fix(acp): keep one answer when a running tool reports progress

apps/server/scripts/acp-mock-agent.ts

@@ -19,6 +19,8 @@ const emitToolCalls = process.env.T3_ACP_EMIT_TOOL_CALLS === "1";
 const emitInterleavedAssistantToolCalls =
   process.env.T3_ACP_EMIT_INTERLEAVED_ASSISTANT_TOOL_CALLS === "1";
 const emitGenericToolPlaceholders = process.env.T3_ACP_EMIT_GENERIC_TOOL_PLACEHOLDERS === "1";
+const emitBackgroundToolDuringAnswer =
+  process.env.T3_ACP_EMIT_BACKGROUND_TOOL_DURING_ANSWER === "1";
 const emitAskQuestion = process.env.T3_ACP_EMIT_ASK_QUESTION === "1";
 const emitXAiAskUserQuestion = process.env.T3_ACP_EMIT_XAI_ASK_USER_QUESTION === "1";
 const emitXAiExitPlanMode = process.env.T3_ACP_EMIT_XAI_EXIT_PLAN_MODE === "1";
@@ -941,6 +943,44 @@ const program = Effect.gen(function* () {
         return yield* Effect.never;
       }
 
+      if (emitBackgroundToolDuringAnswer) {
+        // A command backgrounded earlier reports progress and then finishes
+        // while the next answer is still streaming.
+        const toolCallId = "background-1";
+        const say = (text: string) =>
+          agent.client.sessionUpdate({
+            sessionId: requestedSessionId,
+            update: { sessionUpdate: "agent_message_chunk", content: { type: "text", text } },
+          });
+        const progress = (status: "in_progress" | "completed", stdout: string) =>
+          agent.client.sessionUpdate({
+            sessionId: requestedSessionId,
+            update: {
+              sessionUpdate: "tool_call_update",
+              toolCallId,
+              status,
+              rawOutput: { stdout },
+            },
+          });
+        yield* agent.client.sessionUpdate({
+          sessionId: requestedSessionId,
+          update: {
+            sessionUpdate: "tool_call",
+            toolCallId,
+            title: "Terminal",
+            kind: "execute",
+            status: "in_progress",
+            rawInput: { command: "sleep 3 && echo done" },
+          },
+        });
+        yield* say("| a | b |\n|---|---|\n| 1 ");
+        yield* progress("in_progress", ".");
+        yield* say("| x |\n");
+        yield* progress("completed", "done");
+        yield* say("| 2 | y |");
+        return { stopReason: "end_turn" };
+      }
+
       if (emitInterleavedAssistantToolCalls) {
         const toolCallId = "tool-call-1";
 

apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts

@@ -824,6 +824,52 @@ describe("AcpSessionRuntime", () => {
     ),
   );
 
+  it.effect("keeps one answer when an earlier tool reports progress mid-stream", () =>
+    Effect.gen(function* () {
+      const runtime = yield* AcpSessionRuntime.AcpSessionRuntime;
+      yield* runtime.start();
+      yield* runtime.prompt({ prompt: [{ type: "text", text: "hi" }] });
+
+      const notes = Array.from(yield* Stream.runCollect(Stream.take(runtime.getEvents(), 7)));
+      // The coalesced progress tick emits nothing and the completion lands
+      // between deltas; neither splits the markdown table across items.
+      expect(notes.map((note) => note._tag)).toEqual([
+        "ToolCallUpdated",
+        "AssistantItemStarted",
+        "ContentDelta",
+        "ContentDelta",
+        "ToolCallUpdated",
+        "ContentDelta",
+        "AssistantItemCompleted",
+      ]);
+      const itemIds = new Set(
+        notes.flatMap((note) =>
+          note._tag === "ContentDelta" ||
+          note._tag === "AssistantItemStarted" ||
+          note._tag === "AssistantItemCompleted"
+            ? [note.itemId]
+            : [],
+        ),
+      );
+      expect(itemIds.size).toBe(1);
+    }).pipe(
+      Effect.provide(
+        AcpSessionRuntime.layer({
+          spawn: {
+            command: mockAgentCommand,
+            args: mockAgentArgs,
+            env: { T3_ACP_EMIT_BACKGROUND_TOOL_DURING_ANSWER: "1" },
+          },
+          cwd: process.cwd(),
+          clientInfo: { name: "t3-test", version: "0.0.0" },
+          authMethodId: "test",
+        }),
+      ),
+      Effect.scoped,
+      Effect.provide(NodeServices.layer),
+    ),
+  );
+
   it.effect("emits status-only tool updates through completion", () =>
     Effect.gen(function* () {
       const runtime = yield* AcpSessionRuntime.AcpSessionRuntime;

apps/server/src/provider/acp/AcpSessionRuntime.ts

@@ -1202,11 +1202,7 @@ const handleSessionUpdate = ({
     }
     for (const event of parsed.events) {
       if (event._tag === "ToolCallUpdated") {
-        yield* closeActiveAssistantSegment({
-          queue,
-          assistantSegmentRef,
-        });
-        const { merged, decision } = yield* Ref.modify(toolCallsRef, (current) => {
+        const { merged, decision, isNew } = yield* Ref.modify(toolCallsRef, (current) => {
           const tracked = current.get(event.toolCall.toolCallId);
           const previous = tracked?.state;
           const nextToolCall = mergeToolCallState(previous, event.toolCall);
@@ -1228,11 +1224,16 @@ const handleSessionUpdate = ({
               skippedSinceEmit: decision.skippedSinceEmit,
             });
           }
-          return [{ merged: nextToolCall, decision }, next] as const;
+          return [{ merged: nextToolCall, decision, isNew: tracked === undefined }, next] as const;
         });
         if (!decision.emit) {
           continue;
         }
+        // A new tool call is a boundary in the prose. Progress on a call that
+        // is already shown, such as a background command finishing, is not.
+        if (isNew) {
+          yield* closeActiveAssistantSegment({ queue, assistantSegmentRef });
+        }
         yield* Queue.offer(queue, {
           _tag: "ToolCallUpdated",
           toolCall: merged,