feat(session): add per-message created_at timestamps

- Persistence layer (jsonl.go addMsg/SetHistory) normalizes CreatedAt
  when missing so the invariant is guaranteed at the storage boundary
- API layer (session.go) exposes created_at on all transcript message
  types with session.updated fallback for legacy messages
- Frontend uses per-message timestamps when available
- messagesContentEqual ignores CreatedAt for tail-matching after
  JSONL roundtrip

Fixes #2787

pkg/agent/turn_state.go

@@ -511,10 +511,25 @@ func (ts *turnState) restoreSession(agent *AgentInstance) error {
 	return agent.Sessions.Save(ts.sessionKey)
 }
 
+// messagesContentEqual compares two message slices by content only, ignoring CreatedAt.
+// JSON roundtrip loses the monotonic clock portion of time.Time, so direct
+// reflect.DeepEqual would always differ on messages that roundtripped through
+// the JSONL store.
+func messagesContentEqual(a, b []providers.Message) bool {
+	for i := range a {
+		aCopy, bCopy := a[i], b[i]
+		aCopy.CreatedAt, bCopy.CreatedAt = nil, nil
+		if !reflect.DeepEqual(aCopy, bCopy) {
+			return false
+		}
+	}
+	return true
+}
+
 func matchingTurnMessageTail(history, persisted []providers.Message) int {
 	maxMatch := min(len(history), len(persisted))
 	for size := maxMatch; size > 0; size-- {
-		if reflect.DeepEqual(history[len(history)-size:], persisted[len(persisted)-size:]) {
+		if messagesContentEqual(history[len(history)-size:], persisted[len(persisted)-size:]) {
 			return size
 		}
 	}

pkg/memory/jsonl.go

@@ -561,6 +561,12 @@ func (s *JSONLStore) addMsg(sessionKey string, msg providers.Message) error {
 	l.Lock()
 	defer l.Unlock()
 
+	now := time.Now()
+
+	if msg.CreatedAt == nil {
+		msg.CreatedAt = &now
+	}
+
 	// Append the message as a single JSON line.
 	line, err := json.Marshal(msg)
 	if err != nil {
@@ -598,7 +604,6 @@ func (s *JSONLStore) addMsg(sessionKey string, msg providers.Message) error {
 	if err != nil {
 		return err
 	}
-	now := time.Now()
 	if meta.Count == 0 && meta.CreatedAt.IsZero() {
 		meta.CreatedAt = now
 	}
@@ -726,6 +731,12 @@ func (s *JSONLStore) SetHistory(
 	meta.Count = len(history)
 	meta.UpdatedAt = now
 
+	for i := range history {
+		if history[i].CreatedAt == nil {
+			history[i].CreatedAt = &now
+		}
+	}
+
 	// Write meta BEFORE rewriting the JSONL file. If we crash between
 	// the two writes, meta has Skip=0 and the old file is still intact,
 	// so GetHistory reads from line 1 — returning "too many" messages

pkg/memory/jsonl_test.go

@@ -1032,6 +1032,137 @@ func TestMultipleSessions_Isolation(t *testing.T) {
 	}
 }
 
+func TestStore_SetsCreatedAtWhenNil(t *testing.T) {
+	type writeOp struct {
+		name string
+		fn   func(store *JSONLStore, key string) (expectedCount int)
+	}
+
+	ops := []writeOp{
+		{
+			name: "AddMessage",
+			fn: func(store *JSONLStore, key string) int {
+				if err := store.AddMessage(context.Background(), key, "user", "hello"); err != nil {
+					t.Fatalf("AddMessage: %v", err)
+				}
+				return 1
+			},
+		},
+		{
+			name: "AddFullMessage",
+			fn: func(store *JSONLStore, key string) int {
+				if err := store.AddFullMessage(context.Background(), key, providers.Message{
+					Role:    "user",
+					Content: "hello from full",
+				}); err != nil {
+					t.Fatalf("AddFullMessage: %v", err)
+				}
+				return 1
+			},
+		},
+		{
+			name: "SetHistory",
+			fn: func(store *JSONLStore, key string) int {
+				if err := store.SetHistory(context.Background(), key, []providers.Message{
+					{Role: "user", Content: "msg1"},
+					{Role: "assistant", Content: "msg2"},
+				}); err != nil {
+					t.Fatalf("SetHistory: %v", err)
+				}
+				return 2
+			},
+		},
+	}
+
+	for _, op := range ops {
+		t.Run(op.name, func(t *testing.T) {
+			store := newTestStore(t)
+			key := "s1"
+
+			before := time.Now().Add(-time.Second)
+			expectedCount := op.fn(store, key)
+			after := time.Now().Add(time.Second)
+
+			history, err := store.GetHistory(context.Background(), key)
+			if err != nil {
+				t.Fatalf("GetHistory: %v", err)
+			}
+			if len(history) != expectedCount {
+				t.Fatalf("expected %d messages, got %d", expectedCount, len(history))
+			}
+			for i := range history {
+				if history[i].CreatedAt == nil || history[i].CreatedAt.IsZero() {
+					t.Errorf("message %d CreatedAt is zero — not set by %s", i, op.name)
+				}
+				if history[i].CreatedAt.Before(before) || history[i].CreatedAt.After(after) {
+					t.Errorf("message %d CreatedAt %v outside expected window [%v, %v]", i, history[i].CreatedAt, before, after)
+				}
+			}
+		})
+	}
+}
+
+func TestStore_PreservesExistingCreatedAt(t *testing.T) {
+	t1 := time.Date(2026, 1, 1, 10, 0, 0, 0, time.UTC)
+	t2 := time.Date(2026, 1, 1, 11, 0, 0, 0, time.UTC)
+
+	type writeOp struct {
+		name      string
+		fn        func(store *JSONLStore, key string)
+		wantTimes []time.Time
+	}
+
+	ops := []writeOp{
+		{
+			name: "AddFullMessage",
+			fn: func(store *JSONLStore, key string) {
+				if err := store.AddFullMessage(context.Background(), key, providers.Message{
+					Role:      "user",
+					Content:   "custom time",
+					CreatedAt: &t1,
+				}); err != nil {
+					t.Fatalf("AddFullMessage: %v", err)
+				}
+			},
+			wantTimes: []time.Time{t1},
+		},
+		{
+			name: "SetHistory",
+			fn: func(store *JSONLStore, key string) {
+				if err := store.SetHistory(context.Background(), key, []providers.Message{
+					{Role: "user", Content: "msg1", CreatedAt: &t1},
+					{Role: "assistant", Content: "msg2", CreatedAt: &t2},
+				}); err != nil {
+					t.Fatalf("SetHistory: %v", err)
+				}
+			},
+			wantTimes: []time.Time{t1, t2},
+		},
+	}
+
+	for _, op := range ops {
+		t.Run(op.name, func(t *testing.T) {
+			store := newTestStore(t)
+			key := "s1"
+
+			op.fn(store, key)
+
+			history, err := store.GetHistory(context.Background(), key)
+			if err != nil {
+				t.Fatalf("GetHistory: %v", err)
+			}
+			if len(history) != len(op.wantTimes) {
+				t.Fatalf("expected %d messages, got %d", len(op.wantTimes), len(history))
+			}
+			for i, want := range op.wantTimes {
+				if history[i].CreatedAt == nil || !history[i].CreatedAt.Equal(want) {
+					t.Errorf("message %d CreatedAt = %v, want %v (should preserve caller-provided time)", i, history[i].CreatedAt, want)
+				}
+			}
+		})
+	}
+}
+
 func BenchmarkAddMessage(b *testing.B) {
 	dir := b.TempDir()
 	store, err := NewJSONLStore(dir)

pkg/providers/protocoltypes/types.go

@@ -1,5 +1,7 @@
 package protocoltypes
 
+import "time"
+
 type ToolCall struct {
 	ID               string         `json:"id"`
 	Type             string         `json:"type,omitempty"`
@@ -81,6 +83,7 @@ type Attachment struct {
 type Message struct {
 	Role             string         `json:"role"`
 	Content          string         `json:"content"`
+	CreatedAt        *time.Time     `json:"created_at,omitempty"`
 	Media            []string       `json:"media,omitempty"`
 	Attachments      []Attachment   `json:"attachments,omitempty"`
 	ReasoningContent string         `json:"reasoning_content,omitempty"`

web/backend/api/session.go

@@ -50,6 +50,7 @@ type sessionChatMessage struct {
 	Role        string                  `json:"role"`
 	Content     string                  `json:"content"`
 	Kind        string                  `json:"kind,omitempty"`
+	CreatedAt   *time.Time              `json:"created_at,omitempty"`
 	Media       []string                `json:"media,omitempty"`
 	Attachments []sessionChatAttachment `json:"attachments,omitempty"`
 	ToolCalls   []utils.VisibleToolCall `json:"tool_calls,omitempty"`
@@ -510,6 +511,7 @@ func sessionTranscriptMessages(
 			chatMsg := sessionChatMessage{
 				Role:        "user",
 				Content:     msg.Content,
+				CreatedAt:   msg.CreatedAt,
 				Media:       append([]string(nil), msg.Media...),
 				Attachments: attachments,
 			}
@@ -530,8 +532,9 @@ func sessionTranscriptMessages(
 			toolCallsMsg, hasToolCallsMsg := assistantToolCallsMessage(
 				msg.ToolCalls,
 				toolFeedbackMaxArgsLength,
+				msg.CreatedAt,
 			)
-			visibleToolMessages := visibleAssistantToolMessages(msg.ToolCalls)
+			visibleToolMessages := visibleAssistantToolMessages(msg.ToolCalls, msg.CreatedAt)
 
 			// Pico web chat can persist both visible `message` tool output and a
 			// later plain assistant reply in the same turn. Hide only the fixed
@@ -556,6 +559,7 @@ func sessionTranscriptMessages(
 			chatMsg := sessionChatMessage{
 				Role:        "assistant",
 				Content:     content,
+				CreatedAt:   msg.CreatedAt,
 				Media:       append([]string(nil), msg.Media...),
 				Attachments: attachments,
 			}
@@ -685,12 +689,14 @@ func assistantThoughtMessage(msg providers.Message) (sessionChatMessage, bool) {
 		Role:      "assistant",
 		Content:   reasoning,
 		Kind:      "thought",
+		CreatedAt: msg.CreatedAt,
 	}, true
 }
 
 func assistantToolCallsMessage(
 	toolCalls []providers.ToolCall,
 	toolFeedbackMaxArgsLength int,
+	createdAt *time.Time,
 ) (sessionChatMessage, bool) {
 	if len(toolCalls) == 0 {
 		return sessionChatMessage{}, false
@@ -707,6 +713,7 @@ func assistantToolCallsMessage(
 	return sessionChatMessage{
 		Role:      "assistant",
 		Kind:      "tool_calls",
+		CreatedAt: createdAt,
 		ToolCalls: visibleToolCalls,
 	}, true
 }
@@ -718,7 +725,7 @@ func visibleAssistantToolArgsPreview(
 	return utils.VisibleToolCallArgumentsPreview(tc, toolFeedbackMaxArgsLength)
 }
 
-func visibleAssistantToolMessages(toolCalls []providers.ToolCall) []sessionChatMessage {
+func visibleAssistantToolMessages(toolCalls []providers.ToolCall, createdAt *time.Time) []sessionChatMessage {
 	if len(toolCalls) == 0 {
 		return nil
 	}
@@ -736,6 +743,7 @@ func visibleAssistantToolMessages(toolCalls []providers.ToolCall) []sessionChatM
 		messages = append(messages, sessionChatMessage{
 			Role:      "assistant",
 			Content:   content,
+			CreatedAt: createdAt,
 		})
 	}
 
@@ -918,6 +926,11 @@ func (h *Handler) handleGetSession(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	for i := range sess.Messages {
+		if sess.Messages[i].CreatedAt == nil {
+			sess.Messages[i].CreatedAt = &sess.Updated
+		}
+	}
 	messages := detailSessionMessages(sess.Messages, toolFeedbackMaxArgsLength)
 
 	w.Header().Set("Content-Type", "application/json")

web/frontend/src/api/sessions.ts

@@ -14,6 +14,7 @@ export interface SessionDetail {
   messages: {
     role: "user" | "assistant"
     content: string
+    created_at?: string
     kind?: "normal" | "thought" | "tool_calls"
     media?: string[]
     attachments?: {

web/frontend/src/components/chat/assistant-message.tsx

@@ -132,6 +132,10 @@ export function AssistantMessage({
                 )}
                 <span>{collapsedLabel}</span>
               </div>
+              <div className="flex items-center gap-2">
+                {formattedTimestamp && (
+                  <span className="opacity-50">{formattedTimestamp}</span>
+                )}
                 <IconChevronDown
                   className={cn(
                     "size-3.5 opacity-0 transition-all duration-200 group-hover:opacity-100",
@@ -139,6 +143,7 @@ export function AssistantMessage({
                   )}
                 />
               </div>
+            </div>
           )}
           {(!isCollapsedBlock || isExpanded) && isToolCalls && hasToolCalls && (
             <div className="space-y-3 px-3 pt-0 pb-3">

web/frontend/src/components/chat/chat-page.tsx

@@ -346,6 +346,7 @@ export function ChatPage() {
                   <UserMessage
                     content={msg.content}
                     attachments={msg.attachments}
+                    timestamp={msg.timestamp}
                   />
                 )}
               </div>

web/frontend/src/components/chat/user-message.tsx

@@ -1,17 +1,25 @@
+import { formatMessageTime } from "@/hooks/use-pico-chat"
 import { cn } from "@/lib/utils"
 import type { ChatAttachment } from "@/store/chat"
 
 interface UserMessageProps {
   content: string
   attachments?: ChatAttachment[]
+  timestamp?: string | number
 }
 
-export function UserMessage({ content, attachments = [] }: UserMessageProps) {
+export function UserMessage({
+  content,
+  attachments = [],
+  timestamp = "",
+}: UserMessageProps) {
   const hasText = content.trim().length > 0
   const isCommand = content.trim().startsWith("/")
   const imageAttachments = attachments.filter(
     (attachment) => attachment.type === "image",
   )
+  const formattedTimestamp =
+    timestamp !== "" ? formatMessageTime(timestamp) : ""
 
   return (
     <div className="flex w-full flex-col items-end gap-1.5">
@@ -49,6 +57,10 @@ export function UserMessage({ content, attachments = [] }: UserMessageProps) {
           )}
         </div>
       )}
+
+      {formattedTimestamp && (
+        <span className="px-1 text-[12px] text-zinc-400">{formattedTimestamp}</span>
+      )}
     </div>
   )
 }

web/frontend/src/features/chat/history.ts

@@ -43,8 +43,6 @@ export async function loadSessionMessages(
   sessionId: string,
 ): Promise<ChatMessage[]> {
   const detail = await getSessionHistory(sessionId)
-  const fallbackTime = detail.updated
-
   return detail.messages.map((message, index) => ({
     id: `hist-${index}-${Date.now()}`,
     role: message.role,
@@ -58,7 +56,7 @@ export async function loadSessionMessages(
       media: message.media,
       attachments: message.attachments,
     }),
-    timestamp: fallbackTime,
+    timestamp: message.created_at ?? detail.updated,
   }))
 }