diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index 6c9453e2c..3ae6bdf77 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -706,6 +706,9 @@ export class AgentManager {
       throw new Error(`Agent with id ${agentId} already exists`);
     }
 
+    // Inform the session of its managed agent ID for MCP parent-child relationships
+    session.setManagedAgentId?.(agentId);
+
     const managed = {
       id: agentId,
       provider: config.provider,
diff --git a/packages/server/src/server/agent/agent-sdk-types.ts b/packages/server/src/server/agent/agent-sdk-types.ts
index 205b8f607..e29392282 100644
--- a/packages/server/src/server/agent/agent-sdk-types.ts
+++ b/packages/server/src/server/agent/agent-sdk-types.ts
@@ -180,6 +180,12 @@ export interface AgentSession {
   describePersistence(): AgentPersistenceHandle | null;
   interrupt(): Promise<void>;
   close(): Promise<void>;
+  /**
+   * Set the managed agent ID for this session. This is called by AgentManager
+   * after registration to allow the session to include its ID in MCP requests
+   * (for parent-child agent relationships).
+   */
+  setManagedAgentId?(agentId: string): void;
 }
 
 export interface AgentClient {
diff --git a/packages/server/src/server/agent/mcp-server.ts b/packages/server/src/server/agent/mcp-server.ts
index 7d4e7cd8e..f538c9140 100644
--- a/packages/server/src/server/agent/mcp-server.ts
+++ b/packages/server/src/server/agent/mcp-server.ts
@@ -28,6 +28,11 @@ import { WaitForAgentTracker } from "./wait-for-agent-tracker.js";
 export interface AgentMcpServerOptions {
   agentManager: AgentManager;
   agentRegistry: AgentRegistry;
+  /**
+   * ID of the agent that is connecting to this MCP server.
+   * When set, create_agent will auto-inject this as parentAgentId.
+   */
+  callerAgentId?: string;
 }
 
 const AgentProviderEnum = z.enum(
@@ -166,7 +171,7 @@ async function serializeSnapshotWithMetadata(
 export async function createAgentMcpServer(
   options: AgentMcpServerOptions
 ): Promise<McpServer> {
-  const { agentManager, agentRegistry } = options;
+  const { agentManager, agentRegistry, callerAgentId } = options;
   const waitTracker = new WaitForAgentTracker();
 
   const server = new McpServer({
@@ -267,12 +272,14 @@ export async function createAgentMcpServer(
 
       const provider: AgentProvider = agentType ?? "claude";
       const normalizedTitle = title?.trim() ?? null;
+      // Use explicit parentAgentId if provided, otherwise default to caller agent ID
+      const resolvedParentAgentId = parentAgentId ?? callerAgentId;
       const snapshot = await agentManager.createAgent({
         provider,
         cwd: resolvedCwd,
         modeId: initialMode,
         title: normalizedTitle ?? undefined,
-        parentAgentId,
+        parentAgentId: resolvedParentAgentId,
       });
 
       if (initialPrompt) {
diff --git a/packages/server/src/server/agent/providers/claude-agent.ts b/packages/server/src/server/agent/providers/claude-agent.ts
index 51c7afaa5..e82dbf1f2 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -389,6 +389,7 @@ class ClaudeAgentSession implements AgentSession {
   private cancelCurrentTurn: (() => void) | null = null;
   private cachedRuntimeInfo: AgentRuntimeInfo | null = null;
   private lastOptionsModel: string | null = null;
+  private managedAgentId: string | null = null;
 
   constructor(
     config: ClaudeAgentConfig,
@@ -650,6 +651,10 @@ class ClaudeAgentSession implements AgentSession {
     this.input = null;
   }
 
+  setManagedAgentId(agentId: string): void {
+    this.managedAgentId = agentId;
+  }
+
   private async ensureQuery(): Promise<Query> {
     if (this.query) {
       return this.query;
@@ -685,11 +690,16 @@ class ClaudeAgentSession implements AgentSession {
 
     // Always include the agent-control MCP server so agents can launch other agents
     const agentControlConfig = this.agentControlMcp ?? DEFAULT_AGENT_CONTROL_MCP;
+    // Merge base headers with the caller agent ID header for parent-child relationships
+    const agentControlHeaders: Record<string, string> = {
+      ...agentControlConfig.headers,
+      ...(this.managedAgentId ? { "X-Caller-Agent-Id": this.managedAgentId } : {}),
+    };
     const defaultMcpServers: Record<string, ClaudeMcpServerConfig> = {
       "agent-control": {
         type: "http",
         url: agentControlConfig.url,
-        ...(agentControlConfig.headers ? { headers: agentControlConfig.headers } : {}),
+        ...(Object.keys(agentControlHeaders).length > 0 ? { headers: agentControlHeaders } : {}),
       },
       playwright: {
         type: "stdio",
diff --git a/packages/server/src/server/agent/providers/codex-agent.ts b/packages/server/src/server/agent/providers/codex-agent.ts
index 28f8393fe..7ea209e8e 100644
--- a/packages/server/src/server/agent/providers/codex-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-agent.ts
@@ -636,6 +636,12 @@ class CodexAgentSession implements AgentSession {
     this.pendingPermissions.clear();
   }
 
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  setManagedAgentId(_agentId: string): void {
+    // Codex agents don't currently use MCP for agent-control
+    // This is a no-op but satisfies the AgentSession interface
+  }
+
   private drainHistoryEvents(): AgentStreamEvent[] {
     if (!this.historyEvents.length) {
       return [];
diff --git a/packages/server/src/server/index.ts b/packages/server/src/server/index.ts
index fb9466d46..06abf0224 100644
--- a/packages/server/src/server/index.ts
+++ b/packages/server/src/server/index.ts
@@ -116,11 +116,13 @@ async function main() {
 
   const agentMcpTransports: AgentMcpTransportMap = new Map();
 
-  const createAgentMcpTransport = async () => {
+  const createAgentMcpTransport = async (callerAgentId?: string) => {
     // Create a NEW McpServer instance per session (not shared across sessions)
+    // Pass the caller agent ID so create_agent can auto-set parentAgentId
     const agentMcpServer = await createAgentMcpServer({
       agentManager,
       agentRegistry,
+      callerAgentId,
     });
 
     const transport = new StreamableHTTPServerTransport({
@@ -183,7 +185,9 @@ async function main() {
           });
           return;
         }
-        transport = await createAgentMcpTransport();
+        // Extract caller agent ID from header (sent by agents when connecting)
+        const callerAgentId = req.header("X-Caller-Agent-Id");
+        transport = await createAgentMcpTransport(callerAgentId);
       }
 
       await transport.handleRequest(
diff --git a/plan.md b/plan.md
index c38a29bb7..c66863531 100644
--- a/plan.md
+++ b/plan.md
@@ -345,13 +345,14 @@ Files requiring modification:
   - If issues found: add fix tasks + re-test task.
   - **Done (2025-12-21 22:30)**: PARTIAL PASS with issues found. Fixed infinite loop bug (added `useShallow` to `childAgents` selector). Agent screen loads correctly. Sub-Agents menu section shows "No sub-agents". Parent agent successfully created child via MCP `create_agent`. However, child appears on homepage because `parentAgentId` not set - MCP server doesn't auto-inject calling agent's ID. Fix task added.
 
-- [ ] **Fix**: Auto-inject parentAgentId in MCP create_agent tool.
+- [x] **Fix**: Auto-inject parentAgentId in MCP create_agent tool.
 
   - The MCP server needs to know which agent is calling it
   - Explore passing agent ID context when MCP transport is created
   - Or: Have each agent's MCP session be scoped to that agent
   - Update `create_agent` handler to automatically set `parentAgentId`
   - Run typecheck after changes.
+  - **Done (2025-12-21 23:45)**: Implemented end-to-end parent-child agent ID injection. Added `setManagedAgentId()` to `AgentSession` interface (optional). `ClaudeAgentSession` stores the ID and includes it as `X-Caller-Agent-Id` header when connecting to agent-control MCP. `AgentManager.registerSession()` calls `setManagedAgentId()` after registration. MCP server extracts the header and auto-injects it as `parentAgentId` in `create_agent` handler. Typecheck passes.
 
 - [ ] **Test**: Re-verify parent/child hierarchy after MCP fix.