diff --git a/AutoMark/scripts/mods/AutoMark/mark/servo_skull_mark.lua b/AutoMark/scripts/mods/AutoMark/mark/servo_skull_mark.lua
index 92fb617..719aa41 100644
--- a/AutoMark/scripts/mods/AutoMark/mark/servo_skull_mark.lua
+++ b/AutoMark/scripts/mods/AutoMark/mark/servo_skull_mark.lua
@@ -245,9 +245,9 @@ local function find_hackable_target_unit_omni()
                     end
                     parts[#parts + 1] = tostring(k) .. "=" .. vs
                 end
-                printf("[MOD][AutoMark][DIAG] %s", "OMNI-HACK-DIAG template: " .. table.concat(parts, " "))
+                mod:diag("OMNI-HACK-DIAG template: " .. table.concat(parts, " "))
             else
-                printf("[MOD][AutoMark][DIAG] %s", "OMNI-HACK-DIAG template missing: " .. tostring(tpl))
+                mod:diag("OMNI-HACK-DIAG template missing: " .. tostring(tpl))
             end
         else
             local keys = {}
@@ -255,7 +255,7 @@ local function find_hackable_target_unit_omni()
                 keys[#keys + 1] = tostring(k)
             end
             table.sort(keys)
-            printf("[MOD][AutoMark][DIAG] %s", "OMNI-HACK-DIAG no templates table, system keys: " .. table.concat(keys, ","))
+            mod:diag("OMNI-HACK-DIAG no templates table, system keys: " .. table.concat(keys, ","))
         end
     end
 
@@ -285,7 +285,7 @@ local function find_hackable_target_unit_omni()
     if diag_total then
         mod.omni_hack_diag_counter = (mod.omni_hack_diag_counter or 0) + 1
         if mod.omni_hack_diag_counter % 10 == 1 then
-            printf("[MOD][AutoMark][DIAG] %s", string.format("OMNI-HACK-DIAG breed_units=%d hackable=%d selected=%s dist=%s", diag_total, diag_hackable, tostring(selected_unit), tostring(selected_distance)))
+            mod:diag(string.format("OMNI-HACK-DIAG breed_units=%d hackable=%d selected=%s dist=%s", diag_total, diag_hackable, tostring(selected_unit), tostring(selected_distance)))
         end
     end
     return selected_unit, selected_distance
diff --git a/AutoMark/scripts/mods/AutoMark/targeting/custom_targeting.lua b/AutoMark/scripts/mods/AutoMark/targeting/custom_targeting.lua
index 30b7e1d..b46f93b 100644
--- a/AutoMark/scripts/mods/AutoMark/targeting/custom_targeting.lua
+++ b/AutoMark/scripts/mods/AutoMark/targeting/custom_targeting.lua
@@ -588,7 +588,7 @@ function mod:find_auto_mark_target_unit(min_range, max_range, tag_name, tag_cont
     local omni_mark = mod_settings.omni_mark
     if mod_settings.debug_mode and not mod.omni_diag_settings_logged then
         mod.omni_diag_settings_logged = true
-        printf("[MOD][AutoMark][DIAG] %s", string.format("OMNI-DIAG settings: omni_mark=%s ignore_los=%s debug=%s", tostring(omni_mark), tostring(mod_settings.omni_mark_ignore_los), tostring(mod_settings.debug_mode)))
+        mod:diag(string.format("OMNI-DIAG settings: omni_mark=%s ignore_los=%s debug=%s", tostring(omni_mark), tostring(mod_settings.omni_mark_ignore_los), tostring(mod_settings.debug_mode)))
     end
     local ray_origin, forward, right, up = smart_targeting_extension:_targeting_parameters()
     local hits, num_hits, hits_are_units
@@ -628,7 +628,7 @@ function mod:find_auto_mark_target_unit(min_range, max_range, tag_name, tag_cont
             for _, n in ipairs(ordered_names) do
                 name_parts[#name_parts + 1] = n .. "x" .. counts[n]
             end
-            printf("[MOD][AutoMark][DIAG] %s", string.format("OMNI-DIAG candidates=%d breeds=[%s]", num_hits, table.concat(name_parts, ",")))
+            mod:diag(string.format("OMNI-DIAG candidates=%d breeds=[%s]", num_hits, table.concat(name_parts, ",")))
         end
     else
         hits, num_hits = PhysicsWorld_raycast(smart_targeting_extension._physics_world, ray_origin, forward, max_range, "all", "collision_filter", COLLISION_FILTER)
@@ -849,7 +849,7 @@ function mod:find_auto_mark_target_unit(min_range, max_range, tag_name, tag_cont
             parts[#parts + 1] = k .. "=" .. v
         end
         table.sort(parts)
-        printf("[MOD][AutoMark][DIAG] %s", string.format("OMNI-DIAG pass[%s] best=%s rejects=[%s]", tostring(tag_name), tostring(best_unit_breed_name), table.concat(parts, ",")))
+        mod:diag(string.format("OMNI-DIAG pass[%s] best=%s rejects=[%s]", tostring(tag_name), tostring(best_unit_breed_name), table.concat(parts, ",")))
         omni_rejects = nil
     end
 
diff --git a/AutoMark/scripts/mods/AutoMark/utils/utils.lua b/AutoMark/scripts/mods/AutoMark/utils/utils.lua
index 35dfee4..511dfe3 100644
--- a/AutoMark/scripts/mods/AutoMark/utils/utils.lua
+++ b/AutoMark/scripts/mods/AutoMark/utils/utils.lua
@@ -91,3 +91,23 @@ local ok, err = pcall(mod.dump_breeds, mod)
 if not ok then
     mod:print_debug("breed dump error:", tostring(err))
 end
+
+-- debug-mode diagnostics: DMF mod:info also pushes on-screen notifications and the
+-- sandbox has no printf, so append to a plain text file instead (Mods.lua.io again).
+-- never call unguarded: mod:diag swallows its own failures.
+function mod:diag(...)
+    local args = { ... }
+    local ok_diag, err_diag = pcall(function()
+        local file = Mods.lua.io.open("./../mods/AutoMark/diag_log.txt", "a")
+        if file then
+            for i = 1, #args do
+                args[i] = tostring(args[i])
+            end
+            file:write(table.concat(args, " ") .. "\n")
+            file:close()
+        end
+    end)
+    if not ok_diag then
+        mod:print_debug("diag error:", tostring(err_diag))
+    end
+end