diff --git a/src/agents/pi-embedded-subscribe.test.ts b/src/agents/pi-embedded-subscribe.test.ts
index c857fa1e1..f81d347e5 100644
--- a/src/agents/pi-embedded-subscribe.test.ts
+++ b/src/agents/pi-embedded-subscribe.test.ts
@@ -231,6 +231,132 @@ describe("subscribeEmbeddedPiSession", () => {
     expect(subscription.assistantTexts).toEqual(["Hello block"]);
   });
 
+  it("does not append when text_end content is a prefix of deltas", () => {
+    let handler: ((evt: unknown) => void) | undefined;
+    const session: StubSession = {
+      subscribe: (fn) => {
+        handler = fn;
+        return () => {};
+      },
+    };
+
+    const onBlockReply = vi.fn();
+
+    const subscription = subscribeEmbeddedPiSession({
+      session: session as unknown as Parameters<
+        typeof subscribeEmbeddedPiSession
+      >[0]["session"],
+      runId: "run",
+      onBlockReply,
+      blockReplyBreak: "text_end",
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_delta",
+        delta: "Hello world",
+      },
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_end",
+        content: "Hello",
+      },
+    });
+
+    expect(onBlockReply).toHaveBeenCalledTimes(1);
+    expect(subscription.assistantTexts).toEqual(["Hello world"]);
+  });
+
+  it("does not append when text_end content is already contained", () => {
+    let handler: ((evt: unknown) => void) | undefined;
+    const session: StubSession = {
+      subscribe: (fn) => {
+        handler = fn;
+        return () => {};
+      },
+    };
+
+    const onBlockReply = vi.fn();
+
+    const subscription = subscribeEmbeddedPiSession({
+      session: session as unknown as Parameters<
+        typeof subscribeEmbeddedPiSession
+      >[0]["session"],
+      runId: "run",
+      onBlockReply,
+      blockReplyBreak: "text_end",
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_delta",
+        delta: "Hello world",
+      },
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_end",
+        content: "world",
+      },
+    });
+
+    expect(onBlockReply).toHaveBeenCalledTimes(1);
+    expect(subscription.assistantTexts).toEqual(["Hello world"]);
+  });
+
+  it("appends suffix when text_end content extends deltas", () => {
+    let handler: ((evt: unknown) => void) | undefined;
+    const session: StubSession = {
+      subscribe: (fn) => {
+        handler = fn;
+        return () => {};
+      },
+    };
+
+    const onBlockReply = vi.fn();
+
+    const subscription = subscribeEmbeddedPiSession({
+      session: session as unknown as Parameters<
+        typeof subscribeEmbeddedPiSession
+      >[0]["session"],
+      runId: "run",
+      onBlockReply,
+      blockReplyBreak: "text_end",
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_delta",
+        delta: "Hello",
+      },
+    });
+
+    handler?.({
+      type: "message_update",
+      message: { role: "assistant" },
+      assistantMessageEvent: {
+        type: "text_end",
+        content: "Hello world",
+      },
+    });
+
+    expect(onBlockReply).toHaveBeenCalledTimes(1);
+    expect(subscription.assistantTexts).toEqual(["Hello world"]);
+  });
+
   it("does not duplicate when text_end repeats full content", () => {
     let handler: ((evt: unknown) => void) | undefined;
     const session: StubSession = {