diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 181b5ea..58477b6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -24,4 +24,5 @@ jobs:
           cache: npm
       - run: npm ci
       - run: npm test
+      - run: npm run test:types
       - run: npm pack --dry-run
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7401543..dc2739f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,14 @@
 
 All notable changes to `hy-event-store` are documented in this file.
 
-## 1.4.0 (unreleased)
+## Unreleased
+
+### Added
+
+- Add first-party TypeScript declarations for event payloads, state keys,
+  actions, and action return values, plus compile-time regression coverage.
+
+## 1.4.0 - 2026-08-19
 
 ### Added
 
diff --git a/README.md b/README.md
index 73b7816..eaf1083 100644
--- a/README.md
+++ b/README.md
@@ -26,12 +26,30 @@ Mini Program build workflows.
 npm install hy-event-store
 ~~~
 
-The package currently exposes a CommonJS entry point:
+The package exposes a CommonJS entry point and ships first-party TypeScript
+declarations:
 
 ~~~js
 const { HYEventBus, HYEventStore } = require("hy-event-store")
 ~~~
 
+TypeScript users can add event and state types without changing the runtime API:
+
+~~~ts
+const bus = new HYEventBus<{ ready: []; signedIn: [user: { name: string }] }>()
+bus.on("signedIn", user => console.log(user.name))
+
+const store = new HYEventStore({
+  state: { count: 0 },
+  actions: {
+    increment(state, amount: number) {
+      state.count += amount
+      return state.count
+    }
+  }
+})
+~~~
+
 ## Quick start
 
 ### Event bus
@@ -181,9 +199,9 @@ The distributed package is CommonJS and has no runtime dependencies. CI
 validates the package on Node.js 18 through 26. Browser and Mini Program
 projects should use their normal npm-package build or bundling workflow.
 
-Native ESM exports, TypeScript declarations, a read-only state selector, and
-an opt-in full snapshot mode for <code>onStates</code> are planned design
-improvements; see the [roadmap](ROADMAP.md).
+Native ESM exports, a read-only state selector, and an opt-in full snapshot
+mode for <code>onStates</code> are planned design improvements; see the
+[roadmap](ROADMAP.md).
 
 ## Development
 
diff --git a/README.zh-CN.md b/README.zh-CN.md
index 4a3e50d..d66e3fb 100644
--- a/README.zh-CN.md
+++ b/README.zh-CN.md
@@ -22,12 +22,29 @@
 npm install hy-event-store
 ~~~
 
-当前包提供 CommonJS 入口：
+当前包提供 CommonJS 入口，并内置第一方 TypeScript 声明：
 
 ~~~js
 const { HYEventBus, HYEventStore } = require("hy-event-store")
 ~~~
 
+TypeScript 用户无需改变运行时 API，即可补充事件和状态类型：
+
+~~~ts
+const bus = new HYEventBus<{ ready: []; signedIn: [user: { name: string }] }>()
+bus.on("signedIn", user => console.log(user.name))
+
+const store = new HYEventStore({
+  state: { count: 0 },
+  actions: {
+    increment(state, amount: number) {
+      state.count += amount
+      return state.count
+    }
+  }
+})
+~~~
+
 ## 快速开始
 
 ### 事件总线
@@ -164,7 +181,7 @@ store.offStates(["count", "status"], renderProfile)
 
 发布包为 CommonJS，且没有运行时依赖。CI 在 Node.js 18 至 26 上验证。浏览器和小程序项目应通过各自的 npm 包构建或打包流程使用本包。
 
-原生 ESM 导出、TypeScript 声明、只读状态选择器与 <code>onStates</code> 的完整快照模式属于后续设计项，详见[路线图](ROADMAP.md)。
+原生 ESM 导出、只读状态选择器与 <code>onStates</code> 的完整快照模式属于后续设计项，详见[路线图](ROADMAP.md)。
 
 ## 开发
 
diff --git a/ROADMAP.md b/ROADMAP.md
index ac7a5ad..8a84108 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -21,7 +21,7 @@ keeping the public API compatible.
 
 These items need API design or compatibility research before implementation:
 
-- Add first-party TypeScript declarations
+- Add first-party TypeScript declarations and compile-time coverage
   ([#25](https://github.com/coderwhy/hy-event-store/issues/25)).
 - Document and test CommonJS, ESM, and Mini Program import paths
   ([#27](https://github.com/coderwhy/hy-event-store/issues/27)).
diff --git a/Test/types.test.ts b/Test/types.test.ts
new file mode 100644
index 0000000..3dba997
--- /dev/null
+++ b/Test/types.test.ts
@@ -0,0 +1,57 @@
+import { HYEventBus, HYEventStore } from ".."
+
+type AppEvents = {
+  ready: []
+  signedIn: [user: { id: string; name: string }]
+}
+
+const bus = new HYEventBus<AppEvents>()
+
+bus.on("signedIn", user => {
+  user.id.toUpperCase()
+  user.name.toUpperCase()
+})
+bus.emit("ready")
+bus.emit("signedIn", { id: "1", name: "coderwhy" })
+
+// @ts-expect-error An event payload must match its declared event tuple.
+bus.emit("signedIn", "coderwhy")
+
+const store = new HYEventStore({
+  state: {
+    count: 0,
+    status: "idle" as "idle" | "ready"
+  },
+  actions: {
+    increment(state, amount: number) {
+      state.count += amount
+      return state.count
+    },
+    async load(state, label: string) {
+      state.status = "ready"
+      return `${label}:${state.status}`
+    }
+  }
+})
+
+store.onState("count", count => {
+  count.toFixed()
+})
+
+store.onStates(["count", "status"], change => {
+  change.count?.toFixed()
+  change.status?.toUpperCase()
+})
+
+store.setState("count", 1)
+store.setState("status", "ready")
+
+const nextCount: number = store.dispatch("increment", 2)
+const loaded: Promise<string> = store.dispatch("load", "home")
+
+// @ts-expect-error State keys must exist in the declared state object.
+store.setState("missing", 1)
+// @ts-expect-error State values must match their declared key type.
+store.setState("count", "1")
+// @ts-expect-error Action arguments are inferred from the action signature.
+store.dispatch("increment", "2")
diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..0f4f5ca
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,106 @@
+export type EventMap = Record<string, unknown[]>;
+
+export type EventCallback<Payload extends unknown[] = unknown[]> = (
+  ...payload: Payload
+) => void;
+
+export class HYEventBus<Events extends EventMap = EventMap> {
+  on<EventName extends Extract<keyof Events, string>>(
+    eventName: EventName,
+    eventCallback: EventCallback<Events[EventName]>,
+    thisArg?: unknown
+  ): this;
+
+  once<EventName extends Extract<keyof Events, string>>(
+    eventName: EventName,
+    eventCallback: EventCallback<Events[EventName]>,
+    thisArg?: unknown
+  ): this;
+
+  emit<EventName extends Extract<keyof Events, string>>(
+    eventName: EventName,
+    ...payload: Events[EventName]
+  ): this;
+
+  off<EventName extends Extract<keyof Events, string>>(
+    eventName: EventName,
+    eventCallback: EventCallback<Events[EventName]>
+  ): this;
+
+  clear(): this;
+
+  hasEvent(eventName: string): boolean;
+}
+
+export type StoreAction<
+  State extends object,
+  Args extends unknown[] = unknown[],
+  Result = unknown
+> = (state: State, ...args: Args) => Result;
+
+export type StoreActions<State extends object> = Record<
+  string,
+  StoreAction<State, any[], any>
+>;
+
+export interface HYEventStoreOptions<
+  State extends object,
+  Actions extends StoreActions<State> = Record<never, never>
+> {
+  state: State;
+  actions?: Actions & StoreActions<State>;
+}
+
+type StateKey<State extends object> = Extract<keyof State, string>;
+type StateChange<State extends object, Key extends StateKey<State>> = Partial<
+  Pick<State, Key>
+>;
+type ActionArguments<State extends object, Action> = Action extends (
+  state: State,
+  ...args: infer Args
+) => unknown
+  ? Args
+  : never;
+type ActionResult<Action> = Action extends (...args: any[]) => infer Result
+  ? Result
+  : never;
+
+export class HYEventStore<
+  State extends object,
+  Actions extends StoreActions<State> = Record<never, never>
+> {
+  constructor(options: HYEventStoreOptions<State, Actions>);
+
+  state: State;
+  actions: Actions;
+
+  onState<Key extends StateKey<State>>(
+    stateKey: Key,
+    stateCallback: (value: State[Key]) => void
+  ): void;
+
+  offState<Key extends StateKey<State>>(
+    stateKey: Key,
+    stateCallback: (value: State[Key]) => void
+  ): void;
+
+  onStates<Keys extends readonly StateKey<State>[]>(
+    stateKeys: Keys,
+    stateCallback: (change: StateChange<State, Keys[number]>) => void
+  ): void;
+
+  offStates<Keys extends readonly StateKey<State>[]>(
+    stateKeys: Keys,
+    stateCallback: (change: StateChange<State, Keys[number]>) => void
+  ): void;
+
+  setState<Key extends StateKey<State>>(
+    stateKey: Key,
+    stateValue: State[Key]
+  ): void;
+
+  dispatch<ActionName extends Extract<keyof Actions, string>>(
+    actionName: ActionName,
+    ...args: ActionArguments<State, Actions[ActionName]>
+  ): ActionResult<Actions[ActionName]>;
+}
diff --git a/package-lock.json b/package-lock.json
index 8ba9dac..d9eac33 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -7,7 +7,24 @@
     "": {
       "name": "hy-event-store",
       "version": "1.4.0",
-      "license": "MIT"
+      "license": "MIT",
+      "devDependencies": {
+        "typescript": "^5.7.3"
+      }
+    },
+    "node_modules/typescript": {
+      "version": "5.9.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",
+      "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
     }
   }
 }
diff --git a/package.json b/package.json
index 0ee2a68..adf64aa 100644
--- a/package.json
+++ b/package.json
@@ -3,14 +3,20 @@
   "version": "1.4.0",
   "description": "A zero-dependency event bus and global state store for Vue, React, Mini Programs, and vanilla JavaScript.",
   "main": "src/index.js",
+  "types": "index.d.ts",
   "files": [
     "src",
     "CHANGELOG.md",
-    "README.zh-CN.md"
+    "README.zh-CN.md",
+    "index.d.ts"
   ],
   "scripts": {
     "test": "node --test Test/*.test.js",
-    "test:coverage": "node --test --experimental-test-coverage Test/*.test.js"
+    "test:coverage": "node --test --experimental-test-coverage Test/*.test.js",
+    "test:types": "tsc --noEmit --strict --skipLibCheck --module commonjs --moduleResolution node --target es2020 Test/types.test.ts"
+  },
+  "devDependencies": {
+    "typescript": "^5.7.3"
   },
   "author": "coderwhy",
   "license": "MIT",