diff --git a/.changes/973-chat-message-time-defaults.md b/.changes/973-chat-message-time-defaults.md
new file mode 100644
index 00000000..1a6e5496
--- /dev/null
+++ b/.changes/973-chat-message-time-defaults.md
@@ -0,0 +1,5 @@
+---
+category: Fixed
+---
+
+- **Chat message time defaults** (#973) — validates `chat message list-all` time ranges locally and defaults omitted message-query time bounds without creating reversed windows.
diff --git a/internal/helpers/chat.go b/internal/helpers/chat.go
index a1fef36d..d9352996 100644
--- a/internal/helpers/chat.go
+++ b/internal/helpers/chat.go
@@ -471,6 +471,17 @@ func chatMessageListAllArgs(cmd *cobra.Command) (map[string]any, error) {
 	if err != nil {
 		return nil, err
 	}
+	startMs, err := parseISOTimeToMillis("start", startRaw)
+	if err != nil {
+		return nil, err
+	}
+	endMs, err := parseISOTimeToMillis("end", endRaw)
+	if err != nil {
+		return nil, err
+	}
+	if err := validateTimeRange(startMs, endMs); err != nil {
+		return nil, err
+	}
 	cursor, _ := cmd.Flags().GetString("cursor")
 	return map[string]any{
 		"startTime": startRaw,
diff --git a/internal/helpers/chat_message_pagination_test.go b/internal/helpers/chat_message_pagination_test.go
index 8911b5e1..221bffa3 100644
--- a/internal/helpers/chat_message_pagination_test.go
+++ b/internal/helpers/chat_message_pagination_test.go
@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"io"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -341,6 +342,41 @@ func TestChatMessagePaginationDefaultsEndFromNowWhenOnlyStartProvided(t *testing
 	}
 }
 
+func TestChatMessageListAllRejectsInvalidTimeRanges(t *testing.T) {
+	tests := []struct {
+		name    string
+		args    []string
+		wantErr string
+	}{
+		{
+			name:    "invalid start",
+			args:    []string{"message", "list-all", "--start", "not-a-time", "--end", "2020-01-01T00:00:00+08:00"},
+			wantErr: "cannot parse time for --start",
+		},
+		{
+			name:    "end before start",
+			args:    []string{"message", "list-all", "--start", "2021-01-01T00:00:00+08:00", "--end", "2020-01-01T00:00:00+08:00"},
+			wantErr: "--end must be after --start",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			caller := &chatMessagePaginationCaller{}
+			_, err := executeChatMessagePaginationCommand(t, caller, tt.args...)
+			if err == nil {
+				t.Fatal("command succeeded, want validation error")
+			}
+			if !strings.Contains(err.Error(), tt.wantErr) {
+				t.Fatalf("error = %v, want containing %q", err, tt.wantErr)
+			}
+			if len(caller.calls) != 0 {
+				t.Fatalf("calls = %#v, want no MCP call", caller.calls)
+			}
+		})
+	}
+}
+
 func numericArgAsInt64(t *testing.T, value any) int64 {
 	t.Helper()
 	switch v := value.(type) {