From 2cedc443a50ca18ddde4532c75203640e0372871 Mon Sep 17 00:00:00 2001
From: Bohdan Triapitsyn <artmore@protonmail.com>
Date: Thu, 16 Apr 2026 19:15:40 +0300
Subject: [PATCH] fix(sync): keep deltas after initial part.updated coalescing

---
 .../src/sync/__tests__/event-pipeline.test.js | 31 +++++++++++++++++++
 packages/ui/src/sync/debug.ts                 |  2 +-
 packages/ui/src/sync/event-pipeline.ts        |  5 ---
 3 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/packages/ui/src/sync/__tests__/event-pipeline.test.js b/packages/ui/src/sync/__tests__/event-pipeline.test.js
index cbc32e85..afabd19a 100644
--- a/packages/ui/src/sync/__tests__/event-pipeline.test.js
+++ b/packages/ui/src/sync/__tests__/event-pipeline.test.js
@@ -336,6 +336,37 @@ describe('createEventPipeline', () => {
     expect(received[1].payload.type).toBe('message.part.updated');
   });
 
+  it('keeps text delta after an initial part.updated when no newer part.updated replaced it', async () => {
+    const received = await runPipelineWithEvents([
+      {
+        directory: 'dir-a',
+        payload: {
+          type: 'message.part.updated',
+          properties: {
+            part: { id: 'part-1', type: 'text', messageID: 'msg-1' },
+          },
+        },
+      },
+      {
+        directory: 'dir-a',
+        payload: {
+          type: 'message.part.delta',
+          properties: {
+            messageID: 'msg-1',
+            partID: 'part-1',
+            field: 'text',
+            delta: 'hello',
+          },
+        },
+      },
+    ]);
+
+    expect(received).toHaveLength(2);
+    expect(received[0].payload.type).toBe('message.part.updated');
+    expect(received[1].payload.type).toBe('message.part.delta');
+    expect(received[1].payload.properties.delta).toBe('hello');
+  });
+
   it('coalesces message.part.updated events for the same part', async () => {
     installDomStubs();
 
diff --git a/packages/ui/src/sync/debug.ts b/packages/ui/src/sync/debug.ts
index f9dd69ba..f4191854 100644
--- a/packages/ui/src/sync/debug.ts
+++ b/packages/ui/src/sync/debug.ts
@@ -80,4 +80,4 @@ export const syncDebug = {
     eventApplied: (eventType: string, sessionID?: string, messageID?: string) =>
       log("dispatch", "event → applied", { eventType, sessionID, messageID }),
   },
-} as const
\ No newline at end of file
+} as const
diff --git a/packages/ui/src/sync/event-pipeline.ts b/packages/ui/src/sync/event-pipeline.ts
index 5732ec2b..e91676b5 100644
--- a/packages/ui/src/sync/event-pipeline.ts
+++ b/packages/ui/src/sync/event-pipeline.ts
@@ -286,11 +286,6 @@ export function createEventPipeline(input: EventPipelineInput) {
             }
             d.coalesced.set(k, d.queue.length)
           }
-          if (normalizedPayload.type === "message.part.updated") {
-            const part = (normalizedPayload.properties as { part: { messageID: string; id: string } }).part
-            d.staleDeltas.add(deltaKey(part.messageID, part.id, "text"))
-            d.staleDeltas.add(deltaKey(part.messageID, part.id, "output"))
-          }
           d.queue.push(normalizedPayload)
           scheduleDir(routedDirectory)