diff --git a/src/agents/embedded-agent-subscribe.handlers.messages.test.ts b/src/agents/embedded-agent-subscribe.handlers.messages.test.ts
index dab3a1320d7e..470805bcb628 100644
--- a/src/agents/embedded-agent-subscribe.handlers.messages.test.ts
+++ b/src/agents/embedded-agent-subscribe.handlers.messages.test.ts
@@ -478,6 +478,141 @@ describe("handleMessageUpdate text signatures", () => {
     ]);
   });
 
+  it("uses incremental deltas for same-item phased streams", () => {
+    const onAgentEvent = vi.fn();
+    const context = createMessageUpdateContext({ onAgentEvent });
+    const signature = JSON.stringify({ v: 1, id: "item-final", phase: "final_answer" });
+    const partial = {
+      role: "assistant",
+      phase: "final_answer",
+      content: [
+        {
+          type: "text",
+          textSignature: signature,
+          get text() {
+            throw new Error("full partial text should not be read");
+          },
+        },
+      ],
+    };
+
+    const createPhasedDelta = (delta: string) =>
+      ({
+        type: "message_update",
+        message: { role: "assistant", content: [] },
+        assistantMessageEvent: {
+          type: "text_delta",
+          delta,
+          partial,
+        },
+      }) as never;
+
+    handleMessageUpdate(context, createPhasedDelta("Hello"));
+    handleMessageUpdate(context, createPhasedDelta(" world"));
+
+    expect(onAgentEvent.mock.calls.map(([event]) => event)).toMatchObject([
+      {
+        stream: "assistant",
+        data: { text: "Hello", delta: "Hello", phase: "final_answer" },
+      },
+      {
+        stream: "assistant",
+        data: { text: "Hello world", delta: " world", phase: "final_answer" },
+      },
+    ]);
+  });
+
+  it("keeps same-item phased stream deltas on the user-visible sanitizer path", () => {
+    const onAgentEvent = vi.fn();
+    const context = createMessageUpdateContext({ onAgentEvent });
+    const signature = JSON.stringify({ v: 1, id: "item-final", phase: "final_answer" });
+    const partial = {
+      role: "assistant",
+      phase: "final_answer",
+      content: [
+        {
+          type: "text",
+          textSignature: signature,
+          get text() {
+            throw new Error("full partial text should not be read");
+          },
+        },
+      ],
+    };
+
+    const createPhasedDelta = (delta: string) =>
+      ({
+        type: "message_update",
+        message: { role: "assistant", content: [] },
+        assistantMessageEvent: {
+          type: "text_delta",
+          delta,
+          partial,
+        },
+      }) as never;
+
+    handleMessageUpdate(context, createPhasedDelta("Visible\n<tool_call>{"));
+    handleMessageUpdate(
+      context,
+      createPhasedDelta('"name":"read","arguments":{"file_path":"secret.md"}}</tool_call>'),
+    );
+    handleMessageUpdate(context, createPhasedDelta("\nDone."));
+
+    expect(onAgentEvent.mock.calls.map(([event]) => event)).toMatchObject([
+      {
+        stream: "assistant",
+        data: { text: "Visible", delta: "Visible", phase: "final_answer" },
+      },
+      {
+        stream: "assistant",
+        data: { text: "Visible\n\nDone.", delta: "\n\nDone.", phase: "final_answer" },
+      },
+    ]);
+  });
+
+  it("keeps sanitizer context when a same-item phased stream starts hidden", () => {
+    const onAgentEvent = vi.fn();
+    const context = createMessageUpdateContext({ onAgentEvent });
+    const signature = JSON.stringify({ v: 1, id: "item-final", phase: "final_answer" });
+    const partial = {
+      role: "assistant",
+      phase: "final_answer",
+      content: [
+        {
+          type: "text",
+          textSignature: signature,
+          get text() {
+            throw new Error("full partial text should not be read");
+          },
+        },
+      ],
+    };
+
+    const createPhasedDelta = (delta: string) =>
+      ({
+        type: "message_update",
+        message: { role: "assistant", content: [] },
+        assistantMessageEvent: {
+          type: "text_delta",
+          delta,
+          partial,
+        },
+      }) as never;
+
+    handleMessageUpdate(context, createPhasedDelta("<tool_call>{"));
+    handleMessageUpdate(
+      context,
+      createPhasedDelta('"name":"read","arguments":{"file_path":"secret.md"}}</tool_call>\nDone.'),
+    );
+
+    expect(onAgentEvent.mock.calls.map(([event]) => event)).toMatchObject([
+      {
+        stream: "assistant",
+        data: { text: "Done.", delta: "Done.", phase: "final_answer" },
+      },
+    ]);
+  });
+
   it("treats phased textSignature item changes as assistant-message boundaries", () => {
     const flushBlockReplyBuffer = vi.fn();
     const resetAssistantMessageState = vi.fn();
diff --git a/src/agents/embedded-agent-subscribe.handlers.messages.ts b/src/agents/embedded-agent-subscribe.handlers.messages.ts
index c96da4e97645..d4deb74a9f90 100644
--- a/src/agents/embedded-agent-subscribe.handlers.messages.ts
+++ b/src/agents/embedded-agent-subscribe.handlers.messages.ts
@@ -38,6 +38,7 @@ import {
   extractThinkingFromTaggedStream,
   extractThinkingFromTaggedText,
   promoteThinkingTagsToBlocks,
+  sanitizeAssistantVisibleStreamText,
 } from "./embedded-agent-utils.js";
 import type { AgentEvent, AgentMessage } from "./runtime/index.js";
 
@@ -218,6 +219,22 @@ function resolveStreamVisibleText(params: {
   return { rawText, visibleText: rawText.trim() };
 }
 
+function resolveTextAppendDelta(previousText: string, nextText: string): string {
+  if (!nextText) {
+    return "";
+  }
+  if (!previousText) {
+    return nextText;
+  }
+  if (nextText.startsWith(previousText)) {
+    return nextText.slice(previousText.length);
+  }
+  if (previousText.startsWith(nextText)) {
+    return "";
+  }
+  return nextText;
+}
+
 function copyPartialBlockState(
   target: EmbeddedAgentSubscribeState["partialBlockState"],
   source: EmbeddedAgentSubscribeState["partialBlockState"],
@@ -647,9 +664,11 @@ export function handleMessageUpdate(
     !deliveryPhase &&
     Boolean(streamItemId) &&
     isOpenAiResponsesAssistantMessage(partialAssistant);
+  let streamItemChanged = false;
   if ((deliveryPhase || isPhasePendingOpenAiResponsesTextItem) && streamItemId) {
     const previousStreamItemId = ctx.state.lastAssistantStreamItemId;
     if (previousStreamItemId && previousStreamItemId !== streamItemId) {
+      streamItemChanged = true;
       void ctx.flushBlockReplyBuffer({ assistantMessageIndex: ctx.state.assistantMessageIndex });
       ctx.resetAssistantMessageState(ctx.state.assistantTexts.length);
       void ctx.params.onAssistantMessageStart?.();
@@ -677,11 +696,29 @@ export function handleMessageUpdate(
   }
   const wasThinking = ctx.state.partialBlockState.thinking;
   let visibleDelta = "";
-  let next = shouldUsePhaseAwareBlockReply
+  const shouldReadPhaseAwarePartialText =
+    shouldUsePhaseAwareBlockReply && (streamItemChanged || evtType === "text_end" || !chunk);
+  let next = shouldReadPhaseAwarePartialText
     ? coerceChatContentText(extractAssistantVisibleText(partialAssistant)).trim()
     : "";
   let nextRawStreamText = next;
-  if (!next && deliveryPhase !== "final_answer") {
+  let shouldPersistRawStreamText = false;
+  if (shouldUsePhaseAwareBlockReply && !next && deliveryPhase === "final_answer" && chunk) {
+    visibleDelta = ctx.stripBlockTags(chunk, ctx.state.partialBlockState, {
+      final: evtType === "text_end",
+    });
+    const streamVisibleText = resolveStreamVisibleText({
+      previousRawText: ctx.state.lastStreamedAssistant ?? "",
+      visibleDelta,
+    });
+    const previousVisibleText = sanitizeAssistantVisibleStreamText(
+      ctx.state.lastStreamedAssistant ?? "",
+    ).trim();
+    next = sanitizeAssistantVisibleStreamText(streamVisibleText.rawText).trim();
+    visibleDelta = resolveTextAppendDelta(previousVisibleText, next);
+    nextRawStreamText = streamVisibleText.rawText;
+    shouldPersistRawStreamText = true;
+  } else if (!next && deliveryPhase !== "final_answer") {
     const pendingTagFragment = ctx.state.partialBlockState.pendingTagFragment;
     const shouldRecomputeFullStream = Boolean(pendingTagFragment) || REASONING_TAG_RE.test(chunk);
     if (shouldRecomputeFullStream) {
@@ -800,6 +837,8 @@ export function handleMessageUpdate(
       ctx.emitAssistantStreamData(data, { emitPartialReply: true });
       ctx.state.emittedAssistantUpdate = true;
     }
+  } else if (shouldPersistRawStreamText) {
+    ctx.state.lastStreamedAssistant = nextRawStreamText;
   }
 
   if (
diff --git a/src/agents/embedded-agent-utils.ts b/src/agents/embedded-agent-utils.ts
index a3672c5a1aeb..df14ce57bbe7 100644
--- a/src/agents/embedded-agent-utils.ts
+++ b/src/agents/embedded-agent-utils.ts
@@ -40,6 +40,10 @@ function sanitizeAssistantText(text: string): string {
   return sanitizeAssistantVisibleText(text);
 }
 
+export function sanitizeAssistantVisibleStreamText(text: string): string {
+  return sanitizeUserFacingText(sanitizeAssistantText(text), { errorContext: false });
+}
+
 function finalizeAssistantExtraction(msg: AssistantMessage, extracted: string): string {
   const errorContext = msg.stopReason === "error";
   return sanitizeUserFacingText(extracted, { errorContext });