diff --git a/.changeset/busy-rice-smoke.md b/.changeset/busy-rice-smoke.md
new file mode 100644
index 000000000..69badd88c
--- /dev/null
+++ b/.changeset/busy-rice-smoke.md
@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+---
+
+tasks - disallow requesting a null TTL
diff --git a/docs/migration-SKILL.md b/docs/migration-SKILL.md
index e5de064da..518c1fbc6 100644
--- a/docs/migration-SKILL.md
+++ b/docs/migration-SKILL.md
@@ -434,11 +434,30 @@ const tool = await client.callTool({ name: 'my-tool', arguments: {} });
 
 Remove unused schema imports: `CallToolResultSchema`, `CompatibilityCallToolResultSchema`, `ElicitResultSchema`, `CreateMessageResultSchema`, etc., when they were only used in `request()`/`send()`/`callTool()` calls.
 
-## 12. Client Behavioral Changes
+## 12. Experimental: `TaskCreationParams.ttl` no longer accepts `null`
+
+`TaskCreationParams.ttl` changed from `z.union([z.number(), z.null()]).optional()` to `z.number().optional()`. Per the MCP spec, `null` TTL (unlimited lifetime) is only valid in server responses (`Task.ttl`), not in client requests. Omit `ttl` to let the server decide.
+
+| v1 | v2 |
+|---|---|
+| `task: { ttl: null }` | `task: {}` (omit ttl) |
+| `task: { ttl: 60000 }` | `task: { ttl: 60000 }` (unchanged) |
+
+Type changes in handler context:
+
+| Type | v1 | v2 |
+|---|---|---|
+| `TaskContext.requestedTtl` | `number \| null \| undefined` | `number \| undefined` |
+| `CreateTaskServerContext.task.requestedTtl` | `number \| null \| undefined` | `number \| undefined` |
+| `TaskServerContext.task.requestedTtl` | `number \| null \| undefined` | `number \| undefined` |
+
+> These task APIs are `@experimental` and may change without notice.
+
+## 13. Client Behavioral Changes
 
 `Client.listPrompts()`, `listResources()`, `listResourceTemplates()`, `listTools()` now return empty results when the server lacks the corresponding capability (instead of sending the request). Set `enforceStrictCapabilities: true` in `ClientOptions` to throw an error instead.
 
-## 13. Runtime-Specific JSON Schema Validators (Enhancement)
+## 14. Runtime-Specific JSON Schema Validators (Enhancement)
 
 The SDK now auto-selects the appropriate JSON Schema validator based on runtime:
 
@@ -462,7 +481,7 @@ new McpServer({ name: 'server', version: '1.0.0' }, {});
 
 Access validators via `_shims` export: `import { DefaultJsonSchemaValidator } from '@modelcontextprotocol/server/_shims';`
 
-## 14. Migration Steps (apply in this order)
+## 15. Migration Steps (apply in this order)
 
 1. Update `package.json`: `npm uninstall @modelcontextprotocol/sdk`, install the appropriate v2 packages
 2. Replace all imports from `@modelcontextprotocol/sdk/...` using the import mapping tables (sections 3-4), including `StreamableHTTPServerTransport` → `NodeStreamableHTTPServerTransport`
diff --git a/docs/migration.md b/docs/migration.md
index 64540bc92..993bc440e 100644
--- a/docs/migration.md
+++ b/docs/migration.md
@@ -742,6 +742,46 @@ try {
 }
 ```
 
+### Experimental: `TaskCreationParams.ttl` no longer accepts `null`
+
+The `ttl` field in `TaskCreationParams` (used when requesting the server to create a task) no longer accepts `null`. Per the MCP spec, `null` TTL (meaning unlimited lifetime) is only valid in server responses (`Task.ttl`), not in client requests. Clients should omit `ttl` to let the server decide the lifetime.
+
+This also narrows the type of `requestedTtl` in `TaskContext`, `CreateTaskServerContext`, and `TaskServerContext` from `number | null | undefined` to `number | undefined`.
+
+**Before (v1):**
+
+```typescript
+// Requesting unlimited lifetime by passing null
+const result = await client.callTool({
+    name: 'long-task',
+    arguments: {},
+    task: { ttl: null }
+});
+
+// Handler context had number | null | undefined
+server.setRequestHandler('tools/call', async (request, ctx) => {
+    const ttl: number | null | undefined = ctx.task?.requestedTtl;
+});
+```
+
+**After (v2):**
+
+```typescript
+// Omit ttl to let the server decide (server may return null for unlimited)
+const result = await client.callTool({
+    name: 'long-task',
+    arguments: {},
+    task: {}
+});
+
+// Handler context is now number | undefined
+server.setRequestHandler('tools/call', async (request, ctx) => {
+    const ttl: number | undefined = ctx.task?.requestedTtl;
+});
+```
+
+> **Note:** These task APIs are marked `@experimental` and may change without notice.
+
 ## Enhancements
 
 ### Automatic JSON Schema validator selection by runtime
diff --git a/packages/core/src/experimental/tasks/interfaces.ts b/packages/core/src/experimental/tasks/interfaces.ts
index 8b3459b7c..a9458db34 100644
--- a/packages/core/src/experimental/tasks/interfaces.ts
+++ b/packages/core/src/experimental/tasks/interfaces.ts
@@ -25,7 +25,7 @@ import type {
  * @experimental
  */
 export type CreateTaskServerContext = ServerContext & {
-    task: { store: RequestTaskStore; requestedTtl?: number | null };
+    task: { store: RequestTaskStore; requestedTtl?: number };
 };
 
 /**
@@ -33,7 +33,7 @@ export type CreateTaskServerContext = ServerContext & {
  * @experimental
  */
 export type TaskServerContext = ServerContext & {
-    task: { id: string; store: RequestTaskStore; requestedTtl?: number | null };
+    task: { id: string; store: RequestTaskStore; requestedTtl?: number };
 };
 
 /**
@@ -136,7 +136,7 @@ export interface TaskMessageQueue {
  */
 export interface CreateTaskOptions {
     /**
-     * Time in milliseconds to keep task results available after completion.
+     * Duration in milliseconds to retain task from creation.
      * If `null`, the task has unlimited lifetime until manually cleaned up.
      */
     ttl?: number | null;
diff --git a/packages/core/src/shared/protocol.ts b/packages/core/src/shared/protocol.ts
index b82731582..a5c117036 100644
--- a/packages/core/src/shared/protocol.ts
+++ b/packages/core/src/shared/protocol.ts
@@ -258,7 +258,7 @@ export interface RequestTaskStore {
 export type TaskContext = {
     id?: string;
     store: RequestTaskStore;
-    requestedTtl?: number | null;
+    requestedTtl?: number;
 };
 
 /**
diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index bcb5f4dad..da760cdbe 100644
--- a/packages/core/src/types/types.ts
+++ b/packages/core/src/types/types.ts
@@ -76,10 +76,9 @@ export const CursorSchema = z.string();
  */
 export const TaskCreationParamsSchema = z.looseObject({
     /**
-     * Time in milliseconds to keep task results available after completion.
-     * If `null`, the task has unlimited lifetime until manually cleaned up.
+     * Requested duration in milliseconds to retain task from creation.
      */
-    ttl: z.union([z.number(), z.null()]).optional(),
+    ttl: z.number().optional(),
 
     /**
      * Time in milliseconds to wait between task status requests.
@@ -752,7 +751,7 @@ export const TaskSchema = z.object({
     taskId: z.string(),
     status: TaskStatusSchema,
     /**
-     * Time in milliseconds to keep task results available after completion.
+     * Duration in milliseconds to retain task from creation.
      * If `null`, the task has unlimited lifetime until manually cleaned up.
      */
     ttl: z.union([z.number(), z.null()]),
diff --git a/packages/core/test/experimental/inMemory.test.ts b/packages/core/test/experimental/inMemory.test.ts
index ea6a3a8dc..6c9b6af34 100644
--- a/packages/core/test/experimental/inMemory.test.ts
+++ b/packages/core/test/experimental/inMemory.test.ts
@@ -488,17 +488,16 @@ describe('InMemoryTaskStore', () => {
             expect(task).toBeNull();
         });
 
-        it('should support null TTL for unlimited lifetime', async () => {
-            // Test that null TTL means unlimited lifetime
-            const taskParams: TaskCreationParams = {
-                ttl: null
-            };
+        it('should support omitted TTL for unlimited lifetime', async () => {
+            // Test that omitting TTL means unlimited lifetime (server returns null)
+            // Per spec: clients omit ttl to let server decide, server returns null for unlimited
+            const taskParams: TaskCreationParams = {};
             const createdTask = await store.createTask(taskParams, 2222, {
                 method: 'tools/call',
                 params: {}
             });
 
-            // The returned task should have null TTL
+            // The returned task should have null TTL (unlimited)
             expect(createdTask.ttl).toBeNull();
 
             // Task should not be cleaned up even after a long time
diff --git a/test/integration/test/experimental/tasks/task.test.ts b/test/integration/test/experimental/tasks/task.test.ts
index 848e22c98..d2aca2cc0 100644
--- a/test/integration/test/experimental/tasks/task.test.ts
+++ b/test/integration/test/experimental/tasks/task.test.ts
@@ -1,5 +1,5 @@
-import { isTerminal } from '@modelcontextprotocol/core';
-import type { Task } from '@modelcontextprotocol/server';
+import type { Task } from '@modelcontextprotocol/core';
+import { isTerminal, TaskCreationParamsSchema } from '@modelcontextprotocol/core';
 import { describe, expect, it } from 'vitest';
 
 describe('Task utility functions', () => {
@@ -115,3 +115,30 @@ describe('Task Schema Validation', () => {
         }
     });
 });
+
+describe('TaskCreationParams Schema Validation', () => {
+    it('should accept ttl as a number', () => {
+        const result = TaskCreationParamsSchema.safeParse({ ttl: 60_000 });
+        expect(result.success).toBe(true);
+    });
+
+    it('should accept missing ttl (optional)', () => {
+        const result = TaskCreationParamsSchema.safeParse({});
+        expect(result.success).toBe(true);
+    });
+
+    it('should reject null ttl (not allowed in request, only response)', () => {
+        const result = TaskCreationParamsSchema.safeParse({ ttl: null });
+        expect(result.success).toBe(false);
+    });
+
+    it('should accept pollInterval as a number', () => {
+        const result = TaskCreationParamsSchema.safeParse({ pollInterval: 1000 });
+        expect(result.success).toBe(true);
+    });
+
+    it('should accept both ttl and pollInterval', () => {
+        const result = TaskCreationParamsSchema.safeParse({ ttl: 60_000, pollInterval: 1000 });
+        expect(result.success).toBe(true);
+    });
+});