diff --git a/apps/x/apps/renderer/src/App.tsx b/apps/x/apps/renderer/src/App.tsx
index 8963a5cb..2952b7d0 100644
--- a/apps/x/apps/renderer/src/App.tsx
+++ b/apps/x/apps/renderer/src/App.tsx
@@ -33,6 +33,7 @@ import { AppsView } from '@/components/apps/apps-view';
 import { EmailView } from '@/components/email-view';
 import { WorkspaceView } from '@/components/workspace-view';
 import { CodingRunBlock } from '@/components/coding-run';
+import { SubAgentBlock } from '@/components/sub-agent-block';
 import { KnowledgeView, type KnowledgeViewMode } from '@/components/knowledge-view';
 import { GoogleDocPickerDialog } from '@/components/google-doc-picker-dialog';
 import { ChatHistoryView } from '@/components/chat-history-view';
@@ -5950,6 +5951,16 @@ function App() {
           />
         )
       }
+      if (item.name === 'spawn-agent') {
+        return (
+          <SubAgentBlock
+            key={item.id}
+            item={item}
+            open={isToolOpenForTab(tabId, item.id)}
+            onOpenChange={(open) => setToolOpenForTab(tabId, item.id, open)}
+          />
+        )
+      }
       const appActionData = getAppActionCardData(item)
       if (appActionData) {
         return <AppActionCard key={item.id} data={appActionData} status={item.status} />
diff --git a/apps/x/apps/renderer/src/components/sub-agent-block.tsx b/apps/x/apps/renderer/src/components/sub-agent-block.tsx
new file mode 100644
index 00000000..14deb928
--- /dev/null
+++ b/apps/x/apps/renderer/src/components/sub-agent-block.tsx
@@ -0,0 +1,92 @@
+import { useEffect, useState } from 'react'
+import { Bot } from 'lucide-react'
+import { Tool, ToolContent, ToolHeader } from '@/components/ai-elements/tool'
+import { CompactConversation } from '@/components/compact-conversation'
+import { fetchAgentRunTranscript, type AgentRunTranscript } from '@/lib/agent-transcript'
+import { toToolState, type ToolCall } from '@/lib/chat-conversation'
+
+// Rendered for a spawn-agent tool call: a collapsed status card that expands
+// into the child turn's live transcript. The child is a standalone turn
+// (sessionId null) whose events never reach the session bus, so while it
+// runs we poll sessions:getTurn via fetchAgentRunTranscript — the file is
+// local and append-only, making this cheap — and do one final fetch when the
+// parent tool call settles.
+
+const POLL_MS = 1000
+
+function useChildTranscript(
+  childTurnId: string | undefined,
+  running: boolean,
+  open: boolean,
+): AgentRunTranscript | null {
+  const [transcript, setTranscript] = useState<AgentRunTranscript | null>(null)
+  useEffect(() => {
+    if (!childTurnId || !open) return
+    let alive = true
+    const fetchOnce = async () => {
+      try {
+        const next = await fetchAgentRunTranscript(childTurnId)
+        if (alive) setTranscript(next)
+      } catch {
+        // Child file may not be readable yet; the next tick retries.
+      }
+    }
+    void fetchOnce()
+    if (!running) return () => { alive = false }
+    const timer = setInterval(() => void fetchOnce(), POLL_MS)
+    return () => {
+      alive = false
+      clearInterval(timer)
+    }
+  }, [childTurnId, running, open])
+  return transcript
+}
+
+export function SubAgentBlock({
+  item,
+  open,
+  onOpenChange,
+}: {
+  item: ToolCall
+  open: boolean
+  onOpenChange: (open: boolean) => void
+}) {
+  const input = item.input as
+    | { name?: string; agent_id?: string; task?: string }
+    | undefined
+  const agentName = item.subAgent?.agentName ?? input?.agent_id ?? input?.name ?? 'subagent'
+  const task = item.subAgent?.task ?? input?.task ?? ''
+  const running = item.status === 'pending' || item.status === 'running'
+  const transcript = useChildTranscript(item.subAgent?.childTurnId, running, open)
+
+  return (
+    <Tool open={open} onOpenChange={onOpenChange}>
+      <ToolHeader
+        title={`Sub-agent: ${agentName}`}
+        type="tool-spawn-agent"
+        state={toToolState(item.status)}
+      />
+      <ToolContent>
+        <div className="flex flex-col gap-3 px-4 pb-4">
+          {task && (
+            <div className="flex items-start gap-2 rounded-2xl bg-secondary/50 px-3 py-2 text-sm text-muted-foreground">
+              <Bot className="mt-0.5 size-4 shrink-0" />
+              <span className="whitespace-pre-wrap">{task}</span>
+            </div>
+          )}
+          {transcript ? (
+            <CompactConversation items={transcript.items} />
+          ) : (
+            <div className="px-1 text-sm text-muted-foreground">
+              {item.subAgent
+                ? 'Loading sub-agent transcript…'
+                : running
+                  ? 'Starting sub-agent…'
+                  : 'No sub-agent transcript was recorded.'}
+            </div>
+          )}
+        </div>
+      </ToolContent>
+    </Tool>
+  )
+}
diff --git a/apps/x/apps/renderer/src/lib/chat-conversation.ts b/apps/x/apps/renderer/src/lib/chat-conversation.ts
index 4e2db5de..ff3206a1 100644
--- a/apps/x/apps/renderer/src/lib/chat-conversation.ts
+++ b/apps/x/apps/renderer/src/lib/chat-conversation.ts
@@ -34,6 +34,8 @@ export interface ToolCall {
   // code_agent_run only: structured ACP stream items + the in-flight permission ask.
   codeRunEvents?: CodeRunEvent[]
   pendingCodePermission?: { requestId: string; ask: PermissionAsk } | null
+  // spawn-agent only: the durable parent→child link recorded as tool progress.
+  subAgent?: { childTurnId: string; agentName: string; task: string }
 }
 
 export interface ErrorMessage {
@@ -668,6 +670,7 @@ export const isToolGroup = (item: GroupedConversationItem): item is ToolGroup =>
 const isPlainToolCall = (item: ConversationItem): item is ToolCall => {
   if (!isToolCall(item)) return false
   if (item.name === 'code_agent_run') return false // rich standalone block, never grouped
+  if (item.name === 'spawn-agent') return false // rich standalone block, never grouped
   if (getWebSearchCardData(item)) return false
   if (getComposioConnectCardData(item)) return false
   if (getAppActionCardData(item)) return false
diff --git a/apps/x/apps/renderer/src/lib/session-chat/turn-view.test.ts b/apps/x/apps/renderer/src/lib/session-chat/turn-view.test.ts
index 8bcf8c21..d4fc78a6 100644
--- a/apps/x/apps/renderer/src/lib/session-chat/turn-view.test.ts
+++ b/apps/x/apps/renderer/src/lib/session-chat/turn-view.test.ts
@@ -309,6 +309,49 @@ describe('buildTurnConversation', () => {
     expect(settled.status).toBe('completed')
   })
 
+  it('derives the sub-agent child link from spawn-agent progress', () => {
+    const state = reduceTurn([
+      created(T1, S1),
+      requested(T1, 0),
+      completed(
+        T1,
+        0,
+        assistantCalls(
+          toolCallPart('sa1', 'spawn-agent', { task: 'research X', instructions: 'You research.' }),
+        ),
+      ),
+      invocation(T1, 'sa1', 'spawn-agent'),
+      {
+        type: 'tool_progress',
+        turnId: T1,
+        ts: TS,
+        toolCallId: 'sa1',
+        source: 'sync',
+        progress: {
+          kind: 'subagent',
+          childTurnId: 'child-turn-1',
+          agentName: 'researcher',
+          task: 'research X',
+        } as never,
+      },
+    ])
+    const tool = buildTurnConversation(state).filter(isToolCall)[0]
+    expect(tool.subAgent).toEqual({
+      childTurnId: 'child-turn-1',
+      agentName: 'researcher',
+      task: 'research X',
+    })
+
+    // Without the progress entry the link is simply absent.
+    const bare = reduceTurn([
+      created(T1, S1),
+      requested(T1, 0),
+      completed(T1, 0, assistantCalls(toolCallPart('sa2', 'spawn-agent', { task: 't' }))),
+      invocation(T1, 'sa2', 'spawn-agent'),
+    ])
+    expect(buildTurnConversation(bare).filter(isToolCall)[0].subAgent).toBeUndefined()
+  })
+
   it('renders user attachments and a failed turn as an error item', () => {
     const input = {
       role: 'user' as const,
diff --git a/apps/x/apps/renderer/src/lib/session-chat/turn-view.ts b/apps/x/apps/renderer/src/lib/session-chat/turn-view.ts
index 8db36e6a..cfef1249 100644
--- a/apps/x/apps/renderer/src/lib/session-chat/turn-view.ts
+++ b/apps/x/apps/renderer/src/lib/session-chat/turn-view.ts
@@ -241,6 +241,33 @@ function codeRunViewOf(
   }
 }
 
+// spawn-agent's durable trail in tool_progress: one 'subagent' entry recorded
+// the moment the child turn exists (see the spawn-agent branch in
+// real-tool-registry.ts). It is the parent→child link the card uses to fetch
+// and render the child transcript.
+function subAgentViewOf(tc: ToolCallState): Pick<ToolCall, 'subAgent'> {
+  for (const p of tc.progress) {
+    const entry = p.progress
+    if (!entry || typeof entry !== 'object' || Array.isArray(entry)) continue
+    const { kind, childTurnId, agentName, task } = entry as {
+      kind?: unknown
+      childTurnId?: unknown
+      agentName?: unknown
+      task?: unknown
+    }
+    if (kind === 'subagent' && typeof childTurnId === 'string') {
+      return {
+        subAgent: {
+          childTurnId,
+          agentName: typeof agentName === 'string' ? agentName : 'subagent',
+          task: typeof task === 'string' ? task : '',
+        },
+      }
+    }
+  }
+  return {}
+}
+
 // One turn's contribution to the conversation: the user input, then per
 // completed model call its text and tool calls (with live status/results).
 export function buildTurnConversation(state: TurnState): ConversationItem[] {
@@ -295,6 +322,7 @@ export function buildTurnConversation(state: TurnState): ConversationItem[] {
           status: tc ? toolStatus(tc) : 'running',
           timestamp: ts(),
           ...(tc ? codeRunViewOf(tc) : {}),
+          ...(tc ? subAgentViewOf(tc) : {}),
         } satisfies ToolCall)
       }
     }
diff --git a/apps/x/packages/core/docs/turn-runtime-design.md b/apps/x/packages/core/docs/turn-runtime-design.md
index 1c61a7b8..f4169b64 100644
--- a/apps/x/packages/core/docs/turn-runtime-design.md
+++ b/apps/x/packages/core/docs/turn-runtime-design.md
@@ -1895,6 +1895,17 @@ tool handler may create and execute child turns, forward progress, and return a
 parent tool result. No subflow or parent-turn concept is added to this initial
 turn schema.
 
+Implemented (v1) as the `spawn-agent` builtin: a sync tool whose handler
+(`RealToolRegistry`) runs the child as a standalone headless turn
+(`runSpawnedAgent`), records `{kind: "subagent", childTurnId}` as durable
+tool progress (the only parent→child link), and returns the child's final
+text plus a status envelope. `RequestedAgent` is a union of by-id and inline
+variants; inline agents resolve through `InlineAgentResolver`. Depth is
+capped at 1: both resolvers strip the spawn tool from children, and the
+handler refuses child-shaped parents. Parallel fan-out comes from concurrent
+sync-tool execution (§10.5), not from async suspension; async (restart
+survivability for long children) remains future work.
+
 ### 29.3 Reliability enhancements
 
 - External-input idempotency.
diff --git a/apps/x/packages/core/src/agents/headless.ts b/apps/x/packages/core/src/agents/headless.ts
index d974a5fd..0f17eae4 100644
--- a/apps/x/packages/core/src/agents/headless.ts
+++ b/apps/x/packages/core/src/agents/headless.ts
@@ -1,6 +1,7 @@
 import type { z } from "zod";
 import type { AssistantMessage } from "@x/shared/dist/message.js";
 import {
+    type RequestedAgent,
     reduceTurn,
     type TurnState,
 } from "@x/shared/dist/turns.js";
@@ -26,7 +27,10 @@ export class HeadlessRunError extends Error {
 }
 
 export interface HeadlessAgentOptions {
-    agentId: string;
+    agentId?: string;
+    // Full agent request, used verbatim when set (inline agents, composition
+    // overrides). Takes precedence over agentId/model/provider.
+    agent?: z.infer<typeof RequestedAgent>;
     message: string;
     // Model id; when set without provider, the app-default provider applies.
     model?: string;
@@ -121,22 +125,29 @@ export class HeadlessAgentRunner implements IHeadlessAgentRunner {
     }
 
     async start(options: HeadlessAgentOptions): Promise<HeadlessAgentHandle> {
-        let modelOverride: { provider: string; model: string } | undefined;
-        if (options.model && options.provider) {
-            modelOverride = { provider: options.provider, model: options.model };
-        } else if (options.model || options.provider) {
-            const defaults = await this.defaultModelResolver.resolve();
-            modelOverride = {
-                provider: options.provider ?? defaults.provider,
-                model: options.model ?? defaults.model,
+        let agent = options.agent;
+        if (!agent) {
+            if (!options.agentId) {
+                throw new Error("headless agent needs an agentId or an agent request");
+            }
+            let modelOverride: { provider: string; model: string } | undefined;
+            if (options.model && options.provider) {
+                modelOverride = { provider: options.provider, model: options.model };
+            } else if (options.model || options.provider) {
+                const defaults = await this.defaultModelResolver.resolve();
+                modelOverride = {
+                    provider: options.provider ?? defaults.provider,
+                    model: options.model ?? defaults.model,
+                };
+            }
+            agent = {
+                agentId: options.agentId,
+                ...(modelOverride ? { overrides: { model: modelOverride } } : {}),
             };
         }
 
         const turnId = await this.turnRuntime.createTurn({
-            agent: {
-                agentId: options.agentId,
-                ...(modelOverride ? { overrides: { model: modelOverride } } : {}),
-            },
+            agent,
             sessionId: null,
             context: [],
             input: { role: "user", content: options.message },
diff --git a/apps/x/packages/core/src/agents/spawn-agent.test.ts b/apps/x/packages/core/src/agents/spawn-agent.test.ts
new file mode 100644
index 00000000..55e093f7
--- /dev/null
+++ b/apps/x/packages/core/src/agents/spawn-agent.test.ts
@@ -0,0 +1,222 @@
+import { describe, expect, it } from "vitest";
+import type { z } from "zod";
+import type { TurnEvent, TurnState } from "@x/shared/dist/turns.js";
+import type { ITurnRuntime } from "../turns/api.js";
+import type {
+    HeadlessAgentHandle,
+    HeadlessAgentOptions,
+    HeadlessAgentResult,
+    IHeadlessAgentRunner,
+} from "./headless.js";
+import { runSpawnedAgent } from "./spawn-agent.js";
+
+const TS = "2026-07-07T10:00:00Z";
+
+function parentCreated(
+    overrides: {
+        requested?: z.infer<typeof TurnEvent> extends never ? never : unknown;
+    } & Record<string, unknown> = {},
+): Array<z.infer<typeof TurnEvent>> {
+    return [
+        {
+            type: "turn_created",
+            schemaVersion: 1,
+            turnId: "parent-1",
+            ts: TS,
+            sessionId: null,
+            agent: {
+                requested: (overrides.requested ?? { agentId: "copilot" }) as never,
+                resolved: {
+                    agentId: "copilot",
+                    systemPrompt: "s",
+                    model: { provider: "parent-p", model: "parent-m" },
+                    tools: [],
+                },
+            },
+            context: [],
+            input: { role: "user", content: "hi" },
+            config: {
+                autoPermission: true,
+                humanAvailable: false,
+                maxModelCalls: 20,
+            },
+        } as z.infer<typeof TurnEvent>,
+    ];
+}
+
+function fakeServices(opts: {
+    parentEvents?: Array<z.infer<typeof TurnEvent>>;
+    childResult?: Partial<HeadlessAgentResult>;
+    startError?: string;
+}) {
+    const started: HeadlessAgentOptions[] = [];
+    const turnRuntime = {
+        getTurn: async () => ({
+            turnId: "parent-1",
+            events: opts.parentEvents ?? parentCreated(),
+        }),
+    } as unknown as ITurnRuntime;
+    const headlessRunner: IHeadlessAgentRunner = {
+        start: async (options: HeadlessAgentOptions): Promise<HeadlessAgentHandle> => {
+            if (opts.startError) {
+                throw new Error(opts.startError);
+            }
+            started.push(options);
+            const result: HeadlessAgentResult = {
+                outcome: {
+                    status: "completed",
+                    output: { role: "assistant", content: "answer" },
+                    finishReason: "stop",
+                    usage: { totalTokens: 7 },
+                },
+                state: { modelCalls: [{}, {}] } as unknown as TurnState,
+                summary: "answer",
+                ...opts.childResult,
+            };
+            return { turnId: "child-1", done: Promise.resolve(result) };
+        },
+        run: async () => {
+            throw new Error("unused");
+        },
+    };
+    return { services: { turnRuntime, headlessRunner }, started };
+}
+
+const signal = new AbortController().signal;
+
+describe("runSpawnedAgent", () => {
+    it("runs an inline child on the parent's model and returns the result envelope", async () => {
+        const { services, started } = fakeServices({});
+        const progress: unknown[] = [];
+        const result = await runSpawnedAgent(
+            { task: "find things", name: "researcher", instructions: "You research." },
+            {
+                parentTurnId: "parent-1",
+                signal,
+                services,
+                onChildStarted: async (info) => {
+                    progress.push(info);
+                },
+            },
+        );
+        expect(started[0].agent).toEqual({
+            inline: {
+                name: "researcher",
+                instructions: "You research.",
+                model: { provider: "parent-p", model: "parent-m" },
+            },
+        });
+        expect(started[0].maxModelCalls).toBe(20);
+        expect(started[0].signal).toBe(signal);
+        expect(progress).toEqual([
+            { childTurnId: "child-1", agentName: "researcher", task: "find things" },
+        ]);
+        expect(result).toEqual({
+            isError: false,
+            output: {
+                status: "completed",
+                result: "answer",
+                childTurnId: "child-1",
+                agent: "researcher",
+                modelCalls: 2,
+                usage: { totalTokens: 7 },
+            },
+        });
+    });
+
+    it("marks by-id children with the subagent composition flag", async () => {
+        const { services, started } = fakeServices({});
+        await runSpawnedAgent(
+            { task: "t", agent_id: "background-task-agent", max_model_calls: 5 },
+            { parentTurnId: "parent-1", signal, services },
+        );
+        expect(started[0].agent).toEqual({
+            agentId: "background-task-agent",
+            overrides: {
+                model: { provider: "parent-p", model: "parent-m" },
+                composition: { subagent: true },
+            },
+        });
+        expect(started[0].maxModelCalls).toBe(5);
+    });
+
+    it("requires exactly one of agent_id and instructions", async () => {
+        const { services } = fakeServices({});
+        for (const input of [
+            { task: "t" },
+            { task: "t", agent_id: "a", instructions: "b" },
+        ]) {
+            const result = await runSpawnedAgent(input, {
+                parentTurnId: "parent-1",
+                signal,
+                services,
+            });
+            expect(result.isError).toBe(true);
+            expect(result.output).toMatch(/exactly one/);
+        }
+    });
+
+    it("rejects a model-call budget above the cap at the schema boundary", async () => {
+        const { services } = fakeServices({});
+        const result = await runSpawnedAgent(
+            { task: "t", instructions: "x", max_model_calls: 50 },
+            { parentTurnId: "parent-1", signal, services },
+        );
+        expect(result.isError).toBe(true);
+        expect(result.output).toMatch(/invalid input/);
+    });
+
+    it("refuses to spawn from a child parent (depth cap)", async () => {
+        for (const requested of [
+            { inline: { name: "c", instructions: "x" } },
+            { agentId: "copilot", overrides: { composition: { subagent: true } } },
+        ]) {
+            const { services, started } = fakeServices({
+                parentEvents: parentCreated({ requested }),
+            });
+            const result = await runSpawnedAgent(
+                { task: "t", instructions: "x" },
+                { parentTurnId: "parent-1", signal, services },
+            );
+            expect(result.isError).toBe(true);
+            expect(result.output).toMatch(/cannot spawn further/);
+            expect(started).toHaveLength(0);
+        }
+    });
+
+    it("reports resolution failures conversationally", async () => {
+        const { services } = fakeServices({ startError: "agent not found: nope" });
+        const result = await runSpawnedAgent(
+            { task: "t", agent_id: "nope" },
+            { parentTurnId: "parent-1", signal, services },
+        );
+        expect(result).toEqual({
+            isError: true,
+            output: "spawn-agent: agent not found: nope",
+        });
+    });
+
+    it("returns child failure as an error envelope with the partial answer", async () => {
+        const { services } = fakeServices({
+            childResult: {
+                outcome: {
+                    status: "failed",
+                    error: "model exploded",
+                    usage: {},
+                },
+                summary: "got halfway",
+            },
+        });
+        const result = await runSpawnedAgent(
+            { task: "t", instructions: "x" },
+            { parentTurnId: "parent-1", signal, services },
+        );
+        expect(result.isError).toBe(true);
+        expect(result.output).toMatchObject({
+            status: "failed",
+            error: "model exploded",
+            partialResult: "got halfway",
+            childTurnId: "child-1",
+        });
+    });
+});
diff --git a/apps/x/packages/core/src/agents/spawn-agent.ts b/apps/x/packages/core/src/agents/spawn-agent.ts
new file mode 100644
index 00000000..4466e62c
--- /dev/null
+++ b/apps/x/packages/core/src/agents/spawn-agent.ts
@@ -0,0 +1,253 @@
+import { z } from "zod";
+import {
+    DEFAULT_MAX_MODEL_CALLS,
+    type JsonValue,
+    type ModelDescriptor,
+    type RequestedAgent,
+    type ToolResultData,
+    isInlineAgentRequest,
+    reduceTurn,
+} from "@x/shared/dist/turns.js";
+
+// The spawn-agent tool: runs a sub-agent as a standalone headless child turn
+// and returns its final answer. The input schema and description live here
+// (imported by the BuiltinTools catalog entry); execution resolves runtime
+// services lazily so this module stays import-cycle-free.
+
+export const SpawnAgentInput = z.object({
+    task: z
+        .string()
+        .describe(
+            "The task for the sub-agent. It starts with NO other context — include everything it needs (facts, constraints, expected output format).",
+        ),
+    agent_id: z
+        .string()
+        .optional()
+        .describe(
+            "Run a stored agent by id. Mutually exclusive with `instructions`.",
+        ),
+    name: z
+        .string()
+        .optional()
+        .describe("Short display name for an inline agent (e.g. 'researcher')."),
+    instructions: z
+        .string()
+        .optional()
+        .describe(
+            "System instructions for an agent constructed on the fly. Mutually exclusive with `agent_id`.",
+        ),
+    model: z
+        .string()
+        .optional()
+        .describe("Model id for the sub-agent; defaults to the current model."),
+    provider: z
+        .string()
+        .optional()
+        .describe("Provider for `model`; defaults to the current provider."),
+    tools: z
+        .array(z.string())
+        .optional()
+        .describe(
+            "Builtin tool names for an inline agent. Omit for the default headless profile (files, web, search, knowledge).",
+        ),
+    max_model_calls: z
+        .number()
+        .int()
+        .min(1)
+        .max(DEFAULT_MAX_MODEL_CALLS)
+        .optional()
+        .describe(
+            `Model-call budget for the sub-agent (default and cap: ${DEFAULT_MAX_MODEL_CALLS}).`,
+        ),
+});
+
+export const SPAWN_AGENT_DESCRIPTION =
+    "Launch a sub-agent that works on a task in its own isolated, headless turn and returns its final answer. " +
+    "Issue several spawn-agent calls in ONE response to run sub-agents in parallel — use this to fan out independent research, analysis, or file work. " +
+    "Provide either `agent_id` (a stored agent) or `instructions` (construct a specialist on the fly, optionally with `name` and `tools`). " +
+    "The sub-agent cannot ask the user questions and cannot spawn further sub-agents; give it a complete, self-contained task.";
+
+export interface SpawnedAgentCallbacks {
+    parentTurnId: string;
+    signal: AbortSignal;
+    // Invoked as soon as the child turn exists — the caller records the
+    // durable parent→child link before the child settles.
+    onChildStarted?: (info: {
+        childTurnId: string;
+        agentName: string;
+        task: string;
+    }) => Promise<void>;
+    // Test seam; production resolves these from the DI container.
+    services?: {
+        turnRuntime: import("../turns/api.js").ITurnRuntime;
+        headlessRunner: import("./headless.js").IHeadlessAgentRunner;
+    };
+}
+
+// Runs one spawned child to completion. Never throws for task-level problems
+// (bad input, unknown agent/tool, child failure) — those come back as
+// isError results so the parent model can react conversationally.
+export async function runSpawnedAgent(
+    rawInput: unknown,
+    opts: SpawnedAgentCallbacks,
+): Promise<z.infer<typeof ToolResultData>> {
+    const parsed = SpawnAgentInput.safeParse(rawInput);
+    if (!parsed.success) {
+        return spawnError(`invalid input: ${parsed.error.message}`);
+    }
+    const input = parsed.data;
+    if (!input.agent_id === !input.instructions) {
+        return spawnError(
+            "provide exactly one of `agent_id` or `instructions`",
+        );
+    }
+
+    // Lazy: this module is imported by the BuiltinTools catalog, which the
+    // DI container's bridges import at startup.
+    const { turnRuntime, headlessRunner } =
+        opts.services ?? (await resolveServices());
+
+    let parentModel: z.infer<typeof ModelDescriptor> | undefined;
+    try {
+        const parent = reduceTurn(
+            (await turnRuntime.getTurn(opts.parentTurnId)).events,
+        );
+        const parentAgent = parent.definition.agent;
+        // Defense in depth for the depth-1 cap: resolvers already strip the
+        // spawn tool from children, but a child that somehow holds it must
+        // still not recurse.
+        if (hasSubagentFlag(parentAgent.requested)) {
+            return spawnError("sub-agents cannot spawn further sub-agents");
+        }
+        parentModel = parentAgent.resolved.model;
+    } catch {
+        // Parent unreadable (legacy runs path): fall back to app defaults.
+        parentModel = undefined;
+    }
+
+    const model: z.infer<typeof ModelDescriptor> | undefined = input.model
+        ? {
+              provider: input.provider ?? parentModel?.provider ?? "",
+              model: input.model,
+          }
+        : parentModel;
+    if (model && !model.provider) {
+        return spawnError(
+            "`model` was set but no provider could be determined; pass `provider` too",
+        );
+    }
+
+    const agentName = input.agent_id ?? input.name ?? "subagent";
+    const agent: z.infer<typeof RequestedAgent> = input.agent_id
+        ? {
+              agentId: input.agent_id,
+              overrides: {
+                  ...(model ? { model } : {}),
+                  composition: { subagent: true },
+              },
+          }
+        : {
+              inline: {
+                  name: agentName,
+                  instructions: input.instructions!,
+                  ...(model ? { model } : {}),
+                  ...(input.tools ? { tools: input.tools } : {}),
+              },
+          };
+
+    const maxModelCalls = Math.min(
+        input.max_model_calls ?? DEFAULT_MAX_MODEL_CALLS,
+        DEFAULT_MAX_MODEL_CALLS,
+    );
+
+    let handle: Awaited<ReturnType<typeof headlessRunner.start>>;
+    try {
+        handle = await headlessRunner.start({
+            agent,
+            message: input.task,
+            maxModelCalls,
+            signal: opts.signal,
+        });
+    } catch (error) {
+        // Resolution failures (unknown agent id, unknown tool name) reject
+        // createTurn before any turn file exists.
+        return spawnError(errorText(error));
+    }
+
+    await opts.onChildStarted?.({
+        childTurnId: handle.turnId,
+        agentName,
+        task: input.task,
+    });
+
+    const result = await handle.done;
+    const base = {
+        childTurnId: handle.turnId,
+        agent: agentName,
+        modelCalls: result.state.modelCalls.length,
+        usage: result.outcome.usage as JsonValue,
+    };
+    if (result.outcome.status === "completed") {
+        return {
+            output: {
+                status: "completed",
+                result: result.summary ?? "",
+                ...base,
+            },
+            isError: false,
+        };
+    }
+    return {
+        output: {
+            status: result.outcome.status,
+            error:
+                result.outcome.status === "failed"
+                    ? result.outcome.error
+                    : `sub-agent turn was ${result.outcome.status}`,
+            // A partial answer is often still useful to the parent.
+            ...(result.summary ? { partialResult: result.summary } : {}),
+            ...base,
+        },
+        isError: true,
+    };
+}
+
+async function resolveServices(): Promise<
+    NonNullable<SpawnedAgentCallbacks["services"]>
+> {
+    const { default: container } = await import("../di/container.js");
+    return {
+        turnRuntime:
+            container.resolve<import("../turns/api.js").ITurnRuntime>(
+                "turnRuntime",
+            ),
+        headlessRunner: container.resolve<
+            import("./headless.js").IHeadlessAgentRunner
+        >("headlessAgentRunner"),
+    };
+}
+
+function spawnError(message: string): z.infer<typeof ToolResultData> {
+    return { output: `spawn-agent: ${message}`, isError: true };
+}
+
+// True for any child-shaped parent: inline agents only exist as spawned
+// children, and by-id children carry the subagent composition flag.
+function hasSubagentFlag(
+    requested: z.infer<typeof RequestedAgent>,
+): boolean {
+    if (isInlineAgentRequest(requested)) {
+        return true;
+    }
+    const composition = requested.overrides?.composition;
+    return (
+        typeof composition === "object" &&
+        composition !== null &&
+        !Array.isArray(composition) &&
+        composition.subagent === true
+    );
+}
+
+function errorText(error: unknown): string {
+    return error instanceof Error ? error.message : String(error);
+}
diff --git a/apps/x/packages/core/src/application/lib/builtin-tools.ts b/apps/x/packages/core/src/application/lib/builtin-tools.ts
index edd04270..0190c309 100644
--- a/apps/x/packages/core/src/application/lib/builtin-tools.ts
+++ b/apps/x/packages/core/src/application/lib/builtin-tools.ts
@@ -16,6 +16,8 @@ import { composioAccountsRepo } from "../../composio/repo.js";
 import { executeAction as executeComposioAction, isConfigured as isComposioConfigured, searchTools as searchComposioTools } from "../../composio/client.js";
 import { CURATED_TOOLKITS, CURATED_TOOLKIT_SLUGS } from "@x/shared/dist/composio.js";
 import { RowboatAppManifestSchema } from "@x/shared/dist/rowboat-app.js";
+import { SPAWN_AGENT_TOOL_NAME } from "@x/shared/dist/turns.js";
+import { SPAWN_AGENT_DESCRIPTION, SpawnAgentInput } from "../../agents/spawn-agent.js";
 import { BrowserControlInputSchema, type BrowserControlInput } from "@x/shared/dist/browser-control.js";
 import { BackgroundTaskSchema, TriggersSchema } from "@x/shared/dist/background-task.js";
 import type { CodeModeManager } from "../../code-mode/acp/manager.js";
@@ -2011,4 +2013,27 @@ export const BuiltinTools: z.infer<typeof BuiltinToolsSchema> = {
             }
         },
     },
+
+    [SPAWN_AGENT_TOOL_NAME]: {
+        description: SPAWN_AGENT_DESCRIPTION,
+        inputSchema: SpawnAgentInput,
+        // Legacy runs-runtime path only: the turn runtime intercepts
+        // builtin:spawn-agent in RealToolRegistry with a dedicated handler
+        // that also records the parent→child link as durable tool progress.
+        execute: async (input: unknown, ctx?: ToolContext) => {
+            const { runSpawnedAgent } = await import("../../agents/spawn-agent.js");
+            const result = await runSpawnedAgent(input, {
+                parentTurnId: ctx?.runId ?? "",
+                signal: ctx?.signal ?? new AbortController().signal,
+            });
+            if (result.isError) {
+                throw new Error(
+                    typeof result.output === "string"
+                        ? result.output
+                        : JSON.stringify(result.output),
+                );
+            }
+            return result.output;
+        },
+    },
 };
diff --git a/apps/x/packages/core/src/di/container.ts b/apps/x/packages/core/src/di/container.ts
index 306ebc33..2ba9ec84 100644
--- a/apps/x/packages/core/src/di/container.ts
+++ b/apps/x/packages/core/src/di/container.ts
@@ -43,6 +43,8 @@ import type { IModelRegistry } from "../turns/model-registry.js";
 import type { IToolRegistry } from "../turns/tool-registry.js";
 import type { IPermissionChecker, IPermissionClassifier } from "../turns/permission.js";
 import { RealAgentResolver } from "../turns/bridges/real-agent-resolver.js";
+import { InlineAgentResolver } from "../turns/bridges/inline-agent-resolver.js";
+import { DispatchingAgentResolver } from "../turns/bridges/agent-resolver-dispatch.js";
 import { RealModelRegistry } from "../turns/bridges/real-model-registry.js";
 import { RealToolRegistry } from "../turns/bridges/real-tool-registry.js";
 import { RealPermissionChecker } from "../turns/bridges/real-permission-checker.js";
@@ -123,7 +125,13 @@ container.register({
     ).singleton(),
     lifecycleBus: asClass<ITurnLifecycleBus>(EmitterTurnLifecycleBus).singleton(),
     usageReporter: asClass<IUsageReporter>(RealUsageReporter).singleton(),
-    agentResolver: asFunction<IAgentResolver>(() => new RealAgentResolver()).singleton(),
+    agentResolver: asFunction<IAgentResolver>(
+        () =>
+            new DispatchingAgentResolver(
+                new RealAgentResolver(),
+                new InlineAgentResolver(),
+            ),
+    ).singleton(),
     modelRegistry: asFunction<IModelRegistry>(() => new RealModelRegistry()).singleton(),
     toolRegistry: asFunction<IToolRegistry>(() => new RealToolRegistry()).singleton(),
     permissionChecker: asFunction<IPermissionChecker>(() => new RealPermissionChecker()).singleton(),
diff --git a/apps/x/packages/core/src/sessions/sessions.ts b/apps/x/packages/core/src/sessions/sessions.ts
index e4c262db..e996be14 100644
--- a/apps/x/packages/core/src/sessions/sessions.ts
+++ b/apps/x/packages/core/src/sessions/sessions.ts
@@ -14,6 +14,8 @@ import {
     type ModelDescriptor,
     type ToolResultData,
     deriveTurnStatus,
+    inlineAgentId,
+    isInlineAgentRequest,
     reduceTurn,
 } from "@x/shared/dist/turns.js";
 import type { IMonotonicallyIncreasingIdGenerator } from "../application/lib/id-gen.js";
@@ -189,7 +191,9 @@ export class SessionsImpl implements ISessions {
                     ts: this.clock.now(),
                     turnId,
                     sessionSeq: state.turns.length + 1,
-                    agentId: config.agent.agentId,
+                    agentId: isInlineAgentRequest(config.agent)
+                        ? inlineAgentId(config.agent.inline.name)
+                        : config.agent.agentId,
                     model: await this.resolvedModelOf(turnId),
                 },
             ];
diff --git a/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.test.ts b/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.test.ts
new file mode 100644
index 00000000..4b24d62d
--- /dev/null
+++ b/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.test.ts
@@ -0,0 +1,37 @@
+import { describe, expect, it } from "vitest";
+import type { ResolvedAgent } from "@x/shared/dist/turns.js";
+import type { z } from "zod";
+import { DispatchingAgentResolver } from "./agent-resolver-dispatch.js";
+import type { InlineAgentResolver } from "./inline-agent-resolver.js";
+import type { RealAgentResolver } from "./real-agent-resolver.js";
+
+function stub(agentId: string) {
+    return {
+        resolve: async () =>
+            ({
+                agentId,
+                systemPrompt: "s",
+                model: { provider: "p", model: "m" },
+                tools: [],
+            }) satisfies z.infer<typeof ResolvedAgent>,
+    };
+}
+
+describe("DispatchingAgentResolver", () => {
+    it("routes by-id requests to the by-id resolver and inline to the inline resolver", async () => {
+        const resolver = new DispatchingAgentResolver(
+            stub("from-by-id") as unknown as RealAgentResolver,
+            stub("from-inline") as unknown as InlineAgentResolver,
+        );
+        expect((await resolver.resolve({ agentId: "copilot" })).agentId).toBe(
+            "from-by-id",
+        );
+        expect(
+            (
+                await resolver.resolve({
+                    inline: { name: "x", instructions: "y" },
+                })
+            ).agentId,
+        ).toBe("from-inline");
+    });
+});
diff --git a/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.ts b/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.ts
new file mode 100644
index 00000000..95b4f6e4
--- /dev/null
+++ b/apps/x/packages/core/src/turns/bridges/agent-resolver-dispatch.ts
@@ -0,0 +1,27 @@
+import type { z } from "zod";
+import {
+    type RequestedAgent,
+    type ResolvedAgent,
+    isInlineAgentRequest,
+} from "@x/shared/dist/turns.js";
+import type { IAgentResolver } from "../agent-resolver.js";
+import type { InlineAgentResolver } from "./inline-agent-resolver.js";
+import type { RealAgentResolver } from "./real-agent-resolver.js";
+
+// The only IAgentResolver implementation: narrows the RequestedAgent union
+// exactly once and hands each variant to a resolver that never sees the
+// other's concerns (loadAgent/composition vs. catalog validation/defaults).
+export class DispatchingAgentResolver implements IAgentResolver {
+    constructor(
+        private readonly byId: RealAgentResolver,
+        private readonly inline: InlineAgentResolver,
+    ) {}
+
+    resolve(
+        requested: z.infer<typeof RequestedAgent>,
+    ): Promise<z.infer<typeof ResolvedAgent>> {
+        return isInlineAgentRequest(requested)
+            ? this.inline.resolve(requested)
+            : this.byId.resolve(requested);
+    }
+}
diff --git a/apps/x/packages/core/src/turns/bridges/builtin-descriptors.ts b/apps/x/packages/core/src/turns/bridges/builtin-descriptors.ts
new file mode 100644
index 00000000..1c6cc153
--- /dev/null
+++ b/apps/x/packages/core/src/turns/bridges/builtin-descriptors.ts
@@ -0,0 +1,43 @@
+import { z } from "zod";
+import type { JsonValue, ToolDescriptor } from "@x/shared/dist/turns.js";
+import type { BuiltinTools } from "../../application/lib/builtin-tools.js";
+
+type BuiltinToolDef = (typeof BuiltinTools)[string];
+
+// The one place a builtin catalog entry becomes a persisted ToolDescriptor.
+// Shared by the by-id and inline agent resolvers so both produce
+// byte-identical descriptors (which is what makes agent-snapshot inheritance
+// work across turns).
+export function builtinToolDescriptor(
+    name: string,
+    builtin: BuiltinToolDef,
+): z.infer<typeof ToolDescriptor> {
+    return {
+        toolId: `builtin:${name}`,
+        name,
+        description: builtin.description,
+        inputSchema: toJsonSchema(builtin.inputSchema),
+        execution: "sync",
+        requiresHuman: false,
+    };
+}
+
+export function toJsonSchema(schema: unknown): JsonValue {
+    try {
+        return toJsonValue(z.toJSONSchema(schema as z.ZodType)) ?? {
+            type: "object",
+            properties: {},
+        };
+    } catch {
+        // An exotic zod schema must not break the whole turn.
+        return { type: "object", properties: {} };
+    }
+}
+
+export function toJsonValue(value: unknown): JsonValue | undefined {
+    try {
+        return JSON.parse(JSON.stringify(value)) as JsonValue;
+    } catch {
+        return undefined;
+    }
+}
diff --git a/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.test.ts b/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.test.ts
new file mode 100644
index 00000000..ff69be97
--- /dev/null
+++ b/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.test.ts
@@ -0,0 +1,101 @@
+import { describe, expect, it } from "vitest";
+import { z } from "zod";
+import type { BuiltinTools } from "../../application/lib/builtin-tools.js";
+import { InlineAgentResolver } from "./inline-agent-resolver.js";
+
+const fakeBuiltins = {
+    "web-search": {
+        description: "Search the web",
+        inputSchema: z.object({ query: z.string() }),
+        execute: async () => null,
+    },
+    "file-readText": {
+        description: "Read a file",
+        inputSchema: z.object({ path: z.string() }),
+        execute: async () => null,
+    },
+    "flaky-tool": {
+        description: "Sometimes available",
+        inputSchema: z.object({}),
+        execute: async () => null,
+        isAvailable: async () => false,
+    },
+    "spawn-agent": {
+        description: "Spawn",
+        inputSchema: z.object({}),
+        execute: async () => null,
+    },
+    executeCommand: {
+        description: "Run a command",
+        inputSchema: z.object({ command: z.string() }),
+        execute: async () => null,
+    },
+} as unknown as typeof BuiltinTools;
+
+function makeResolver() {
+    return new InlineAgentResolver({
+        builtins: fakeBuiltins,
+        defaultModel: async () => ({ model: "m-default", provider: "p-default" }),
+    });
+}
+
+describe("InlineAgentResolver", () => {
+    it("materializes the spec verbatim: inline agentId, instructions as prompt, spec model", async () => {
+        const resolved = await makeResolver().resolve({
+            inline: {
+                name: "researcher",
+                instructions: "You research things.",
+                model: { provider: "p1", model: "m1" },
+                tools: ["web-search"],
+            },
+        });
+        expect(resolved.agentId).toBe("inline:researcher");
+        expect(resolved.systemPrompt).toBe("You research things.");
+        expect(resolved.model).toEqual({ provider: "p1", model: "m1" });
+        expect(resolved.tools).toEqual([
+            expect.objectContaining({
+                toolId: "builtin:web-search",
+                name: "web-search",
+                execution: "sync",
+                requiresHuman: false,
+            }),
+        ]);
+    });
+
+    it("falls back to the app-default model when the spec has none", async () => {
+        const resolved = await makeResolver().resolve({
+            inline: { name: "a", instructions: "x", tools: [] },
+        });
+        expect(resolved.model).toEqual({ provider: "p-default", model: "m-default" });
+    });
+
+    it("rejects unknown builtin names loudly", async () => {
+        await expect(
+            makeResolver().resolve({
+                inline: { name: "a", instructions: "x", tools: ["nope"] },
+            }),
+        ).rejects.toThrowError(/unknown builtin tool: nope/);
+    });
+
+    it("strips spawn-agent (depth cap) and skips unavailable tools", async () => {
+        const resolved = await makeResolver().resolve({
+            inline: {
+                name: "a",
+                instructions: "x",
+                tools: ["spawn-agent", "flaky-tool", "file-readText"],
+            },
+        });
+        expect(resolved.tools.map((t) => t.name)).toEqual(["file-readText"]);
+    });
+
+    it("default profile is the catalog minus the headless/child exclusions", async () => {
+        const resolved = await makeResolver().resolve({
+            inline: { name: "a", instructions: "x" },
+        });
+        const names = resolved.tools.map((t) => t.name);
+        expect(names).toEqual(["web-search", "file-readText"]);
+        // Excluded by policy, not by absence:
+        expect(names).not.toContain("executeCommand");
+        expect(names).not.toContain("spawn-agent");
+    });
+});
diff --git a/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.ts b/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.ts
new file mode 100644
index 00000000..fc2ae4e9
--- /dev/null
+++ b/apps/x/packages/core/src/turns/bridges/inline-agent-resolver.ts
@@ -0,0 +1,95 @@
+import type { z } from "zod";
+import {
+    type InlineAgentRequest,
+    type ResolvedAgent,
+    SPAWN_AGENT_TOOL_NAME,
+    type ToolDescriptor,
+    inlineAgentId,
+} from "@x/shared/dist/turns.js";
+import { ResolvedAgent as ResolvedAgentSchema } from "@x/shared/dist/turns.js";
+import { BuiltinTools } from "../../application/lib/builtin-tools.js";
+import { getDefaultModelAndProvider } from "../../models/defaults.js";
+import { builtinToolDescriptor } from "./builtin-descriptors.js";
+
+// Default tool profile for inline agents that omit `tools`: every builtin
+// except the ones that make no sense headlessly or in a child. Mirrors the
+// background-task agent's exclusions (no interactive approval surface) plus
+// the task/session launchers — an ephemeral child should do its own work,
+// not schedule more.
+const DEFAULT_PROFILE_EXCLUDED = new Set([
+    "executeCommand", // headless: no interactive approval
+    "code_agent_run", // headless: needs interactive permission UI
+    "launch-code-task",
+    "run-background-task-agent",
+    "create-background-task",
+    "patch-background-task",
+    "run-live-note-agent",
+    SPAWN_AGENT_TOOL_NAME,
+]);
+
+export interface InlineAgentResolverDeps {
+    builtins?: typeof BuiltinTools;
+    defaultModel?: () => Promise<{ model: string; provider: string }>;
+}
+
+// Materializes an inline (spawned) agent definition into the same immutable
+// ResolvedAgent snapshot a stored agent gets. Deliberately knows nothing
+// about loadAgent, composition overrides, or copilot context — an inline
+// agent is exactly its persisted spec.
+export class InlineAgentResolver {
+    private readonly builtins: typeof BuiltinTools;
+    private readonly defaultModel: () => Promise<{ model: string; provider: string }>;
+
+    constructor(deps: InlineAgentResolverDeps = {}) {
+        this.builtins = deps.builtins ?? BuiltinTools;
+        this.defaultModel = deps.defaultModel ?? getDefaultModelAndProvider;
+    }
+
+    async resolve(
+        requested: z.infer<typeof InlineAgentRequest>,
+    ): Promise<z.infer<typeof ResolvedAgent>> {
+        const spec = requested.inline;
+
+        let model = spec.model;
+        if (!model) {
+            const fallback = await this.defaultModel();
+            model = { provider: fallback.provider, model: fallback.model };
+        }
+
+        const names =
+            spec.tools ??
+            Object.keys(this.builtins).filter(
+                (name) => !DEFAULT_PROFILE_EXCLUDED.has(name),
+            );
+
+        const tools: Array<z.infer<typeof ToolDescriptor>> = [];
+        for (const name of names) {
+            // Depth is capped at 1: a child never spawns, even when its spec
+            // naively asks for the tool. Stripped rather than rejected so a
+            // model that includes it out of habit still gets a working agent.
+            if (name === SPAWN_AGENT_TOOL_NAME) {
+                continue;
+            }
+            const builtin = this.builtins[name];
+            if (!builtin) {
+                // A typo'd explicit selection should fail the spawn loudly
+                // (createTurn rejects; the spawn tool reports it), not run a
+                // silently under-tooled agent.
+                throw new Error(
+                    `inline agent requested unknown builtin tool: ${name}`,
+                );
+            }
+            if (builtin.isAvailable && !(await builtin.isAvailable())) {
+                continue;
+            }
+            tools.push(builtinToolDescriptor(name, builtin));
+        }
+
+        return ResolvedAgentSchema.parse({
+            agentId: inlineAgentId(spec.name),
+            systemPrompt: spec.instructions,
+            model,
+            tools,
+        });
+    }
+}
diff --git a/apps/x/packages/core/src/turns/bridges/real-agent-resolver.test.ts b/apps/x/packages/core/src/turns/bridges/real-agent-resolver.test.ts
index 974d7502..8bbcbf60 100644
--- a/apps/x/packages/core/src/turns/bridges/real-agent-resolver.test.ts
+++ b/apps/x/packages/core/src/turns/bridges/real-agent-resolver.test.ts
@@ -31,6 +31,11 @@ const fakeBuiltins = {
         execute: async () => null,
         isAvailable: async () => false,
     },
+    "spawn-agent": {
+        description: "Spawn a sub-agent",
+        inputSchema: z.object({ task: z.string() }),
+        execute: async () => null,
+    },
 } as unknown as typeof BuiltinTools;
 
 function makeResolver(agent: z.infer<typeof Agent>, deps: Partial<ConstructorParameters<typeof RealAgentResolver>[0]> = {}) {
@@ -172,6 +177,26 @@ describe("RealAgentResolver", () => {
         expect(b.systemPrompt).toBe(a.systemPrompt);
     });
 
+    it("strips spawn-agent from by-id children (subagent composition flag)", async () => {
+        const agent = makeAgent({
+            tools: {
+                "spawn-agent": { type: "builtin", name: "spawn-agent" },
+                "file-list": { type: "builtin", name: "file-list" },
+            },
+        });
+        const asParent = await makeResolver(agent).resolve({ agentId: "copilot" });
+        expect(asParent.tools.map((t) => t.name)).toEqual([
+            "spawn-agent",
+            "file-list",
+        ]);
+
+        const asChild = await makeResolver(agent).resolve({
+            agentId: "copilot",
+            overrides: { composition: { subagent: true } },
+        });
+        expect(asChild.tools.map((t) => t.name)).toEqual(["file-list"]);
+    });
+
     it("does not load notes/work-dir for non-copilot agents", async () => {
         let notesLoaded = 0;
         const resolver = makeResolver(makeAgent({ name: "background-task-agent" }), {
diff --git a/apps/x/packages/core/src/turns/bridges/real-agent-resolver.ts b/apps/x/packages/core/src/turns/bridges/real-agent-resolver.ts
index 8f2ce1b0..674a5b48 100644
--- a/apps/x/packages/core/src/turns/bridges/real-agent-resolver.ts
+++ b/apps/x/packages/core/src/turns/bridges/real-agent-resolver.ts
@@ -1,9 +1,9 @@
 import { z } from "zod";
 import type { Agent } from "@x/shared/dist/agent.js";
 import {
-    type JsonValue,
-    RequestedAgent,
+    AgentByIdRequest,
     ResolvedAgent,
+    SPAWN_AGENT_TOOL_NAME,
     type ToolDescriptor,
 } from "@x/shared/dist/turns.js";
 import {
@@ -14,7 +14,10 @@ import {
 } from "../../agents/runtime.js";
 import { BuiltinTools } from "../../application/lib/builtin-tools.js";
 import { getDefaultModelAndProvider } from "../../models/defaults.js";
-import type { IAgentResolver } from "../agent-resolver.js";
+import {
+    builtinToolDescriptor,
+    toJsonValue,
+} from "./builtin-descriptors.js";
 
 export const ASK_HUMAN_TOOL = "ask-human";
 
@@ -56,6 +59,9 @@ const CompositionOverrides = z.object({
     codeCwd: z.string().nullable().optional(),
     videoMode: z.boolean().optional(),
     coachMode: z.boolean().optional(),
+    // Set by spawn-agent for by-id children: strips the spawn tool so depth
+    // is capped at 1 regardless of which stored agent is spawned.
+    subagent: z.boolean().optional(),
 });
 
 export interface RealAgentResolverDeps {
@@ -69,8 +75,10 @@ export interface RealAgentResolverDeps {
 // Bridges the existing agent system (loadAgent + dynamic builders, the
 // BuiltinTools catalog, MCP attachments) to the immutable ResolvedAgent
 // snapshot. The composed system prompt is byte-identical to the old
-// runtime's streamAgent assembly for the same inputs.
-export class RealAgentResolver implements IAgentResolver {
+// runtime's streamAgent assembly for the same inputs. Resolves only the
+// by-id RequestedAgent variant; inline agents go through
+// InlineAgentResolver via DispatchingAgentResolver.
+export class RealAgentResolver {
     private readonly load: typeof loadAgent;
     private readonly builtins: typeof BuiltinTools;
     private readonly defaultModel: () => Promise<{ model: string; provider: string }>;
@@ -86,7 +94,7 @@ export class RealAgentResolver implements IAgentResolver {
     }
 
     async resolve(
-        requested: z.infer<typeof RequestedAgent>,
+        requested: z.infer<typeof AgentByIdRequest>,
     ): Promise<z.infer<typeof ResolvedAgent>> {
         const agent = await this.load(requested.agentId);
         if (!agent) {
@@ -127,7 +135,9 @@ export class RealAgentResolver implements IAgentResolver {
             coachMode: composition.coachMode ?? false,
         });
 
-        const tools = await this.resolveTools(agent);
+        const tools = await this.resolveTools(agent, {
+            subagent: composition.subagent ?? false,
+        });
         return ResolvedAgent.parse({
             agentId: requested.agentId,
             systemPrompt,
@@ -138,6 +148,7 @@ export class RealAgentResolver implements IAgentResolver {
 
     private async resolveTools(
         agent: z.infer<typeof Agent>,
+        options: { subagent: boolean },
     ): Promise<Array<z.infer<typeof ToolDescriptor>>> {
         const tools: Array<z.infer<typeof ToolDescriptor>> = [];
         for (const [name, attachment] of Object.entries(agent.tools ?? {})) {
@@ -161,6 +172,14 @@ export class RealAgentResolver implements IAgentResolver {
                 tools.push(ASK_HUMAN_DESCRIPTOR);
                 continue;
             }
+            // Depth cap: a spawned child never spawns, whichever stored
+            // agent it happens to be.
+            if (
+                options.subagent &&
+                attachment.name === SPAWN_AGENT_TOOL_NAME
+            ) {
+                continue;
+            }
             const builtin = this.builtins[attachment.name];
             if (!builtin) {
                 continue;
@@ -169,34 +188,10 @@ export class RealAgentResolver implements IAgentResolver {
                 continue;
             }
             tools.push({
-                toolId: `builtin:${attachment.name}`,
+                ...builtinToolDescriptor(attachment.name, builtin),
                 name,
-                description: builtin.description,
-                inputSchema: toJsonSchema(builtin.inputSchema),
-                execution: "sync",
-                requiresHuman: false,
             });
         }
         return tools;
     }
 }
-
-function toJsonSchema(schema: unknown): JsonValue {
-    try {
-        return toJsonValue(z.toJSONSchema(schema as z.ZodType)) ?? {
-            type: "object",
-            properties: {},
-        };
-    } catch {
-        // An exotic zod schema must not break the whole turn.
-        return { type: "object", properties: {} };
-    }
-}
-
-function toJsonValue(value: unknown): JsonValue | undefined {
-    try {
-        return JSON.parse(JSON.stringify(value)) as JsonValue;
-    } catch {
-        return undefined;
-    }
-}
diff --git a/apps/x/packages/core/src/turns/bridges/real-tool-registry.ts b/apps/x/packages/core/src/turns/bridges/real-tool-registry.ts
index d0048161..c3eb6214 100644
--- a/apps/x/packages/core/src/turns/bridges/real-tool-registry.ts
+++ b/apps/x/packages/core/src/turns/bridges/real-tool-registry.ts
@@ -1,7 +1,11 @@
 import type { ChildProcess } from "child_process";
 import type { z } from "zod";
 import type { ToolAttachment } from "@x/shared/dist/agent.js";
-import type { JsonValue, ToolDescriptor } from "@x/shared/dist/turns.js";
+import {
+    type JsonValue,
+    SPAWN_AGENT_TOOL_NAME,
+    type ToolDescriptor,
+} from "@x/shared/dist/turns.js";
 import { execTool } from "../../application/lib/exec-tool.js";
 import { BuiltinTools } from "../../application/lib/builtin-tools.js";
 import {
@@ -89,6 +93,28 @@ export class RealToolRegistry implements IToolRegistry {
                 >,
             };
         }
+        if (descriptor.toolId === `builtin:${SPAWN_AGENT_TOOL_NAME}`) {
+            // The dedicated agent-tool handler (turn-runtime-design §29.2):
+            // runs the child as a standalone headless turn and records the
+            // parent→child link as durable tool progress. Bypasses execTool
+            // so it gets the runtime's reportProgress channel.
+            return {
+                descriptor: descriptor as { execution: "sync" } & z.infer<
+                    typeof ToolDescriptor
+                >,
+                execute: async (input, ctx: ToolExecutionContext) => {
+                    const { runSpawnedAgent } = await import(
+                        "../../agents/spawn-agent.js"
+                    );
+                    return runSpawnedAgent(input, {
+                        parentTurnId: ctx.turnId,
+                        signal: ctx.signal,
+                        onChildStarted: (info) =>
+                            ctx.reportProgress({ kind: "subagent", ...info }),
+                    });
+                },
+            };
+        }
         if (descriptor.toolId.startsWith("builtin:")) {
             const name = descriptor.toolId.slice("builtin:".length);
             const builtin = this.builtins[name];
diff --git a/apps/x/packages/shared/src/turns.ts b/apps/x/packages/shared/src/turns.ts
index df9a65f4..cb565246 100644
--- a/apps/x/packages/shared/src/turns.ts
+++ b/apps/x/packages/shared/src/turns.ts
@@ -27,7 +27,7 @@ export const ModelDescriptor = z.object({
     model: z.string(),
 });
 
-export const RequestedAgent = z.object({
+export const AgentByIdRequest = z.object({
     agentId: z.string(),
     overrides: z
         .object({
@@ -42,6 +42,42 @@ export const RequestedAgent = z.object({
         .optional(),
 });
 
+// An agent constructed at request time (sub-agents spawned by a parent turn).
+// Persisted verbatim in turn_created.agent.requested, so the definition
+// self-documents in the turn file; the resolver materializes it into the same
+// immutable ResolvedAgent snapshot as a stored agent.
+export const InlineAgentSpec = z.object({
+    name: z.string(),
+    instructions: z.string(),
+    model: ModelDescriptor.optional(),
+    // Builtin tool names; resolution validates against the live catalog and
+    // substitutes the default headless profile when omitted.
+    tools: z.array(z.string()).optional(),
+});
+
+export const InlineAgentRequest = z.object({
+    inline: InlineAgentSpec,
+});
+
+// The builtin that spawns sub-agent turns. Named here (not in core) because
+// resolvers, the tool registry, and the renderer's card dispatch all key on
+// it, and children must never receive it (depth is capped at 1).
+export const SPAWN_AGENT_TOOL_NAME = "spawn-agent";
+
+// The ResolvedAgent.agentId convention for inline agents; also what sessions
+// denormalize into their index for inline-agent turns.
+export function inlineAgentId(name: string): string {
+    return `inline:${name}`;
+}
+
+export const RequestedAgent = z.union([AgentByIdRequest, InlineAgentRequest]);
+
+export function isInlineAgentRequest(
+    requested: z.infer<typeof RequestedAgent>,
+): requested is z.infer<typeof InlineAgentRequest> {
+    return "inline" in requested;
+}
+
 export const ToolDescriptor = z.object({
     toolId: z.string(),
     name: z.string(),