
+        slackService.findUserByEmail(member.email),
+        timeoutPromise
+      ])
+    } catch (err) {
+      console.error('[slackAccess] findUserByEmail failed:', err)
+      return
+    }
+
+    if (!userId) return
+
+    await Member.findByIdAndUpdate(
+      member._id,
+      {
+        slackInvited: true,
+        slackInvitedAt: new Date(),
+        slackUserId: userId
+      },
+      { runValidators: false }
+    )
+
+    await logActivity(
+      member._id,
+      'slack_access_auto_detected',
+      { slackUserId: userId },
+      { visibility: 'member' }
+    )
+  } catch (err) {
+    console.error('[slackAccess] autoFlagPreExistingSlackAccess error:', err)
+  }
+}
diff --git a/tests/server/models/member-slack-fields.test.js b/tests/server/models/member-slack-fields.test.js
new file mode 100644
index 0000000..d64ddac
--- /dev/null
+++ b/tests/server/models/member-slack-fields.test.js
@@ -0,0 +1,67 @@
+// Spec: docs/specs/wave-based-slack-onboarding.md
+// Test plan: docs/specs/wave-based-slack-onboarding-tests.md §1
+//
+// SCAFFOLD: `describe.skip` until the schema migration lands. Tests use the
+// schema's path metadata only — no DB connection required.
+
+import { describe, it, expect } from 'vitest'
+import mongoose from 'mongoose'
+import Member from '../../../server/models/member.js'
+
+describe.skip('Member schema — Slack fields (post-migration)', () => {
+  it('does not define slackInviteStatus (1.2)', () => {
+    expect(Member.schema.path('slackInviteStatus')).toBeUndefined()
+  })
+
+  it('defines slackInvited as Boolean with default false (1.1)', () => {
+    const path = Member.schema.path('slackInvited')
+    expect(path).toBeDefined()
+    expect(path.instance).toBe('Boolean')
+    expect(path.defaultValue).toBe(false)
+  })
+
+  it('defines slackInvitedAt as an optional Date (1.3)', () => {
+    const path = Member.schema.path('slackInvitedAt')
+    expect(path).toBeDefined()
+    expect(path.instance).toBe('Date')
+    expect(path.isRequired).toBeFalsy()
+  })
+
+  it('retains slackUserId as String (1.4)', () => {
+    const path = Member.schema.path('slackUserId')
+    expect(path).toBeDefined()
+    expect(path.instance).toBe('String')
+  })
+
+  it('does not auto-stamp slackInvitedAt via pre-save hook (1.5)', () => {
+    // Constructing a doc and flipping slackInvited should NOT set slackInvitedAt
+    // — call sites are responsible (project convention).
+    const doc = new Member({
+      email: 't@example.com',
+      name: 'T',
+      circle: 'community',
+      contributionAmount: 0
+    })
+    doc.slackInvited = true
+    expect(doc.slackInvitedAt).toBeUndefined()
+  })
+
+  it('new member defaults: slackInvited false, slackInvitedAt unset (1.1)', () => {
+    const doc = new Member({
+      email: 'new@example.com',
+      name: 'New',
+      circle: 'community',
+      contributionAmount: 0
+    })
+    expect(doc.slackInvited).toBe(false)
+    expect(doc.slackInvitedAt).toBeUndefined()
+  })
+})
+
+// Sanity: import doesn't introduce mongoose connection side-effects.
+describe('mongoose import sanity', () => {
+  it('imports without error', () => {
+    expect(mongoose).toBeDefined()
+    expect(Member).toBeDefined()
+  })
+})
diff --git a/tests/server/utils/slackAccess.test.js b/tests/server/utils/slackAccess.test.js
new file mode 100644
index 0000000..03db079
--- /dev/null
+++ b/tests/server/utils/slackAccess.test.js
@@ -0,0 +1,134 @@
+// Spec: docs/specs/wave-based-slack-onboarding.md
+// Test plan: docs/specs/wave-based-slack-onboarding-tests.md §2
+//
+// SCAFFOLD: `describe.skip` until the helper at server/utils/slackAccess.js
+// is created. Unskip during implementation (TDD).
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+
+vi.mock('../../../server/models/member.js', () => ({
+  default: {
+    findByIdAndUpdate: vi.fn().mockResolvedValue(null)
+  }
+}))
+
+vi.mock('../../../server/utils/slack.ts', () => ({
+  getSlackService: vi.fn()
+}))
+
+// `logActivity` is stubbed globally in tests/server/setup.js
+
+describe('autoFlagPreExistingSlackAccess (server/utils/slackAccess.js)', () => {
+  let autoFlagPreExistingSlackAccess
+  let Member
+  let getSlackService
+  let findUserByEmail
+
+  beforeEach(async () => {
+    vi.clearAllMocks()
+    vi.useFakeTimers()
+
+    findUserByEmail = vi.fn()
+    getSlackService = (await import('../../../server/utils/slack.ts')).getSlackService
+    getSlackService.mockReturnValue({ findUserByEmail })
+
+    Member = (await import('../../../server/models/member.js')).default
+    autoFlagPreExistingSlackAccess = (
+      await import('../../../server/utils/slackAccess.js')
+    ).autoFlagPreExistingSlackAccess
+
+    globalThis.logActivity.mockClear?.()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  const baseMember = () => ({
+    _id: 'member-abc',
+    email: 'pat@example.com',
+    name: 'Pat Example',
+    slackInvited: false
+  })
+
+  it('flags member when Slack lookup finds the email (2.1)', async () => {
+    findUserByEmail.mockResolvedValue('U123')
+
+    await autoFlagPreExistingSlackAccess(baseMember())
+
+    expect(Member.findByIdAndUpdate).toHaveBeenCalledWith(
+      'member-abc',
+      expect.objectContaining({
+        slackInvited: true,
+        slackInvitedAt: expect.any(Date),
+        slackUserId: 'U123'
+      }),
+      expect.objectContaining({ runValidators: false })
+    )
+    expect(globalThis.logActivity).toHaveBeenCalledWith(
+      'member-abc',
+      'slack_access_auto_detected',
+      expect.any(Object),
+      expect.any(Object)
+    )
+  })
+
+  it('no-ops when lookup returns no match (2.2)', async () => {
+    findUserByEmail.mockResolvedValue(null)
+
+    await autoFlagPreExistingSlackAccess(baseMember())
+
+    expect(Member.findByIdAndUpdate).not.toHaveBeenCalled()
+    expect(globalThis.logActivity).not.toHaveBeenCalled()
+  })
+
+  it('returns silently when Slack service is not configured (2.3)', async () => {
+    getSlackService.mockReturnValue(null)
+
+    await expect(
+      autoFlagPreExistingSlackAccess(baseMember())
+    ).resolves.not.toThrow()
+    expect(Member.findByIdAndUpdate).not.toHaveBeenCalled()
+  })
+
+  it('swallows Slack API errors and does not throw to caller (2.4)', async () => {
+    findUserByEmail.mockRejectedValue(new Error('slack down'))
+
+    await expect(
+      autoFlagPreExistingSlackAccess(baseMember())
+    ).resolves.not.toThrow()
+    expect(Member.findByIdAndUpdate).not.toHaveBeenCalled()
+  })
+
+  it('returns within timeout if Slack lookup hangs (2.5)', async () => {
+    findUserByEmail.mockImplementation(() => new Promise(() => { /* never resolves */ }))
+
+    const promise = autoFlagPreExistingSlackAccess(baseMember())
+    await vi.advanceTimersByTimeAsync(3500)
+    await expect(promise).resolves.not.toThrow()
+    expect(Member.findByIdAndUpdate).not.toHaveBeenCalled()
+  })
+
+  it('uses findByIdAndUpdate with runValidators:false, never member.save (2.6)', async () => {
+    findUserByEmail.mockResolvedValue('U999')
+    const member = baseMember()
+    member.save = vi.fn() // sentinel — must not be called
+
+    await autoFlagPreExistingSlackAccess(member)
+
+    expect(member.save).not.toHaveBeenCalled()
+    const opts = Member.findByIdAndUpdate.mock.calls.at(-1)?.[2]
+    expect(opts).toMatchObject({ runValidators: false })
+  })
+
+  it('writes activity log entry distinct from manual mark (2.7)', async () => {
+    findUserByEmail.mockResolvedValue('U123')
+    await autoFlagPreExistingSlackAccess(baseMember())
+
+    const [, action] = globalThis.logActivity.mock.calls.at(-1)
+    expect(action).toBe('slack_access_auto_detected')
+    expect(action).not.toBe('slack_invited_manually')
+  })
+
+  it.todo('idempotent: does not re-stamp slackInvitedAt on already-flagged member (2.8)')
+})