diff --git a/src/agents/tools/tts-tool.test.ts b/src/agents/tools/tts-tool.test.ts
index fe9a6c1def9..32418b7a5fa 100644
--- a/src/agents/tools/tts-tool.test.ts
+++ b/src/agents/tools/tts-tool.test.ts
@@ -9,8 +9,15 @@ const { createTtsTool } = await import("./tts-tool.js");
 describe("createTtsTool", () => {
   it("uses SILENT_REPLY_TOKEN in guidance text", () => {
     const tool = createTtsTool();
-
     expect(tool.description).toContain("QUIET_TOKEN");
     expect(tool.description).not.toContain("NO_REPLY");
   });
+
+  it("returns error even when channel arg is provided but no agentChannel", async () => {
+    const tool = createTtsTool();
+    const result = await tool.execute("test-call", { text: "hello world", channel: "telegram" });
+    const firstContent = result.content[0] as { type: string; text: string };
+    expect(firstContent.text).toContain("requires a bound");
+    expect((result.details as Record<string, unknown>).error).toBe("no_channel");
+  });
 });
diff --git a/src/agents/tools/tts-tool.ts b/src/agents/tools/tts-tool.ts
index 03ed3cd9a04..7951250afaf 100644
--- a/src/agents/tools/tts-tool.ts
+++ b/src/agents/tools/tts-tool.ts
@@ -27,11 +27,25 @@ export function createTtsTool(opts?: {
       const params = args as Record<string, unknown>;
       const text = readStringParam(params, "text", { required: true });
       const channel = readStringParam(params, "channel");
+      const resolvedChannel = channel ?? opts?.agentChannel;
+
+      if (!opts?.agentChannel) {
+        return {
+          content: [
+            {
+              type: "text",
+              text: 'TTS requires a bound session channel for audio delivery. Use sessionTarget: "main" instead of "isolated".',
+            },
+          ],
+          details: { error: "no_channel" },
+        };
+      }
+
       const cfg = opts?.config ?? loadConfig();
       const result = await textToSpeech({
         text,
         cfg,
-        channel: channel ?? opts?.agentChannel,
+        channel: resolvedChannel,
       });
 
       if (result.success && result.audioPath) {