diff --git a/README.md b/README.md
index d0fc288..53e7df0 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,8 @@ npm install @lunogram/js-sdk
 ```typescript
 import { Lunogram } from '@lunogram/js-sdk'
 
-const lunogram = new Lunogram('your-api-key')
+// The project UUID is required and scopes every request to that project.
+const lunogram = new Lunogram('your-api-key', 'your-project-uuid')
 
 // Identify a user
 await lunogram.user.upsert({
@@ -46,6 +47,27 @@ await lunogram.organization.upsert({
 })
 ```
 
+## Project Scoping
+
+Every Lunogram Client API request is scoped to a single **project**. You supply
+the project UUID once, when constructing the client, and the SDK injects it into
+every request path automatically — you never pass it per call.
+
+All Client API endpoints live under `/api/client/projects/{projectId}/...`:
+
+| Operation                | Path                                                              |
+| ------------------------ | ----------------------------------------------------------------- |
+| Users                    | `/api/client/projects/{projectId}/users`                          |
+| User events              | `/api/client/projects/{projectId}/users/events`                   |
+| User scheduled           | `/api/client/projects/{projectId}/users/scheduled`                |
+| Organizations            | `/api/client/projects/{projectId}/organizations`                  |
+| Organization members     | `/api/client/projects/{projectId}/organizations/users`            |
+| Organization events      | `/api/client/projects/{projectId}/organizations/events`           |
+| Organization scheduled   | `/api/client/projects/{projectId}/organizations/scheduled`        |
+
+The `projectId` must be a valid UUID; the client throws a `ValidationError` at
+construction time if it is missing, empty, or malformed.
+
 ## Identity Model
 
 Users and organizations are identified by an array of `ExternalID` objects:
@@ -69,7 +91,10 @@ The SDK automatically generates an anonymous identifier for each session. When y
 ```typescript
 import { BrowserClient } from '@lunogram/js-sdk'
 
-const client = new BrowserClient({ apiKey: 'your-api-key' })
+const client = new BrowserClient({
+    apiKey: 'your-api-key',
+    projectId: 'your-project-uuid',
+})
 
 // anonymousId is auto-generated
 client.user.anonymousId
@@ -100,7 +125,7 @@ The SDK is also exposed on `window.Lunogram`:
 
 ```html
 <script>
-    const lunogram = new Lunogram('your-api-key')
+    const lunogram = new Lunogram('your-api-key', 'your-project-uuid')
     lunogram.user.upsert({
         identifier: [{ externalId: 'user-123' }],
         email: 'user@example.com',
@@ -115,6 +140,7 @@ import { Client } from '@lunogram/js-sdk'
 
 const client = new Client({
     apiKey: 'your-api-key',
+    projectId: 'your-project-uuid', // required — scopes all requests to this project
     urlEndpoint: 'https://your-api.com/api', // optional
 })
 
diff --git a/package.json b/package.json
index b721ce7..6eb1070 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,7 @@
     "build:esm": "tsc --project ./tsconfig.module.json",
     "build:cjs": "tsc --project ./tsconfig.json",
     "lint": "eslint . --ext .ts",
+    "test": "vitest run test/url.test.ts",
     "test:e2e": "vitest run test/e2e.test.ts",
     "release:github": "pnpm publish --no-git-checks --registry=https://npm.pkg.github.com",
     "release:npm": "npm publish --access public --registry=https://registry.npmjs.org"
diff --git a/src/core/http.ts b/src/core/http.ts
index 9685aff..142e9f5 100644
--- a/src/core/http.ts
+++ b/src/core/http.ts
@@ -1,5 +1,5 @@
 import { ClientProps } from '../types'
-import { mapKeys } from '../utils'
+import { mapKeys, isUuid } from '../utils'
 import { DefaultEndpoint } from './constants'
 import {
     LunogramError,
@@ -23,10 +23,20 @@ const statusErrorFactories: Partial<Record<number, () => LunogramError>> = {
 export class HttpHandler {
     readonly #apiKey: string
     readonly #baseUrl: string
+    readonly #projectId: string
 
     constructor(props: ClientProps) {
+        const projectId = props.projectId?.trim()
+        if (!projectId) {
+            throw new ValidationError('A non-empty `projectId` is required to create a Lunogram client')
+        }
+        if (!isUuid(projectId)) {
+            throw new ValidationError('`projectId` must be a valid UUID')
+        }
+
         this.#apiKey = props.apiKey
         this.#baseUrl = props.urlEndpoint ?? DefaultEndpoint
+        this.#projectId = projectId
     }
 
     async get<T = unknown>(path: string, data?: unknown): Promise<T> {
@@ -42,7 +52,7 @@ export class HttpHandler {
     }
 
     async #request<T>(method: HttpMethod, path: string, data?: unknown): Promise<T> {
-        const url = `${this.#baseUrl}/client/${path}`
+        const url = `${this.#baseUrl}/client/projects/${this.#projectId}/${path}`
 
         try {
             const response = await fetch(url, {
diff --git a/src/index.ts b/src/index.ts
index c4d8915..0502e96 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -17,10 +17,12 @@ export class Lunogram {
     /**
      * Creates a new Lunogram instance.
      * @param apiKey - Your Lunogram API key
+     * @param projectId - The UUID of your Lunogram project (required). All
+     *                    Client API requests are scoped to this project.
      * @param urlEndpoint - Optional custom API endpoint URL
      */
-    constructor(apiKey: string, urlEndpoint?: string) {
-        this.client = new BrowserClient({ apiKey, urlEndpoint })
+    constructor(apiKey: string, projectId: string, urlEndpoint?: string) {
+        this.client = new BrowserClient({ apiKey, projectId, urlEndpoint })
     }
 
     /**
diff --git a/src/types/common.ts b/src/types/common.ts
index 112edfe..984b65e 100644
--- a/src/types/common.ts
+++ b/src/types/common.ts
@@ -2,5 +2,10 @@ export type JSONValue = string | number | boolean | null | { [key: string]: JSON
 
 export interface ClientProps {
     apiKey: string
+    /**
+     * The UUID of the Lunogram project. Required. Every Client API request is
+     * scoped to this project and issued under `/client/projects/{projectId}/...`.
+     */
+    projectId: string
     urlEndpoint?: string
 }
diff --git a/src/utils/crypto.ts b/src/utils/crypto.ts
index 336105f..b438172 100644
--- a/src/utils/crypto.ts
+++ b/src/utils/crypto.ts
@@ -1,5 +1,9 @@
-import { v4 as uuidv4 } from 'uuid'
+import { v4 as uuidv4, validate as uuidValidate } from 'uuid'
 
 export function generateUuid(): string {
     return uuidv4()
 }
+
+export function isUuid(value: string): boolean {
+    return uuidValidate(value)
+}
diff --git a/test/e2e.test.ts b/test/e2e.test.ts
index f9569f2..e0a50e7 100644
--- a/test/e2e.test.ts
+++ b/test/e2e.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, beforeAll } from 'vitest'
 import { Client } from '../src'
 
 const apiKey = process.env.LUNOGRAM_API_KEY
+const projectId = process.env.LUNOGRAM_PROJECT_ID
 const urlEndpoint = process.env.LUNOGRAM_API_URL
 
 const userId = `test-user-${Date.now()}`
@@ -13,7 +14,10 @@ beforeAll(() => {
     if (!apiKey) {
         throw new Error('LUNOGRAM_API_KEY environment variable is required')
     }
-    client = new Client({ apiKey, urlEndpoint })
+    if (!projectId) {
+        throw new Error('LUNOGRAM_PROJECT_ID environment variable is required')
+    }
+    client = new Client({ apiKey, projectId, urlEndpoint })
 })
 
 describe('Users', () => {
diff --git a/test/url.test.ts b/test/url.test.ts
new file mode 100644
index 0000000..6e1030c
--- /dev/null
+++ b/test/url.test.ts
@@ -0,0 +1,101 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+import { Client, ValidationError } from '../src'
+
+const apiKey = 'test-api-key'
+const projectId = '11111111-1111-4111-8111-111111111111'
+const urlEndpoint = 'https://api.example.com/api'
+
+function mockFetch() {
+    return vi.fn(async () =>
+        new Response(JSON.stringify({}), {
+            status: 200,
+            headers: { 'Content-Type': 'application/json' },
+        }),
+    )
+}
+
+describe('Client API URL construction', () => {
+    let fetchMock: ReturnType<typeof mockFetch>
+
+    beforeEach(() => {
+        fetchMock = mockFetch()
+        vi.stubGlobal('fetch', fetchMock)
+    })
+
+    afterEach(() => {
+        vi.unstubAllGlobals()
+        vi.restoreAllMocks()
+    })
+
+    function lastUrl(): string {
+        const call = fetchMock.mock.calls.at(-1)
+        return String(call?.[0])
+    }
+
+    it('prefixes user requests with /client/projects/{projectId}', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.user.upsert({ identifier: [{ externalId: 'user-1' }] })
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/users`)
+    })
+
+    it('prefixes user events', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.user.events.post([{ name: 'evt', identifier: [{ externalId: 'u' }], data: {} }])
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/users/events`)
+    })
+
+    it('prefixes user scheduled', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.user.schedule.upsert({
+            name: 'r',
+            identifier: [{ externalId: 'u' }],
+            scheduledAt: new Date().toISOString(),
+        })
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/users/scheduled`)
+    })
+
+    it('prefixes organizations', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.organization.upsert({ identifier: [{ externalId: 'org-1' }] })
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/organizations`)
+    })
+
+    it('prefixes organization member operations (path override)', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.organization.addUser({
+            organization: { identifier: [{ externalId: 'org-1' }] },
+            user: { identifier: [{ externalId: 'user-1' }] },
+        })
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/organizations/users`)
+    })
+
+    it('prefixes organization events', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.organization.events.post([{ name: 'evt', identifier: [{ externalId: 'o' }], data: {} }])
+        expect(lastUrl()).toBe(`${urlEndpoint}/client/projects/${projectId}/organizations/events`)
+    })
+
+    it('does not double-prefix on repeated calls', async () => {
+        const client = new Client({ apiKey, projectId, urlEndpoint })
+        await client.user.upsert({ identifier: [{ externalId: 'a' }] })
+        await client.user.upsert({ identifier: [{ externalId: 'b' }] })
+        for (const call of fetchMock.mock.calls) {
+            expect(String(call[0])).toBe(`${urlEndpoint}/client/projects/${projectId}/users`)
+        }
+    })
+})
+
+describe('projectId validation', () => {
+    it('throws when projectId is missing', () => {
+        // @ts-expect-error projectId is required
+        expect(() => new Client({ apiKey, urlEndpoint })).toThrow(ValidationError)
+    })
+
+    it('throws when projectId is empty', () => {
+        expect(() => new Client({ apiKey, projectId: '   ', urlEndpoint })).toThrow(ValidationError)
+    })
+
+    it('throws when projectId is not a UUID', () => {
+        expect(() => new Client({ apiKey, projectId: 'not-a-uuid', urlEndpoint })).toThrow(ValidationError)
+    })
+})