diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index 608e8abdf..6c9453e2c 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -727,6 +727,7 @@ export class AgentManager {
       historyPrimed: false,
       lastUserMessageAt: null,
       attention: { requiresAttention: false },
+      parentAgentId: config.parentAgentId,
     } as ActiveManagedAgent;
 
     this.agents.set(agentId, managed);
diff --git a/packages/server/src/server/agent/mcp-server.ts b/packages/server/src/server/agent/mcp-server.ts
index 61cd14b62..7d4e7cd8e 100644
--- a/packages/server/src/server/agent/mcp-server.ts
+++ b/packages/server/src/server/agent/mcp-server.ts
@@ -220,6 +220,12 @@ export async function createAgentMcpServer(
           .describe(
             "Run agent in background. If false (default), waits for completion or permission request. If true, returns immediately."
           ),
+        parentAgentId: z
+          .string()
+          .optional()
+          .describe(
+            "Optional parent agent ID. When set, this agent is a child of the specified parent agent."
+          ),
       },
       outputSchema: {
         agentId: z.string(),
@@ -246,6 +252,7 @@ export async function createAgentMcpServer(
       worktreeName,
       background = false,
       title,
+      parentAgentId,
     }) => {
       let resolvedCwd = expandPath(cwd);
 
@@ -265,6 +272,7 @@ export async function createAgentMcpServer(
         cwd: resolvedCwd,
         modeId: initialMode,
         title: normalizedTitle ?? undefined,
+        parentAgentId,
       });
 
       if (initialPrompt) {
diff --git a/plan.md b/plan.md
index 5fe488ab3..0f3b51ee2 100644
--- a/plan.md
+++ b/plan.md
@@ -300,12 +300,13 @@ Files requiring modification:
   - Run typecheck after changes.
   - **Done (2025-12-21 19:45)**: Added `parentAgentId?: string` to `ManagedAgentBase` in `agent-manager.ts:97`, `AgentSessionConfig` in `agent-sdk-types.ts:164`, and `AgentSnapshotPayloadSchema` in `messages.ts:248`. Updated `toAgentPayload()` in `agent-projections.ts:83` to include `parentAgentId` in the payload. Typecheck passes.
 
-- [ ] **Implement**: Add parentAgentId to MCP create_agent tool.
+- [x] **Implement**: Add parentAgentId to MCP create_agent tool.
 
   - Add `parentAgentId` to tool input schema in `mcp-server.ts:183`
   - Pass `parentAgentId` to `agentManager.createAgent()` call in `mcp-server.ts:263`
   - The MCP server has access to the calling agent context - use that ID
   - Run typecheck after changes.
+  - **Done (2025-12-21 21:15)**: Added `parentAgentId` optional field to `create_agent` tool input schema in `mcp-server.ts:223-228`. Updated handler to accept `parentAgentId` parameter and pass it to `agentManager.createAgent()` in `mcp-server.ts:275`. Also updated `registerSession()` in `agent-manager.ts:730` to copy `parentAgentId` from config to the managed agent. Typecheck passes.
 
 - [ ] **Implement**: Add parentAgentId to client types and session store.