From 584ab1c2d389270847151398a3a443e004acbc80 Mon Sep 17 00:00:00 2001
From: Kiri110K <kirill.a.chernov@gmail.com>
Date: Sun, 3 May 2026 11:34:18 +0900
Subject: [PATCH] Fix OpenCode raw text delta assembly

---
 .../provider/Layers/OpenCodeAdapter.test.ts   | 89 ++++++++++++++++++-
 .../src/provider/Layers/OpenCodeAdapter.ts    | 15 +---
 2 files changed, 87 insertions(+), 17 deletions(-)

diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
index 370e5c028ff..763a63a5667 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
@@ -565,17 +565,98 @@ it.layer(OpenCodeAdapterTestLayer)("OpenCodeAdapterLive", (it) => {
     }),
   );
 
-  it.effect("deduplicates overlapping assistant text deltas after part updates", () =>
+  it.effect("appends raw assistant text deltas and reconciles part update snapshots", () =>
     Effect.sync(() => {
       const firstUpdate = mergeOpenCodeAssistantText(undefined, "Hello");
       const overlapDelta = appendOpenCodeAssistantTextDelta(firstUpdate.latestText, "lo world");
-      const secondUpdate = mergeOpenCodeAssistantText(overlapDelta.nextText, "Hello world!");
+      const secondUpdate = mergeOpenCodeAssistantText(overlapDelta.nextText, "Hellolo world");
 
       assert.deepEqual(
         [firstUpdate.deltaToEmit, overlapDelta.deltaToEmit, secondUpdate.deltaToEmit],
-        ["Hello", " world", "!"],
+        ["Hello", "lo world", ""],
       );
-      assert.equal(secondUpdate.latestText, "Hello world!");
+      assert.equal(secondUpdate.latestText, "Hellolo world");
+    }),
+  );
+
+  it.effect("does not strip coincidental prefix overlap from OpenCode part deltas", () =>
+    Effect.gen(function* () {
+      const adapter = yield* OpenCodeAdapter;
+      const threadId = asThreadId("thread-opencode-raw-delta");
+      const part = {
+        id: "part-raw-delta",
+        sessionID: "http://127.0.0.1:9999/session",
+        messageID: "msg-raw-delta",
+        type: "text",
+        text: "A B",
+        time: { start: 1 },
+      };
+      runtimeMock.state.subscribedEvents = [
+        {
+          type: "message.updated",
+          properties: {
+            sessionID: "http://127.0.0.1:9999/session",
+            info: {
+              id: "msg-raw-delta",
+              role: "assistant",
+            },
+          },
+        },
+        {
+          type: "message.part.updated",
+          properties: {
+            sessionID: "http://127.0.0.1:9999/session",
+            part,
+            time: 1,
+          },
+        },
+        {
+          type: "message.part.delta",
+          properties: {
+            sessionID: "http://127.0.0.1:9999/session",
+            messageID: "msg-raw-delta",
+            partID: "part-raw-delta",
+            field: "text",
+            delta: "Bonus",
+          },
+        },
+        {
+          type: "message.part.updated",
+          properties: {
+            sessionID: "http://127.0.0.1:9999/session",
+            part: {
+              ...part,
+              text: "A BBonus",
+              time: { start: 1, end: 2 },
+            },
+            time: 2,
+          },
+        },
+      ];
+      const eventsFiber = yield* adapter.streamEvents.pipe(
+        Stream.filter((event) => event.threadId === threadId),
+        Stream.take(5),
+        Stream.runCollect,
+        Effect.forkChild,
+      );
+
+      yield* adapter.startSession({
+        provider: ProviderDriverKind.make("opencode"),
+        threadId,
+        runtimeMode: "full-access",
+      });
+
+      const events = Array.from(yield* Fiber.join(eventsFiber).pipe(Effect.timeout("1 second")));
+      const deltas = events.filter((event) => event.type === "content.delta");
+      assert.deepEqual(
+        deltas.map((event) => (event.type === "content.delta" ? event.payload.delta : "")),
+        ["A B", "Bonus"],
+      );
+      assert.equal(events.at(-1)?.type, "item.completed");
+      const completed = events.at(-1);
+      if (completed?.type === "item.completed") {
+        assert.equal(completed.payload.detail, "A BBonus");
+      }
     }),
   );
 
diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
index a7b179ab1c3..fa99da97b50 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
@@ -302,16 +302,6 @@ function commonPrefixLength(left: string, right: string): number {
   return index;
 }
 
-function suffixPrefixOverlap(text: string, delta: string): number {
-  const maxLength = Math.min(text.length, delta.length);
-  for (let length = maxLength; length > 0; length -= 1) {
-    if (text.endsWith(delta.slice(0, length))) {
-      return length;
-    }
-  }
-  return 0;
-}
-
 function resolveLatestAssistantText(previousText: string | undefined, nextText: string): string {
   if (previousText && previousText.length > nextText.length && previousText.startsWith(nextText)) {
     return previousText;
@@ -340,10 +330,9 @@ export function appendOpenCodeAssistantTextDelta(
   readonly nextText: string;
   readonly deltaToEmit: string;
 } {
-  const deltaToEmit = delta.slice(suffixPrefixOverlap(previousText, delta));
   return {
-    nextText: previousText + deltaToEmit,
-    deltaToEmit,
+    nextText: previousText + delta,
+    deltaToEmit: delta,
   };
 }