diff --git a/src/services/agents/claude-computer-use.ts b/src/services/agents/claude-computer-use.ts
index 874d981..e658454 100644
--- a/src/services/agents/claude-computer-use.ts
+++ b/src/services/agents/claude-computer-use.ts
@@ -9,6 +9,7 @@ import {
   StartClaudeComputerUseTaskParams,
   StartClaudeComputerUseTaskResponse,
 } from "../../types/agents/claude-computer-use";
+import { validateCustomApiKeys } from "./validation";
 
 export class ClaudeComputerUseService extends BaseService {
   /**
@@ -19,6 +20,9 @@ export class ClaudeComputerUseService extends BaseService {
     params: StartClaudeComputerUseTaskParams
   ): Promise<StartClaudeComputerUseTaskResponse> {
     try {
+      validateCustomApiKeys(params.useCustomApiKeys, params.apiKeys, {
+        anthropicBaseUrl: params.apiKeys?.anthropicBaseUrl,
+      });
       return await this.request<StartClaudeComputerUseTaskResponse>("/task/claude-computer-use", {
         method: "POST",
         body: JSON.stringify(params),
diff --git a/src/services/agents/cua.ts b/src/services/agents/cua.ts
index cb2080b..e6d7bcd 100644
--- a/src/services/agents/cua.ts
+++ b/src/services/agents/cua.ts
@@ -9,6 +9,7 @@ import {
   StartCuaTaskParams,
   StartCuaTaskResponse,
 } from "../../types/agents/cua";
+import { validateCustomApiKeys } from "./validation";
 
 export class CuaService extends BaseService {
   /**
@@ -17,6 +18,9 @@ export class CuaService extends BaseService {
    */
   async start(params: StartCuaTaskParams): Promise<StartCuaTaskResponse> {
     try {
+      validateCustomApiKeys(params.useCustomApiKeys, params.apiKeys, {
+        openaiBaseUrl: params.apiKeys?.openaiBaseUrl,
+      });
       return await this.request<StartCuaTaskResponse>("/task/cua", {
         method: "POST",
         body: JSON.stringify(params),
diff --git a/src/services/agents/gemini-computer-use.ts b/src/services/agents/gemini-computer-use.ts
index 48c5bcc..0af9b5a 100644
--- a/src/services/agents/gemini-computer-use.ts
+++ b/src/services/agents/gemini-computer-use.ts
@@ -9,6 +9,7 @@ import {
   StartGeminiComputerUseTaskParams,
   StartGeminiComputerUseTaskResponse,
 } from "../../types/agents/gemini-computer-use";
+import { validateCustomApiKeys } from "./validation";
 
 export class GeminiComputerUseService extends BaseService {
   /**
@@ -19,6 +20,9 @@ export class GeminiComputerUseService extends BaseService {
     params: StartGeminiComputerUseTaskParams
   ): Promise<StartGeminiComputerUseTaskResponse> {
     try {
+      validateCustomApiKeys(params.useCustomApiKeys, params.apiKeys, {
+        googleBaseUrl: params.apiKeys?.googleBaseUrl,
+      });
       return await this.request<StartGeminiComputerUseTaskResponse>("/task/gemini-computer-use", {
         method: "POST",
         body: JSON.stringify(params),
diff --git a/src/services/agents/validation.ts b/src/services/agents/validation.ts
new file mode 100644
index 0000000..c910e51
--- /dev/null
+++ b/src/services/agents/validation.ts
@@ -0,0 +1,26 @@
+import { HyperbrowserError } from "../../client";
+
+const isAbsoluteHttpUrl = (value: string): boolean => {
+  try {
+    const url = new URL(value);
+    return url.protocol === "http:" || url.protocol === "https:";
+  } catch {
+    return false;
+  }
+};
+
+export const validateCustomApiKeys = (
+  useCustomApiKeys: boolean | undefined,
+  apiKeys: object | undefined,
+  baseUrls: Record<string, string | undefined>
+): void => {
+  if (useCustomApiKeys && !apiKeys) {
+    throw new HyperbrowserError("apiKeys must be provided when useCustomApiKeys is true");
+  }
+
+  Object.entries(baseUrls).forEach(([field, value]) => {
+    if (value !== undefined && !isAbsoluteHttpUrl(value)) {
+      throw new HyperbrowserError(`${field} must be an absolute http or https URL`);
+    }
+  });
+};
diff --git a/src/types/agents/claude-computer-use.ts b/src/types/agents/claude-computer-use.ts
index 37181c8..d011bb0 100644
--- a/src/types/agents/claude-computer-use.ts
+++ b/src/types/agents/claude-computer-use.ts
@@ -3,6 +3,7 @@ import { CreateSessionParams } from "../session";
 
 export interface ClaudeComputerUseApiKeys {
   anthropic?: string;
+  anthropicBaseUrl?: string;
 }
 
 export interface StartClaudeComputerUseTaskParams {
diff --git a/src/types/agents/cua.ts b/src/types/agents/cua.ts
index 916b190..51523d4 100644
--- a/src/types/agents/cua.ts
+++ b/src/types/agents/cua.ts
@@ -3,6 +3,7 @@ import { CreateSessionParams } from "../session";
 
 export interface CuaApiKeys {
   openai?: string;
+  openaiBaseUrl?: string;
 }
 
 export interface StartCuaTaskParams {
diff --git a/src/types/agents/gemini-computer-use.ts b/src/types/agents/gemini-computer-use.ts
index 090cfce..4e17677 100644
--- a/src/types/agents/gemini-computer-use.ts
+++ b/src/types/agents/gemini-computer-use.ts
@@ -3,6 +3,7 @@ import { CreateSessionParams } from "../session";
 
 export interface GeminiComputerUseApiKeys {
   google?: string;
+  googleBaseUrl?: string;
 }
 
 export interface StartGeminiComputerUseTaskParams {
diff --git a/tests/sandbox/e2e/computer-use-contract.test.ts b/tests/sandbox/e2e/computer-use-contract.test.ts
new file mode 100644
index 0000000..74fcc8e
--- /dev/null
+++ b/tests/sandbox/e2e/computer-use-contract.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, test, vi } from "vitest";
+import { HyperbrowserError } from "../../../src/client";
+import { ClaudeComputerUseService } from "../../../src/services/agents/claude-computer-use";
+import { CuaService } from "../../../src/services/agents/cua";
+import { GeminiComputerUseService } from "../../../src/services/agents/gemini-computer-use";
+
+describe("computer use agent control contracts", () => {
+  test("CUA forwards custom OpenAI API key and base URL", async () => {
+    const service = new CuaService("test-key", "https://api.example.com", 30_000);
+    const payload = { jobId: "job_123", liveUrl: null };
+    const requestSpy = vi.spyOn(service as any, "request").mockResolvedValue(payload);
+
+    const response = await service.start({
+      task: "go to example.com",
+      useCustomApiKeys: true,
+      apiKeys: {
+        openai: "sk-test",
+        openaiBaseUrl: "https://openai-compatible.example.com/v1",
+      },
+    });
+
+    expect(requestSpy).toHaveBeenCalledWith("/task/cua", {
+      method: "POST",
+      body: JSON.stringify({
+        task: "go to example.com",
+        useCustomApiKeys: true,
+        apiKeys: {
+          openai: "sk-test",
+          openaiBaseUrl: "https://openai-compatible.example.com/v1",
+        },
+      }),
+    });
+    expect(response).toEqual(payload);
+  });
+
+  test("Claude Computer Use forwards custom Anthropic API key and base URL", async () => {
+    const service = new ClaudeComputerUseService("test-key", "https://api.example.com", 30_000);
+    const payload = { jobId: "job_123", liveUrl: null };
+    const requestSpy = vi.spyOn(service as any, "request").mockResolvedValue(payload);
+
+    const response = await service.start({
+      task: "go to example.com",
+      useCustomApiKeys: true,
+      apiKeys: {
+        anthropic: "sk-ant-test",
+        anthropicBaseUrl: "https://anthropic-compatible.example.com",
+      },
+    });
+
+    expect(requestSpy).toHaveBeenCalledWith("/task/claude-computer-use", {
+      method: "POST",
+      body: JSON.stringify({
+        task: "go to example.com",
+        useCustomApiKeys: true,
+        apiKeys: {
+          anthropic: "sk-ant-test",
+          anthropicBaseUrl: "https://anthropic-compatible.example.com",
+        },
+      }),
+    });
+    expect(response).toEqual(payload);
+  });
+
+  test("Gemini Computer Use forwards custom Google API key and base URL", async () => {
+    const service = new GeminiComputerUseService("test-key", "https://api.example.com", 30_000);
+    const payload = { jobId: "job_123", liveUrl: null };
+    const requestSpy = vi.spyOn(service as any, "request").mockResolvedValue(payload);
+
+    const response = await service.start({
+      task: "go to example.com",
+      useCustomApiKeys: true,
+      apiKeys: {
+        google: "google-test",
+        googleBaseUrl: "https://gemini-compatible.example.com",
+      },
+    });
+
+    expect(requestSpy).toHaveBeenCalledWith("/task/gemini-computer-use", {
+      method: "POST",
+      body: JSON.stringify({
+        task: "go to example.com",
+        useCustomApiKeys: true,
+        apiKeys: {
+          google: "google-test",
+          googleBaseUrl: "https://gemini-compatible.example.com",
+        },
+      }),
+    });
+    expect(response).toEqual(payload);
+  });
+
+  test("custom API key mode requires apiKeys", async () => {
+    const service = new CuaService("test-key", "https://api.example.com", 30_000);
+
+    await expect(
+      service.start({
+        task: "go to example.com",
+        useCustomApiKeys: true,
+      })
+    ).rejects.toThrow(HyperbrowserError);
+  });
+
+  test("provider base URLs must be absolute http or https URLs", async () => {
+    const service = new CuaService("test-key", "https://api.example.com", 30_000);
+
+    await expect(
+      service.start({
+        task: "go to example.com",
+        apiKeys: {
+          openaiBaseUrl: "localhost:3000/v1",
+        },
+      })
+    ).rejects.toThrow("openaiBaseUrl must be an absolute http or https URL");
+  });
+});