diff --git a/packages/agent-core/src/agent/tool/index.ts b/packages/agent-core/src/agent/tool/index.ts
index 550cfeba8..5038b2a58 100644
--- a/packages/agent-core/src/agent/tool/index.ts
+++ b/packages/agent-core/src/agent/tool/index.ts
@@ -393,6 +393,9 @@ export class ToolManager {
               log: this.agent.log,
             },
           ),
+        this.agent.subagentHost &&
+          this.agent.type !== 'sub' &&
+          new b.SwarmTool(this.agent.subagentHost, { log: this.agent.log }),
         toolServices?.webSearcher && new b.WebSearchTool(toolServices.webSearcher),
         toolServices?.urlFetcher && new b.FetchURLTool(toolServices.urlFetcher),
       ]
diff --git a/packages/agent-core/src/profile/default/agent.yaml b/packages/agent-core/src/profile/default/agent.yaml
index 82b81bd3e..072904eba 100644
--- a/packages/agent-core/src/profile/default/agent.yaml
+++ b/packages/agent-core/src/profile/default/agent.yaml
@@ -23,6 +23,7 @@ tools:
   - Skill
   - WebSearch
   - Agent
+  - Swarm
   - FetchURL
   - AskUserQuestion
   - EnterPlanMode
diff --git a/packages/agent-core/src/tools/builtin/collaboration/swarm.ts b/packages/agent-core/src/tools/builtin/collaboration/swarm.ts
new file mode 100644
index 000000000..2ab1f0c47
--- /dev/null
+++ b/packages/agent-core/src/tools/builtin/collaboration/swarm.ts
@@ -0,0 +1,90 @@
+/**
+ * SwarmTool — collaboration tool that runs a task as a self-directed agent
+ * swarm.
+ *
+ * Like {@link AgentTool}, this is a "collaboration tool": it uses
+ * `SessionSubagentHost` (injected via the constructor) to create in-process
+ * subagents. The {@link SwarmCoordinator} dynamically decomposes the task into
+ * parallel role-specialized workers, then synthesizes their outputs into one
+ * answer.
+ *
+ * Workers are spawned with an ad-hoc `profileOverride`, and the tool is
+ * registered only on non-sub agents so a swarm worker can never launch another
+ * swarm (recursion guard).
+ */
+
+import { z } from 'zod';
+
+import type { BuiltinTool } from '../../../agent/tool';
+import type { Logger } from '../../../logging';
+import type { ExecutableToolContext, ExecutableToolResult, ToolExecution } from '../../../loop/types';
+import type { SessionSubagentHost } from '../../../session/subagent-host';
+import { toInputJsonSchema } from '../../support/input-schema';
+import { SwarmCoordinator } from '../../../agent/swarm/coordinator';
+
+export const SwarmToolInputSchema = z.object({
+  task: z.string().describe('The high-level task to decompose and run as a parallel agent swarm.'),
+});
+
+export type SwarmToolInput = z.infer<typeof SwarmToolInputSchema>;
+
+const SWARM_DESCRIPTION =
+  'Run a task as a self-directed agent swarm: dynamically decompose it into parallel ' +
+  'role-specialized subagents, then synthesize their outputs into one answer. ' +
+  'Use for broad, parallelizable tasks (research, multi-file analysis). ' +
+  'Subagents run in isolated contexts and cannot themselves launch swarms.';
+
+const DEFAULT_MAX_CONCURRENCY = 4;
+
+export class SwarmTool implements BuiltinTool<SwarmToolInput> {
+  readonly name: string = 'Swarm';
+  readonly description: string = SWARM_DESCRIPTION;
+  readonly parameters: Record<string, unknown> = toInputJsonSchema(SwarmToolInputSchema);
+  private readonly log: Logger | undefined;
+
+  constructor(
+    private readonly subagentHost: SessionSubagentHost,
+    options?: { log?: Logger },
+  ) {
+    this.log = options?.log;
+  }
+
+  resolveExecution(args: SwarmToolInput): ToolExecution {
+    return {
+      description: `Running swarm: ${args.task.slice(0, 60)}`,
+      approvalRule: 'Swarm',
+      execute: (ctx) => this.execution(args, ctx),
+    };
+  }
+
+  private async execution(
+    args: SwarmToolInput,
+    ctx: ExecutableToolContext,
+  ): Promise<ExecutableToolResult> {
+    const coordinator = new SwarmCoordinator({
+      signal: ctx.signal,
+      maxConcurrency: DEFAULT_MAX_CONCURRENCY,
+      onProgress: (text) => ctx.onUpdate?.({ kind: 'status', text }),
+      spawnSubagent: async ({ profileName, systemPrompt, tools, prompt, description, signal }) => {
+        const handle = await this.subagentHost.spawn(profileName, {
+          parentToolCallId: ctx.toolCallId,
+          prompt,
+          description,
+          runInBackground: false,
+          signal,
+          profileOverride: { systemPrompt, tools },
+        });
+        return handle.completion;
+      },
+    });
+
+    try {
+      const output = await coordinator.run(args.task);
+      return { output };
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      this.log?.error(`swarm failed: ${message}`);
+      return { output: `Swarm failed: ${message}`, isError: true };
+    }
+  }
+}
diff --git a/packages/agent-core/src/tools/builtin/index.ts b/packages/agent-core/src/tools/builtin/index.ts
index ebbe0dc71..70c21875d 100644
--- a/packages/agent-core/src/tools/builtin/index.ts
+++ b/packages/agent-core/src/tools/builtin/index.ts
@@ -8,6 +8,7 @@ export * from '../cron/cron-list';
 export * from './collaboration/agent';
 export * from './collaboration/ask-user';
 export * from './collaboration/skill-tool';
+export * from './collaboration/swarm';
 export * from './file/edit';
 export * from './file/glob';
 export * from './file/grep';
diff --git a/packages/agent-core/test/swarm/swarm-tool.test.ts b/packages/agent-core/test/swarm/swarm-tool.test.ts
new file mode 100644
index 000000000..f6b2af8e6
--- /dev/null
+++ b/packages/agent-core/test/swarm/swarm-tool.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it, vi } from 'vitest';
+
+import { SwarmTool } from '../../src/tools/builtin/collaboration/swarm';
+import type { SessionSubagentHost } from '../../src/session/subagent-host';
+
+const PLAN_JSON = JSON.stringify({
+  subtasks: [{ role: 'R', systemPrompt: 'sp', prompt: 'p' }],
+});
+
+function fakeHost(): SessionSubagentHost {
+  const spawn = vi.fn(async (profileName: string) => {
+    const result =
+      profileName === 'swarm-planner'
+        ? PLAN_JSON
+        : profileName === 'swarm-synthesizer'
+          ? 'FINAL'
+          : 'worker-out';
+    return { agentId: 'a', profileName, resumed: false, completion: Promise.resolve({ result }) };
+  });
+  return { spawn } as unknown as SessionSubagentHost;
+}
+
+describe('SwarmTool', () => {
+  it('exposes a task parameter and an approval rule', () => {
+    const tool = new SwarmTool(fakeHost());
+    expect(tool.name).toBe('Swarm');
+    const exec = tool.resolveExecution({ task: 'hello' });
+    expect('approvalRule' in exec && exec.approvalRule).toBe('Swarm');
+  });
+
+  it('runs the coordinator and returns the synthesized output', async () => {
+    const tool = new SwarmTool(fakeHost());
+    const exec = tool.resolveExecution({ task: 'do it' });
+    if (!('execute' in exec)) throw new Error('expected runnable execution');
+    const updates: string[] = [];
+    const result = await exec.execute({
+      turnId: 't1',
+      toolCallId: 'tc1',
+      signal: new AbortController().signal,
+      onUpdate: (u) => {
+        if (u.text !== undefined) updates.push(u.text);
+      },
+    });
+    expect('output' in result && result.output).toBe('FINAL');
+    expect(updates.length).toBeGreaterThan(0);
+  });
+});