diff --git a/CHANGELOG.md b/CHANGELOG.md
index 81e60c3..ffa24b8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,17 @@ Versions prior to v1.3.0 were maintained in a private repository (history unavil
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.8.0] - 2026-06-22
+
+### Added
+
+- **Idempotency support** — mutation endpoints (POST, PUT, PATCH, DELETE) now accept an `idempotencyKey` via `RequestOptions`. The API deduplicates requests sharing the same key within a 24-hour window; subsequent retries with the same key replay the stored response without re-executing side effects.
+  - `RequestOptions.idempotencyKey?: string` — pass `crypto.randomUUID()` once per logical operation and reuse on every retry of that operation.
+  - `IDEMPOTENCY_KEY_HEADER` constant (`'Idempotency-Key'`) exported from the root package — use when inspecting outgoing request headers.
+  - `IDEMPOTENT_REPLAYED_HEADER` constant (`'Idempotent-Replayed'`) exported from the root package — check this response header to detect replayed responses.
+  - A 409 conflict (same key already in flight) surfaces as a `ContioAPIError` with `statusCode: 409`; the `retryAfter` field indicates when to retry.
+  - Idempotency keys are ignored on GET/HEAD requests (naturally idempotent) and multipart/file-upload endpoints.
+
 ## [1.7.1] - 2026-05-21
 
 ### Changed
@@ -307,6 +318,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Error handling**: `ContioAPIError` with structured error information
 - **Retry logic**: Automatic retry with exponential backoff for transient failures
 
+[1.8.0]: https://github.com/Contio-AI/partner-sdk/compare/v1.7.1...v1.8.0
+[1.7.1]: https://github.com/Contio-AI/partner-sdk/compare/v1.7.0...v1.7.1
+[1.7.0]: https://github.com/Contio-AI/partner-sdk/compare/v1.6.0...v1.7.0
 [1.6.0]: https://github.com/Contio-AI/partner-sdk/compare/v1.5.0...v1.6.0
 [1.5.0]: https://github.com/Contio-AI/partner-sdk/compare/v1.4.7...v1.5.0
 [1.4.7]: https://github.com/Contio-AI/partner-sdk/compare/v1.4.6...v1.4.7
diff --git a/package.json b/package.json
index 0b48174..4e6f0fb 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@contio/partner-sdk",
-  "version": "1.7.1",
+  "version": "1.8.0",
   "description": "Official SDK for Contio MeetingOS Partner API",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
diff --git a/src/client/base.ts b/src/client/base.ts
index a094771..66c41ad 100644
--- a/src/client/base.ts
+++ b/src/client/base.ts
@@ -10,7 +10,7 @@ import { ErrorResponse } from '../models/auth';
  * SDK version for User-Agent header.
  * This is updated during the release process.
  */
-export const SDK_VERSION = '1.4.5';
+export const SDK_VERSION = '1.8.0';
 
 export interface ClientConfig {
   baseURL?: string;
@@ -34,6 +34,22 @@ export interface RequestOptions {
    * (e.g., 'America/New_York') or a timezone abbreviation (e.g., 'EST', 'PST').
    */
   timezone?: string;
+  /**
+   * Idempotency key for safe retries on mutation requests (POST, PUT, PATCH, DELETE).
+   * The API deduplicates requests sharing the same key within a 24-hour window —
+   * the first request executes normally and its response is stored; subsequent retries
+   * with the same key replay the stored response without re-executing.
+   *
+   * - Ignored on GET/HEAD requests (they are naturally idempotent).
+   * - Ignored on multipart/file-upload endpoints.
+   * - Must be ≤255 printable ASCII characters.
+   * - A conflict (another request with the same key still in flight) returns a 409
+   *   error whose {@link ContioAPIError.retryAfter} field indicates when to retry.
+   *
+   * Use `crypto.randomUUID()` to generate a unique key per logical operation and
+   * reuse that same key on every retry of that operation.
+   */
+  idempotencyKey?: string;
 }
 
 /**
@@ -41,6 +57,19 @@ export interface RequestOptions {
  */
 export const TIMEZONE_HEADER = 'X-Client-Timezone';
 
+/**
+ * Header name for idempotency keys on mutation requests.
+ * @see {@link RequestOptions.idempotencyKey}
+ */
+export const IDEMPOTENCY_KEY_HEADER = 'Idempotency-Key';
+
+/**
+ * Response header set to `"true"` by the API when the response body is a
+ * replay of a previously captured response (i.e. the idempotency key was
+ * already used and the original response is being returned verbatim).
+ */
+export const IDEMPOTENT_REPLAYED_HEADER = 'Idempotent-Replayed';
+
 export class ContioAPIError extends Error {
   public readonly code: string;
   public readonly statusCode?: number;
@@ -123,7 +152,7 @@ export abstract class BaseClient {
   protected abstract addAuthHeaders(config: InternalAxiosRequestConfig): InternalAxiosRequestConfig | Promise<InternalAxiosRequestConfig>;
 
   /**
-   * Build axios config with optional per-request timezone override
+   * Build axios config with optional per-request overrides (timezone, idempotency key).
    */
   protected buildRequestConfig(options?: RequestOptions, baseConfig?: AxiosRequestConfig): AxiosRequestConfig {
     const config: AxiosRequestConfig = { ...baseConfig };
@@ -135,6 +164,13 @@ export abstract class BaseClient {
       };
     }
 
+    if (options?.idempotencyKey) {
+      config.headers = {
+        ...config.headers,
+        [IDEMPOTENCY_KEY_HEADER]: options.idempotencyKey,
+      };
+    }
+
     return config;
   }
 
diff --git a/src/index.ts b/src/index.ts
index 838bffc..41afd5a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -17,7 +17,7 @@ export { ApiKeyClient } from './auth/apiKey';
 // Export API clients
 export { PartnerUserClient } from './client/user';
 export { PartnerAdminClient } from './client/admin';
-export { BaseClient, ContioAPIError, TIMEZONE_HEADER } from './client/base';
+export { BaseClient, ContioAPIError, TIMEZONE_HEADER, IDEMPOTENCY_KEY_HEADER, IDEMPOTENT_REPLAYED_HEADER } from './client/base';
 export type { ClientConfig, RequestOptions } from './client/base';
 
 // Export all models
diff --git a/tests/base-client.test.ts b/tests/base-client.test.ts
index 658bed9..50dbd59 100644
--- a/tests/base-client.test.ts
+++ b/tests/base-client.test.ts
@@ -6,7 +6,7 @@
 import MockAdapter from 'axios-mock-adapter';
 import { PartnerUserClient } from '../src/client/user';
 import { OAuthClient } from '../src/auth/oauth';
-import { ContioAPIError } from '../src/client/base';
+import { ContioAPIError, IDEMPOTENCY_KEY_HEADER, IDEMPOTENT_REPLAYED_HEADER } from '../src/client/base';
 
 describe('BaseClient', () => {
   let oauthClient: OAuthClient;
@@ -414,6 +414,79 @@ describe('BaseClient', () => {
     });
   });
 
+  describe('Idempotency', () => {
+    it('should send Idempotency-Key header on POST requests', async () => {
+      mockAxios.onPost('/meetings').reply((config) => {
+        expect(config.headers?.[IDEMPOTENCY_KEY_HEADER]).toBe('test-idem-key');
+        return [201, { id: 'm1', title: 'Test' }];
+      });
+
+      await userClient.createMeeting(
+        { title: 'Test', start_time: '2026-06-22T10:00:00Z', end_time: '2026-06-22T11:00:00Z', workspace_id: 'ws1' },
+        { idempotencyKey: 'test-idem-key' },
+      );
+    });
+
+    it('should send Idempotency-Key header on PATCH requests', async () => {
+      mockAxios.onPatch('/meetings/m1').reply((config) => {
+        expect(config.headers?.[IDEMPOTENCY_KEY_HEADER]).toBe('patch-key');
+        return [200, { id: 'm1', title: 'Updated' }];
+      });
+
+      await userClient.updateMeeting('m1', { title: 'Updated' }, { idempotencyKey: 'patch-key' });
+    });
+
+    it('should send Idempotency-Key header on DELETE requests', async () => {
+      mockAxios.onDelete('/meetings/m1/participants/p1').reply((config) => {
+        expect(config.headers?.[IDEMPOTENCY_KEY_HEADER]).toBe('delete-key');
+        return [204, null];
+      });
+
+      await userClient.removeMeetingParticipant('m1', 'p1', { idempotencyKey: 'delete-key' });
+    });
+
+    it('should not send Idempotency-Key header when idempotencyKey is not set', async () => {
+      mockAxios.onPost('/meetings').reply((config) => {
+        expect(config.headers?.[IDEMPOTENCY_KEY_HEADER]).toBeUndefined();
+        return [201, { id: 'm1', title: 'Test' }];
+      });
+
+      await userClient.createMeeting(
+        { title: 'Test', start_time: '2026-06-22T10:00:00Z', end_time: '2026-06-22T11:00:00Z', workspace_id: 'ws1' },
+      );
+    });
+
+    it('should surface retryAfter on 409 in-flight conflict', async () => {
+      const noRetryClient = new PartnerUserClient(oauthClient, { retries: 0 });
+      const noRetryMock = new MockAdapter((noRetryClient as any).axiosInstance);
+
+      noRetryMock.onPost('/meetings').reply(
+        409,
+        { error: 'idempotency_conflict', code: 'idempotency_conflict', message: 'Request in progress' },
+        { 'retry-after': '5' },
+      );
+
+      try {
+        await noRetryClient.createMeeting(
+          { title: 'Test', start_time: '2026-06-22T10:00:00Z', end_time: '2026-06-22T11:00:00Z', workspace_id: 'ws1' },
+          { idempotencyKey: 'conflict-key' },
+        );
+        fail('Should have thrown');
+      } catch (error) {
+        expect(error).toBeInstanceOf(ContioAPIError);
+        const apiError = error as ContioAPIError;
+        expect(apiError.statusCode).toBe(409);
+        expect(apiError.retryAfter).toBe('5');
+      }
+      noRetryMock.reset();
+    });
+
+    it('should export IDEMPOTENCY_KEY_HEADER and IDEMPOTENT_REPLAYED_HEADER constants', () => {
+      expect(IDEMPOTENCY_KEY_HEADER).toBe('Idempotency-Key');
+      expect(IDEMPOTENT_REPLAYED_HEADER).toBe('Idempotent-Replayed');
+    });
+  });
+
   describe('Request interceptor error path', () => {
     it('should reject with the error when request interceptor fails', async () => {
       const noRetryClient = new PartnerUserClient(oauthClient, { retries: 0 });