diff --git a/Core/ClassicAPISelfTest.lua b/Core/ClassicAPISelfTest.lua
new file mode 100644
index 0000000..ffa3819
--- /dev/null
+++ b/Core/ClassicAPISelfTest.lua
@@ -0,0 +1,173 @@
+-- Guda ClassicAPI runtime self-test
+-- Read-only diagnostics for the test branch. Run /gudatest in game.
+-- Lua 5.0 compatible.
+
+local addon = Guda
+
+local SelfTest = {}
+addon.Modules.ClassicAPISelfTest = SelfTest
+
+local function Bool(v)
+    return v and "yes" or "no"
+end
+
+local function ExtractItemID(link)
+    if not link then return nil end
+    local _, _, id = string.find(link, "item:(%d+)")
+    return id and tonumber(id) or nil
+end
+
+local function AddMismatch(state, text)
+    state.mismatches = state.mismatches + 1
+    if state.mismatches <= 12 then
+        addon:Print("|cffff6060Mismatch:|r %s", text)
+    end
+end
+
+local function CompareContainer(state, bagID, label)
+    local api = addon.Modules.ClassicAPI
+    local numSlots = GetContainerNumSlots(bagID) or 0
+
+    for slotID = 1, numSlots do
+        local texture, count, locked, quality, readable =
+            GetContainerItemInfo(bagID, slotID)
+        local link = GetContainerItemLink(bagID, slotID)
+        local vanillaOccupied = texture ~= nil or link ~= nil
+
+        local ok, info = pcall(api.GetContainerItemInfo, api, bagID, slotID)
+        if not ok then
+            AddMismatch(state, format("%s %d:%d ClassicAPI call errored", label, bagID, slotID))
+        else
+            local classicOccupied = info ~= nil
+            state.checked = state.checked + 1
+
+            if vanillaOccupied ~= classicOccupied then
+                AddMismatch(state, format("%s %d:%d occupancy Vanilla=%s ClassicAPI=%s",
+                    label, bagID, slotID, Bool(vanillaOccupied), Bool(classicOccupied)))
+            elseif vanillaOccupied and info then
+                local vanillaID = ExtractItemID(link)
+                local classicID = info.itemID and tonumber(info.itemID) or nil
+
+                if vanillaID and classicID and vanillaID ~= classicID then
+                    AddMismatch(state, format("%s %d:%d itemID %s ~= %s",
+                        label, bagID, slotID, tostring(vanillaID), tostring(classicID)))
+                end
+
+                if (count or 1) ~= (info.stackCount or 1) then
+                    AddMismatch(state, format("%s %d:%d count %s ~= %s",
+                        label, bagID, slotID, tostring(count), tostring(info.stackCount)))
+                end
+
+                if (locked and true or false) ~= (info.isLocked and true or false) then
+                    AddMismatch(state, format("%s %d:%d locked state differs",
+                        label, bagID, slotID))
+                end
+
+                if link and info.hyperlink and link ~= info.hyperlink then
+                    AddMismatch(state, format("%s %d:%d hyperlink differs",
+                        label, bagID, slotID))
+                end
+
+                -- ClassicAPI deliberately leaves cache-dependent fields nil
+                -- when static item data is cold, so compare only when both exist.
+                if texture and info.iconFileID and texture ~= info.iconFileID then
+                    AddMismatch(state, format("%s %d:%d texture differs",
+                        label, bagID, slotID))
+                end
+                if quality ~= nil and info.quality ~= nil and quality ~= info.quality then
+                    AddMismatch(state, format("%s %d:%d quality %s ~= %s",
+                        label, bagID, slotID, tostring(quality), tostring(info.quality)))
+                end
+                if readable ~= nil and info.isReadable ~= nil
+                   and (readable and true or false) ~= (info.isReadable and true or false) then
+                    AddMismatch(state, format("%s %d:%d readable state differs",
+                        label, bagID, slotID))
+                end
+            end
+        end
+    end
+end
+
+function SelfTest:Run()
+    local api = addon.Modules.ClassicAPI
+    addon:Print("|cffffd200=== Guda ClassicAPI self-test ===|r")
+
+    if not api then
+        addon:Print("|cffff6060FAIL: ClassicAPI integration module is missing.|r")
+        return false
+    end
+
+    local status = api:GetStatus()
+    addon:Print("ClassicAPI fast path: %s", status.available and "|cff60ff60ACTIVE|r" or "|cffff6060MISSING|r")
+    addon:Print("C_Container info=%s itemID=%s charges=%s",
+        Bool(status.containerInfo), Bool(status.containerItemID), Bool(status.containerCharges))
+    addon:Print("Frame helpers: EnumerateFrames=%s GetFramesRegisteredForEvent=%s",
+        Bool(type(EnumerateFrames) == "function"),
+        Bool(type(GetFramesRegisteredForEvent) == "function"))
+
+    if not status.available then
+        addon:Print("|cffff6060FAIL: required C_Container fast-path functions are unavailable.|r")
+        return false
+    end
+
+    local state = { checked = 0, mismatches = 0 }
+
+    for bagID = 0, 4 do
+        CompareContainer(state, bagID, "bag")
+    end
+
+    -- Keyring exists on the target 1.12/Turtle client. pcall is inside the
+    -- comparator, so a ClassicAPI incompatibility is reported instead of
+    -- aborting the test.
+    if (GetContainerNumSlots(-2) or 0) > 0 then
+        CompareContainer(state, -2, "keyring")
+    end
+
+    if addon.Modules.BankScanner and addon.Modules.BankScanner.IsBankOpen
+       and addon.Modules.BankScanner:IsBankOpen() then
+        for _, bagID in ipairs(addon.Constants.BANK_BAGS or {}) do
+            CompareContainer(state, bagID, "bank")
+        end
+    else
+        addon:Print("Bank comparison skipped (open a bank and run /gudatest again).")
+    end
+
+    local budget = addon.Modules.Utils and addon.Modules.Utils.GetFrameBudget
+        and addon.Modules.Utils:GetFrameBudget() or nil
+    if budget then
+        addon:Print("Queue frame budget: %.1f ms", budget * 1000)
+    end
+
+    if addon.Modules.InventoryIndex and addon.Modules.InventoryIndex.GetStats then
+        local s = addon.Modules.InventoryIndex:GetStats()
+        addon:Print("Inventory index: current=%d static=%d dirty(current/static)=%s/%s",
+            s.currentItems or 0, s.staticItems or 0,
+            Bool(s.currentDirty), Bool(s.staticDirty))
+    end
+
+    if addon.Modules.ItemButtonPerformance and addon.Modules.ItemButtonPerformance.GetStats then
+        local s = addon.Modules.ItemButtonPerformance:GetStats()
+        addon:Print("ItemButton perf: watcher=%s polling=%s sharedGlows=%d active=%s",
+            Bool(s.cursorWatcherFound), Bool(s.cursorPollingEnabled),
+            s.sharedGlowDrivers or 0, Bool(s.sharedGlowActive))
+    end
+
+    if state.mismatches == 0 then
+        addon:Print("|cff60ff60PASS: %d container slots checked, no API mismatches.|r", state.checked)
+        return true
+    end
+
+    if state.mismatches > 12 then
+        addon:Print("... %d additional mismatch(es) hidden.", state.mismatches - 12)
+    end
+    addon:Print("|cffff6060FAIL: %d mismatch(es) across %d checked slots.|r",
+        state.mismatches, state.checked)
+    return false
+end
+
+SLASH_GUDACLASSICAPITEST1 = "/gudatest"
+SlashCmdList["GUDACLASSICAPITEST"] = function()
+    SelfTest:Run()
+end
+
+addon:Debug("ClassicAPI /gudatest runtime diagnostics loaded")