fix: prevent negative token/cost totals during message removal

JChat/Chat.swift

@@ -139,6 +139,20 @@ final class Chat {
         if verbosityOverride != nil { count += 1 }
         return count
     }
+
+    // MARK: - Usage Totals
+
+    func addUsage(promptTokens: Int, completionTokens: Int, cost: Double) {
+        totalPromptTokens += promptTokens
+        totalCompletionTokens += completionTokens
+        totalCost += cost
+    }
+
+    func removeUsage(promptTokens: Int, completionTokens: Int, cost: Double) {
+        totalPromptTokens = max(0, totalPromptTokens - promptTokens)
+        totalCompletionTokens = max(0, totalCompletionTokens - completionTokens)
+        totalCost = max(0, totalCost - cost)
+    }
 }
 
 @Model

JChat/Documentation/CHANGELOG_INTERNAL.md

@@ -11,3 +11,4 @@
 - Improved Markdown code block readability with stronger contrast, consistent header labels, and always-visible copy action.
 - Replaced abrupt setup blocking with a first-run readiness checklist for API key and default model in `ContentView`.
 - Added a canonical manual regression checklist for chat, setup guardrails, markdown, and settings persistence flows.
+- Hardened chat usage accounting by clamping token/cost totals at zero during delete/regenerate paths.

JChat/Views/ChatViewModel.swift

@@ -161,9 +161,7 @@ class ChatViewModel {
                         if let cachedModel = try? context.fetch(modelDescriptor).first {
                             assistantMessage.cost = cachedModel.calculateCost(promptTokens: prompt, completionTokens: completion)
                         }
-                        chat.totalPromptTokens += prompt
-                        chat.totalCompletionTokens += completion
-                        chat.totalCost += assistantMessage.cost
+                        chat.addUsage(promptTokens: prompt, completionTokens: completion, cost: assistantMessage.cost)
                     case .modelID(let id):
                         assistantMessage.modelID = id
                     case .done:
@@ -214,9 +212,7 @@ class ChatViewModel {
               sorted[index - 1].role == .user else { return }
 
         // Subtract old assistant message stats
-        chat.totalPromptTokens -= message.promptTokens
-        chat.totalCompletionTokens -= message.completionTokens
-        chat.totalCost -= message.cost
+        chat.removeUsage(promptTokens: message.promptTokens, completionTokens: message.completionTokens, cost: message.cost)
 
         // Delete old assistant message
         chat.messages.removeAll { $0.id == message.id }
@@ -304,9 +300,7 @@ class ChatViewModel {
                         if let cachedModel = try? context.fetch(modelDescriptor).first {
                             assistantMessage.cost = cachedModel.calculateCost(promptTokens: prompt, completionTokens: completion)
                         }
-                        chat.totalPromptTokens += prompt
-                        chat.totalCompletionTokens += completion
-                        chat.totalCost += assistantMessage.cost
+                        chat.addUsage(promptTokens: prompt, completionTokens: completion, cost: assistantMessage.cost)
                     case .modelID(let id):
                         assistantMessage.modelID = id
                     case .done:
@@ -335,9 +329,7 @@ class ChatViewModel {
 
     func deleteMessage(_ message: Message, in context: ModelContext) {
         guard let chat = selectedChat else { return }
-        chat.totalPromptTokens -= message.promptTokens
-        chat.totalCompletionTokens -= message.completionTokens
-        chat.totalCost -= message.cost
+        chat.removeUsage(promptTokens: message.promptTokens, completionTokens: message.completionTokens, cost: message.cost)
         chat.messages.removeAll { $0.id == message.id }
         context.delete(message)
         try? context.save()

JChatTests/JChatTests.swift

@@ -64,4 +64,14 @@ struct JChatTests {
         #expect(chat.overrideCount == 0)
     }
 
+    @Test func usageRemovalClampsAtZero() {
+        let chat = Chat()
+        chat.addUsage(promptTokens: 10, completionTokens: 5, cost: 0.12)
+        chat.removeUsage(promptTokens: 99, completionTokens: 99, cost: 1.0)
+
+        #expect(chat.totalPromptTokens == 0)
+        #expect(chat.totalCompletionTokens == 0)
+        #expect(chat.totalCost == 0)
+    }
+
 }