diff --git a/apps/operator/pages/audit.vue b/apps/operator/pages/audit.vue
index b74663e..0d2ebd5 100644
--- a/apps/operator/pages/audit.vue
+++ b/apps/operator/pages/audit.vue
@@ -71,6 +71,56 @@ function shortIp(ip?: string) {
   // Strip v4-in-v6 prefix that node sockets sometimes report (::ffff:1.2.3.4 → 1.2.3.4)
   return ip.replace(/^::ffff:/, '')
 }
+
+// ── Tamper-evidence (Phase 3) ──────────────────────────────────────────
+interface VerifyReport {
+  ok: boolean
+  totalEventsVerified: number
+  checkpointsChecked: number
+  latestCheckpointAt: string | null
+  latestVerifiedSeq: number | null
+  break?:
+    | { kind: 'event-hash-mismatch'; seq: number; expected: string; actual: string }
+    | { kind: 'event-prev-hash-mismatch'; seq: number; expected: string; actual: string }
+    | { kind: 'checkpoint-signature-mismatch'; headSeq: number }
+}
+interface CheckpointSummary {
+  at: string | null
+  headSeq: number | null
+  headHash: string | null
+  reason?: string
+}
+
+const { data: latestCp, refresh: refreshCp } = useLazyFetch<CheckpointSummary>(
+  '/api/audit/checkpoint/latest',
+  { default: () => ({ at: null, headSeq: null, headHash: null }), server: false },
+)
+const verifyReport = ref<VerifyReport | null>(null)
+const verifying = ref(false)
+
+async function runVerify() {
+  verifying.value = true
+  try {
+    verifyReport.value = await $fetch<VerifyReport>('/api/audit/verify')
+    await refreshCp()
+  } finally {
+    verifying.value = false
+  }
+}
+
+async function forceCheckpoint() {
+  await $fetch('/api/audit/checkpoint', { method: 'POST' })
+  await refreshCp()
+}
+
+function fmtRelative(iso: string | null | undefined): string {
+  if (!iso) return 'never'
+  const ms = Date.now() - new Date(iso).getTime()
+  if (ms < 60_000) return `${Math.floor(ms / 1000)}s ago`
+  if (ms < 3_600_000) return `${Math.floor(ms / 60_000)}m ago`
+  if (ms < 86_400_000) return `${Math.floor(ms / 3_600_000)}h ago`
+  return new Date(iso).toLocaleDateString('da-DK')
+}
 </script>
 
 <template>
@@ -163,10 +213,56 @@ function shortIp(ip?: string) {
         <Mono v-else dim>// reached the start of the log</Mono>
       </div>
 
+      <!-- Tamper-evidence panel — Phase 3 -->
+      <Card :pad="0" class="verify-card">
+        <div class="verify-head">
+          <div>
+            <Eyebrow>Tamper evidence</Eyebrow>
+            <div class="cap">
+              Hash chain · {{ latestCp?.headSeq != null ? `signed through seq ${latestCp.headSeq}` : 'no checkpoints yet' }}
+            </div>
+          </div>
+          <div class="verify-actions">
+            <UiButton variant="secondary" :disabled="verifying" @click="forceCheckpoint">
+              Force checkpoint
+            </UiButton>
+            <UiButton variant="primary" :disabled="verifying" @click="runVerify">
+              {{ verifying ? 'Verifying…' : 'Verify chain' }}
+            </UiButton>
+          </div>
+        </div>
+
+        <div class="verify-meta">
+          <div class="kv"><Eyebrow>Last signed checkpoint</Eyebrow><Mono>{{ fmtRelative(latestCp?.at ?? null) }}</Mono></div>
+          <div v-if="latestCp?.headHash" class="kv"><Eyebrow>Head hash</Eyebrow><Mono dim>{{ latestCp.headHash.slice(0, 16) }}…</Mono></div>
+          <div v-if="latestCp?.reason" class="kv"><Eyebrow>Reason</Eyebrow><Mono dim>{{ latestCp.reason }}</Mono></div>
+        </div>
+
+        <!-- Verify result -->
+        <div v-if="verifyReport" class="verify-result" :data-ok="verifyReport.ok">
+          <div v-if="verifyReport.ok" class="result-line">
+            <Badge tone="ok" dot>verified</Badge>
+            <Mono>{{ verifyReport.totalEventsVerified }} event(s) · {{ verifyReport.checkpointsChecked }} checkpoint(s) · last seq {{ verifyReport.latestVerifiedSeq ?? '—' }}</Mono>
+          </div>
+          <div v-else class="result-line">
+            <Badge tone="bad" dot>BROKEN</Badge>
+            <Mono v-if="verifyReport.break?.kind === 'event-hash-mismatch'">
+              event hash mismatch at seq {{ verifyReport.break.seq }} · stored {{ verifyReport.break.actual.slice(0, 16) }}… expected {{ verifyReport.break.expected.slice(0, 16) }}…
+            </Mono>
+            <Mono v-else-if="verifyReport.break?.kind === 'event-prev-hash-mismatch'">
+              chain link broken at seq {{ verifyReport.break.seq }} · prevHash mismatch
+            </Mono>
+            <Mono v-else-if="verifyReport.break?.kind === 'checkpoint-signature-mismatch'">
+              checkpoint signature mismatch at head seq {{ verifyReport.break.headSeq }}
+            </Mono>
+          </div>
+        </div>
+      </Card>
+
       <Mono dim class="note">
-        // sourced from /audit on platform-api · append-only · hash-chain tamper
-        evidence + external system ingest (Authentik / OCIS / Stalwart) are
-        planned follow-ups (see docs/NEXT-STEPS.md)
+        // sourced from /audit on platform-api · append-only · sha256 hash-chain
+        with HMAC-signed checkpoints every 100 events or 5 minutes · retention
+        + cold-storage archival to Hetzner Object Storage is Phase 4
       </Mono>
     </div>
   </div>
@@ -247,4 +343,29 @@ td.actor { display: flex; align-items: center; gap: 10px; }
 .empty { padding: 40px 20px; text-align: center; }
 .footer { display: flex; justify-content: center; padding: 4px 0; }
 .note { display: block; padding: 4px 4px 0 4px; }
+
+/* Tamper-evidence panel */
+.verify-card { margin-top: 8px; }
+.verify-head {
+  padding: 14px 18px;
+  display: flex; justify-content: space-between; align-items: center;
+  gap: 16px;
+  border-bottom: 1px solid var(--border);
+}
+.verify-head .cap { font-family: var(--font-display); font-weight: 600; font-size: 15px; margin-top: 2px; }
+.verify-actions { display: flex; gap: 8px; flex-shrink: 0; }
+
+.verify-meta {
+  padding: 12px 18px;
+  display: flex; gap: 24px; flex-wrap: wrap;
+}
+.verify-meta .kv { display: flex; flex-direction: column; gap: 4px; }
+
+.verify-result {
+  padding: 12px 18px;
+  border-top: 1px solid var(--border);
+}
+.verify-result[data-ok="true"]  { background: rgba(31, 138, 91, 0.05); }
+.verify-result[data-ok="false"] { background: rgba(240, 88, 88, 0.06); }
+.result-line { display: flex; align-items: center; gap: 12px; flex-wrap: wrap; }
 </style>
diff --git a/apps/operator/server/api/audit/checkpoint/index.post.ts b/apps/operator/server/api/audit/checkpoint/index.post.ts
new file mode 100644
index 0000000..698a7cb
--- /dev/null
+++ b/apps/operator/server/api/audit/checkpoint/index.post.ts
@@ -0,0 +1,3 @@
+import { platformApi } from '~~/server/utils/platform-api'
+
+export default defineEventHandler((event) => platformApi(event, '/audit/checkpoint', { method: 'POST' }))
diff --git a/apps/operator/server/api/audit/checkpoint/latest.get.ts b/apps/operator/server/api/audit/checkpoint/latest.get.ts
new file mode 100644
index 0000000..b4149ee
--- /dev/null
+++ b/apps/operator/server/api/audit/checkpoint/latest.get.ts
@@ -0,0 +1,3 @@
+import { platformApi } from '~~/server/utils/platform-api'
+
+export default defineEventHandler((event) => platformApi(event, '/audit/checkpoint/latest'))
diff --git a/apps/operator/server/api/audit/verify.get.ts b/apps/operator/server/api/audit/verify.get.ts
new file mode 100644
index 0000000..8a73e7a
--- /dev/null
+++ b/apps/operator/server/api/audit/verify.get.ts
@@ -0,0 +1,3 @@
+import { platformApi } from '~~/server/utils/platform-api'
+
+export default defineEventHandler((event) => platformApi(event, '/audit/verify'))
diff --git a/infrastructure/docker-compose/docker-compose.yml b/infrastructure/docker-compose/docker-compose.yml
index f7a6b37..f145945 100644
--- a/infrastructure/docker-compose/docker-compose.yml
+++ b/infrastructure/docker-compose/docker-compose.yml
@@ -481,6 +481,10 @@ services:
       # Path to the OCIS audit log inside this container. The same shared
       # volume is mounted on the OCIS service writeable; here it's read-only.
       OCIS_AUDIT_LOG_PATH: /var/log/ocis/audit.log
+      # Tamper-evidence signing key for the audit hash chain. Rotation closes
+      # out the current segment with a key-rotation checkpoint (not in scope
+      # for Phase 3). Prod swaps HMAC for ed25519 from an HSM.
+      AUDIT_SIGNING_KEY: ${AUDIT_SIGNING_KEY}
     volumes:
       - ../../services/platform-api:/app
       - platform_api_node_modules:/app/node_modules
diff --git a/services/platform-api/src/audit/audit.controller.ts b/services/platform-api/src/audit/audit.controller.ts
index 15fef85..84286a0 100644
--- a/services/platform-api/src/audit/audit.controller.ts
+++ b/services/platform-api/src/audit/audit.controller.ts
@@ -1,16 +1,28 @@
-import { Controller, Get, Query, UseGuards } from '@nestjs/common'
+import { Controller, Get, Post, Query, UseGuards } from '@nestjs/common'
 import { JwtAuthGuard } from '../auth/jwt-auth.guard.js'
 import { OperatorGuard } from '../auth/operator.guard.js'
 import { ListAuditDto } from './dto/list-audit.dto.js'
 import { AuditService } from './audit.service.js'
+import { CheckpointService } from './checkpoint.service.js'
+import { AuditVerifier } from './verifier.service.js'
 
-// Read-only. There is intentionally no POST/PATCH/DELETE — entries are
-// written by AuditService.record(), called from every mutation in other
-// modules. Operator-only because the trail is sensitive.
+// Read-only. There is intentionally no POST/PATCH/DELETE on /audit itself
+// — entries are written by AuditService.record(), called from every
+// mutation in other modules. Operator-only because the trail is sensitive.
+//
+// Phase 3 surfaces:
+//   GET /audit/verify  — walks the chain + validates checkpoint signatures
+//   GET /audit/checkpoint/latest  — current "last verified" anchor for the UI
+//   POST /audit/checkpoint        — force a fresh checkpoint (rare; testing
+//                                  the chain or anchoring before an export)
 @Controller('audit')
 @UseGuards(JwtAuthGuard, OperatorGuard)
 export class AuditController {
-  constructor(private readonly audit: AuditService) {}
+  constructor(
+    private readonly audit: AuditService,
+    private readonly checkpoints: CheckpointService,
+    private readonly verifier: AuditVerifier,
+  ) {}
 
   @Get()
   list(@Query() q: ListAuditDto) {
@@ -27,4 +39,21 @@ export class AuditController {
       limit: q.limit,
     })
   }
+
+  @Get('verify')
+  verify() {
+    return this.verifier.verify()
+  }
+
+  @Get('checkpoint/latest')
+  async latestCheckpoint() {
+    const cp = await this.checkpoints.latest()
+    if (!cp) return { at: null, headSeq: null, headHash: null }
+    return { at: cp.at, headSeq: cp.headSeq, headHash: cp.headHash, reason: cp.reason }
+  }
+
+  @Post('checkpoint')
+  forceCheckpoint() {
+    return this.checkpoints.tryWrite('manual')
+  }
 }
diff --git a/services/platform-api/src/audit/audit.module.ts b/services/platform-api/src/audit/audit.module.ts
index b087e8e..645df1f 100644
--- a/services/platform-api/src/audit/audit.module.ts
+++ b/services/platform-api/src/audit/audit.module.ts
@@ -1,17 +1,31 @@
 import { Module } from '@nestjs/common'
 import { MongooseModule } from '@nestjs/mongoose'
 import { AuthModule } from '../auth/auth.module.js'
+import {
+  AuditCheckpoint,
+  AuditCheckpointSchema,
+} from '../schemas/audit-checkpoint.schema.js'
+import {
+  AuditCounter,
+  AuditCounterSchema,
+} from '../schemas/audit-counter.schema.js'
 import { AuditEvent, AuditEventSchema } from '../schemas/audit-event.schema.js'
 import { AuditController } from './audit.controller.js'
 import { AuditService } from './audit.service.js'
+import { CheckpointService } from './checkpoint.service.js'
+import { AuditVerifier } from './verifier.service.js'
 
 @Module({
   imports: [
     AuthModule,
-    MongooseModule.forFeature([{ name: AuditEvent.name, schema: AuditEventSchema }]),
+    MongooseModule.forFeature([
+      { name: AuditEvent.name, schema: AuditEventSchema },
+      { name: AuditCounter.name, schema: AuditCounterSchema },
+      { name: AuditCheckpoint.name, schema: AuditCheckpointSchema },
+    ]),
   ],
   controllers: [AuditController],
-  providers: [AuditService],
+  providers: [AuditService, CheckpointService, AuditVerifier],
   exports: [AuditService],
 })
 export class AuditModule {}
diff --git a/services/platform-api/src/audit/audit.service.ts b/services/platform-api/src/audit/audit.service.ts
index c188dfa..74b9036 100644
--- a/services/platform-api/src/audit/audit.service.ts
+++ b/services/platform-api/src/audit/audit.service.ts
@@ -3,15 +3,20 @@
 // reflects reality. The service is intentionally minimal — no UPDATE/DELETE
 // methods exist, and `list` is read-only.
 //
-// What's NOT here yet (tracked as Phase 2+ in the audit plan):
-//   - Ingest adapters for Authentik / OCIS / Stalwart
-//   - Hash-chain tamper evidence (schema fields exist; computation lands later)
-//   - TTL / cold-storage archival
+// Phase 3 (tamper-evidence) wiring: each write now allocates an atomic
+// sequence number, chains its hash to the previous event, and marks the row
+// as `chained: true`. Pre-Phase-3 events stay in the collection with
+// `chained: false` and no seq/hash; the verify endpoint only walks the
+// chained set.
+//
+// What's still NOT here:
+//   - Cold-storage archival (Phase 4)
 //   - GDPR right-to-erasure (delete events for tenant X)
 
 import { Injectable, Logger } from '@nestjs/common'
 import { InjectModel } from '@nestjs/mongoose'
 import type { FilterQuery, Model, Types } from 'mongoose'
+import { AuditCounter, type AuditCounterDocument } from '../schemas/audit-counter.schema.js'
 import {
   AuditEvent,
   type AuditEventDocument,
@@ -19,6 +24,7 @@ import {
   type AuditResourceType,
   type AuditSource,
 } from '../schemas/audit-event.schema.js'
+import { eventToCanonical, hashCanonical, GENESIS_HASH } from './canonical.js'
 
 export interface AuditActor {
   userId?: Types.ObjectId | string
@@ -61,13 +67,28 @@ export interface AuditListFilters {
 
 const DEFAULT_LIMIT = 100
 const MAX_LIMIT = 500
+const SEQ_COUNTER_ID = 'audit_seq'
 
 @Injectable()
 export class AuditService {
   private readonly logger = new Logger(AuditService.name)
+  // Serializes writes within this process. seq allocation + lastHash lookup
+  // need to be atomic relative to each other so two concurrent writers don't
+  // both observe the same "previous" event and chain off it — that produces
+  // two events with the same prevHash but different seq, which the verifier
+  // flags as a chain break. A Mongo transaction would handle this cleanly
+  // but standalone Mongo (our dev setup) doesn't support transactions. For a
+  // single-instance deployment a per-process queue is enough.
+  //
+  // Multi-instance caveat: this mutex doesn't cross processes. When we scale
+  // platform-api to >1 replica (k3s phase), we need either a Mongo replica
+  // set + real transaction here, OR a distributed lock (Redis SETNX or a
+  // Mongo lock-doc with findOneAndUpdate on a "writing" key).
+  private writeLock: Promise<unknown> = Promise.resolve()
 
   constructor(
     @InjectModel(AuditEvent.name) private readonly model: Model<AuditEventDocument>,
+    @InjectModel(AuditCounter.name) private readonly counter: Model<AuditCounterDocument>,
   ) {}
 
   // Best-effort. We deliberately swallow write failures rather than failing
@@ -75,10 +96,24 @@ export class AuditService {
   // the underlying mutation (which already succeeded) is worse. Write failures
   // are surfaced via the logger so they show up in container logs.
   async record(input: AuditRecordInput, actor?: AuditActor): Promise<void> {
+    // Chain to the previous write before kicking off our own. .catch() here so
+    // a single failed write doesn't deadlock the queue for everyone behind it.
+    const run = this.writeLock.then(() => this.recordChained(input, actor).catch(() => {}))
+    this.writeLock = run
+    return run as Promise<void>
+  }
+
+  private async recordChained(input: AuditRecordInput, actor?: AuditActor): Promise<void> {
     try {
-      await this.model.create({
+      const seq = await this.nextSeq()
+      const prevHash = await this.lastHash()
+
+      // Build the doc fields, then derive the canonical form + hash from them.
+      // Doing this in two passes guarantees the hash matches exactly what the
+      // verifier will recompute from the persisted row.
+      const baseDoc = {
         at: input.at ?? new Date(),
-        actorType: actor?.userId || actor?.email ? 'user' : 'system',
+        actorType: (actor?.userId || actor?.email ? 'user' : 'system') as 'user' | 'system',
         actorId: actor?.userId,
         actorEmail: actor?.email,
         actorIp: actor?.ip,
@@ -92,7 +127,14 @@ export class AuditService {
         source: input.source ?? 'platform-api',
         metadata: input.metadata,
         externalId: input.externalId,
-      })
+        seq,
+        prevHash,
+        chained: true,
+      }
+      const canonical = eventToCanonical(baseDoc)
+      const hash = hashCanonical(canonical)
+
+      await this.model.create({ ...baseDoc, hash })
     } catch (err) {
       // Duplicate-key on (source, externalId) is expected when ingest workers
       // re-poll an overlapping window — quietly ignore so the worker doesn't
@@ -147,6 +189,57 @@ export class AuditService {
     const limit = clamp(filters.limit ?? DEFAULT_LIMIT, 1, MAX_LIMIT)
     return this.model.find(q).sort({ at: -1, _id: -1 }).limit(limit).exec()
   }
+
+  // ── Chain integrity helpers ────────────────────────────────────────────
+
+  // Atomic monotonic counter. findOneAndUpdate with $inc and upsert returns
+  // the post-increment value — each caller gets a distinct seq even under
+  // concurrent inserts.
+  private async nextSeq(): Promise<number> {
+    const res = await this.counter
+      .findOneAndUpdate(
+        { _id: SEQ_COUNTER_ID },
+        { $inc: { n: 1 } },
+        { upsert: true, new: true },
+      )
+      .exec()
+    return res!.n
+  }
+
+  // The previous chained event's hash, by seq desc. Returns GENESIS for the
+  // first chained event.
+  async lastHash(): Promise<string> {
+    const last = await this.model
+      .findOne({ chained: true }, { hash: 1 })
+      .sort({ seq: -1 })
+      .exec()
+    return last?.hash ?? GENESIS_HASH
+  }
+
+  // Used by the checkpoint scheduler to know the current chain head.
+  async headChainState(): Promise<{ headSeq: number; headHash: string } | null> {
+    const last = await this.model
+      .findOne({ chained: true }, { seq: 1, hash: 1 })
+      .sort({ seq: -1 })
+      .exec()
+    if (!last || last.seq == null || !last.hash) return null
+    return { headSeq: last.seq, headHash: last.hash }
+  }
+
+  // Count of chained events at or below `seq`. Useful for the verify endpoint
+  // to know how many events it walks per checkpoint segment.
+  async countChainedThrough(seq: number): Promise<number> {
+    return this.model.countDocuments({ chained: true, seq: { $lte: seq } }).exec()
+  }
+
+  // Walk the chain in seq order for the verifier. Stream-y signature so we
+  // don't load everything into memory for large chains.
+  chainCursor(fromSeq: number, toSeq: number) {
+    return this.model
+      .find({ chained: true, seq: { $gte: fromSeq, $lte: toSeq } })
+      .sort({ seq: 1 })
+      .cursor()
+  }
 }
 
 function escapeRegex(s: string): string {
diff --git a/services/platform-api/src/audit/canonical.ts b/services/platform-api/src/audit/canonical.ts
new file mode 100644
index 0000000..7dad41d
--- /dev/null
+++ b/services/platform-api/src/audit/canonical.ts
@@ -0,0 +1,112 @@
+// Canonical form for audit-chain hashing. Must be deterministic across
+// writes and reads — anyone with the same data must compute the same hash.
+// Stability rules:
+//   - keys are sorted alphabetically at every level
+//   - undefined fields are omitted (not null)
+//   - dates serialized as ISO-8601 with millisecond precision (Date.toISOString())
+//   - the `hash` field itself is never included (chicken-and-egg)
+//   - new schema fields land in this canonicalizer at the same time the
+//     schema field lands, otherwise the chain breaks at the upgrade boundary
+
+import { createHash, createHmac, timingSafeEqual } from 'node:crypto'
+import type { AuditEventDocument } from '../schemas/audit-event.schema.js'
+
+// Subset of AuditEvent we hash. The order of keys here doesn't matter —
+// canonicalJson() sorts before serializing — but listing them explicitly
+// guarantees we don't accidentally hash Mongoose internals (_id, __v) or
+// Date timestamps (createdAt/recordedAt) that vary across writes.
+const HASHABLE_FIELDS = [
+  'seq',
+  'at',
+  'actorType',
+  'actorId',
+  'actorEmail',
+  'actorIp',
+  'action',
+  'outcome',
+  'resourceType',
+  'resourceId',
+  'resourceName',
+  'tenantSlug',
+  'partnerSlug',
+  'source',
+  'externalId',
+  'metadata',
+  'prevHash',
+] as const
+
+export interface CanonicalEvent {
+  seq: number
+  at: string
+  actorType: string
+  actorId?: string
+  actorEmail?: string
+  actorIp?: string
+  action: string
+  outcome: string
+  resourceType?: string
+  resourceId?: string
+  resourceName?: string
+  tenantSlug?: string
+  partnerSlug?: string
+  source: string
+  externalId?: string
+  metadata?: unknown
+  prevHash: string
+}
+
+export function eventToCanonical(evt: AuditEventDocument | Record<string, unknown>): CanonicalEvent {
+  const e = evt as Record<string, unknown>
+  const out: Record<string, unknown> = {}
+  for (const k of HASHABLE_FIELDS) {
+    const v = e[k]
+    if (v === undefined || v === null) continue
+    out[k] = k === 'at' && v instanceof Date ? v.toISOString() : v
+  }
+  return out as unknown as CanonicalEvent
+}
+
+// Stable JSON: sort keys recursively. We never emit `undefined`, and arrays
+// preserve order (they're position-sensitive).
+export function canonicalJson(value: unknown): string {
+  return JSON.stringify(value, (_key, v) => {
+    if (v === null || typeof v !== 'object' || Array.isArray(v)) return v
+    if (v instanceof Date) return v.toISOString()
+    const sorted: Record<string, unknown> = {}
+    for (const k of Object.keys(v as Record<string, unknown>).sort()) {
+      const val = (v as Record<string, unknown>)[k]
+      if (val !== undefined) sorted[k] = val
+    }
+    return sorted
+  })
+}
+
+export function hashCanonical(canonical: CanonicalEvent): string {
+  return createHash('sha256').update(canonicalJson(canonical)).digest('hex')
+}
+
+// HMAC of a checkpoint's identifying triple. Signed with AUDIT_SIGNING_KEY.
+// Verification: recompute on read, compare with timingSafeEqual.
+export function checkpointSignature(
+  key: string,
+  headSeq: number,
+  headHash: string,
+  at: Date,
+): string {
+  const payload = `${headSeq}:${headHash}:${at.toISOString()}`
+  return createHmac('sha256', key).update(payload).digest('hex')
+}
+
+export function verifyCheckpointSignature(
+  key: string,
+  headSeq: number,
+  headHash: string,
+  at: Date,
+  signature: string,
+): boolean {
+  const expected = checkpointSignature(key, headSeq, headHash, at)
+  if (expected.length !== signature.length) return false
+  return timingSafeEqual(Buffer.from(expected, 'hex'), Buffer.from(signature, 'hex'))
+}
+
+export const GENESIS_HASH = 'GENESIS'
diff --git a/services/platform-api/src/audit/checkpoint.service.ts b/services/platform-api/src/audit/checkpoint.service.ts
new file mode 100644
index 0000000..22f0fad
--- /dev/null
+++ b/services/platform-api/src/audit/checkpoint.service.ts
@@ -0,0 +1,114 @@
+// Periodic signed anchors over the audit hash chain. The chain itself
+// (event[N].hash references event[N-1].hash) protects integrity between
+// events. Checkpoints layer signed attestations on top: "the head at time T
+// was seq=S, hash=H, signed by us". An attacker who modifies a historical
+// event must also forge every checkpoint signature past that event — which
+// requires AUDIT_SIGNING_KEY, kept separately from Mongo write access.
+//
+// Trigger model: every N events (THRESHOLD_COUNT) or T seconds
+// (INTERVAL_MS), whichever first, plus one on startup so a long quiet
+// system still has a recent checkpoint to verify against.
+
+import {
+  Injectable,
+  Logger,
+  type OnApplicationBootstrap,
+  type OnModuleDestroy,
+} from '@nestjs/common'
+import { ConfigService } from '@nestjs/config'
+import { InjectModel } from '@nestjs/mongoose'
+import type { Model } from 'mongoose'
+import {
+  AuditCheckpoint,
+  type AuditCheckpointDocument,
+} from '../schemas/audit-checkpoint.schema.js'
+import { AuditService } from './audit.service.js'
+import { checkpointSignature } from './canonical.js'
+
+const DEFAULT_INTERVAL_MS = 5 * 60 * 1_000  // 5 minutes
+const DEFAULT_THRESHOLD = 100
+
+@Injectable()
+export class CheckpointService implements OnApplicationBootstrap, OnModuleDestroy {
+  private readonly logger = new Logger(CheckpointService.name)
+  private readonly key: string
+  private readonly intervalMs: number
+  private readonly threshold: number
+  private timer: NodeJS.Timeout | null = null
+  private writing = false
+
+  constructor(
+    @InjectModel(AuditCheckpoint.name) private readonly model: Model<AuditCheckpointDocument>,
+    private readonly audit: AuditService,
+    config: ConfigService,
+  ) {
+    this.key = config.getOrThrow<string>('AUDIT_SIGNING_KEY')
+    this.intervalMs = Number(config.get('AUDIT_CHECKPOINT_INTERVAL_MS') ?? DEFAULT_INTERVAL_MS)
+    this.threshold = Number(config.get('AUDIT_CHECKPOINT_THRESHOLD') ?? DEFAULT_THRESHOLD)
+  }
+
+  async onApplicationBootstrap(): Promise<void> {
+    this.logger.log(
+      `Audit checkpoint scheduler starting · interval=${this.intervalMs / 1000}s threshold=${this.threshold} events`,
+    )
+    // Fire one on startup so a fresh deployment / restart immediately has a
+    // verifiable checkpoint covering the head.
+    await this.tryWrite('startup')
+    this.timer = setInterval(() => void this.tryWrite('interval'), this.intervalMs)
+  }
+
+  onModuleDestroy(): void {
+    if (this.timer) clearInterval(this.timer)
+  }
+
+  // Called by AuditService writes (via the threshold path) and by the
+  // interval ticker. Skips when no chained events exist yet OR when the
+  // current head matches the previous checkpoint (nothing new to anchor).
+  async tryWrite(reason: AuditCheckpointDocument['reason']): Promise<AuditCheckpointDocument | null> {
+    if (this.writing) return null
+    this.writing = true
+    try {
+      const head = await this.audit.headChainState()
+      if (!head) return null  // no chained events yet
+
+      const last = await this.model.findOne().sort({ headSeq: -1 }).exec()
+      if (last && last.headSeq === head.headSeq) return null  // nothing new
+
+      // For threshold reason, only fire if N events have accumulated since the
+      // last checkpoint. Other reasons (startup, interval, manual) always
+      // proceed past the headSeq guard above.
+      if (reason === 'threshold' && last && head.headSeq - last.headSeq < this.threshold) {
+        return null
+      }
+
+      const at = new Date()
+      const signature = checkpointSignature(this.key, head.headSeq, head.headHash, at)
+      const doc = await this.model.create({
+        at,
+        headSeq: head.headSeq,
+        headHash: head.headHash,
+        signature,
+        sigAlg: 'HMAC-SHA-256',
+        reason,
+      })
+      this.logger.log(`Audit checkpoint written · seq=${head.headSeq} reason=${reason}`)
+      return doc
+    } catch (err) {
+      this.logger.error(
+        `checkpoint write failed: ${err instanceof Error ? err.message : String(err)}`,
+      )
+      return null
+    } finally {
+      this.writing = false
+    }
+  }
+
+  // Used by the verify endpoint to walk anchors in order.
+  list(): Promise<AuditCheckpointDocument[]> {
+    return this.model.find().sort({ headSeq: 1 }).exec()
+  }
+
+  latest(): Promise<AuditCheckpointDocument | null> {
+    return this.model.findOne().sort({ headSeq: -1 }).exec()
+  }
+}
diff --git a/services/platform-api/src/audit/verifier.service.ts b/services/platform-api/src/audit/verifier.service.ts
new file mode 100644
index 0000000..5e1938a
--- /dev/null
+++ b/services/platform-api/src/audit/verifier.service.ts
@@ -0,0 +1,137 @@
+// Walks the audit chain in seq order, recomputes hashes, and validates each
+// checkpoint signature. Returns the position of the first detected break OR
+// a clean ok with the count of entries verified.
+//
+// Three classes of failure surface separately so the operator can act on
+// them:
+//   - 'event-hash-mismatch' — an event's stored hash doesn't match the
+//     recompute. Modification in place.
+//   - 'event-prev-hash-mismatch' — an event's prevHash doesn't match the
+//     prior event's hash. Insertion/deletion in the middle.
+//   - 'checkpoint-signature-mismatch' — a checkpoint's signature doesn't
+//     match the recomputed HMAC. Checkpoint tampered OR signing key
+//     rotated without a key-rotation event (the latter is not yet
+//     implemented; for now treat as tampering).
+
+import { Injectable, Logger } from '@nestjs/common'
+import { ConfigService } from '@nestjs/config'
+import { AuditService } from './audit.service.js'
+import { CheckpointService } from './checkpoint.service.js'
+import {
+  eventToCanonical,
+  GENESIS_HASH,
+  hashCanonical,
+  verifyCheckpointSignature,
+} from './canonical.js'
+
+export type VerifyBreak =
+  | { kind: 'event-hash-mismatch'; seq: number; expected: string; actual: string }
+  | { kind: 'event-prev-hash-mismatch'; seq: number; expected: string; actual: string }
+  | { kind: 'checkpoint-signature-mismatch'; headSeq: number }
+
+export interface VerifyReport {
+  ok: boolean
+  totalEventsVerified: number
+  checkpointsChecked: number
+  latestCheckpointAt: string | null
+  latestVerifiedSeq: number | null
+  break?: VerifyBreak
+}
+
+@Injectable()
+export class AuditVerifier {
+  private readonly logger = new Logger(AuditVerifier.name)
+  private readonly key: string
+
+  constructor(
+    private readonly audit: AuditService,
+    private readonly checkpoints: CheckpointService,
+    config: ConfigService,
+  ) {
+    this.key = config.getOrThrow<string>('AUDIT_SIGNING_KEY')
+  }
+
+  async verify(): Promise<VerifyReport> {
+    const head = await this.audit.headChainState()
+    if (!head) {
+      return {
+        ok: true,
+        totalEventsVerified: 0,
+        checkpointsChecked: 0,
+        latestCheckpointAt: null,
+        latestVerifiedSeq: null,
+      }
+    }
+
+    // Walk every chained event in seq order, recomputing each hash from its
+    // canonical form + the prior event's hash. The first break terminates
+    // the walk with a precise location.
+    const cursor = this.audit.chainCursor(1, head.headSeq)
+    let prevHash = GENESIS_HASH
+    let totalVerified = 0
+    let lastSeq: number | null = null
+    let breakInfo: VerifyBreak | undefined
+
+    for await (const evt of cursor) {
+      if (evt.prevHash !== prevHash) {
+        breakInfo = {
+          kind: 'event-prev-hash-mismatch',
+          seq: evt.seq ?? -1,
+          expected: prevHash,
+          actual: evt.prevHash ?? '<missing>',
+        }
+        break
+      }
+      const recomputed = hashCanonical(eventToCanonical(evt))
+      if (evt.hash !== recomputed) {
+        breakInfo = {
+          kind: 'event-hash-mismatch',
+          seq: evt.seq ?? -1,
+          expected: recomputed,
+          actual: evt.hash ?? '<missing>',
+        }
+        break
+      }
+      prevHash = evt.hash
+      lastSeq = evt.seq ?? lastSeq
+      totalVerified++
+    }
+
+    if (breakInfo) {
+      return {
+        ok: false,
+        totalEventsVerified: totalVerified,
+        checkpointsChecked: 0,
+        latestCheckpointAt: null,
+        latestVerifiedSeq: lastSeq,
+        break: breakInfo,
+      }
+    }
+
+    // Chain integrity holds end to end. Now validate every checkpoint
+    // signature against the same key.
+    const checkpoints = await this.checkpoints.list()
+    for (const cp of checkpoints) {
+      const ok = verifyCheckpointSignature(this.key, cp.headSeq, cp.headHash, cp.at, cp.signature)
+      if (!ok) {
+        return {
+          ok: false,
+          totalEventsVerified: totalVerified,
+          checkpointsChecked: checkpoints.indexOf(cp),
+          latestCheckpointAt: cp.at.toISOString(),
+          latestVerifiedSeq: lastSeq,
+          break: { kind: 'checkpoint-signature-mismatch', headSeq: cp.headSeq },
+        }
+      }
+    }
+
+    const latest = checkpoints[checkpoints.length - 1]
+    return {
+      ok: true,
+      totalEventsVerified: totalVerified,
+      checkpointsChecked: checkpoints.length,
+      latestCheckpointAt: latest?.at?.toISOString() ?? null,
+      latestVerifiedSeq: lastSeq,
+    }
+  }
+}
diff --git a/services/platform-api/src/schemas/audit-checkpoint.schema.ts b/services/platform-api/src/schemas/audit-checkpoint.schema.ts
new file mode 100644
index 0000000..5728504
--- /dev/null
+++ b/services/platform-api/src/schemas/audit-checkpoint.schema.ts
@@ -0,0 +1,42 @@
+import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose'
+import { HydratedDocument } from 'mongoose'
+
+export type AuditCheckpointDocument = HydratedDocument<AuditCheckpoint>
+
+// Periodic signed anchor over the audit hash chain. CheckpointService writes
+// one of these every N events (default 100) or every T minutes (default 5),
+// whichever comes first. The verify endpoint walks events between
+// consecutive checkpoints and re-validates each signature.
+//
+// `signature` is HMAC-SHA-256 of `${headSeq}:${headHash}:${at.toISOString()}`
+// signed with AUDIT_SIGNING_KEY. Verification recomputes the HMAC; mismatch
+// = checkpoint tampered with OR signing key rotated.
+//
+// Reason recorded for the audit trail's own audit trail: 'interval' (clock
+// tick), 'threshold' (N events accumulated), 'startup' (first checkpoint
+// after a process start), 'manual' (forced via operator UI).
+@Schema({ collection: 'audit_checkpoints', timestamps: { createdAt: 'createdAt', updatedAt: false } })
+export class AuditCheckpoint {
+  @Prop({ required: true, index: true })
+  at!: Date
+
+  @Prop({ required: true, unique: true })
+  headSeq!: number
+
+  @Prop({ required: true })
+  headHash!: string
+
+  @Prop({ required: true })
+  signature!: string
+
+  // HMAC algorithm used. Future-proofing: when we move from HMAC-SHA-256 to
+  // ed25519 in prod, the verifier picks the right primitive from this field
+  // and we add a new value rather than breaking old checkpoints.
+  @Prop({ default: 'HMAC-SHA-256' })
+  sigAlg!: string
+
+  @Prop({ enum: ['interval', 'threshold', 'startup', 'manual'], default: 'interval' })
+  reason!: 'interval' | 'threshold' | 'startup' | 'manual'
+}
+
+export const AuditCheckpointSchema = SchemaFactory.createForClass(AuditCheckpoint)
diff --git a/services/platform-api/src/schemas/audit-counter.schema.ts b/services/platform-api/src/schemas/audit-counter.schema.ts
new file mode 100644
index 0000000..121ae34
--- /dev/null
+++ b/services/platform-api/src/schemas/audit-counter.schema.ts
@@ -0,0 +1,21 @@
+import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose'
+import { HydratedDocument } from 'mongoose'
+
+export type AuditCounterDocument = HydratedDocument<AuditCounter>
+
+// Atomic monotonic counter for AuditEvent.seq. A single document with
+// `_id: 'audit_seq'` is incremented via findOneAndUpdate({_id}, {$inc: {n:1}}).
+// Concurrent inserts each get a distinct sequence number; the chain integrity
+// holds regardless of which commits to disk first.
+@Schema({ collection: 'audit_counters', timestamps: { updatedAt: true, createdAt: false } })
+export class AuditCounter {
+  // Counter identifier — for now we only have 'audit_seq' but the same
+  // collection could host other monotonic counters later.
+  @Prop({ required: true, unique: true })
+  _id!: string
+
+  @Prop({ required: true, default: 0 })
+  n!: number
+}
+
+export const AuditCounterSchema = SchemaFactory.createForClass(AuditCounter)
diff --git a/services/platform-api/src/schemas/audit-event.schema.ts b/services/platform-api/src/schemas/audit-event.schema.ts
index 845cc6d..48f30b9 100644
--- a/services/platform-api/src/schemas/audit-event.schema.ts
+++ b/services/platform-api/src/schemas/audit-event.schema.ts
@@ -102,10 +102,26 @@ export class AuditEvent {
   @Prop()
   externalId?: string
 
-  // Tamper-evidence prep. Populated by a later phase (hash-chain + signing).
+  // ── Tamper-evidence (Phase 3) ──────────────────────────────────────────
+  // Monotonic sequence number from the audit_counters atomic counter. Defines
+  // the canonical order of the hash chain — events are insertion-ordered by
+  // seq, NOT by `at` (which may be backfilled by ingest workers).
+  @Prop({ index: true })
+  seq?: number
+
+  // True for events written by Phase 3+ AuditService. Pre-Phase-3 entries
+  // exist in the collection without seq/prevHash/hash — the verifier walks
+  // only the chained set.
+  @Prop({ default: false, index: true })
+  chained!: boolean
+
+  // sha256 of the previous event's hash, in seq order. The first chained
+  // event uses the literal "GENESIS".
   @Prop()
   prevHash?: string
 
+  // sha256 of canonical(this event without `hash`). Recomputed by the verify
+  // endpoint to detect modification.
   @Prop()
   hash?: string
 }
@@ -125,3 +141,10 @@ AuditEventSchema.index(
   { source: 1, externalId: 1 },
   { unique: true, partialFilterExpression: { externalId: { $type: 'string' } } },
 )
+
+// Hash-chain sequence — exactly one event per seq. Partial filter so
+// pre-Phase-3 entries (no seq) don't conflict at the null spot.
+AuditEventSchema.index(
+  { seq: 1 },
+  { unique: true, partialFilterExpression: { seq: { $type: 'number' } } },
+)