diff --git a/extensions/acpx/src/runtime-internals/events.test.ts b/extensions/acpx/src/runtime-internals/events.test.ts
index 018d1a3c3c7..0736ba4063c 100644
--- a/extensions/acpx/src/runtime-internals/events.test.ts
+++ b/extensions/acpx/src/runtime-internals/events.test.ts
@@ -5,9 +5,24 @@ function jsonLine(payload: unknown): string {
   return JSON.stringify(payload);
 }
 
+function beginPrompt(projector: PromptStreamProjector, id = "req-1") {
+  projector.ingestLine(
+    jsonLine({
+      jsonrpc: "2.0",
+      id,
+      method: "session/prompt",
+      params: {
+        sessionId: "session-1",
+        prompt: [{ type: "text", text: "hello" }],
+      },
+    }),
+  );
+}
+
 describe("PromptStreamProjector", () => {
   it("maps agent message chunks to output deltas", () => {
     const projector = new PromptStreamProjector();
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -34,6 +49,7 @@ describe("PromptStreamProjector", () => {
 
   it("preserves leading spaces in streamed output chunks", () => {
     const projector = new PromptStreamProjector();
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -60,6 +76,7 @@ describe("PromptStreamProjector", () => {
 
   it("maps agent thought chunks to thought deltas", () => {
     const projector = new PromptStreamProjector();
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -86,6 +103,7 @@ describe("PromptStreamProjector", () => {
 
   it("maps tool call updates to tool_call events", () => {
     const projector = new PromptStreamProjector();
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -108,19 +126,53 @@ describe("PromptStreamProjector", () => {
     });
   });
 
-  it("maps prompt response stop reasons to done events", () => {
+  it("ignores replayed updates before current prompt starts", () => {
     const projector = new PromptStreamProjector();
-    projector.ingestLine(
+    const replayed = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
-        id: "req-1",
-        method: "session/prompt",
+        method: "session/update",
         params: {
           sessionId: "session-1",
-          prompt: [{ type: "text", text: "hello" }],
+          update: {
+            sessionUpdate: "agent_message_chunk",
+            content: {
+              type: "text",
+              text: "old turn",
+            },
+          },
         },
       }),
     );
+    beginPrompt(projector, "req-2");
+    const current = projector.ingestLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "agent_message_chunk",
+            content: {
+              type: "text",
+              text: "new turn",
+            },
+          },
+        },
+      }),
+    );
+
+    expect(replayed).toBeNull();
+    expect(current).toEqual({
+      type: "text_delta",
+      text: "new turn",
+      stream: "output",
+    });
+  });
+
+  it("maps prompt response stop reasons to done events", () => {
+    const projector = new PromptStreamProjector();
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -139,17 +191,7 @@ describe("PromptStreamProjector", () => {
 
   it("maps json-rpc errors to runtime errors", () => {
     const projector = new PromptStreamProjector();
-    projector.ingestLine(
-      jsonLine({
-        jsonrpc: "2.0",
-        id: "req-1",
-        method: "session/prompt",
-        params: {
-          sessionId: "session-1",
-          prompt: [{ type: "text", text: "hello" }],
-        },
-      }),
-    );
+    beginPrompt(projector);
     const event = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -170,17 +212,7 @@ describe("PromptStreamProjector", () => {
 
   it("ignores non-prompt response errors", () => {
     const projector = new PromptStreamProjector();
-    projector.ingestLine(
-      jsonLine({
-        jsonrpc: "2.0",
-        id: 3,
-        method: "session/prompt",
-        params: {
-          sessionId: "session-1",
-          prompt: [{ type: "text", text: "hello" }],
-        },
-      }),
-    );
+    beginPrompt(projector, "3");
     const loadError = projector.ingestLine(
       jsonLine({
         jsonrpc: "2.0",
@@ -200,11 +232,25 @@ describe("PromptStreamProjector", () => {
         },
       }),
     );
+    const trailingReplay = projector.ingestLine(
+      jsonLine({
+        jsonrpc: "2.0",
+        method: "session/update",
+        params: {
+          sessionId: "session-1",
+          update: {
+            sessionUpdate: "agent_message_chunk",
+            content: { type: "text", text: "should be ignored" },
+          },
+        },
+      }),
+    );
 
     expect(loadError).toBeNull();
     expect(promptDone).toEqual({
       type: "done",
       stopReason: "end_turn",
     });
+    expect(trailingReplay).toBeNull();
   });
 });
diff --git a/extensions/acpx/src/runtime-internals/events.ts b/extensions/acpx/src/runtime-internals/events.ts
index bb13e2f3c78..2e66076f595 100644
--- a/extensions/acpx/src/runtime-internals/events.ts
+++ b/extensions/acpx/src/runtime-internals/events.ts
@@ -180,11 +180,6 @@ export class PromptStreamProjector {
       return null;
     }
 
-    const updateEvent = parseSessionUpdateEvent(parsed);
-    if (updateEvent) {
-      return updateEvent;
-    }
-
     if (asTrimmedString(parsed.method) === "session/prompt") {
       const id = normalizeJsonRpcId(parsed.id);
       if (id) {
@@ -193,8 +188,13 @@ export class PromptStreamProjector {
       return null;
     }
 
+    const updateEvent = parseSessionUpdateEvent(parsed);
+    if (updateEvent) {
+      return this.promptRequestIds.size > 0 ? updateEvent : null;
+    }
+
     if (Object.hasOwn(parsed, "error")) {
-      if (!this.shouldHandlePromptResponse(parsed)) {
+      if (!this.consumePromptResponse(parsed)) {
         return null;
       }
       const error = isRecord(parsed.error) ? parsed.error : null;
@@ -211,7 +211,7 @@ export class PromptStreamProjector {
     }
 
     const stopReason = parsePromptStopReason(parsed);
-    if (!stopReason || !this.shouldHandlePromptResponse(parsed)) {
+    if (!stopReason || !this.consumePromptResponse(parsed)) {
       return null;
     }
 
@@ -221,11 +221,15 @@ export class PromptStreamProjector {
     };
   }
 
-  private shouldHandlePromptResponse(message: Record<string, unknown>): boolean {
+  private consumePromptResponse(message: Record<string, unknown>): boolean {
     const id = normalizeJsonRpcId(message.id);
     if (!id) {
       return false;
     }
-    return this.promptRequestIds.has(id);
+    if (!this.promptRequestIds.has(id)) {
+      return false;
+    }
+    this.promptRequestIds.delete(id);
+    return true;
   }
 }