fix(orchestration): show follow-up assistant replies after Cursor session resume

apps/server/src/orchestration/Layers/ProjectionPipeline.ts

 import { 
 import { 
@@ -54,6 +54,7 @@ import {
   parseThreadSegmentFromAttachmentId,
   toSafeThreadAttachmentSegment,
 } from "../../attachmentStore.ts";
+import { mergeThreadMessageProjection } from "../threadMessageProjection.ts";
 
 export const ORCHESTRATION_PROJECTOR_NAMES = {
   projects: "projection.projects",
@@ -816,18 +817,22 @@ const makeOrchestrationProjectionPipeline = Effect.fn("makeOrchestrationProjecti
             messageId: event.payload.messageId,
           });
           const previousMessage = Option.getOrUndefined(existingMessage);
-          const nextText = Option.match(existingMessage, {
-            onNone: () => event.payload.text,
-            onSome: (message) => {
-              if (event.payload.streaming) {
-                return `${message.text}${event.payload.text}`;
-              }
-              if (event.payload.text.length === 0) {
-                return message.text;
-              }
-              return event.payload.text;
+          const mergedMessage = mergeThreadMessageProjection(
+            previousMessage
+              ? {
+                  text: previousMessage.text,
+                  turnId: previousMessage.turnId,
+                  createdAt: previousMessage.createdAt,
+                }
+              : undefined,
+            {
+              text: event.payload.text,
+              turnId: event.payload.turnId,
+              streaming: event.payload.streaming,
+              createdAt: event.payload.createdAt,
+              updatedAt: event.payload.updatedAt,
             },
-          });
+          );
           const nextAttachments =
             event.payload.attachments !== undefined
               ? yield* materializeAttachmentsForProjection({
@@ -837,13 +842,13 @@ const makeOrchestrationProjectionPipeline = Effect.fn("makeOrchestrationProjecti
           yield* projectionThreadMessageRepository.upsert({
             messageId: event.payload.messageId,
             threadId: event.payload.threadId,
-            turnId: event.payload.turnId,
+            turnId: mergedMessage.turnId,
             role: event.payload.role,
-            text: nextText,
+            text: mergedMessage.text,
             ...(nextAttachments !== undefined ? { attachments: [...nextAttachments] } : {}),
             isStreaming: event.payload.streaming,
-            createdAt: previousMessage?.createdAt ?? event.payload.createdAt,
-            updatedAt: event.payload.updatedAt,
+            createdAt: mergedMessage.createdAt,
+            updatedAt: mergedMessage.updatedAt,
           });
           return;
         }

apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts

@@ -38,6 +38,10 @@ import {
   type ProviderRuntimeIngestionShape,
 } from "../Services/ProviderRuntimeIngestion.ts";
 import { ServerSettingsService } from "../../serverSettings.ts";
+import {
+  assistantSegmentBaseKeyFromRuntimeItem,
+  assistantSegmentMessageId,
+} from "../assistantMessageIds.ts";
 
 const providerTurnKey = (threadId: ThreadId, turnId: TurnId) => `${threadId}:${turnId}`;
 
@@ -194,14 +198,9 @@ function proposedPlanIdFromEvent(event: ProviderRuntimeEvent, threadId: ThreadId
 }
 
 function assistantSegmentBaseKeyFromEvent(event: ProviderRuntimeEvent): string {
-  return String(event.itemId ?? event.turnId ?? event.eventId);
+  return assistantSegmentBaseKeyFromRuntimeItem(event.itemId, event.turnId, event.eventId);
 }
 
-function assistantSegmentMessageId(baseKey: string, segmentIndex: number): MessageId {
-  return MessageId.make(
-    segmentIndex === 0 ? `assistant:${baseKey}` : `assistant:${baseKey}:segment:${segmentIndex}`,
-  );
-}
 function buildContextWindowActivityPayload(
   event: ProviderRuntimeEvent,
 ): ThreadTokenUsageSnapshot | undefined {
@@ -756,11 +755,11 @@ const make = Effect.gen(function* () {
             onNone: () => ({
               baseKey: input.baseKey,
               nextSegmentIndex: 1,
-              activeMessageId: assistantSegmentMessageId(input.baseKey, 0),
+              activeMessageId: assistantSegmentMessageId(input.baseKey, 0, input.turnId),
             }),
             onSome: (state) => {
               const segmentIndex = state.baseKey === input.baseKey ? state.nextSegmentIndex : 0;
-              const messageId = assistantSegmentMessageId(input.baseKey, segmentIndex);
+              const messageId = assistantSegmentMessageId(input.baseKey, segmentIndex, input.turnId);
               return {
                 baseKey: input.baseKey,
                 nextSegmentIndex: state.baseKey === input.baseKey ? state.nextSegmentIndex + 1 : 1,
@@ -781,7 +780,7 @@ const make = Effect.gen(function* () {
   }) =>
     Effect.gen(function* () {
       if (!input.turnId) {
-        return assistantSegmentMessageId(assistantSegmentBaseKeyFromEvent(input.event), 0);
+        return assistantSegmentMessageId(assistantSegmentBaseKeyFromEvent(input.event), 0, undefined);
       }
 
       const activeMessageId = yield* getActiveAssistantMessageIdForTurn(
@@ -1459,8 +1458,14 @@ const make = Effect.gen(function* () {
       const assistantCompletion =
         event.type === "item.completed" && event.payload.itemType === "assistant_message"
           ? {
-              messageId: MessageId.make(
-                `assistant:${event.itemId ?? event.turnId ?? event.eventId}`,
+              messageId: assistantSegmentMessageId(
+                assistantSegmentBaseKeyFromRuntimeItem(
+                  event.itemId,
+                  event.turnId,
+                  event.eventId,
+                ),
+                0,
+                toTurnId(event.turnId),
               ),
               fallbackText: event.payload.detail,
             }
@@ -1634,8 +1639,14 @@ const make = Effect.gen(function* () {
           if (hasCheckpointForTurn(checkpointContext.checkpoints, turnId)) {
             // Already tracked; no-op.
           } else {
-            const assistantMessageId = MessageId.make(
-              `assistant:${event.itemId ?? event.turnId ?? event.eventId}`,
+            const assistantMessageId = assistantSegmentMessageId(
+              assistantSegmentBaseKeyFromRuntimeItem(
+                event.itemId,
+                event.turnId,
+                event.eventId,
+              ),
+              0,
+              turnId,
             );
             yield* orchestrationEngine.dispatch({
               type: "thread.turn.diff.complete",

apps/server/src/orchestration/assistantMessageIds.test.ts

@@ -0,0 +1,22 @@
+import { MessageId, TurnId } from "@t3tools/contracts";
+import { describe, expect, it } from "vitest";
+
+import { assistantSegmentMessageId } from "./assistantMessageIds.ts";
+
+describe("assistantMessageIds", () => {
+  it("scopes assistant message ids by turn so resumed sessions do not collide", () => {
+    const baseKey = "assistant:session-1:segment:0";
+    const turnOne = TurnId.make("turn-1");
+    const turnTwo = TurnId.make("turn-2");
+
+    expect(assistantSegmentMessageId(baseKey, 0, turnOne)).toBe(
+      MessageId.make("assistant:turn-1:assistant:session-1:segment:0"),
+    );
+    expect(assistantSegmentMessageId(baseKey, 0, turnTwo)).toBe(
+      MessageId.make("assistant:turn-2:assistant:session-1:segment:0"),
+    );
+    expect(assistantSegmentMessageId(baseKey, 0, turnOne)).not.toBe(
+      assistantSegmentMessageId(baseKey, 0, turnTwo),
+    );
+  });
+});

apps/server/src/orchestration/assistantMessageIds.ts

@@ -0,0 +1,22 @@
+import { MessageId, type TurnId } from "@t3tools/contracts";
+
+export function assistantSegmentBaseKeyFromRuntimeItem(
+  itemId: string | undefined,
+  turnId: string | undefined,
+  eventId: string,
+): string {
+  return String(itemId ?? turnId ?? eventId);
+}
+
+export function assistantSegmentMessageId(
+  baseKey: string,
+  segmentIndex: number,
+  turnId?: TurnId,
+): MessageId {
+  const scopedBase = turnId ? `${turnId}:${baseKey}` : baseKey;
+  return MessageId.make(
+    segmentIndex === 0
+      ? `assistant:${scopedBase}`
+      : `assistant:${scopedBase}:segment:${segmentIndex}`,
+  );
+}

apps/server/src/orchestration/projector.ts

@@ -9,6 +9,7 @@ import * as Effect from "effect/Effect";
 import * as Schema from "effect/Schema";
 
 import { toProjectorDecodeError, type OrchestrationProjectorDecodeError } from "./Errors.ts";
+import { mergeThreadMessageProjection } from "./threadMessageProjection.ts";
 import {
   MessageSentPayloadSchema,
   ProjectCreatedPayload,
@@ -413,20 +414,33 @@ export function projectEvent(
         const messages = existingMessage
           ? thread.messages.map((entry) =>
               entry.id === message.id
-                ? {
-                    ...entry,
-                    text: message.streaming
-                      ? `${entry.text}${message.text}`
-                      : message.text.length > 0
-                        ? message.text
-                        : entry.text,
-                    streaming: message.streaming,
-                    updatedAt: message.updatedAt,
-                    turnId: message.turnId,
-                    ...(message.attachments !== undefined
-                      ? { attachments: message.attachments }
-                      : {}),
-                  }
+                ? (() => {
+                    const mergedMessage = mergeThreadMessageProjection(
+                      {
+                        text: entry.text,
+                        turnId: entry.turnId,
+                        createdAt: entry.createdAt,
+                      },
+                      {
+                        text: message.text,
+                        turnId: message.turnId,
+                        streaming: message.streaming,
+                        createdAt: message.createdAt,
+                        updatedAt: message.updatedAt,
+                      },
+                    );
+                    return {
+                      ...entry,
+                      text: mergedMessage.text,
+                      streaming: message.streaming,
+                      createdAt: mergedMessage.createdAt,
+                      updatedAt: mergedMessage.updatedAt,
+                      turnId: mergedMessage.turnId,
+                      ...(message.attachments !== undefined
+                        ? { attachments: message.attachments }
+                        : {}),
+                    };
+                  })()
                 : entry,
             )
           : [...thread.messages, message];

apps/server/src/orchestration/threadMessageProjection.test.ts

@@ -0,0 +1,90 @@
+import { TurnId } from "@t3tools/contracts";
+import { describe, expect, it } from "vitest";
+
+import { assistantMessageTurnChanged, mergeThreadMessageProjection } from "./threadMessageProjection.ts";
+
+describe("threadMessageProjection", () => {
+  it("detects turn changes on reused assistant message ids", () => {
+    expect(
+      assistantMessageTurnChanged(
+        { text: "old", turnId: TurnId.make("turn-1"), createdAt: "2026-07-01T21:00:00.000Z" },
+        TurnId.make("turn-2"),
+      ),
+    ).toBe(true);
+  });
+
+  it("resets text and createdAt when a reused message id starts a new turn", () => {
+    const merged = mergeThreadMessageProjection(
+      {
+        text: "first turn reply",
+        turnId: TurnId.make("turn-1"),
+        createdAt: "2026-07-01T21:00:00.000Z",
+      },
+      {
+        text: "second turn reply",
+        turnId: TurnId.make("turn-2"),
+        streaming: false,
+        createdAt: "2026-07-02T01:44:30.000Z",
+        updatedAt: "2026-07-02T01:44:30.000Z",
+      },
+    );
+
+    expect(merged.text).toBe("second turn reply");
+    expect(merged.createdAt).toBe("2026-07-02T01:44:30.000Z");
+    expect(merged.turnId).toBe(TurnId.make("turn-2"));
+  });
+
+  it("appends streaming deltas within the same turn", () => {
+    const merged = mergeThreadMessageProjection(
+      {
+        text: "hello",
+        turnId: TurnId.make("turn-1"),
+        createdAt: "2026-07-01T21:00:00.000Z",
+      },
+      {
+        text: " world",
+        turnId: TurnId.make("turn-1"),
+        streaming: true,
+        createdAt: "2026-07-01T21:00:01.000Z",
+        updatedAt: "2026-07-01T21:00:01.000Z",
+      },
+    );
+
+    expect(merged.text).toBe("hello world");
+    expect(merged.createdAt).toBe("2026-07-01T21:00:00.000Z");
+  });
+
+  it("persists first attachments-only messages with empty text", () => {
+    const merged = mergeThreadMessageProjection(undefined, {
+      text: "",
+      turnId: TurnId.make("turn-1"),
+      streaming: false,
+      createdAt: "2026-07-02T01:44:30.000Z",
+      updatedAt: "2026-07-02T01:44:30.000Z",
+    });
+
+    expect(merged.text).toBe("");
+    expect(merged.createdAt).toBe("2026-07-02T01:44:30.000Z");
+    expect(merged.turnId).toBe(TurnId.make("turn-1"));
+  });
+
+  it("starts fresh streaming text when the turn changes", () => {
+    const merged = mergeThreadMessageProjection(
+      {
+        text: "first turn reply",
+        turnId: TurnId.make("turn-1"),
+        createdAt: "2026-07-01T21:00:00.000Z",
+      },
+      {
+        text: "Checking",
+        turnId: TurnId.make("turn-2"),
+        streaming: true,
+        createdAt: "2026-07-02T01:44:30.000Z",
+        updatedAt: "2026-07-02T01:44:30.000Z",
+      },
+    );
+
+    expect(merged.text).toBe("Checking");
+    expect(merged.createdAt).toBe("2026-07-02T01:44:30.000Z");
+  });
+});

apps/server/src/orchestration/threadMessageProjection.ts

@@ -0,0 +1,56 @@
+import type { TurnId } from "@t3tools/contracts";
+
+export interface ThreadMessageProjectionSlice {
+  readonly text: string;
+  readonly turnId: TurnId | null;
+  readonly createdAt: string;
+}
+
+export interface ThreadMessageProjectionIncoming {
+  readonly text: string;
+  readonly turnId: TurnId | null;
+  readonly streaming: boolean;
+  readonly createdAt: string;
+  readonly updatedAt: string;
+}
+
+export function assistantMessageTurnChanged(
+  previousMessage: ThreadMessageProjectionSlice | undefined,
+  nextTurnId: TurnId | null,
+): boolean {
+  if (!previousMessage) {
+    return false;
+  }
+  return previousMessage.turnId !== nextTurnId;
+}
+
+export function mergeThreadMessageProjection(
+  previousMessage: ThreadMessageProjectionSlice | undefined,
+  incoming: ThreadMessageProjectionIncoming,
+): ThreadMessageProjectionSlice & { readonly updatedAt: string } {
+  const turnChanged = assistantMessageTurnChanged(previousMessage, incoming.turnId);
+
+  const nextText = (() => {
+    if (!previousMessage || turnChanged) {
+      if (incoming.streaming || incoming.text.length > 0) {
+        return incoming.text;
+      }
+      return turnChanged ? "" : (previousMessage?.text ?? "");
+    }
+
+    if (incoming.streaming) {
+      return `${previousMessage.text}${incoming.text}`;
+    }
+    if (incoming.text.length === 0) {
+      return previousMessage.text;
+    }
+    return incoming.text;
+  })();
+
+  return {
+    text: nextText,
+    turnId: incoming.turnId,
+    createdAt: turnChanged ? incoming.createdAt : (previousMessage?.createdAt ?? incoming.createdAt),
+    updatedAt: incoming.updatedAt,
+  };
+}