fix(server): ACP agents keep the kept conversation after Edit from here

apps/server/src/orchestration-v2/CommandPolicy.ts

@@ -312,9 +312,11 @@ const ensureNativeFork: CommandPolicyV2Shape["ensureNativeFork"] = (input) => {
 };
 
 const ensureRollback: CommandPolicyV2Shape["ensureRollback"] = (input) => {
+  // Without native truncation, the next turn replays the kept conversation as a handoff.
   if (
     !input.capabilities.threads.canRollbackThread ||
-    !input.capabilities.checkpointing.providerCanRollbackConversation
+    (!input.capabilities.checkpointing.providerCanRollbackConversation &&
+      !input.capabilities.context.canConsumeHandoffSummaries)
   ) {
     return Effect.fail(
       unsupported(input, "rollback", "providerInstanceId conversation rollback is unavailable"),

apps/server/src/orchestration-v2/ProviderTurnStartService.test.ts

@@ -759,19 +759,6 @@ function makeLocalCommandHarness(input: {
     ),
   );
   const fallbackEnsureThread = vi.fn(() => Effect.succeed(providerThread));
-  const resumeFallbackSession = {
-    driver: providerThread.driver,
-    resumeThread: () =>
-      Effect.fail(
-        input.resumeFailure ??
-          new ProviderAdapter.ProviderAdapterEventStreamError({
-            driver: providerThread.driver,
-            providerSessionId,
-            cause: "native thread is gone",
-          }),
-      ),
-    ensureThread: fallbackEnsureThread,
-  };
   const providerSession = {
     id: providerSessionId,
     driver: providerThread.driver,
@@ -790,6 +777,20 @@ function makeLocalCommandHarness(input: {
     updatedAt: now,
     lastError: null,
   };
+  const resumeFallbackSession = {
+    driver: providerThread.driver,
+    providerSession,
+    resumeThread: () =>
+      Effect.fail(
+        input.resumeFailure ??
+          new ProviderAdapter.ProviderAdapterEventStreamError({
+            driver: providerThread.driver,
+            providerSessionId,
+            cause: "native thread is gone",
+          }),
+      ),
+    ensureThread: fallbackEnsureThread,
+  };
   const startTurn = vi.fn((_turn: ProviderAdapter.ProviderAdapterV2TurnInput) => Effect.void);
   const sideChatThread = {
     ...providerThread,

apps/server/src/orchestration-v2/ProviderTurnStartService.ts

@@ -9,6 +9,7 @@ import {
   type OrchestrationV2ProviderThread,
   type OrchestrationV2Run,
   type OrchestrationV2RunAttempt,
+  type OrchestrationV2ThreadProjection,
   type OrchestrationV2TurnItem,
   isCompactionCommandMessage,
   parseGoalCommand,
@@ -115,6 +116,53 @@ const refusedBeforePrompt = (error: unknown): boolean =>
   PROMPT_REFUSAL_TAGS.some((tag) => Predicate.isTagged(error, tag)) ||
   (Predicate.hasProperty(error, "cause") && refusedBeforePrompt(error.cause));
 
+/**
+ * Whether a non-truncating provider's native session lost the conversation kept before `run`.
+ * Rollback resets it: the newest accepted turn was rolled back (works without legacy-missing
+ * native ids) or ran under another native id (rollback of runs the provider never accepted).
+ */
+const nativeSessionMissesKeptTurns = (
+  projection: Pick<
+    OrchestrationV2ThreadProjection,
+    "thread" | "runs" | "attempts" | "providerTurns" | "contextHandoffs"
+  >,
+  providerThread: OrchestrationV2ProviderThread,
+  run: OrchestrationV2Run,
+): boolean => {
+  const nativeId = providerThread.nativeThreadRef?.nativeId;
+  if (nativeId === undefined) return false;
+  const acceptedAttemptIds = new Set(projection.providerTurns.map((turn) => turn.runAttemptId));
+  const accepted = projection.attempts.filter((attempt) => acceptedAttemptIds.has(attempt.id));
+  const runsById = new Map(projection.runs.map((source) => [source.id, source]));
+  const ordinalOf = (attempt: OrchestrationV2RunAttempt) =>
+    runsById.get(attempt.runId)?.ordinal ?? 0;
+  const newest = accepted
+    .filter((attempt) => attempt.providerThreadId === providerThread.id)
+    .toSorted((a, b) => ordinalOf(a) - ordinalOf(b) || a.attemptOrdinal - b.attemptOrdinal)
+    .at(-1);
+  if (newest === undefined) return false;
+  // Inherited fork history is kept even when every local run was rolled back.
+  const keepsHistory =
+    (projection.thread.forkedFrom?.type === "run" && projection.thread.lineage.sideChat !== true) ||
+    accepted.some((attempt) => {
+      const source = runsById.get(attempt.runId);
+      return (
+        source !== undefined && source.ordinal < run.ordinal && source.status !== "rolled_back"
+      );
+    });
+  return (
+    keepsHistory &&
+    (runsById.get(newest.runId)?.status === "rolled_back" ||
+      (newest.nativeThreadId !== undefined && newest.nativeThreadId !== nativeId)) &&
+    !projection.contextHandoffs.some(
+      (handoff) =>
+        handoff.toProviderThreadId === providerThread.id &&
+        handoff.delivery?.nativeThreadId === nativeId &&
+        handoff.delivery.status !== "pending",
+    )
+  );
+};
+
 export interface ProviderTurnStartServiceV2Shape {
   readonly start: (input: {
     readonly threadId: ThreadId;
@@ -1089,13 +1137,22 @@ export const layer: Layer.Layer<
             coveredRunOrdinals: { from: 1, to: Math.max(1, run.ordinal - 1) },
             strategy: "full_thread_summary",
             runs: projection.runs,
-            items: (yield* projectionStore.getTurnStartHistory(input.threadId)).filter(
-              (item) =>
-                item.runId === null ||
-                projection.runs.some(
-                  (source) => source.id === item.runId && source.ordinal < run.ordinal,
-                ),
-            ),
+            items: [
+              ...(yield* ContextHandoffService.readInheritedHandoffItems(
+                projectionStore,
+                projection.thread,
+              )),
+              ...(yield* projectionStore.getTurnStartHistory(input.threadId)).filter(
+                (item) =>
+                  item.runId === null ||
+                  projection.runs.some(
+                    (source) =>
+                      source.id === item.runId &&
+                      source.ordinal < run.ordinal &&
+                      source.status !== "rolled_back",
+                  ),
+              ),
+            ],
             createdAt,
           });
           return { handoff, transferId, createdAt };
@@ -1260,15 +1317,21 @@ export const layer: Layer.Layer<
             handoff.delivery?.nativeThreadId === providerThread.nativeThreadRef?.nativeId &&
             handoff.delivery?.status === "pending",
         );
+        // A provider that cannot truncate rolls back into a fresh native session holding none of the kept turns.
+        const nativeSessionLacksKeptTurns =
+          !session.providerSession.capabilities.checkpointing.providerCanRollbackConversation &&
+          nativeSessionMissesKeptTurns(projection, providerThread, run);
         const resumed = yield* Effect.result(
-          uncertainDelivery
+          uncertainDelivery || nativeSessionLacksKeptTurns
             ? Effect.fail(
                 new ProviderAdapter.ProviderAdapterTurnStartError({
                   driver: session.driver,
                   threadId: projection.thread.id,
                   providerThreadId: providerThread.id,
                   runId,
-                  cause: "Uncertain native history injection",
+                  cause: uncertainDelivery
+                    ? "Uncertain native history injection"
+                    : "Native session holds none of the kept conversation",
                 }),
               )
             : session.resumeThread({
@@ -1348,9 +1411,11 @@ export const layer: Layer.Layer<
           runId,
           reason: uncertainDelivery
             ? "uncertain_history_delivery"
-            : resumed._tag === "Success"
-              ? "handoff_budget_exhausted"
-              : "resume_failed",
+            : nativeSessionLacksKeptTurns
+              ? "native_session_lacks_kept_turns"
+              : resumed._tag === "Success"
+                ? "handoff_budget_exhausted"
+                : "resume_failed",
           errorTag: resumed._tag === "Failure" ? resumed.failure._tag : "ContextHandoffBudgetError",
         });
         const replacement = yield* loadFromProvider(