From 58591c37a48ee4439fc8c7c2d138b003c2df2dae Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Wed, 13 May 2026 16:27:38 +0100
Subject: [PATCH] fix(tui): emit v4 embedded chat deltas

(cherry picked from commit a6d878376b15a4d6f6f63cb4093816b4c20c0b4b)
---
 src/tui/embedded-backend.test.ts | 61 ++++++++++++++++++++++++++++++++
 src/tui/embedded-backend.ts      | 22 ++++++++++++
 2 files changed, 83 insertions(+)

diff --git a/src/tui/embedded-backend.test.ts b/src/tui/embedded-backend.test.ts
index 49e1a2ce4db..f4155bae133 100644
--- a/src/tui/embedded-backend.test.ts
+++ b/src/tui/embedded-backend.test.ts
@@ -207,6 +207,7 @@ describe("EmbeddedTuiBackend", () => {
           runId: "run-local-1",
           sessionKey: "agent:main:main",
           state: "delta",
+          deltaText: "hello",
           message: {
             role: "assistant",
             content: [{ type: "text", text: "hello" }],
@@ -303,6 +304,65 @@ describe("EmbeddedTuiBackend", () => {
     });
   });
 
+  it("marks local embedded replacement deltas", async () => {
+    const { EmbeddedTuiBackend } = await import("./embedded-backend.js");
+    const pending = deferred<{
+      payloads: Array<{ text: string }>;
+      meta: Record<string, unknown>;
+    }>();
+    agentCommandFromIngressMock.mockReturnValueOnce(pending.promise);
+
+    const backend = new EmbeddedTuiBackend();
+    const events: Array<{ event: string; payload: unknown }> = [];
+    backend.onEvent = (evt) => {
+      events.push({ event: evt.event, payload: evt.payload });
+    };
+
+    backend.start();
+    await backend.sendChat({
+      sessionKey: "agent:main:main",
+      message: "replace",
+      runId: "run-local-replace",
+    });
+
+    registeredListener?.({
+      runId: "run-local-replace",
+      stream: "assistant",
+      data: { text: "Hello world" },
+    });
+    registeredListener?.({
+      runId: "run-local-replace",
+      stream: "assistant",
+      data: { text: "Goodbye world" },
+    });
+
+    pending.resolve({ payloads: [{ text: "Goodbye world" }], meta: {} });
+    await flushMicrotasks();
+
+    const chatPayloads = events
+      .filter((entry) => entry.event === "chat")
+      .map(
+        (entry) =>
+          entry.payload as {
+            state?: string;
+            deltaText?: string;
+            replace?: boolean;
+          },
+      );
+    expect(
+      chatPayloads
+        .filter((payload) => payload.state === "delta")
+        .map((payload) => ({
+          state: payload.state,
+          deltaText: payload.deltaText,
+          replace: payload.replace,
+        })),
+    ).toEqual([
+      { state: "delta", deltaText: "Hello world", replace: undefined },
+      { state: "delta", deltaText: "Goodbye world", replace: true },
+    ]);
+  });
+
   it("keeps a fallback response deliverable after a retryable lifecycle error", async () => {
     const { EmbeddedTuiBackend } = await import("./embedded-backend.js");
     const pending = deferred<{
@@ -496,6 +556,7 @@ describe("EmbeddedTuiBackend", () => {
           runId: "run-tool-first",
           sessionKey: "agent:main:main",
           state: "delta",
+          deltaText: "",
           message: {
             role: "assistant",
             content: [{ type: "text", text: "" }],
diff --git a/src/tui/embedded-backend.ts b/src/tui/embedded-backend.ts
index 55b6bf86a4a..bab0c2c11dc 100644
--- a/src/tui/embedded-backend.ts
+++ b/src/tui/embedded-backend.ts
@@ -60,6 +60,7 @@ type LocalRunState = {
   sessionKey: string;
   controller: AbortController;
   buffer: string;
+  lastBroadcastText?: string;
   isBtw: boolean;
   question?: string;
   finalSent: boolean;
@@ -106,6 +107,16 @@ function timeoutSecondsFromMs(timeoutMs?: number): string | undefined {
   return String(Math.max(0, Math.ceil(timeoutMs / 1000)));
 }
 
+function resolveDeltaPayload(text: string, previousText: string | undefined) {
+  if (previousText === undefined) {
+    return { deltaText: text };
+  }
+  if (!text.startsWith(previousText)) {
+    return { deltaText: text, replace: true as const };
+  }
+  return { deltaText: text.slice(previousText.length) };
+}
+
 export class EmbeddedTuiBackend implements TuiBackend {
   readonly connection = { url: "local embedded" };
 
@@ -396,11 +407,17 @@ export class EmbeddedTuiBackend implements TuiBackend {
     if (!text || projected.suppress) {
       return;
     }
+    const deltaPayload = resolveDeltaPayload(text, run.lastBroadcastText);
+    if (!deltaPayload.deltaText && !deltaPayload.replace) {
+      return;
+    }
     run.registered = true;
+    run.lastBroadcastText = text;
     this.emit("chat", {
       runId,
       sessionKey: run.sessionKey,
       state: "delta",
+      ...deltaPayload,
       message: {
         role: "assistant",
         content: [{ type: "text", text }],
@@ -416,6 +433,7 @@ export class EmbeddedTuiBackend implements TuiBackend {
     }
     run.finalSent = true;
     run.registered = true;
+    run.lastBroadcastText = undefined;
     const projected = projectLiveAssistantBufferedText(run.buffer.trim(), {
       suppressLeadFragments: false,
     });
@@ -445,6 +463,7 @@ export class EmbeddedTuiBackend implements TuiBackend {
     }
     run.finalSent = true;
     run.registered = true;
+    run.lastBroadcastText = undefined;
     this.emit("chat", {
       runId,
       sessionKey: run.sessionKey,
@@ -459,6 +478,7 @@ export class EmbeddedTuiBackend implements TuiBackend {
     }
     run.finalSent = true;
     run.registered = true;
+    run.lastBroadcastText = undefined;
     this.emit("chat", {
       runId,
       sessionKey: run.sessionKey,
@@ -472,10 +492,12 @@ export class EmbeddedTuiBackend implements TuiBackend {
       return;
     }
     run.registered = true;
+    run.lastBroadcastText = "";
     this.emit("chat", {
       runId,
       sessionKey: run.sessionKey,
       state: "delta",
+      deltaText: "",
       message: {
         role: "assistant",
         content: [{ type: "text", text: "" }],