Limit tool call coalescing to prepend boundary

packages/app/src/timeline/session-stream-reducers.test.ts

@@ -601,6 +601,84 @@ describe("processTimelineResponse", () => {
     ]);
   });
 
+  it("does not coalesce tool call lifecycle rows away from the prepend boundary", () => {
+    const callId = "toolu_not_boundary";
+    const currentTail = hydrateStreamState(
+      [
+        {
+          event: makeTimelineEvent("current chunk"),
+          timestamp: new Date(3000),
+        },
+        {
+          event: {
+            type: "timeline",
+            provider: "claude",
+            item: makeToolCallTimelineEntry(4, callId, "completed", {
+              type: "read",
+              filePath: "/tmp/example.ts",
+            }).item,
+          } as AgentStreamEventPayload,
+          timestamp: new Date(4000),
+        },
+      ],
+      { source: "canonical" },
+    );
+    const existingCursor: TimelineCursor = {
+      epoch: "epoch-1",
+      startSeq: 3,
+      endSeq: 5,
+    };
+
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      currentTail,
+      currentCursor: existingCursor,
+      payload: {
+        ...baseTimelineInput.payload,
+        direction: "before",
+        epoch: "epoch-1",
+        startCursor: { seq: 1 },
+        endCursor: { seq: 2 },
+        entries: [
+          makeToolCallTimelineEntry(1, callId, "running", {
+            type: "unknown",
+            input: { file_path: "/tmp/example.ts" },
+            output: null,
+          }),
+          makeTimelineEntry(2, "older chunk "),
+        ],
+      },
+    });
+
+    expect(
+      result.tail.map((item) => ({
+        kind: item.kind,
+        id: item.id,
+        status: item.kind === "tool_call" ? item.payload.data.status : null,
+        text: item.kind === "assistant_message" ? item.text : null,
+      })),
+    ).toEqual([
+      {
+        kind: "tool_call",
+        id: `agent_tool_${callId}`,
+        status: "running",
+        text: null,
+      },
+      {
+        kind: "assistant_message",
+        id: expect.any(String),
+        status: null,
+        text: "older chunk current chunk",
+      },
+      {
+        kind: "tool_call",
+        id: `agent_tool_${callId}`,
+        status: "completed",
+        text: null,
+      },
+    ]);
+  });
+
   it("requests canonical catch-up when a projected entry overlaps unseen seqs", () => {
     const existingCursor: TimelineCursor = {
       epoch: "epoch-1",

packages/app/src/timeline/session-stream-reducers.ts

@@ -5,8 +5,9 @@ import { useSessionStore } from "@/stores/session-store";
 import type { StreamItem } from "@/types/stream";
 import {
   applyStreamEvent,
-  coalesceAgentToolCallItems,
   hydrateStreamState,
+  isAgentToolCallItem,
+  mergeAgentToolCallItem,
   reduceStreamUpdate,
 } from "@/types/stream";
 
@@ -351,11 +352,26 @@ function mergePrependedCanonicalTail(olderTail: StreamItem[], currentTail: Strea
 
   const olderLast = olderTail.at(-1);
   const currentFirst = currentTail[0];
-  if (olderLast?.kind !== "assistant_message" || currentFirst?.kind !== "assistant_message") {
-    return coalesceAgentToolCallItems([...olderTail, ...currentTail]);
+
+  if (
+    olderLast &&
+    currentFirst &&
+    isAgentToolCallItem(olderLast) &&
+    isAgentToolCallItem(currentFirst) &&
+    olderLast.payload.data.callId === currentFirst.payload.data.callId
+  ) {
+    return [
+      ...olderTail.slice(0, -1),
+      mergeAgentToolCallItem(olderLast, currentFirst.payload.data, currentFirst.timestamp),
+      ...currentTail.slice(1),
+    ];
   }
 
-  return coalesceAgentToolCallItems([
+  if (olderLast?.kind !== "assistant_message" || currentFirst?.kind !== "assistant_message") {
+    return [...olderTail, ...currentTail];
+  }
+
+  return [
     ...olderTail.slice(0, -1),
     {
       ...olderLast,
@@ -363,7 +379,7 @@ function mergePrependedCanonicalTail(olderTail: StreamItem[], currentTail: Strea
       timestamp: currentFirst.timestamp,
     },
     ...currentTail.slice(1),
-  ]);
+  ];
 }
 
 function applyTimelineIncrementalPath(args: {

packages/app/src/types/stream.ts

@@ -438,7 +438,7 @@ function mergeAgentToolCallStatus(
   return "running";
 }
 
-function mergeAgentToolCallItem(
+export function mergeAgentToolCallItem(
   existing: AgentToolCallItem,
   data: AgentToolCallData,
   timestamp: Date,
@@ -515,28 +515,6 @@ function appendAgentToolCall(
   return [...state, item];
 }
 
-export function coalesceAgentToolCallItems(items: StreamItem[]): StreamItem[] {
-  const state: StreamItem[] = [];
-  const toolCallIndexByCallId = new Map<string, number>();
-  for (const item of items) {
-    if (!isAgentToolCallItem(item)) {
-      state.push(item);
-      continue;
-    }
-    const existingIndex = toolCallIndexByCallId.get(item.payload.data.callId);
-    if (existingIndex === undefined) {
-      toolCallIndexByCallId.set(item.payload.data.callId, state.length);
-      state.push(item);
-      continue;
-    }
-    const existing = state[existingIndex];
-    if (existing && isAgentToolCallItem(existing)) {
-      state[existingIndex] = mergeAgentToolCallItem(existing, item.payload.data, item.timestamp);
-    }
-  }
-  return state;
-}
-
 function appendActivityLog(state: StreamItem[], entry: ActivityLogItem): StreamItem[] {
   const index = state.findIndex((existing) => existing.id === entry.id);
   if (index >= 0) {