diff --git a/packages/server/src/server/agent/agent-stream-coalescer.test.ts b/packages/server/src/server/agent/agent-stream-coalescer.test.ts
index c48a5ec8e4..1cdf4bf375 100644
--- a/packages/server/src/server/agent/agent-stream-coalescer.test.ts
+++ b/packages/server/src/server/agent/agent-stream-coalescer.test.ts
@@ -723,6 +723,57 @@ describe("AgentStreamCoalescer", () => {
     ]);
   });
 
+  test("keeps a known tool call detail when a later update in the window has none", () => {
+    const { coalescer, flushes } = createHarness();
+    primeLeadingEdge(coalescer, flushes);
+
+    coalescer.handle(
+      "agent-1",
+      timeline({
+        type: "tool_call",
+        callId: "agent-call",
+        name: "Task",
+        status: "running",
+        error: null,
+        detail: {
+          type: "sub_agent",
+          subAgentType: "general-purpose",
+          description: "List files",
+          log: "",
+          actions: [],
+        },
+      }),
+    );
+    coalescer.handle(
+      "agent-1",
+      timeline({
+        type: "tool_call",
+        callId: "agent-call",
+        name: "Agent",
+        status: "completed",
+        error: null,
+        detail: { type: "unknown", input: { description: "List files" }, output: null },
+      }),
+    );
+
+    expect(flushes.map((flush) => flush.item)).toEqual([
+      {
+        type: "tool_call",
+        callId: "agent-call",
+        name: "Agent",
+        status: "completed",
+        error: null,
+        detail: {
+          type: "sub_agent",
+          subAgentType: "general-purpose",
+          description: "List files",
+          log: "",
+          actions: [],
+        },
+      },
+    ]);
+  });
+
   test("preserves mixed text and tool call arrival order within a flush", async () => {
     const { coalescer, flushes } = createHarness();
     primeLeadingEdge(coalescer, flushes);
diff --git a/packages/server/src/server/agent/agent-stream-coalescer.ts b/packages/server/src/server/agent/agent-stream-coalescer.ts
index ca52a5479e..c11d968228 100644
--- a/packages/server/src/server/agent/agent-stream-coalescer.ts
+++ b/packages/server/src/server/agent/agent-stream-coalescer.ts
@@ -1,4 +1,5 @@
 import type { AgentProvider, AgentStreamEvent, AgentTimelineItem } from "./agent-sdk-types.js";
+import { mergeToolCallItems } from "./timeline-projection.js";
 
 export const AGENT_STREAM_COALESCE_DEFAULT_WINDOW_MS = 60;
 
@@ -183,9 +184,16 @@ export class AgentStreamCoalescer {
     }
 
     const existingIndex = buffer.toolCallEntryIndexes.get(event.item.callId);
+    const existing = existingIndex !== undefined ? buffer.entries[existingIndex] : undefined;
+    // Merge rather than replace, so an update that carries less (an unknown detail after a
+    // labeled one) cannot erase what the timeline would have kept had both been flushed.
+    let item = event.item;
+    if (existing?.kind === "tool_call") {
+      item = mergeToolCallItems(existing.item, event.item);
+    }
     const entry: PendingToolCallEntry = {
       kind: "tool_call",
-      item: event.item,
+      item,
       provider: event.provider,
       ...(event.turnId !== undefined ? { turnId: event.turnId } : {}),
     };
diff --git a/packages/server/src/server/agent/timeline-projection.ts b/packages/server/src/server/agent/timeline-projection.ts
index d8128a6a2b..1f72cdc498 100644
--- a/packages/server/src/server/agent/timeline-projection.ts
+++ b/packages/server/src/server/agent/timeline-projection.ts
@@ -87,7 +87,8 @@ function mergeToolCallDetail(existing: ToolCallDetail, incoming: ToolCallDetail)
   return incoming;
 }
 
-function mergeToolCallItems(
+/** Fold a later update for the same tool call into the one it follows. */
+export function mergeToolCallItems(
   existing: Extract<AgentTimelineItem, { type: "tool_call" }>,
   incoming: Extract<AgentTimelineItem, { type: "tool_call" }>,
 ): Extract<AgentTimelineItem, { type: "tool_call" }> {