fix: inherit custom tools in subagents

.changeset/subagent-user-tools.md

@@ -0,0 +1,6 @@
+---
+"@moonshot-ai/agent-core": patch
+"@moonshot-ai/kimi-code": patch
+---
+
+Allow subagents to use custom tools registered on their parent agent.

packages/agent-core/src/agent/tool/index.ts

@@ -121,6 +121,17 @@ export class ToolManager {
     this.enabledTools.delete(name);
   }
 
+  inheritUserTools(parent: ToolManager): void {
+    for (const tool of parent.userTools.values()) {
+      if (!parent.enabledTools.has(tool.name)) continue;
+      this.registerUserTool({
+        name: tool.name,
+        description: tool.description,
+        parameters: tool.parameters,
+      });
+    }
+  }
+
   registerMcpServer(
     serverName: string,
     client: MCPClient,

packages/agent-core/src/session/subagent-host.ts

@@ -287,6 +287,7 @@ export class SessionSubagentHost {
 
     const context = await prepareSystemPromptContext(child.kaos);
     child.useProfile(profile, context);
+    child.tools.inheritUserTools(parent.tools);
   }
 
   private async triggerSubagentStart(

packages/agent-core/test/session/subagent-host.test.ts

@@ -16,6 +16,7 @@ import { SessionSubagentHost } from '../../src/session/subagent-host';
 import { abortError, userCancellationReason } from '../../src/utils/abort';
 import { testAgent, type AgentTestContext } from '../agent/harness/agent';
 import { createFakeKaos } from '../tools/fixtures/fake-kaos';
+import { executeTool } from '../tools/fixtures/execute-tool';
 
 // Git context collection is exercised in git-context.test.ts; here it is
 // mocked so subagent-host tests stay deterministic and assert only the
@@ -223,6 +224,62 @@ describe('SessionSubagentHost', () => {
     ]);
   });
 
+  it('inherits active parent user tools when spawning a subagent', async () => {
+    const parent = testAgent();
+    parent.configure();
+    await parent.rpc.registerTool(lookupToolRegistration());
+    parent.newEvents();
+
+    const summary =
+      'Investigated the delegated task thoroughly, used the inherited custom lookup surface where appropriate, and returned a detailed summary that lets the parent agent continue without repeating the work. '.repeat(
+        2,
+      );
+    const child = testAgent();
+    child.mockNextResponse({
+      type: 'text',
+      text: summary,
+    });
+    const session = fakeSession(parent.agent, child.agent);
+    const host = new SessionSubagentHost(session, 'main');
+
+    const handle = await host.spawn('coder', {
+      parentToolCallId: 'call_agent',
+      prompt: 'Use the available lookup tool',
+      description: 'Use lookup',
+      runInBackground: false,
+      signal,
+    });
+
+    await expect(handle.completion).resolves.toMatchObject({
+      result: summary.trim(),
+    });
+    expect(child.llmCalls[0]?.tools.map((tool) => tool.name)).toContain('Lookup');
+    expect(child.agent.tools.data()).toContainEqual({
+      name: 'Lookup',
+      description: 'Look up a short test value.',
+      active: true,
+      source: 'user',
+    });
+
+    const lookupTool = child.agent.tools.loopTools.find((tool) => tool.name === 'Lookup');
+    expect(lookupTool).toBeDefined();
+
+    const execution = executeTool(lookupTool!, {
+      turnId: '0',
+      toolCallId: 'call_lookup',
+      args: { query: 'moon' },
+      signal,
+    });
+    const routedTo = await Promise.race([
+      child.untilToolCall({ output: 'moon-result' }).then(() => 'child'),
+      parent.untilToolCall({ output: 'moon-result' }).then(() => 'parent'),
+      new Promise<'timeout'>((resolve) => setTimeout(() => resolve('timeout'), 50)),
+    ]);
+
+    expect(routedTo).toBe('child');
+    await expect(execution).resolves.toMatchObject({ output: 'moon-result' });
+  });
+
   it('falls back to bundled subagent profiles when the parent profile is missing', async () => {
     const parent = testAgent();
     parent.configure();
@@ -1084,6 +1141,21 @@ function contextProfile(): ResolvedAgentProfile {
   };
 }
 
+function lookupToolRegistration() {
+  return {
+    name: 'Lookup',
+    description: 'Look up a short test value.',
+    parameters: {
+      type: 'object',
+      properties: {
+        query: { type: 'string' },
+      },
+      required: ['query'],
+      additionalProperties: false,
+    },
+  };
+}
+
 function profile(input: {
   readonly name: string;
   readonly tools: readonly string[];