diff --git a/packages/kap-server/src/routes/prompts.ts b/packages/kap-server/src/routes/prompts.ts
index 111b5bf59..8bd2a435d 100644
--- a/packages/kap-server/src/routes/prompts.ts
+++ b/packages/kap-server/src/routes/prompts.ts
@@ -6,6 +6,7 @@
  */
 
 import {
+  IAgentLifecycleService,
   IAgentPromptLegacyService,
   ISessionLifecycleService,
   isKimiError,
@@ -25,7 +26,7 @@ import { z } from 'zod';
 
 import { errEnvelope, okEnvelope } from '../envelope';
 import { defineRoute } from '../middleware/defineRoute';
-import { ensureMainAgent } from '../transport/mainAgent';
+import { ensureMainAgent, MAIN_AGENT_ID } from '../transport/mainAgent';
 import { parseActionSuffix } from './action-suffix';
 
 interface PromptRouteHost {
@@ -55,7 +56,11 @@ const validationDetailsSchema = z.array(z.object({ path: z.string(), message: z.
 const authProviderDetailsSchema = z.object({ provider_id: z.string() });
 const authModelDetailsSchema = z.object({ model_id: z.string(), provider_id: z.string() }).partial();
 
-async function resolveLegacy(core: Scope, sessionId: string): Promise<IAgentPromptLegacyService> {
+async function resolveLegacy(
+  core: Scope,
+  sessionId: string,
+  agentId?: string,
+): Promise<IAgentPromptLegacyService> {
   // `resume` (not `get`) so a persisted-but-cold session — created by a previous
   // process, by v1, or closed in this one — is loaded from disk instead of
   // being reported as `session.not_found`. Mirrors the snapshot route. Returns
@@ -64,7 +69,17 @@ async function resolveLegacy(core: Scope, sessionId: string): Promise<IAgentProm
   if (session === undefined) {
     throw new KimiError('session.not_found', `session ${sessionId} does not exist`);
   }
-  const agent = await ensureMainAgent(session);
+  // A prompt may target a forked side-channel agent (e.g. `/btw`) via
+  // `body.agent_id`. Default to `main` when absent; only `main` is
+  // auto-created — any other id must already exist (forked beforehand), or it
+  // is reported as `agent.not_found`.
+  const agent =
+    agentId === undefined || agentId === MAIN_AGENT_ID
+      ? await ensureMainAgent(session)
+      : session.accessor.get(IAgentLifecycleService).getHandle(agentId);
+  if (agent === undefined) {
+    throw new KimiError('agent.not_found', `agent ${agentId} does not exist`);
+  }
   return agent.accessor.get(IAgentPromptLegacyService);
 }
 
@@ -116,7 +131,7 @@ export function registerPromptsRoutes(app: PromptRouteHost, core: Scope): void {
     async (req, reply) => {
       try {
         const { session_id } = req.params;
-        const legacy = await resolveLegacy(core, session_id);
+        const legacy = await resolveLegacy(core, session_id, req.body.agent_id);
         const result = await legacy.submit(req.body);
         reply.send(okEnvelope(result, req.id));
       } catch (error) {
diff --git a/packages/kap-server/test/prompts.test.ts b/packages/kap-server/test/prompts.test.ts
index 0a6eb1af4..2a53f5830 100644
--- a/packages/kap-server/test/prompts.test.ts
+++ b/packages/kap-server/test/prompts.test.ts
@@ -3,6 +3,7 @@ import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 
 import {
+  IAgentContextMemoryService,
   IAgentLifecycleService,
   ISessionLifecycleService,
 } from '@moonshot-ai/agent-core-v2';
@@ -179,4 +180,57 @@ describe('server-v2 /api/v1 prompts', () => {
     expect(list.body.data.active).toBeNull();
     expect(list.body.data.queued).toEqual([]);
   });
+
+  it('routes a submitted prompt to the agent named by agent_id (BTW side channel)', async () => {
+    const id = await createSession(home as string);
+    await createMainAgent(id);
+
+    // Fork the main agent into a side-channel child the way `/btw` does.
+    const session = server!.core.accessor.get(ISessionLifecycleService).get(id);
+    if (session === undefined) throw new Error(`session ${id} not found`);
+    const lifecycle = session.accessor.get(IAgentLifecycleService);
+    const child = await lifecycle.fork('main');
+
+    const submitted = await call<PromptItemWire>('POST', `/api/v1/sessions/${id}/prompts`, {
+      content: [{ type: 'text', text: 'side question' }],
+      agent_id: child.id,
+    });
+    expect(submitted.body.code).toBe(0);
+
+    // The user message is appended to the target agent's context before the turn
+    // runs, so it persists even after the (model-less) turn settles — a durable
+    // signal of which agent actually received the prompt.
+    const contextHasUserText = (
+      handle: { accessor: { get: typeof child.accessor.get } },
+      text: string,
+    ): boolean =>
+      handle.accessor
+        .get(IAgentContextMemoryService)
+        .get()
+        .some(
+          (m) =>
+            m.role === 'user' &&
+            m.content.some((p) => p.type === 'text' && p.text === text),
+        );
+
+    // The side-channel child received the prompt.
+    expect(contextHasUserText(child, 'side question')).toBe(true);
+
+    // The main agent must NOT have received it — previously the route ignored
+    // agent_id and always targeted main, so the reply landed in the main view.
+    const main = lifecycle.getHandle('main');
+    expect(main).toBeDefined();
+    expect(contextHasUserText(main!, 'side question')).toBe(false);
+  });
+
+  it('returns 40401 when agent_id names an unknown agent', async () => {
+    const id = await createSession(home as string);
+    await createMainAgent(id);
+
+    const { body } = await call<null>('POST', `/api/v1/sessions/${id}/prompts`, {
+      content: [{ type: 'text', text: 'hello' }],
+      agent_id: 'agent_does_not_exist',
+    });
+    expect(body.code).toBe(40401);
+  });
 });