feat: context fill grade in chat UI

MLXServer/Server/PromptBuilder.swift

@@ -96,7 +96,7 @@ enum PromptBuilder {
             additionalContext: additionalContext
         )
 
-        let estimatedPromptTokens = (instructions.count + chatMessages.reduce(0) { $0 + $1.content.count }) * 10 / 35
+        let estimatedPromptTokens = estimatePromptTokens(instructions: instructions, chatMessages: chatMessages)
 
         return PreparedPrompt(
             instructions: instructions,
@@ -111,6 +111,13 @@ enum PromptBuilder {
         )
     }
 
+    static func estimatePromptTokens(instructions: String, chatMessages: [Chat.Message]) -> Int {
+        let characterCount = instructions.count + chatMessages.reduce(0) { partial, message in
+            partial + message.content.count
+        }
+        return max(0, characterCount * 10 / 35)
+    }
+
     private static func imageFingerprint(_ source: String) -> UInt64 {
         var hash: UInt64 = 14_695_981_039_346_656_037
         for byte in source.utf8 {

MLXServer/ViewModels/ChatViewModel.swift

@@ -49,6 +49,34 @@ final class ChatViewModel {
         hasUnsavedChanges ? "\(documentDisplayName) *" : documentDisplayName
     }
 
+    var currentContextLength: Int {
+        modelManager.currentModel?.contextLength ?? 0
+    }
+
+    var estimatedPromptTokens: Int {
+        let draft = inputText.trimmingCharacters(in: .whitespacesAndNewlines)
+        var chatMessages = conversation.messages.compactMap(historyMessage(from:))
+        if !draft.isEmpty {
+            chatMessages.append(Chat.Message(role: .user, content: draft))
+        }
+        return PromptBuilder.estimatePromptTokens(
+            instructions: effectiveSystemPrompt,
+            chatMessages: chatMessages
+        )
+    }
+
+    var contextUsedTokens: Int {
+        if isGenerating && (promptTokens > 0 || generationTokens > 0) {
+            return promptTokens + generationTokens
+        }
+        return estimatedPromptTokens
+    }
+
+    var contextFillRatio: Double {
+        guard currentContextLength > 0 else { return 0 }
+        return min(max(Double(contextUsedTokens) / Double(currentContextLength), 0), 1)
+    }
+
     /// Ensure a ChatSession exists for the current model.
     private func ensureSession() {
         guard let container = modelManager.modelContainer else { return }

MLXServer/Views/StatusBarView.swift

@@ -31,6 +31,10 @@ struct StatusBarView: View {
                 .font(.caption)
                 .foregroundStyle(.secondary)
 
+            if let model = modelManager.currentModel, model.contextLength > 0 {
+                contextFillView(totalContext: model.contextLength)
+            }
+
             Spacer()
 
             // GPU memory
@@ -78,4 +82,43 @@ struct StatusBarView: View {
         .padding(.vertical, 4)
         .background(.bar)
     }
+
+    @ViewBuilder
+    private func contextFillView(totalContext: Int) -> some View {
+        let usedTokens = viewModel.contextUsedTokens
+        let ratio = viewModel.contextFillRatio
+        let percent = Int((ratio * 100).rounded())
+
+        HStack(spacing: 6) {
+            Capsule()
+                .fill(.quaternary)
+                .frame(width: 48, height: 6)
+                .overlay(alignment: .leading) {
+                    Capsule()
+                        .fill(contextFillColor(for: ratio))
+                        .frame(width: max(4, 48 * ratio), height: 6)
+                }
+
+            Text("Ctx \(percent)%")
+                .font(.caption.monospacedDigit())
+                .foregroundStyle(.secondary)
+        }
+        .help("Approximate context usage: \(formatTokenCount(usedTokens)) of \(formatTokenCount(totalContext)) tokens")
+    }
+
+    private func contextFillColor(for ratio: Double) -> Color {
+        if ratio >= 0.9 { return .red }
+        if ratio >= 0.7 { return .orange }
+        return .blue
+    }
+
+    private func formatTokenCount(_ count: Int) -> String {
+        if count >= 1_000_000 {
+            return String(format: "%.1fM", Double(count) / 1_000_000)
+        }
+        if count >= 1_000 {
+            return String(format: "%.1fk", Double(count) / 1_000)
+        }
+        return "\(count)"
+    }
 }

MLXServerTests/Server/PromptBuilderTests.swift

@@ -61,6 +61,20 @@ final class PromptBuilderTests: XCTestCase {
         XCTAssertEqual(prepared.additionalContext?["enable_thinking"] as? Bool, legacy.additionalContext?["enable_thinking"] as? Bool)
     }
 
+    func testEstimatePromptTokensMatchesSharedCharacterHeuristic() {
+        let messages = [
+            Chat.Message(role: .user, content: "1234567890"),
+            Chat.Message(role: .assistant, content: "abcdefghij")
+        ]
+
+        let estimated = PromptBuilder.estimatePromptTokens(
+            instructions: "system12345",
+            chatMessages: messages
+        )
+
+        XCTAssertEqual(estimated, 8)
+    }
+
     func testBuildAggregatesInstructionsAndMessages() {
         let request = APIChatCompletionRequest(
             model: "gemma",