diff --git a/src/__tests__/resources/advertisers.test.ts b/src/__tests__/resources/advertisers.test.ts
index 604df65..81a534d 100644
--- a/src/__tests__/resources/advertisers.test.ts
+++ b/src/__tests__/resources/advertisers.test.ts
@@ -15,6 +15,7 @@ describe('AdvertisersResource', () => {
       version: 'v2',
       persona: 'buyer' as const,
       debug: false,
+      validate: undefined,
       request: jest.fn(),
       connect: jest.fn(),
       disconnect: jest.fn(),
diff --git a/src/__tests__/resources/campaigns.test.ts b/src/__tests__/resources/campaigns.test.ts
index 9304761..613f322 100644
--- a/src/__tests__/resources/campaigns.test.ts
+++ b/src/__tests__/resources/campaigns.test.ts
@@ -15,6 +15,7 @@ describe('CampaignsResource', () => {
       version: 'v2',
       persona: 'buyer' as const,
       debug: false,
+      validate: undefined,
       request: jest.fn(),
       connect: jest.fn(),
       disconnect: jest.fn(),
diff --git a/src/__tests__/resources/products.test.ts b/src/__tests__/resources/products.test.ts
index 2498612..556e542 100644
--- a/src/__tests__/resources/products.test.ts
+++ b/src/__tests__/resources/products.test.ts
@@ -15,6 +15,7 @@ describe('BundleProductsResource', () => {
       version: 'v2',
       persona: 'buyer' as const,
       debug: false,
+      validate: undefined,
       request: jest.fn(),
       connect: jest.fn(),
       disconnect: jest.fn(),
diff --git a/src/__tests__/validation.test.ts b/src/__tests__/validation.test.ts
new file mode 100644
index 0000000..d3ebf55
--- /dev/null
+++ b/src/__tests__/validation.test.ts
@@ -0,0 +1,95 @@
+import { z } from 'zod';
+import { Scope3ApiError } from '../adapters/base';
+import {
+  validateInput,
+  validateResponse,
+  shouldValidateInput,
+  shouldValidateResponse,
+} from '../validation';
+
+const testSchema = z.object({
+  name: z.string(),
+  count: z.number(),
+});
+
+describe('shouldValidateInput', () => {
+  it('returns true for true', () => {
+    expect(shouldValidateInput(true)).toBe(true);
+  });
+
+  it('returns true for "input"', () => {
+    expect(shouldValidateInput('input')).toBe(true);
+  });
+
+  it('returns false for "response"', () => {
+    expect(shouldValidateInput('response')).toBe(false);
+  });
+
+  it('returns false for false', () => {
+    expect(shouldValidateInput(false)).toBe(false);
+  });
+
+  it('returns false for undefined', () => {
+    expect(shouldValidateInput(undefined)).toBe(false);
+  });
+});
+
+describe('shouldValidateResponse', () => {
+  it('returns true for true', () => {
+    expect(shouldValidateResponse(true)).toBe(true);
+  });
+
+  it('returns true for "response"', () => {
+    expect(shouldValidateResponse('response')).toBe(true);
+  });
+
+  it('returns false for "input"', () => {
+    expect(shouldValidateResponse('input')).toBe(false);
+  });
+
+  it('returns false for false', () => {
+    expect(shouldValidateResponse(false)).toBe(false);
+  });
+
+  it('returns false for undefined', () => {
+    expect(shouldValidateResponse(undefined)).toBe(false);
+  });
+});
+
+describe('validateInput', () => {
+  it('returns parsed data on valid input', () => {
+    const result = validateInput(testSchema, { name: 'test', count: 5 });
+    expect(result).toEqual({ name: 'test', count: 5 });
+  });
+
+  it('throws Scope3ApiError with status 400 on invalid input', () => {
+    expect(() => validateInput(testSchema, { name: 123 })).toThrow(Scope3ApiError);
+    try {
+      validateInput(testSchema, { name: 123 });
+    } catch (e) {
+      const err = e as Scope3ApiError;
+      expect(err.status).toBe(400);
+      expect(err.message).toContain('Input validation failed');
+      expect(err.details?.validationErrors).toBeDefined();
+    }
+  });
+});
+
+describe('validateResponse', () => {
+  it('returns parsed data on valid response', () => {
+    const result = validateResponse(testSchema, { name: 'test', count: 5 });
+    expect(result).toEqual({ name: 'test', count: 5 });
+  });
+
+  it('throws Scope3ApiError with status 502 on invalid response', () => {
+    expect(() => validateResponse(testSchema, { bad: 'data' })).toThrow(Scope3ApiError);
+    try {
+      validateResponse(testSchema, { bad: 'data' });
+    } catch (e) {
+      const err = e as Scope3ApiError;
+      expect(err.status).toBe(502);
+      expect(err.message).toContain('Response validation failed');
+      expect(err.details?.validationErrors).toBeDefined();
+    }
+  });
+});
diff --git a/src/adapters/base.ts b/src/adapters/base.ts
index a6d24c1..3a8dbfa 100644
--- a/src/adapters/base.ts
+++ b/src/adapters/base.ts
@@ -3,6 +3,7 @@
  */
 
 import type { ApiVersion, Persona, Scope3ClientConfig } from '../types';
+import type { ValidateMode } from '../validation';
 
 /**
  * HTTP methods supported by the adapter
@@ -31,6 +32,8 @@ export interface BaseAdapter {
   readonly persona: Persona;
   /** Whether debug mode is enabled */
   readonly debug: boolean;
+  /** Validation mode */
+  readonly validate: ValidateMode | undefined;
 
   /**
    * Make an API request
diff --git a/src/adapters/mcp.ts b/src/adapters/mcp.ts
index 455158f..314ab83 100644
--- a/src/adapters/mcp.ts
+++ b/src/adapters/mcp.ts
@@ -6,6 +6,7 @@
 import { Client } from '@modelcontextprotocol/sdk/client/index.js';
 import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
 import type { ApiVersion, Persona, Scope3ClientConfig } from '../types';
+import type { ValidateMode } from '../validation';
 import {
   BaseAdapter,
   HttpMethod,
@@ -29,6 +30,7 @@ export class McpAdapter implements BaseAdapter {
   readonly version: ApiVersion;
   readonly persona: Persona;
   readonly debug: boolean;
+  readonly validate: ValidateMode | undefined;
 
   private readonly apiKey: string;
   private mcpClient: Client;
@@ -42,6 +44,7 @@ export class McpAdapter implements BaseAdapter {
     this.version = resolveVersion(config);
     this.persona = resolvePersona(config);
     this.debug = config.debug ?? false;
+    this.validate = config.validate;
 
     // Initialize MCP client
     this.mcpClient = new Client(
diff --git a/src/adapters/rest.ts b/src/adapters/rest.ts
index f9f4695..6094bd9 100644
--- a/src/adapters/rest.ts
+++ b/src/adapters/rest.ts
@@ -4,6 +4,7 @@
  */
 
 import type { ApiVersion, Persona, Scope3ClientConfig } from '../types';
+import type { ValidateMode } from '../validation';
 import {
   BaseAdapter,
   HttpMethod,
@@ -24,6 +25,7 @@ export class RestAdapter implements BaseAdapter {
   readonly version: ApiVersion;
   readonly persona: Persona;
   readonly debug: boolean;
+  readonly validate: ValidateMode | undefined;
 
   private readonly apiKey: string;
   private readonly timeout: number;
@@ -34,6 +36,7 @@ export class RestAdapter implements BaseAdapter {
     this.version = resolveVersion(config);
     this.persona = resolvePersona(config);
     this.debug = config.debug ?? false;
+    this.validate = config.validate;
     this.timeout = config.timeout ?? 30000;
 
     if (this.debug) {
diff --git a/src/index.ts b/src/index.ts
index 6584b09..9b3780e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -50,6 +50,10 @@ export type { ParsedSkill, SkillCommand, SkillParameter, SkillExample } from './
 export { WebhookServer } from './webhook-server';
 export type { WebhookEvent, WebhookHandler, WebhookServerConfig } from './webhook-server';
 
+// Validation
+export { validateInput, validateResponse } from './validation';
+export type { ValidateMode } from './validation';
+
 // Schemas (auto-generated from OpenAPI spec)
 export * from './schemas';
 
diff --git a/src/resources/bundles.ts b/src/resources/bundles.ts
index d54bfc1..9913af7 100644
--- a/src/resources/bundles.ts
+++ b/src/resources/bundles.ts
@@ -11,6 +11,13 @@ import type {
   BrowseProductsInput,
   ApiResponse,
 } from '../types';
+import { discoverySchemas } from '../schemas/registry';
+import {
+  shouldValidateInput,
+  shouldValidateResponse,
+  validateInput,
+  validateResponse,
+} from '../validation';
 import { BundleProductsResource } from './products';
 
 /**
@@ -25,6 +32,9 @@ export class BundlesResource {
    * @returns Created bundle with bundleId
    */
   async create(data: CreateBundleInput): Promise<ApiResponse<Bundle>> {
+    if (shouldValidateInput(this.adapter.validate)) {
+      validateInput(discoverySchemas.discoverInput, data);
+    }
     return this.adapter.request<ApiResponse<Bundle>>('POST', '/bundles', data);
   }
 
@@ -38,7 +48,7 @@ export class BundlesResource {
     bundleId: string,
     params?: DiscoverProductsParams
   ): Promise<ApiResponse<DiscoverProductsResponse>> {
-    return this.adapter.request<ApiResponse<DiscoverProductsResponse>>(
+    const result = await this.adapter.request<ApiResponse<DiscoverProductsResponse>>(
       'GET',
       `/bundles/${validateResourceId(bundleId)}/discover-products`,
       undefined,
@@ -54,6 +64,10 @@ export class BundlesResource {
         },
       }
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(discoverySchemas.discoverResponse, result.data);
+    }
+    return result;
   }
 
   /**
@@ -62,11 +76,18 @@ export class BundlesResource {
    * @returns Discovered product groups with auto-created bundleId
    */
   async browseProducts(data: BrowseProductsInput): Promise<ApiResponse<DiscoverProductsResponse>> {
-    return this.adapter.request<ApiResponse<DiscoverProductsResponse>>(
+    if (shouldValidateInput(this.adapter.validate)) {
+      validateInput(discoverySchemas.discoverInput, data);
+    }
+    const result = await this.adapter.request<ApiResponse<DiscoverProductsResponse>>(
       'POST',
       '/bundles/discover-products',
       data
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(discoverySchemas.discoverResponse, result.data);
+    }
+    return result;
   }
 
   /**
diff --git a/src/resources/campaigns.ts b/src/resources/campaigns.ts
index 58f5788..6adbc83 100644
--- a/src/resources/campaigns.ts
+++ b/src/resources/campaigns.ts
@@ -14,6 +14,8 @@ import type {
   PaginatedApiResponse,
   ApiResponse,
 } from '../types';
+import { campaignSchemas } from '../schemas/registry';
+import { shouldValidateResponse, validateResponse } from '../validation';
 
 /**
  * Resource for managing campaigns (Buyer persona)
@@ -44,10 +46,14 @@ export class CampaignsResource {
    * @returns Campaign details
    */
   async get(id: string): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>(
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
       'GET',
       `/campaigns/${validateResourceId(id)}`
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
@@ -56,7 +62,15 @@ export class CampaignsResource {
    * @returns Created campaign
    */
   async createDiscovery(data: CreateDiscoveryCampaignInput): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>('POST', '/campaigns/discovery', data);
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
+      'POST',
+      '/campaigns/discovery',
+      data
+    );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
@@ -69,11 +83,15 @@ export class CampaignsResource {
     id: string,
     data: UpdateDiscoveryCampaignInput
   ): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>(
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
       'PUT',
       `/campaigns/discovery/${validateResourceId(id)}`,
       data
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
@@ -82,7 +100,15 @@ export class CampaignsResource {
    * @returns Created campaign
    */
   async createPerformance(data: CreatePerformanceCampaignInput): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>('POST', '/campaigns/performance', data);
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
+      'POST',
+      '/campaigns/performance',
+      data
+    );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
@@ -95,11 +121,15 @@ export class CampaignsResource {
     id: string,
     data: UpdatePerformanceCampaignInput
   ): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>(
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
       'PUT',
       `/campaigns/performance/${validateResourceId(id)}`,
       data
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
@@ -108,7 +138,15 @@ export class CampaignsResource {
    * @returns Created campaign
    */
   async createAudience(data: CreateAudienceCampaignInput): Promise<ApiResponse<Campaign>> {
-    return this.adapter.request<ApiResponse<Campaign>>('POST', '/campaigns/audience', data);
+    const result = await this.adapter.request<ApiResponse<Campaign>>(
+      'POST',
+      '/campaigns/audience',
+      data
+    );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(campaignSchemas.response, result.data);
+    }
+    return result;
   }
 
   /**
diff --git a/src/resources/products.ts b/src/resources/products.ts
index 8a64f0e..7461ea8 100644
--- a/src/resources/products.ts
+++ b/src/resources/products.ts
@@ -10,6 +10,13 @@ import type {
   RemoveBundleProductsInput,
   ApiResponse,
 } from '../types';
+import { discoverySchemas } from '../schemas/registry';
+import {
+  shouldValidateInput,
+  shouldValidateResponse,
+  validateInput,
+  validateResponse,
+} from '../validation';
 
 /**
  * Resource for managing products within a bundle
@@ -25,10 +32,14 @@ export class BundleProductsResource {
    * @returns Bundle products response with product list and budget context
    */
   async list(): Promise<ApiResponse<BundleProductsResponse>> {
-    return this.adapter.request<ApiResponse<BundleProductsResponse>>(
+    const result = await this.adapter.request<ApiResponse<BundleProductsResponse>>(
       'GET',
       `/bundles/${this.bundleId}/products`
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(discoverySchemas.sessionProductsResponse, result.data);
+    }
+    return result;
   }
 
   /**
@@ -37,11 +48,18 @@ export class BundleProductsResource {
    * @returns Updated bundle products response
    */
   async add(data: AddBundleProductsInput): Promise<ApiResponse<BundleProductsResponse>> {
-    return this.adapter.request<ApiResponse<BundleProductsResponse>>(
+    if (shouldValidateInput(this.adapter.validate)) {
+      validateInput(discoverySchemas.addProductsInput, data);
+    }
+    const result = await this.adapter.request<ApiResponse<BundleProductsResponse>>(
       'POST',
       `/bundles/${this.bundleId}/products`,
       data
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(discoverySchemas.sessionProductsResponse, result.data);
+    }
+    return result;
   }
 
   /**
@@ -49,6 +67,9 @@ export class BundleProductsResource {
    * @param data Product IDs to remove
    */
   async remove(data: RemoveBundleProductsInput): Promise<void> {
+    if (shouldValidateInput(this.adapter.validate)) {
+      validateInput(discoverySchemas.removeProductsInput, data);
+    }
     await this.adapter.request<void>('DELETE', `/bundles/${this.bundleId}/products`, data);
   }
 }
diff --git a/src/resources/reporting.ts b/src/resources/reporting.ts
index 014c4ba..4c91073 100644
--- a/src/resources/reporting.ts
+++ b/src/resources/reporting.ts
@@ -4,6 +4,8 @@
 
 import type { BaseAdapter } from '../adapters/base';
 import type { ReportingParams } from '../types';
+import { reportingSchemas } from '../schemas/registry';
+import { shouldValidateResponse, validateResponse } from '../validation';
 
 /**
  * Resource for accessing reporting data (Buyer persona)
@@ -17,7 +19,7 @@ export class ReportingResource {
    * @returns Reporting response (summary or timeseries depending on view param)
    */
   async get<T = unknown>(params?: ReportingParams): Promise<T> {
-    return this.adapter.request<T>('GET', '/reporting/metrics', undefined, {
+    const result = await this.adapter.request<T>('GET', '/reporting/metrics', undefined, {
       params: {
         view: params?.view,
         days: params?.days,
@@ -28,5 +30,9 @@ export class ReportingResource {
         demo: params?.demo,
       },
     });
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(reportingSchemas.response, result);
+    }
+    return result;
   }
 }
diff --git a/src/resources/sales-agents.ts b/src/resources/sales-agents.ts
index 7342714..e78ad5d 100644
--- a/src/resources/sales-agents.ts
+++ b/src/resources/sales-agents.ts
@@ -8,6 +8,13 @@ import type {
   ListSalesAgentsParams,
   RegisterSalesAgentAccountInput,
 } from '../types';
+import { salesAgentSchemas } from '../schemas/registry';
+import {
+  shouldValidateInput,
+  shouldValidateResponse,
+  validateInput,
+  validateResponse,
+} from '../validation';
 
 /**
  * Resource for managing sales agents (Buyer persona)
@@ -21,7 +28,7 @@ export class SalesAgentsResource {
    * @returns Sales agents with account info
    */
   async list(params?: ListSalesAgentsParams): Promise<unknown> {
-    return this.adapter.request<unknown>('GET', '/sales-agents', undefined, {
+    const result = await this.adapter.request<unknown>('GET', '/sales-agents', undefined, {
       params: {
         status: params?.status,
         relationship: params?.relationship,
@@ -30,6 +37,10 @@ export class SalesAgentsResource {
         offset: params?.offset,
       },
     });
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(salesAgentSchemas.listResponse, result);
+    }
+    return result;
   }
 
   /**
@@ -42,10 +53,17 @@ export class SalesAgentsResource {
     agentId: string,
     data: RegisterSalesAgentAccountInput
   ): Promise<SalesAgentAccount> {
-    return this.adapter.request<SalesAgentAccount>(
+    if (shouldValidateInput(this.adapter.validate)) {
+      validateInput(salesAgentSchemas.registerAccountInput, data);
+    }
+    const result = await this.adapter.request<SalesAgentAccount>(
       'POST',
       `/sales-agents/${validateResourceId(agentId)}/accounts`,
       data
     );
+    if (shouldValidateResponse(this.adapter.validate)) {
+      validateResponse(salesAgentSchemas.accountResponse, result);
+    }
+    return result;
   }
 }
diff --git a/src/schemas/registry.ts b/src/schemas/registry.ts
new file mode 100644
index 0000000..ad4f88b
--- /dev/null
+++ b/src/schemas/registry.ts
@@ -0,0 +1,34 @@
+import { schemas } from './buyer';
+
+export const advertiserSchemas = {
+  createInput: schemas.CreateAdvertiserBody,
+  updateInput: schemas.UpdateAdvertiserBody,
+};
+
+export const campaignSchemas = {
+  createInput: schemas.CreateCampaignBody,
+  updateInput: schemas.UpdateCampaignBody,
+  executeInput: schemas.ExecuteCampaignBody,
+  response: schemas.Campaign,
+  listResponse: schemas.CampaignListResponse,
+  statusChangeResponse: schemas.CampaignStatusChangeResponse,
+};
+
+export const discoverySchemas = {
+  discoverInput: schemas.DiscoverProductsBody,
+  discoverResponse: schemas.DiscoverProductsResponse,
+  addProductsInput: schemas.AddProductsRequest,
+  removeProductsInput: schemas.RemoveProductsRequest,
+  sessionProductsResponse: schemas.SessionProductsResponse,
+};
+
+export const reportingSchemas = {
+  response: schemas.ReportingMetricsResponse,
+};
+
+export const salesAgentSchemas = {
+  registerAccountInput: schemas.RegisterSalesAgentAccountBody,
+  response: schemas.Agent,
+  listResponse: schemas.AgentList,
+  accountResponse: schemas.AgentAccount,
+};
diff --git a/src/types/index.ts b/src/types/index.ts
index 629f7b5..ec226c4 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -38,6 +38,8 @@ export interface Scope3ClientConfig {
   timeout?: number;
   /** Enable debug logging */
   debug?: boolean;
+  /** Enable runtime validation with Zod schemas: true (both), 'input', or 'response' */
+  validate?: boolean | 'input' | 'response';
 }
 
 // ============================================================================
diff --git a/src/validation.ts b/src/validation.ts
new file mode 100644
index 0000000..04f1314
--- /dev/null
+++ b/src/validation.ts
@@ -0,0 +1,36 @@
+import { type ZodSchema, type ZodError } from 'zod';
+import { Scope3ApiError } from './adapters/base';
+
+export type ValidateMode = boolean | 'input' | 'response';
+
+export function shouldValidateInput(mode: ValidateMode | undefined): boolean {
+  return mode === true || mode === 'input';
+}
+
+export function shouldValidateResponse(mode: ValidateMode | undefined): boolean {
+  return mode === true || mode === 'response';
+}
+
+export function validateInput<T>(schema: ZodSchema<T>, data: unknown): T {
+  const result = schema.safeParse(data);
+  if (!result.success) {
+    throw new Scope3ApiError(400, `Input validation failed: ${formatZodError(result.error)}`, {
+      validationErrors: result.error.issues,
+    });
+  }
+  return result.data;
+}
+
+export function validateResponse<T>(schema: ZodSchema<T>, data: unknown): T {
+  const result = schema.safeParse(data);
+  if (!result.success) {
+    throw new Scope3ApiError(502, `Response validation failed: ${formatZodError(result.error)}`, {
+      validationErrors: result.error.issues,
+    });
+  }
+  return result.data;
+}
+
+function formatZodError(error: ZodError): string {
+  return error.issues.map((i) => `${i.path.join('.')}: ${i.message}`).join('; ');
+}