Fix side chat activation latency

plugins/side-chat/server.test.ts

@@ -92,6 +92,26 @@ describe("resolveReplySeedText", () => {
 });
 
 describe("createSideChat rpc", () => {
+  it("limits the reply-seed lookup to the latest timeline segment", async () => {
+    const timeline = vi.fn(async () =>
+      timelineResult([conversationRow("latest answer")]),
+    );
+    const fork = vi.fn(async () => makeThreadResponse({ id: "thr_fork" }));
+    const { harness } = await loadPlugin({ timeline, fork });
+
+    await harness.callRpc("createSideChat", {
+      sourceThreadId: "thr_src",
+      sourceSeqEnd: 7,
+      anchorText: "latest answer",
+    });
+
+    expect(timeline).toHaveBeenCalledWith({
+      threadId: "thr_src",
+      includeNestedRows: "true",
+      segmentLimit: "1",
+    });
+  });
+
   it("forks hidden+isolated with a seed when the anchor is an earlier message", async () => {
     const fork = vi.fn(async () => makeThreadResponse({ id: "thr_fork" }));
     const { harness } = await loadPlugin({

plugins/side-chat/server.ts

@@ -176,9 +176,14 @@ export const sideChatRpcContract = defineRpcContract({
 export default async function plugin(bb: BbPluginApi) {
   bb.rpc.register(sideChatRpcContract, {
     async createSideChat({ sourceThreadId, sourceSeqEnd, anchorText }) {
+      // The seed rule reads only the source's *last* conversation message, and
+      // every timeline segment is anchored at a user message — so the newest
+      // segment always holds it. Without this the lookup projects the default
+      // 20 segments of nested rows on the activation path.
       const timeline = await bb.sdk.threads.timeline({
         threadId: sourceThreadId,
         includeNestedRows: "true",
+        segmentLimit: "1",
       });
       const seedText = resolveReplySeedText({
         anchorText,