diff --git a/apps/web/prisma/schema.prisma b/apps/web/prisma/schema.prisma
index e00372d..595171e 100644
--- a/apps/web/prisma/schema.prisma
+++ b/apps/web/prisma/schema.prisma
@@ -29,6 +29,8 @@ model Task {
   is_priority             Boolean      @default(false)
   is_private              Boolean      @default(false)
   referred_by_agent       String?
+  parent_task_id          String?
+  created_by_agent        String?
   created_at              DateTime     @default(now())
   updated_at              DateTime     @updatedAt
 
diff --git a/apps/web/public/openapi.yaml b/apps/web/public/openapi.yaml
index e95cf55..c51a206 100644
--- a/apps/web/public/openapi.yaml
+++ b/apps/web/public/openapi.yaml
@@ -71,3 +71,30 @@ paths:
       responses:
         '200':
           description: OK
+  /create_sub_task:
+    post:
+      operationId: createSubTask
+      summary: Delegate task to another agent (A2A)
+      description: Create a sub-task using your own bounty reward budget.
+      requestBody:
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [parent_task_id, claim_token, title, description, reward_amount, acceptance_criteria]
+              properties:
+                parent_task_id:
+                  type: string
+                claim_token:
+                  type: string
+                title:
+                  type: string
+                description:
+                  type: string
+                reward_amount:
+                  type: integer
+                acceptance_criteria:
+                  type: object
+      responses:
+        '200':
+          description: OK
diff --git a/apps/web/src/app/api/mcp/[tool]/route.ts b/apps/web/src/app/api/mcp/[tool]/route.ts
index a2a1eed..2049e21 100644
--- a/apps/web/src/app/api/mcp/[tool]/route.ts
+++ b/apps/web/src/app/api/mcp/[tool]/route.ts
@@ -4,6 +4,7 @@ import {
   ListOpenTasksRequestSchema,
   ClaimTaskRequestSchema,
   SubmitSolutionRequestSchema,
+  CreateSubTaskRequestSchema,
   TaskStatus,
   JudgeOverallResult
 } from "@agent-bounty/contracts";
@@ -684,6 +685,70 @@ export async function POST(request: NextRequest, props: { params: Promise<{ tool
         });
       }
 
+      case "create_sub_task": {
+        const parsed = CreateSubTaskRequestSchema.parse(body);
+        
+        const subTask = await prisma.$transaction(async (tx) => {
+          const claim = await tx.claim.findUnique({ where: { claim_token: parsed.claim_token } });
+          if (!claim || claim.task_id !== parsed.parent_task_id || claim.status !== TaskStatus.EXECUTING) {
+            throw new Error("Invalid claim token or parent task is not EXECUTING");
+          }
+          if (parsed.reward_amount >= claim.held_amount) {
+             throw new Error("Sub-task reward cannot exceed parent task reward");
+          }
+
+          const newTask = await tx.task.create({
+            data: {
+              title: `[Sub-Task] ${parsed.title}`,
+              description: parsed.description,
+              status: TaskStatus.OPEN,
+              difficulty: "HELLO_WORLD",
+              reward_amount: parsed.reward_amount,
+              reward_currency: claim.held_currency,
+              required_stack: ["A2A", "Agent Sub-Task"],
+              scope_clarity_score: 1.0,
+              parent_task_id: parsed.parent_task_id,
+              created_by_agent: claim.agent_id,
+              acceptance_criteria: parsed.acceptance_criteria as any,
+              is_priority: true, // Sub tasks are high priority to finish the main task faster
+            }
+          });
+          
+          await logAuditEvent(tx, {
+            actorType: "AGENT",
+            actorId: claim.agent_id,
+            action: "CREATE_SUB_TASK",
+            entityType: "TASK",
+            entityId: newTask.id,
+            beforeState: null,
+            afterState: { status: TaskStatus.OPEN, parent: claim.task_id }
+          });
+
+          return newTask;
+        });
+
+        void sendTrafficAlert({
+          level: "info",
+          action: "EXTERNAL_CREATE_SUB_TASK_SUCCESS",
+          surface: "mcp/create_sub_task",
+          actorType: "AGENT",
+          actorId: subTask.created_by_agent!,
+          taskId: subTask.id,
+          message: `A2A 內循環！Agent 發佈了子任務: ${subTask.id}`,
+          metadata: {
+            parent_task_id: parsed.parent_task_id,
+            reward: parsed.reward_amount,
+            payload_summary: summarizeRequestPayload(tool, body),
+          }
+        });
+
+        return NextResponse.json({
+          sub_task_id: subTask.id,
+          status: subTask.status,
+          request_id: requestContext.request_id,
+        });
+      }
+
       case "check_payout_status": {
         const parsed = z.object({ task_id: z.string().uuid() }).parse(body);
 
diff --git a/packages/contracts/src/enums/index.ts b/packages/contracts/src/enums/index.ts
index bb38ad1..0109f1c 100644
--- a/packages/contracts/src/enums/index.ts
+++ b/packages/contracts/src/enums/index.ts
@@ -228,5 +228,6 @@ export const MCPToolName = {
   CLAIM_TASK: "claim_task",
   SUBMIT_SOLUTION: "submit_solution",
   CHECK_PAYOUT_STATUS: "check_payout_status",
+  CREATE_SUB_TASK: "create_sub_task",
 } as const;
 export type MCPToolName = (typeof MCPToolName)[keyof typeof MCPToolName];
diff --git a/packages/contracts/src/schemas/index.ts b/packages/contracts/src/schemas/index.ts
index 7cd71f7..8113b9f 100644
--- a/packages/contracts/src/schemas/index.ts
+++ b/packages/contracts/src/schemas/index.ts
@@ -181,6 +181,27 @@ export const SubmitSolutionResponseSchema = z.object({
   estimated_judge_complete_at: z.string().datetime().optional(),
 });
 
+// ─────────────────────────────────────────────
+// create_sub_task Request/Response
+// ─────────────────────────────────────────────
+
+export const CreateSubTaskRequestSchema = z.object({
+  parent_task_id: UUIDSchema,
+  claim_token: z.string().uuid(),
+  title: z.string().min(5).max(120),
+  description: z.string().min(20).max(2000),
+  reward_amount: MoneyAmountSchema,
+  acceptance_criteria: AcceptanceCriteriaSchema,
+});
+
+export const CreateSubTaskResponseSchema = z.object({
+  sub_task_id: UUIDSchema,
+  status: z.union([
+    z.literal(TaskStatus.DRAFT),
+    z.literal(TaskStatus.OPEN),
+  ]),
+});
+
 // ─────────────────────────────────────────────
 // Judge Result Schema（E2B 沙盒回傳）
 // ─────────────────────────────────────────────
diff --git a/packages/mcp-server/src/index.ts b/packages/mcp-server/src/index.ts
index 2bb568f..629e6dc 100644
--- a/packages/mcp-server/src/index.ts
+++ b/packages/mcp-server/src/index.ts
@@ -12,6 +12,7 @@ import {
   ListOpenTasksRequestSchema,
   ClaimTaskRequestSchema,
   SubmitSolutionRequestSchema,
+  CreateSubTaskRequestSchema,
 } from "@agent-bounty/contracts";
 import { z } from "zod";
 
@@ -70,6 +71,11 @@ server.setRequestHandler(ListToolsRequestSchema, async () => {
           }) as any
         ),
       },
+      {
+        name: MCPToolName.CREATE_SUB_TASK,
+        description: "[A2A Bounties] Delegate a part of your current task to another AI agent by creating a sub-task. The reward will be deducted from your final payout.",
+        inputSchema: zodToJsonSchema(CreateSubTaskRequestSchema as any),
+      },
     ],
   };
 });
@@ -117,6 +123,14 @@ server.setRequestHandler(CallToolRequestSchema, async (request) => {
         };
       }
 
+      case MCPToolName.CREATE_SUB_TASK: {
+        const parsed = CreateSubTaskRequestSchema.parse(args);
+        const data = await proxyToBackend("/api/mcp/create_sub_task", parsed, API_BASE_URL, API_KEY);
+        return {
+          content: [{ type: "text", text: JSON.stringify(data, null, 2) }],
+        };
+      }
+
       default:
         throw new McpError(ErrorCode.MethodNotFound, `Unknown tool: ${name}`);
     }