fix(acp): drop [MODE_UPDATE] agent text echoes from chat

Author: SDSLeon

src/supervisor/agents/acp/canonicalMapping.test.ts

@@ -41,6 +41,25 @@ describe("mapAcpSessionUpdate", () => {
     expect((second[0] as { delta: string }).delta).toBe(" world");
   });
 
+  it("drops [MODE_UPDATE] agent text echoes — mode is chosen in the launcher, not chat", () => {
+    // Gemini's ACP server emits `[MODE_UPDATE] <mode>` as a fresh
+    // agent_message_chunk every time a session starts (or switches) into a
+    // specific approval mode. The user already picked that mode in the
+    // launcher UI; replaying it as a chat message on every turn is pure
+    // noise, so the mapper must drop the chunk before opening an assistant
+    // item.
+    const state = createAcpMapperState("t-mode");
+    const events = mapAcpSessionUpdate(
+      note({
+        sessionUpdate: "agent_message_chunk",
+        content: { type: "text", text: "[MODE_UPDATE] yolo" },
+      }),
+      state,
+    );
+    expect(events).toEqual([]);
+    expect(state.openAssistantItemId).toBeUndefined();
+  });
+
   it("drops user_message_chunk echoes — supervisor/renderer own the user_message item", () => {
     // Some ACP servers (Copilot) echo the user's prompt back as
     // `user_message_chunk` updates after we send `session/prompt`. The

src/supervisor/agents/acp/canonicalMapping.ts

@@ -169,6 +169,19 @@ export function mapAcpSessionUpdate(
 
   switch (update.sessionUpdate) {
     case "agent_message_chunk": {
+      const content = (update as { content?: ContentBlock }).content;
+      // Gemini echoes `[MODE_UPDATE] <mode>` as an agent text chunk whenever the
+      // session is launched (or switched) into a specific approval mode. The
+      // user already chose the mode in the launcher; surfacing the echo as
+      // chat noise on every turn is just clutter. Drop it before we open an
+      // assistant item so the chat stays clean.
+      if (
+        !state.openAssistantItemId &&
+        content?.type === "text" &&
+        /^\[MODE_UPDATE\]/.test(content.text)
+      ) {
+        break;
+      }
       // Open an assistant item on first chunk; emit deltas thereafter.
       if (!state.openAssistantItemId) {
         // Close any prior reasoning/user items — assistant is starting fresh.
@@ -181,7 +194,6 @@ export function mapAcpSessionUpdate(
           itemType: "assistant_message",
         });
       }
-      const content = (update as { content?: ContentBlock }).content;
       if (content) {
         if (content.type === "text") {
           events.push({