diff --git a/pkg/runtime/compactor/compactor.go b/pkg/runtime/compactor/compactor.go
index e4addce95..cd0de6d53 100644
--- a/pkg/runtime/compactor/compactor.go
+++ b/pkg/runtime/compactor/compactor.go
@@ -153,26 +153,93 @@ func RunLLM(ctx context.Context, args LLMArgs) (*Result, error) {
 // [maxKeepTokens] window. Used by the runtime when a hook supplies
 // its own summary so the kept-tail policy stays consistent across
 // the two strategies.
-func ComputeFirstKeptEntry(sess *session.Session, a *agent.Agent) int {
-	return mapToSessionIndex(sess, compaction.SplitIndexForKeep(nonSystemMessages(sess, a), maxKeepTokens))
+func ComputeFirstKeptEntry(sess *session.Session, _ *agent.Agent) int {
+	refs := nonSystemMessageRefs(sess)
+	messages := messagesFromRefs(refs)
+	splitIdx := compaction.SplitIndexForKeep(messages, maxKeepTokens)
+	splitIdx = skipSyntheticKeepBoundary(refs, splitIdx)
+	return firstKeptEntryFromRefs(refs, splitIdx)
+}
+
+const syntheticSessionIndex = -1
+
+type messageRef struct {
+	message      chat.Message
+	sessionIndex int
 }
 
-// nonSystemMessages returns the agent-visible messages in sess with
-// the system entries filtered out. Both the LLM strategy (via
-// [extractMessages]) and the hook-supplied path (via
-// [ComputeFirstKeptEntry]) operate on this same shape, which is also
-// what [compaction.SplitIndexForKeep] expects.
-func nonSystemMessages(sess *session.Session, a *agent.Agent) []chat.Message {
-	var messages []chat.Message
-	for _, msg := range sess.GetMessages(a) {
-		if msg.Role == chat.MessageRoleSystem {
+// nonSystemMessageRefs reconstructs the compactable conversation as the latest
+// summary (if any) followed by the messages visible after that summary, while
+// retaining provenance back to sess.Messages for real messages. The provenance
+// is critical after a prior compaction: the previous summary is synthetic, so
+// indexes in the reconstructed message list no longer line up with raw session
+// indexes.
+func nonSystemMessageRefs(sess *session.Session) []messageRef {
+	items := sess.SnapshotItems()
+
+	lastSummaryIndex := -1
+	for i := len(items) - 1; i >= 0; i-- {
+		if items[i].Summary != "" {
+			lastSummaryIndex = i
+			break
+		}
+	}
+
+	var refs []messageRef
+	if lastSummaryIndex >= 0 {
+		refs = append(refs, messageRef{
+			message: chat.Message{
+				Role:      chat.MessageRoleUser,
+				Content:   "Session Summary: " + items[lastSummaryIndex].Summary,
+				CreatedAt: time.Now().Format(time.RFC3339),
+			},
+			sessionIndex: syntheticSessionIndex,
+		})
+	}
+
+	startIndex := lastSummaryIndex + 1
+	if lastSummaryIndex >= 0 {
+		kept := items[lastSummaryIndex].FirstKeptEntry
+		if kept >= 0 && kept < lastSummaryIndex {
+			startIndex = kept
+		}
+	}
+
+	for i := startIndex; i < len(items); i++ {
+		item := items[i]
+		if !item.IsMessage() || item.Message.Message.Role == chat.MessageRoleSystem {
 			continue
 		}
-		messages = append(messages, msg)
+		refs = append(refs, messageRef{message: item.Message.Message, sessionIndex: i})
+	}
+
+	return refs
+}
+
+func messagesFromRefs(refs []messageRef) []chat.Message {
+	messages := make([]chat.Message, len(refs))
+	for i, ref := range refs {
+		messages[i] = ref.message
 	}
 	return messages
 }
 
+func skipSyntheticKeepBoundary(refs []messageRef, splitIdx int) int {
+	for splitIdx < len(refs) && refs[splitIdx].sessionIndex == syntheticSessionIndex {
+		splitIdx++
+	}
+	return splitIdx
+}
+
+func firstKeptEntryFromRefs(refs []messageRef, splitIdx int) int {
+	for i := splitIdx; i < len(refs); i++ {
+		if refs[i].sessionIndex >= 0 {
+			return refs[i].sessionIndex
+		}
+	}
+	return -1
+}
+
 // extractMessages returns the messages to send to the compaction
 // model, plus the index (into sess.Messages) of the first message
 // that is kept verbatim after compaction. The caller is responsible
@@ -188,8 +255,9 @@ func nonSystemMessages(sess *session.Session, a *agent.Agent) []chat.Message {
 // If the conversation tail itself doesn't fit in
 // (contextLimit − MaxSummaryTokens − prompt-overhead), older messages
 // are dropped from the front of the to-compact list to make room.
-func extractMessages(sess *session.Session, a *agent.Agent, contextLimit int64, additionalPrompt string) ([]chat.Message, int) {
-	messages := nonSystemMessages(sess, a)
+func extractMessages(sess *session.Session, _ *agent.Agent, contextLimit int64, additionalPrompt string) ([]chat.Message, int) {
+	refs := nonSystemMessageRefs(sess)
+	messages := messagesFromRefs(refs)
 	// Clear Cost and CacheControl on our local copy of the conversation.
 	// Cost is per-message bookkeeping that's already accumulated into
 	// sess.TotalCost(); leaving it set would double-count when the
@@ -204,7 +272,8 @@ func extractMessages(sess *session.Session, a *agent.Agent, contextLimit int64,
 	}
 
 	splitIdx := compaction.SplitIndexForKeep(messages, maxKeepTokens)
-	firstKeptEntry := mapToSessionIndex(sess, splitIdx)
+	splitIdx = skipSyntheticKeepBoundary(refs, splitIdx)
+	firstKeptEntry := firstKeptEntryFromRefs(refs, splitIdx)
 	messages = messages[:splitIdx]
 
 	systemPromptMessage := chat.Message{
diff --git a/pkg/runtime/compactor/compactor_test.go b/pkg/runtime/compactor/compactor_test.go
index 301dfaa0f..48e36ca83 100644
--- a/pkg/runtime/compactor/compactor_test.go
+++ b/pkg/runtime/compactor/compactor_test.go
@@ -165,10 +165,10 @@ func TestComputeFirstKeptEntry(t *testing.T) {
 
 	a := agent.New("test", "")
 
-	t.Run("empty session returns 0", func(t *testing.T) {
+	t.Run("empty session returns sentinel", func(t *testing.T) {
 		t.Parallel()
 		sess := session.New()
-		assert.Equal(t, 0, ComputeFirstKeptEntry(sess, a))
+		assert.Equal(t, -1, ComputeFirstKeptEntry(sess, a))
 	})
 
 	t.Run("short conversation: split at end (compact everything)", func(t *testing.T) {
@@ -178,10 +178,62 @@ func TestComputeFirstKeptEntry(t *testing.T) {
 			session.NewMessageItem(&session.Message{Message: chat.Message{Role: chat.MessageRoleUser, Content: "hi"}}),
 			session.NewMessageItem(&session.Message{Message: chat.Message{Role: chat.MessageRoleAssistant, Content: "hello"}}),
 		}))
-		assert.Equal(t, len(sess.Messages), ComputeFirstKeptEntry(sess, a))
+		assert.Equal(t, -1, ComputeFirstKeptEntry(sess, a))
 	})
 }
 
+func TestComputeFirstKeptEntry_AfterPriorCompactionAllowsZeroKeptEntry(t *testing.T) {
+	t.Parallel()
+
+	var items []session.Item
+	for i := range 4 {
+		items = append(items, session.NewMessageItem(&session.Message{
+			Message: chat.Message{
+				Role:    chat.MessageRoleUser,
+				Content: strings.Repeat(string(rune('a'+i)), 40000), // ~10k tokens each
+			},
+		}))
+	}
+	items = append(items, session.Item{
+		Summary:        "previous summary",
+		FirstKeptEntry: 0,
+	})
+
+	sess := session.New(session.WithMessages(items))
+	a := agent.New("test", "")
+
+	assert.Equal(t, 3, ComputeFirstKeptEntry(sess, a))
+}
+
+func TestComputeFirstKeptEntry_AfterPriorCompactionUsesRawSessionIndexes(t *testing.T) {
+	t.Parallel()
+
+	var items []session.Item
+	for i := range 10 {
+		items = append(items, session.NewMessageItem(&session.Message{
+			Message: chat.Message{
+				Role:    chat.MessageRoleUser,
+				Content: strings.Repeat(string(rune('a'+i)), 40000), // ~10k tokens each
+			},
+		}))
+	}
+	items = append(items, session.Item{
+		Summary:        "previous summary",
+		FirstKeptEntry: 8,
+	})
+
+	sess := session.New(session.WithMessages(items))
+	a := agent.New("test", "")
+
+	// The reconstructed conversation seen by compaction is:
+	//   [synthetic previous summary, raw message 8, raw message 9]
+	// With ~10k-token raw messages and a 20k keep window, the kept suffix
+	// starts at reconstructed index 2, which must map back to raw session
+	// index 9. A simple filtered-index-to-raw-index mapping would return 2
+	// and resurrect old messages that were already covered by the summary.
+	assert.Equal(t, 9, ComputeFirstKeptEntry(sess, a))
+}
+
 func TestMapToSessionIndex(t *testing.T) {
 	t.Parallel()
 
diff --git a/pkg/runtime/session_compaction_test.go b/pkg/runtime/session_compaction_test.go
index a1c067b40..2e72676cc 100644
--- a/pkg/runtime/session_compaction_test.go
+++ b/pkg/runtime/session_compaction_test.go
@@ -74,7 +74,7 @@ func TestSessionGetMessages_SummaryWithoutFirstKeptEntry(t *testing.T) {
 		session.NewMessageItem(&session.Message{
 			Message: chat.Message{Role: chat.MessageRoleAssistant, Content: "m2"},
 		}),
-		{Summary: "This is a summary"},
+		{Summary: "This is a summary", FirstKeptEntry: -1},
 		session.NewMessageItem(&session.Message{
 			Message: chat.Message{Role: chat.MessageRoleUser, Content: "m3"},
 		}),
diff --git a/pkg/session/branch.go b/pkg/session/branch.go
index a7442a1ec..bfc60c51a 100644
--- a/pkg/session/branch.go
+++ b/pkg/session/branch.go
@@ -52,7 +52,7 @@ func cloneSessionItem(item Item) (Item, error) {
 		}
 		return Item{SubSession: clonedSub}, nil
 	case item.Summary != "":
-		return Item{Summary: item.Summary, Cost: item.Cost}, nil
+		return Item{Summary: item.Summary, FirstKeptEntry: item.FirstKeptEntry, Cost: item.Cost}, nil
 	default:
 		return Item{}, errors.New("cannot clone empty session item")
 	}
diff --git a/pkg/session/branch_test.go b/pkg/session/branch_test.go
index 33e9384af..9e2e036a0 100644
--- a/pkg/session/branch_test.go
+++ b/pkg/session/branch_test.go
@@ -85,10 +85,11 @@ func TestCloneSessionItem(t *testing.T) {
 	})
 
 	t.Run("summary item clones successfully", func(t *testing.T) {
-		item := Item{Summary: "test summary"}
+		item := Item{Summary: "test summary", FirstKeptEntry: 2}
 		cloned, err := cloneSessionItem(item)
 		require.NoError(t, err)
 		assert.Equal(t, "test summary", cloned.Summary)
+		assert.Equal(t, 2, cloned.FirstKeptEntry)
 	})
 }
 
diff --git a/pkg/session/migrations.go b/pkg/session/migrations.go
index df5eeb8fd..8881d3a8e 100644
--- a/pkg/session/migrations.go
+++ b/pkg/session/migrations.go
@@ -398,7 +398,7 @@ func getAllMigrations() []Migration {
 			ID:          21,
 			Name:        "021_add_first_kept_entry_column",
 			Description: "Add first_kept_entry column to session_items for compaction-preserved messages",
-			UpSQL:       `ALTER TABLE session_items ADD COLUMN first_kept_entry INTEGER DEFAULT 0`,
+			UpSQL:       `ALTER TABLE session_items ADD COLUMN first_kept_entry INTEGER DEFAULT -1`,
 		},
 	}
 }
diff --git a/pkg/session/session.go b/pkg/session/session.go
index cd2585cbf..621c60cfd 100644
--- a/pkg/session/session.go
+++ b/pkg/session/session.go
@@ -354,6 +354,14 @@ func (s *Session) snapshotItems() []Item {
 	return items
 }
 
+// SnapshotItems returns a copy of s.Messages safe to use without holding
+// s.mu. Each Message value is deep-copied so concurrent UpdateMessage calls
+// cannot mutate the snapshot; non-Message fields (Summary, SubSession, Cost,
+// FirstKeptEntry) are shallow-copied since they are not mutated in place.
+func (s *Session) SnapshotItems() []Item {
+	return s.snapshotItems()
+}
+
 // cloneChatMessage returns a deep copy of a chat.Message, duplicating
 // all slice and pointer fields that would otherwise alias the original.
 func cloneChatMessage(m chat.Message) chat.Message {
@@ -902,7 +910,7 @@ func buildSessionSummaryMessages(items []Item) ([]chat.Message, int) {
 	startIndex := lastSummaryIndex + 1
 	if lastSummaryIndex >= 0 {
 		kept := items[lastSummaryIndex].FirstKeptEntry
-		if kept > 0 && kept < lastSummaryIndex {
+		if kept >= 0 && kept < lastSummaryIndex {
 			startIndex = kept
 		}
 	}
diff --git a/pkg/session/session_test.go b/pkg/session/session_test.go
index 9212598e9..3029f6684 100644
--- a/pkg/session/session_test.go
+++ b/pkg/session/session_test.go
@@ -128,7 +128,7 @@ func TestGetMessagesWithSummary(t *testing.T) {
 		Content: "first response",
 	}))
 
-	s.Messages = append(s.Messages, Item{Summary: "This is a summary of the conversation so far"})
+	s.Messages = append(s.Messages, Item{Summary: "This is a summary of the conversation so far", FirstKeptEntry: -1})
 
 	s.AddMessage(NewAgentMessage("", &chat.Message{
 		Role:    chat.MessageRoleUser,
@@ -161,6 +161,32 @@ func TestGetMessagesWithSummary(t *testing.T) {
 	assert.Equal(t, 3, userAssistantMessages, "should only include messages after summary")
 }
 
+func TestGetMessagesWithSummaryAndZeroFirstKeptEntry(t *testing.T) {
+	testAgent := &agent.Agent{}
+
+	s := New(WithMessages([]Item{
+		NewMessageItem(&Message{Message: chat.Message{Role: chat.MessageRoleUser, Content: "m1"}}),
+		NewMessageItem(&Message{Message: chat.Message{Role: chat.MessageRoleAssistant, Content: "m2"}}),
+		NewMessageItem(&Message{Message: chat.Message{Role: chat.MessageRoleUser, Content: "m3"}}),
+		{Summary: "summary", FirstKeptEntry: 0},
+	}))
+
+	messages := s.GetMessages(testAgent)
+
+	var conversationMessages []chat.Message
+	for _, msg := range messages {
+		if msg.Role != chat.MessageRoleSystem {
+			conversationMessages = append(conversationMessages, msg)
+		}
+	}
+
+	require.Len(t, conversationMessages, 4)
+	assert.Equal(t, "Session Summary: summary", conversationMessages[0].Content)
+	assert.Equal(t, "m1", conversationMessages[1].Content)
+	assert.Equal(t, "m2", conversationMessages[2].Content)
+	assert.Equal(t, "m3", conversationMessages[3].Content)
+}
+
 func TestGetMessages_Instructions(t *testing.T) {
 	testAgent := agent.New("root", "instructions")
 
@@ -201,7 +227,7 @@ func TestGetMessages_CacheControlWithSummary(t *testing.T) {
 	)
 
 	s := New()
-	s.Messages = append(s.Messages, Item{Summary: "Test summary"})
+	s.Messages = append(s.Messages, Item{Summary: "Test summary", FirstKeptEntry: -1})
 
 	extra := chat.Message{
 		Role:    chat.MessageRoleSystem,
diff --git a/pkg/session/store.go b/pkg/session/store.go
index 7dedd5a39..73a2fd756 100644
--- a/pkg/session/store.go
+++ b/pkg/session/store.go
@@ -689,7 +689,7 @@ type sessionItemRow struct {
 // loadSession when resolving sub-sessions inside a transaction.
 func (s *SQLiteSessionStore) loadSessionItems(ctx context.Context, q querier, sessionID string) ([]Item, error) {
 	rows, err := q.QueryContext(ctx,
-		`SELECT position, item_type, agent_name, message_json, implicit, subsession_id, summary_text, COALESCE(first_kept_entry, 0)
+		`SELECT position, item_type, agent_name, message_json, implicit, subsession_id, summary_text, COALESCE(first_kept_entry, -1)
 		 FROM session_items WHERE session_id = ? ORDER BY position`, sessionID)
 	if err != nil {
 		return nil, err