diff --git a/src/agents/acp-spawn.ts b/src/agents/acp-spawn.ts
index 91eeae3436a..71f5e510ac4 100644
--- a/src/agents/acp-spawn.ts
+++ b/src/agents/acp-spawn.ts
@@ -1070,7 +1070,9 @@ export async function spawnAcpDirect(
     return createAcpSpawnFailure({
       status: "error",
       errorCode: "thread_required",
-      error: 'mode="session" requires thread=true so the ACP session can stay bound to a thread.',
+      error:
+        'sessions_spawn(runtime="acp", mode="session") requires thread=true so the ACP session can stay bound to a channel thread. ' +
+        'Retry with { mode: "session", thread: true } on a channel that exposes threads (e.g. Discord, Slack, Telegram topics), or use mode="run" for one-shot work.',
     });
   }
 
diff --git a/src/agents/subagent-spawn.mode-session-diagnostics.test.ts b/src/agents/subagent-spawn.mode-session-diagnostics.test.ts
new file mode 100644
index 00000000000..3cae57392b5
--- /dev/null
+++ b/src/agents/subagent-spawn.mode-session-diagnostics.test.ts
@@ -0,0 +1,136 @@
+import os from "node:os";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import type { SubagentLifecycleHookRunner } from "../plugins/hooks.js";
+import {
+  createSubagentSpawnTestConfig,
+  loadSubagentSpawnModuleForTest,
+} from "./subagent-spawn.test-helpers.js";
+
+type SubagentSpawningEvent = Parameters<SubagentLifecycleHookRunner["runSubagentSpawning"]>[0];
+
+describe('spawnSubagentDirect mode="session" diagnostics (#67400)', () => {
+  const callGatewayMock = vi.fn();
+  let spawnSubagentDirect: typeof import("./subagent-spawn.js").spawnSubagentDirect;
+  let resetSubagentRegistryForTests: typeof import("./subagent-registry.js").resetSubagentRegistryForTests;
+
+  beforeEach(async () => {
+    callGatewayMock.mockReset();
+    ({ spawnSubagentDirect, resetSubagentRegistryForTests } = await loadSubagentSpawnModuleForTest({
+      callGatewayMock,
+      loadConfig: () => createSubagentSpawnTestConfig(os.tmpdir()),
+      workspaceDir: os.tmpdir(),
+    }));
+    resetSubagentRegistryForTests();
+  });
+
+  it("names usable alternatives before a thread retry", async () => {
+    const result = await spawnSubagentDirect(
+      {
+        task: "persistent planning session",
+        mode: "session",
+      },
+      {
+        agentSessionKey: "agent:main:main",
+        agentChannel: "webchat",
+      },
+    );
+
+    expect(result.status).toBe("error");
+    if (result.status === "error") {
+      expect(result.error).toContain("thread: true");
+      expect(result.error).toContain('mode="run"');
+      expect(result.error).toContain("sessions_send");
+    }
+  });
+
+  it("rejects thread=true with actionable guidance when no hook is registered", async () => {
+    const result = await spawnSubagentDirect(
+      {
+        task: "persistent planning session",
+        mode: "session",
+        thread: true,
+      },
+      {
+        agentSessionKey: "agent:main:main",
+        agentChannel: "webchat",
+      },
+    );
+
+    expect(result.status).toBe("error");
+    if (result.status === "error") {
+      expect(result.error).toContain("not running on a channel");
+      expect(result.error).toContain('mode="run"');
+      expect(result.error).toContain("sessions_send");
+    }
+  });
+});
+
+describe('spawnSubagentDirect mode="session" with registered thread hooks (#67400)', () => {
+  const callGatewayMock = vi.fn();
+  let spawnSubagentDirect: typeof import("./subagent-spawn.js").spawnSubagentDirect;
+  let resetSubagentRegistryForTests: typeof import("./subagent-registry.js").resetSubagentRegistryForTests;
+
+  beforeEach(async () => {
+    callGatewayMock.mockReset();
+    ({ spawnSubagentDirect, resetSubagentRegistryForTests } = await loadSubagentSpawnModuleForTest({
+      callGatewayMock,
+      loadConfig: () => createSubagentSpawnTestConfig(os.tmpdir()),
+      workspaceDir: os.tmpdir(),
+      hookRunner: {
+        hasHooks: () => true,
+        runSubagentSpawning: async (event: SubagentSpawningEvent) => {
+          const requesterChannel = event.requester?.channel;
+          if (requesterChannel !== "discord") {
+            return undefined;
+          }
+          return {
+            status: "ok" as const,
+            threadBindingReady: true,
+          };
+        },
+      },
+    }));
+    resetSubagentRegistryForTests();
+  });
+
+  it("names thread=true and the non-thread alternatives when hooks are registered", async () => {
+    const result = await spawnSubagentDirect(
+      {
+        task: "persistent planning session",
+        mode: "session",
+      },
+      {
+        agentSessionKey: "agent:main:main",
+        agentChannel: "discord",
+      },
+    );
+
+    expect(result.status).toBe("error");
+    if (result.status === "error") {
+      expect(result.error).toContain("thread: true");
+      expect(result.error).toContain('mode="run"');
+      expect(result.error).toContain("sessions_send");
+    }
+  });
+
+  it("rejects thread=true with actionable guidance when hooks do not bind the requester channel", async () => {
+    const result = await spawnSubagentDirect(
+      {
+        task: "persistent planning session",
+        mode: "session",
+        thread: true,
+      },
+      {
+        agentSessionKey: "agent:main:main",
+        agentChannel: "webchat",
+      },
+    );
+
+    expect(result.status).toBe("error");
+    if (result.status === "error") {
+      expect(result.error).toContain("not running on a channel");
+      expect(result.error).toContain('mode="run"');
+      expect(result.error).toContain("sessions_send");
+    }
+  });
+});
diff --git a/src/agents/subagent-spawn.ts b/src/agents/subagent-spawn.ts
index ce3773aec79..9710cee6726 100644
--- a/src/agents/subagent-spawn.ts
+++ b/src/agents/subagent-spawn.ts
@@ -475,6 +475,21 @@ function summarizeError(err: unknown): string {
   return "error";
 }
 
+function buildThreadBindingUnavailableError(mode: SpawnSubagentMode): string {
+  if (mode === "session") {
+    return (
+      'sessions_spawn(mode="session") is only available on channels that expose thread bindings (e.g. Discord threads, Slack threads, Telegram forum topics). ' +
+      "This request is not running on a channel that can bind a subagent thread. " +
+      'Use mode="run" for one-shot subagent work, or sessions_send(sessionKey=...) to keep talking to a persistent session without thread binding.'
+    );
+  }
+  return (
+    "thread=true is only available on channels that expose thread bindings (e.g. Discord threads, Slack threads, Telegram forum topics). " +
+    "This request is not running on a channel that can bind a subagent thread. " +
+    "Retry without thread=true, or re-run sessions_spawn from a channel that supports threads."
+  );
+}
+
 async function ensureThreadBindingForSubagentSpawn(params: {
   hookRunner: SubagentLifecycleHookRunner | null;
   childSessionKey: string;
@@ -491,17 +506,15 @@ async function ensureThreadBindingForSubagentSpawn(params: {
 }): Promise<
   { status: "ok"; deliveryOrigin?: DeliveryContext } | { status: "error"; error: string }
 > {
-  const hookRunner = params.hookRunner;
-  if (!hookRunner?.hasHooks("subagent_spawning")) {
+  if (!params.hookRunner?.hasHooks("subagent_spawning")) {
     return {
       status: "error",
-      error:
-        "thread=true is unavailable because no channel plugin registered subagent_spawning hooks.",
+      error: buildThreadBindingUnavailableError(params.mode),
     };
   }
 
   try {
-    const result = await hookRunner.runSubagentSpawning(
+    const result = await params.hookRunner.runSubagentSpawning(
       {
         childSessionKey: params.childSessionKey,
         agentId: params.agentId,
@@ -522,6 +535,12 @@ async function ensureThreadBindingForSubagentSpawn(params: {
         error: error || "Failed to prepare thread binding for this subagent session.",
       };
     }
+    if (!result) {
+      return {
+        status: "error",
+        error: buildThreadBindingUnavailableError(params.mode),
+      };
+    }
     if (result?.status !== "ok" || !result.threadBindingReady) {
       return {
         status: "error",
@@ -578,7 +597,9 @@ export async function spawnSubagentDirect(
   if (spawnMode === "session" && !requestThreadBinding) {
     return {
       status: "error",
-      error: 'mode="session" requires thread=true so the subagent can stay bound to a thread.',
+      error:
+        'sessions_spawn(mode="session") requires thread=true so the subagent can stay bound to a channel thread. ' +
+        'Retry with { mode: "session", thread: true } on a channel that supports threads, use mode="run" for one-shot work, or use sessions_send(sessionKey=...) to keep talking to a persistent session without thread binding.',
     };
   }
   const cleanup =