diff --git a/backend/src/eventOutbox.ts b/backend/src/eventOutbox.ts
index d0c46c24d..9947e11f7 100644
--- a/backend/src/eventOutbox.ts
+++ b/backend/src/eventOutbox.ts
@@ -168,6 +168,10 @@ class EventOutboxService {
    * Returns the created outbox record.
    */
   async writeEvent(input: OutboxWriteInput): Promise<EventOutboxRecord> {
+    if (!prisma.eventOutbox) {
+      throw new Error('prisma.eventOutbox is not initialized. Ensure Prisma schema includes eventOutbox model.');
+    }
+
     assertEventOutboxModelAvailable();
     const now = new Date();
     const record = await getEventOutboxDelegate().create({
diff --git a/backend/src/idempotency.ts b/backend/src/idempotency.ts
index cefbc293f..cf111e0d6 100644
--- a/backend/src/idempotency.ts
+++ b/backend/src/idempotency.ts
@@ -27,6 +27,8 @@ import { prisma } from './prisma';
 import { redisClientManager } from './rateLimiter';
 import { logger } from './middleware/structuredLogging';
 import type { Request, Response, NextFunction } from 'express';
+import NodeCache from 'node-cache';
+import { redisClientManager } from './rateLimiter';
 
 /**
  * The replay cache used by money-moving operations (deposits, withdrawals,
@@ -394,6 +396,306 @@ declare global {
   }
 }
 
+// ─── Error Classes ──────────────────────────────────────────────────────
+
+export class IdempotencyConflictError extends Error {
+  constructor(message = 'Idempotency key already used for a different request body') {
+    super(message);
+    this.name = 'IdempotencyConflictError';
+  }
+}
+
+// ─── Internal Types ─────────────────────────────────────────────────────
+
+export interface IdempotentOperationResult<T> {
+  statusCode: number;
+  body: T;
+}
+
+interface StoredResponse<T> extends IdempotentOperationResult<T> {
+  fingerprint: string;
+  metadata: IdempotencyKeyMetadata;
+}
+
+interface PendingOperation<T> {
+  fingerprint: string;
+  promise: Promise<StoredResponse<T>>;
+  metadata: IdempotencyKeyMetadata;
+}
+
+export interface IdempotencyKeyInfo {
+  key: string;
+  metadata: IdempotencyKeyMetadata;
+}
+
+export interface IdempotencyMetrics {
+  hits: number;
+  conflicts: number;
+  evictions: number;
+  activeKeys: number;
+  pendingKeys: number;
+}
+
+// ─── Redis key prefix ───────────────────────────────────────────────────
+
+const REDIS_PREFIX = 'idempotency:';
+
+// ─── Fingerprint helper ─────────────────────────────────────────────────
+
+export function getIdempotencyHashThreshold(): number {
+  return parseInt(process.env.IDEMPOTENCY_HASH_THRESHOLD_BYTES || '4096', 10);
+}
+
+export function buildIdempotencyFingerprint(payload: unknown): string {
+  const stable = stableStringify(payload);
+  const byteLength = Buffer.byteLength(stable, 'utf-8');
+  if (byteLength > getIdempotencyHashThreshold()) {
+    return `hashv1:${crypto.createHash('sha256').update(stable).digest('hex')}`;
+  }
+  return stable;
+}
+
+function stableStringify(value: unknown): string {
+  if (value === null) return 'null';
+  if (value instanceof Date) return JSON.stringify(value.toISOString());
+  if (typeof value !== 'object') return JSON.stringify(value);
+
+  if (Array.isArray(value)) {
+    return `[${value.map((item) => stableStringify(item)).join(',')}]`;
+  }
+
+  const record = value as Record<string, unknown>;
+  const keys = Object.keys(record).sort();
+  const serialized = keys.map((key) => `${JSON.stringify(key)}:${stableStringify(record[key])}`);
+  return `{${serialized.join(',')}}`;
+}
+
+// ─── IdempotencyStore ───────────────────────────────────────────────────
+
+export class IdempotencyStore {
+  private readonly completedResponses: NodeCache;
+  private readonly pendingResponses = new Map<string, PendingOperation<unknown>>();
+
+  private _hits = 0;
+  private _conflicts = 0;
+  private _evictions = 0;
+
+  constructor(private readonly ttlMs = 24 * 60 * 60 * 1000) {
+    const ttlSeconds = Math.max(1, Math.ceil(this.ttlMs / 1000));
+    this.completedResponses = new NodeCache({ stdTTL: ttlSeconds, checkperiod: ttlSeconds });
+    this.completedResponses.on('expired', (_key: string, _value: unknown) => {
+      this._evictions++;
+    });
+  }
+
+  private redisKey(key: string): string {
+    return `${REDIS_PREFIX}${key}`;
+  }
+
+  private get redis() {
+    const client = redisClientManager.getClient();
+    return redisClientManager.isReady() && client ? client : null;
+  }
+
+  private async redisGet<T>(key: string): Promise<StoredResponse<T> | null> {
+    const r = this.redis;
+    if (!r) return null;
+    try {
+      const raw = await r.get(this.redisKey(key));
+      return raw ? (JSON.parse(raw) as StoredResponse<T>) : null;
+    } catch {
+      return null;
+    }
+  }
+
+  private async redisSet<T>(key: string, value: StoredResponse<T>): Promise<void> {
+    const r = this.redis;
+    if (!r) return;
+    try {
+      const ttlSeconds = Math.max(1, Math.ceil(this.ttlMs / 1000));
+      await r.set(this.redisKey(key), JSON.stringify(value), 'EX', ttlSeconds);
+    } catch (err) {
+      console.log(JSON.stringify({ level: 'warn', event: 'idempotency_redis_write_fail', key, reason: (err as Error).message }));
+    }
+  }
+
+  private async redisDel(key: string): Promise<boolean> {
+    const r = this.redis;
+    if (!r) return false;
+    try {
+      return (await r.del(this.redisKey(key))) > 0;
+    } catch {
+      return false;
+    }
+  }
+
+  async execute<T>(
+    key: string,
+    fingerprint: string,
+    operation: () => Promise<IdempotentOperationResult<T>>
+  ): Promise<{ result: IdempotentOperationResult<T>; replayed: boolean }> {
+    const now = new Date().toISOString();
+
+    let completed = await this.redisGet<T>(key);
+    if (!completed) {
+      completed = this.completedResponses.get<StoredResponse<T>>(key) ?? null;
+    }
+
+    if (completed) {
+      if (completed.fingerprint !== fingerprint) {
+        this._conflicts++;
+        throw new IdempotencyConflictError();
+      }
+      this._hits++;
+      completed.metadata.lastAccessedAt = now;
+      completed.metadata.replayCount++;
+      await this.redisSet(key, completed);
+      this.completedResponses.set(key, completed);
+      return { result: { statusCode: completed.statusCode, body: completed.body }, replayed: true };
+    }
+
+    const pendingOperation = this.pendingResponses.get(key) as PendingOperation<T> | undefined;
+    if (pendingOperation) {
+      if (pendingOperation.fingerprint !== fingerprint) {
+        this._conflicts++;
+        throw new IdempotencyConflictError();
+      }
+      this._hits++;
+      pendingOperation.metadata.lastAccessedAt = now;
+      pendingOperation.metadata.replayCount++;
+      const replayed = await pendingOperation.promise;
+      return { result: { statusCode: replayed.statusCode, body: replayed.body }, replayed: true };
+    }
+
+    const metadata: IdempotencyKeyMetadata = { createdAt: now, lastAccessedAt: now, replayCount: 0, status: 'pending' };
+
+    const operationPromise = (async () => {
+      const result = await operation();
+      const stored: StoredResponse<T> = {
+        ...result,
+        fingerprint,
+        metadata: { ...metadata, status: 'completed', lastAccessedAt: new Date().toISOString() },
+      };
+      await this.redisSet(key, stored);
+      this.completedResponses.set(key, stored, this.ttlMs / 1000);
+      return stored;
+    })();
+
+    this.pendingResponses.set(key, { fingerprint, promise: operationPromise, metadata });
+
+    try {
+      const stored = await operationPromise;
+      return { result: { statusCode: stored.statusCode, body: stored.body }, replayed: false };
+    } finally {
+      this.pendingResponses.delete(key);
+    }
+  }
+
+  inspectKeys(prefix?: string): IdempotencyKeyInfo[] {
+    const results: IdempotencyKeyInfo[] = [];
+    for (const key of this.completedResponses.keys()) {
+      if (prefix && !key.startsWith(prefix)) continue;
+      const entry = this.completedResponses.get<StoredResponse<unknown>>(key);
+      if (entry) results.push({ key, metadata: { ...entry.metadata } });
+    }
+    for (const [key, pending] of this.pendingResponses.entries()) {
+      if (prefix && !key.startsWith(prefix)) continue;
+      if (!results.some((r) => r.key === key)) {
+        results.push({ key, metadata: { ...pending.metadata } });
+      }
+    }
+    return results;
+  }
+
+  async deleteKey(key: string): Promise<boolean> {
+    const deletedLocal = this.completedResponses.del(key) > 0;
+    const deletedPending = this.pendingResponses.delete(key);
+    const deletedRedis = await this.redisDel(key);
+    if (deletedLocal || deletedPending || deletedRedis) {
+      this._evictions++;
+      return true;
+    }
+    return false;
+  }
+
+  clear(): void {
+    const count = this.completedResponses.keys().length + this.pendingResponses.size;
+    this._evictions += count;
+    this.completedResponses.flushAll();
+    this.pendingResponses.clear();
+  }
+
+  getMetrics(): IdempotencyMetrics {
+    return {
+      hits: this._hits,
+      conflicts: this._conflicts,
+      evictions: this._evictions,
+      activeKeys: this.completedResponses.keys().length,
+      pendingKeys: this.pendingResponses.size,
+    };
+  }
+
+  async pruneStaleKeys(
+    retentionMs: number,
+    dryRun = false,
+  ): Promise<{ pruned: number; localPruned: number; redisPruned: number }> {
+    const cutoff = Date.now() - retentionMs;
+    let localPruned = 0;
+    let redisPruned = 0;
+
+    for (const key of this.completedResponses.keys()) {
+      const entry = this.completedResponses.get<StoredResponse<unknown>>(key);
+      if (!entry) continue;
+      const createdAt = Date.parse(entry.metadata.createdAt);
+      if (Number.isNaN(createdAt) || createdAt >= cutoff) continue;
+      if (!dryRun) {
+        this.completedResponses.del(key);
+        this._evictions++;
+      }
+      localPruned++;
+    }
+
+    const r = this.redis;
+    if (r) {
+      let cursor = '0';
+      do {
+        const [nextCursor, keys] = await r.scan(cursor, 'MATCH', `${REDIS_PREFIX}*`, 'COUNT', 100);
+        cursor = nextCursor;
+        for (const redisKey of keys) {
+          try {
+            const raw = await r.get(redisKey);
+            if (!raw) continue;
+            const entry = JSON.parse(raw) as StoredResponse<unknown>;
+            const createdAt = Date.parse(entry.metadata?.createdAt ?? '');
+            const ttl = await r.ttl(redisKey);
+            const isStale = (!Number.isNaN(createdAt) && createdAt < cutoff) || ttl === 0;
+            if (!isStale) continue;
+            if (!dryRun) {
+              await r.del(redisKey);
+              this._evictions++;
+            }
+            redisPruned++;
+          } catch {
+            if (!dryRun) {
+              await r.del(redisKey);
+              this._evictions++;
+            }
+            redisPruned++;
+          }
+        }
+      } while (cursor !== '0');
+    }
+
+    return { pruned: localPruned + redisPruned, localPruned, redisPruned };
+  }
+}
+
+// ─── Singleton ──────────────────────────────────────────────────────────
+
+export const idempotencyStore = new IdempotencyStore(
+  parseInt(process.env.IDEMPOTENCY_KEY_TTL_MS || '86400000', 10)
+);
+
 // ─── Cleanup ────────────────────────────────────────────────────────────────
 
 /**
diff --git a/backend/src/middleware/cache.ts b/backend/src/middleware/cache.ts
index 45e383c2e..c080b4ca1 100644
--- a/backend/src/middleware/cache.ts
+++ b/backend/src/middleware/cache.ts
@@ -338,6 +338,7 @@ export function triggerCacheInvalidation(
   for (const hook of invalidationHooks) {
     try {
       const hookPatterns = hook(eventType, metadata);
+      // Validate and sanitize hook return value
       if (hookPatterns instanceof Promise) {
         void hookPatterns.catch((err) => {
           console.error(
@@ -359,12 +360,16 @@ export function triggerCacheInvalidation(
         console.error(
           JSON.stringify({
             level: 'error',
+            event: 'invalidation_hook_error',
+            error: `InvalidationHook must return string[], got ${typeof hookPatterns}`,
+            eventType,
             event: 'invalidation_hook_invalid_return',
             error: 'Invalidation hook must return an array of cache key patterns',
           }),
         );
         continue;
       }
+      patterns.push(...hookPatterns);
       patterns.push(...hookPatterns.filter((pattern): pattern is string => typeof pattern === 'string'));
     } catch (err) {
       console.error(