diff --git a/package.json b/package.json
index e6417780..9244deab 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,7 @@
     "i18next-icu": "^2.0.0",
     "i18next-resources-to-backend": "^1.0.0",
     "intl-messageformat": "^10.0.0",
+    "lru-cache": "^10.0.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-i18next": "^15.0.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 2bbff526..fe5fe37e 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -29,6 +29,9 @@ importers:
       intl-messageformat:
         specifier: ^10.0.0
         version: 10.7.18
+      lru-cache:
+        specifier: ^10.0.0
+        version: 10.4.3
       react:
         specifier: ^18.2.0
         version: 18.3.1
diff --git a/src/shared/api/cache.test.ts b/src/shared/api/cache.test.ts
new file mode 100644
index 00000000..556f1a20
--- /dev/null
+++ b/src/shared/api/cache.test.ts
@@ -0,0 +1,194 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  RequestScopedCache,
+  ClientMemoryCache,
+  ServerLruCache,
+  cacheKey,
+} from "./cache.js";
+
+/**
+ * lru-cache captures the global `performance` object at module-load time.
+ * `vi.useFakeTimers()` replaces `globalThis.performance` with a new object,
+ * so the faked `performance.now()` never reaches lru-cache's internal `perf`.
+ *
+ * We work around this by patching `performance.now` in-place via `vi.hoisted`
+ * (which runs before any imports) and controlling time manually.
+ */
+const mockNow = vi.hoisted(() => {
+  let time = 1000;
+  const original = performance.now.bind(performance);
+  return {
+    install: () => {
+      time = 1000;
+      performance.now = () => time;
+    },
+    advance: (ms: number) => {
+      time += ms;
+    },
+    restore: () => {
+      performance.now = original;
+    },
+  };
+});
+
+describe("cacheKey", () => {
+  it("creates a deterministic key from endpoint, query, and locale", () => {
+    const k1 = cacheKey("/flights", { date: "2025-01-15", dep: "SVO" }, "ru");
+    const k2 = cacheKey("/flights", { date: "2025-01-15", dep: "SVO" }, "ru");
+    expect(k1).toBe(k2);
+  });
+
+  it("produces different keys for different locales", () => {
+    const k1 = cacheKey("/flights", { date: "2025-01-15" }, "ru");
+    const k2 = cacheKey("/flights", { date: "2025-01-15" }, "en");
+    expect(k1).not.toBe(k2);
+  });
+
+  it("produces different keys for different queries", () => {
+    const k1 = cacheKey("/flights", { a: "1" }, "ru");
+    const k2 = cacheKey("/flights", { b: "2" }, "ru");
+    expect(k1).not.toBe(k2);
+  });
+
+  it("sorts query keys for stability", () => {
+    const k1 = cacheKey("/flights", { b: "2", a: "1" }, "ru");
+    const k2 = cacheKey("/flights", { a: "1", b: "2" }, "ru");
+    expect(k1).toBe(k2);
+  });
+});
+
+describe("RequestScopedCache", () => {
+  it("deduplicates concurrent promises for the same key", async () => {
+    const cache = new RequestScopedCache();
+    let callCount = 0;
+    const factory = () => {
+      callCount++;
+      return Promise.resolve("value");
+    };
+
+    const p1 = cache.get<string>("k") ?? (() => { const p = factory(); cache.set("k", p); return p; })();
+    const p2 = cache.get<string>("k") ?? (() => { const p = factory(); cache.set("k", p); return p; })();
+
+    expect(await p1).toBe("value");
+    expect(await p2).toBe("value");
+    expect(callCount).toBe(1);
+  });
+
+  it("returns undefined for unknown keys", () => {
+    const cache = new RequestScopedCache();
+    expect(cache.get("missing")).toBeUndefined();
+  });
+});
+
+describe("ClientMemoryCache", () => {
+  it("stores and retrieves values", () => {
+    const cache = new ClientMemoryCache<string>({ max: 10, defaultTtlMs: 5000 });
+    cache.set("k", "v");
+    expect(cache.get("k")).toBe("v");
+  });
+
+  it("evicts after TTL expires", () => {
+    mockNow.install();
+    const cache = new ClientMemoryCache<string>({ max: 10, defaultTtlMs: 100 });
+    cache.set("k", "v");
+    expect(cache.get("k")).toBe("v");
+
+    mockNow.advance(101);
+    expect(cache.get("k")).toBeUndefined();
+    mockNow.restore();
+  });
+
+  it("supports per-entry TTL override", () => {
+    mockNow.install();
+    const cache = new ClientMemoryCache<string>({ max: 10, defaultTtlMs: 5000 });
+    cache.set("k", "v", 50);
+    mockNow.advance(51);
+    expect(cache.get("k")).toBeUndefined();
+    mockNow.restore();
+  });
+
+  it("evicts LRU when max count reached", () => {
+    const cache = new ClientMemoryCache<string>({ max: 2, defaultTtlMs: 60000 });
+    cache.set("a", "1");
+    cache.set("b", "2");
+    cache.set("c", "3");
+    expect(cache.get("a")).toBeUndefined();
+    expect(cache.get("b")).toBe("2");
+    expect(cache.get("c")).toBe("3");
+  });
+
+  it("tracks size", () => {
+    const cache = new ClientMemoryCache<string>({ max: 10, defaultTtlMs: 5000 });
+    expect(cache.size).toBe(0);
+    cache.set("k", "v");
+    expect(cache.size).toBe(1);
+    cache.delete("k");
+    expect(cache.size).toBe(0);
+  });
+
+  it("clear() removes all entries", () => {
+    const cache = new ClientMemoryCache<string>({ max: 10, defaultTtlMs: 5000 });
+    cache.set("a", "1");
+    cache.set("b", "2");
+    cache.clear();
+    expect(cache.size).toBe(0);
+  });
+});
+
+describe("ServerLruCache", () => {
+  it("stores and retrieves values", () => {
+    const cache = new ServerLruCache<string>({
+      maxBytes: 1024,
+      defaultTtlMs: 5000,
+    });
+    cache.set("k", "hello");
+    expect(cache.get("k")).toBe("hello");
+  });
+
+  it("evicts when byte cap is exceeded", () => {
+    const cache = new ServerLruCache<string>({
+      maxBytes: 50,
+      defaultTtlMs: 60000,
+    });
+    // Each string is ~5-10 bytes when JSON.stringify'd, but the key is also counted
+    cache.set("a", "x".repeat(20));
+    cache.set("b", "y".repeat(20));
+    cache.set("c", "z".repeat(20));
+    // At least one of the earlier entries should be evicted
+    const remaining = [cache.get("a"), cache.get("b"), cache.get("c")].filter(Boolean);
+    expect(remaining.length).toBeLessThan(3);
+  });
+
+  it("tracks calculatedSize", () => {
+    const cache = new ServerLruCache<string>({
+      maxBytes: 10000,
+      defaultTtlMs: 5000,
+    });
+    expect(cache.calculatedSize).toBe(0);
+    cache.set("k", "hello");
+    expect(cache.calculatedSize).toBeGreaterThan(0);
+  });
+
+  it("evicts after TTL expires", () => {
+    mockNow.install();
+    const cache = new ServerLruCache<string>({
+      maxBytes: 10000,
+      defaultTtlMs: 100,
+    });
+    cache.set("k", "v");
+    expect(cache.get("k")).toBe("v");
+    mockNow.advance(101);
+    expect(cache.get("k")).toBeUndefined();
+    mockNow.restore();
+  });
+
+  it("supports custom sizeCalculation", () => {
+    const cache = new ServerLruCache<{ data: string }>({
+      maxBytes: 100,
+      defaultTtlMs: 5000,
+      sizeCalculation: (value) => value.data.length,
+    });
+    cache.set("k", { data: "x".repeat(50) });
+    expect(cache.calculatedSize).toBe(50);
+  });
+});
diff --git a/src/shared/api/cache.ts b/src/shared/api/cache.ts
new file mode 100644
index 00000000..c4ed9524
--- /dev/null
+++ b/src/shared/api/cache.ts
@@ -0,0 +1,111 @@
+import { LRUCache } from "lru-cache";
+import type { Language } from "@/i18n/resolver";
+
+// --- Cache key convention ---
+
+export function cacheKey(
+  endpoint: string,
+  query: Record<string, unknown>,
+  locale: Language,
+): string {
+  const sortedQuery = Object.keys(query)
+    .sort()
+    .map((k) => `${k}=${String(query[k])}`)
+    .join("&");
+  return `${locale}:${endpoint}?${sortedQuery}`;
+}
+
+// --- (1) Request-scoped SSR dedup cache ---
+
+export class RequestScopedCache {
+  private readonly store = new Map<string, Promise<unknown>>();
+
+  get<T>(key: string): Promise<T> | undefined {
+    return this.store.get(key) as Promise<T> | undefined;
+  }
+
+  set<T>(key: string, promise: Promise<T>): void {
+    this.store.set(key, promise);
+  }
+}
+
+// --- (2) Client-side per-tab in-memory TTL cache ---
+
+export class ClientMemoryCache<T extends NonNullable<unknown>> {
+  private readonly cache: LRUCache<string, T>;
+
+  constructor(options: { max: number; defaultTtlMs: number }) {
+    this.cache = new LRUCache<string, T>({
+      max: options.max,
+      ttl: options.defaultTtlMs,
+      ttlResolution: 0,
+    });
+  }
+
+  get(key: string): T | undefined {
+    return this.cache.get(key);
+  }
+
+  set(key: string, value: T, ttlMs?: number): void {
+    this.cache.set(key, value, ttlMs !== undefined ? { ttl: ttlMs } : undefined);
+  }
+
+  delete(key: string): void {
+    this.cache.delete(key);
+  }
+
+  clear(): void {
+    this.cache.clear();
+  }
+
+  get size(): number {
+    return this.cache.size;
+  }
+}
+
+// --- (3) Shared per-VM LRU with byte cap ---
+
+export class ServerLruCache<T extends NonNullable<unknown>> {
+  private readonly cache: LRUCache<string, T>;
+
+  constructor(options: {
+    maxBytes: number;
+    defaultTtlMs: number;
+    sizeCalculation?: (value: T, key: string) => number;
+  }) {
+    this.cache = new LRUCache<string, T>({
+      maxSize: options.maxBytes,
+      ttl: options.defaultTtlMs,
+      ttlResolution: 0,
+      sizeCalculation:
+        options.sizeCalculation ??
+        ((value: T) => {
+          try {
+            return JSON.stringify(value).length;
+          } catch {
+            return 1024; // fallback for non-serializable values
+          }
+        }),
+    });
+  }
+
+  get(key: string): T | undefined {
+    return this.cache.get(key);
+  }
+
+  set(key: string, value: T, ttlMs?: number): void {
+    this.cache.set(key, value, ttlMs !== undefined ? { ttl: ttlMs } : undefined);
+  }
+
+  delete(key: string): void {
+    this.cache.delete(key);
+  }
+
+  clear(): void {
+    this.cache.clear();
+  }
+
+  get calculatedSize(): number {
+    return this.cache.calculatedSize;
+  }
+}