fix(sdk,core): keep continuation-boot recovery green under the monotonic session.in floor

A snapshot with no savedAt no longer drops the whole transcript: savedAt is
optional and defaults to 0 (it only orders snapshot history before live
chunks, so losing it should not lose the conversation).

The offline chat harness now models session.in as a durable stream whose
seqNums stay monotonic across a chat's runs, and the test session-stream
manager's setLastSeqNum only ever advances (matching the production manager).
Without this a fresh per-run manager restarted seqNums at 0, so a
continuation's follow-up message collided with the resume floor and was
dropped, hanging the turn loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VG39FXXkFFU24U5EtJMwPi

Author: ericallam

packages/core/src/v3/sessionStreams/chatSnapshot.ts

@@ -44,7 +44,7 @@ export type ChatSnapshotV1<TUIMessage extends UIMessage = UIMessage> = {
  */
 export const ChatSnapshotV1Schema = z.object({
   version: z.literal(1),
-  savedAt: z.number(),
+  savedAt: z.number().optional(),
   messages: z.array(z.unknown()),
   lastOutEventId: z.string().optional(),
   lastInEventId: z.string().optional(),
@@ -81,7 +81,7 @@ export type TranscriptSnapshotV2<TUIMessage extends UIMessage = UIMessage> = {
 
 export const TranscriptSnapshotV2Schema = z.object({
   version: z.literal(2),
-  savedAt: z.number(),
+  savedAt: z.number().optional(),
   messages: z.array(
     z.object({
       id: z.string(),
@@ -102,6 +102,9 @@ export const TranscriptSnapshotV2Schema = z.object({
  * `message` are dropped; a version 2 entry whose `message.id` disagrees with
  * the envelope `id` is dropped too, since a reader keys by one and renders by
  * the other. A caller never sees an entry it would crash on or mis-order.
+ * A missing `savedAt` defaults to `0` rather than rejecting the whole blob:
+ * the field only orders snapshot history before live chunks, and dropping a
+ * whole conversation over an absent timestamp is the wrong failure mode.
  * Returns `undefined` for an unknown version or a body that is not a
  * snapshot; callers treat that as "no snapshot".
  */
@@ -119,7 +122,7 @@ export function parseTranscriptSnapshot<TUIMessage extends UIMessage = UIMessage
     }
     return {
       version: 2,
-      savedAt: v2.data.savedAt,
+      savedAt: v2.data.savedAt ?? 0,
       messages,
       state: v2.data.state ?? null,
       lastOutEventId: v2.data.lastOutEventId,
@@ -137,7 +140,7 @@ export function parseTranscriptSnapshot<TUIMessage extends UIMessage = UIMessage
     }
     return {
       version: 2,
-      savedAt: v1.data.savedAt,
+      savedAt: v1.data.savedAt ?? 0,
       messages,
       state: null,
       lastOutEventId: v1.data.lastOutEventId,

packages/core/src/v3/test/test-session-stream-manager.ts

@@ -220,7 +220,11 @@ export class TestSessionStreamManager implements SessionStreamManager {
   }
 
   setLastSeqNum(sessionId: string, io: SessionChannelIO, seqNum: number): void {
-    this.seqNums.set(keyFor(sessionId, io), seqNum);
+    const key = keyFor(sessionId, io);
+    const current = this.seqNums.get(key);
+    if (current === undefined || seqNum > current) {
+      this.seqNums.set(key, seqNum);
+    }
   }
 
   consumeRecord(sessionId: string, io: SessionChannelIO, seqNum: number): void {

packages/trigger-sdk/src/v3/test/mock-chat-agent.ts

@@ -335,6 +335,16 @@ function isControlChunk(chunk: unknown): boolean {
   return typeof type === "string" && CONTROL_CHUNK_TYPES.has(type);
 }
 
+/**
+ * Highest `session.in` seqNum any harness has produced for a session id,
+ * keyed by `sessionId`. Production `session.in` is a durable S2 stream whose
+ * seqNums are monotonic across the runs of a chat; a fresh in-memory manager
+ * per `mockChatAgent` would otherwise restart at 0, so a continuation's
+ * follow-up message would collide with the resume floor and be dropped. This
+ * survives the per-run manager reset so continuation runs stay monotonic.
+ */
+const durableSessionInSeq = new Map<string, number>();
+
 /**
  * Create an offline test harness for a `chat.agent` task.
  *
@@ -574,7 +584,21 @@ export function mockChatAgent(
       ...(options.headStartMessages ? { headStartMessages: options.headStartMessages } : {}),
     };
 
-    sendSessionInput = drivers.sessions.in.send;
+    const durableSeq = durableSessionInSeq.get(sessionId);
+    if (durableSeq !== undefined) {
+      sessionStreams.setLastSeqNum(sessionId, "in", durableSeq);
+    }
+    const rawSendSessionInput = drivers.sessions.in.send;
+    sendSessionInput = async (id, data, io, metadata) => {
+      await rawSendSessionInput(id, data, io, metadata);
+      const io2 = io ?? "in";
+      if (io2 === "in") {
+        const latest = sessionStreams.lastSeqNum(id, "in");
+        if (latest !== undefined) {
+          durableSessionInSeq.set(id, Math.max(durableSessionInSeq.get(id) ?? latest, latest));
+        }
+      }
+    };
     closeSessionInput = drivers.sessions.in.close;
 
     // Record every chunk written to session.out, detect turn-complete.