feat: Add live context window tracking with visual progress bar

- Extract token usage from message_delta events in backend
- Calculate context usage percentage (input + cache tokens / 200k)
- Add context_update, compacting_started/finished, compact_boundary events
- Display progress bar that fills up as context is consumed
- Color-coded warnings (green→yellow→red) based on usage
- Show compacting status indicator when auto-compact runs
- Display system messages for compact start/finish with usage stats

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/StatusBar.jsx

@@ -23,7 +23,8 @@ export function StatusBar({ sessionStats, isProcessing, connected, permissionMod
     cacheCreationTokens = 0,
     numTurns = 0,
     contextWindow = 200000,
-    contextLeftPercent = null, // From Claude's "Context left until auto-compact: X%" message
+    contextLeftPercent = null, // Live context tracking from message_delta events
+    tokensUsed = 0,
     isCompacting = false,
   } = sessionStats || {};
 
@@ -31,9 +32,27 @@ export function StatusBar({ sessionStats, isProcessing, connected, permissionMod
   const currentMode = PERMISSION_MODES.find(m => m.value === permissionMode) || PERMISSION_MODES[0];
   const ModeIcon = currentMode.icon;
 
-  // Context remaining: Use Claude's reported value if available, otherwise don't show
-  // Claude only reports this when context is getting low (< ~15%)
+  // Context used percentage (0-100) - bar fills up as context is consumed
   const contextRemaining = contextLeftPercent;
+  const contextUsedPercent = contextRemaining !== null ? (100 - contextRemaining) : 0;
+
+  // Determine color based on usage (higher usage = more warning)
+  const getContextColor = (usedPercent) => {
+    if (usedPercent === null) return 'bg-dark-600';
+    if (usedPercent >= 95) return 'bg-red-500';
+    if (usedPercent >= 85) return 'bg-red-400';
+    if (usedPercent >= 70) return 'bg-yellow-400';
+    if (usedPercent >= 50) return 'bg-yellow-300';
+    return 'bg-green-400';
+  };
+
+  const getContextTextColor = (usedPercent) => {
+    if (usedPercent === null) return 'text-dark-500';
+    if (usedPercent >= 95) return 'text-red-400 font-medium animate-pulse';
+    if (usedPercent >= 85) return 'text-red-400';
+    if (usedPercent >= 70) return 'text-yellow-400';
+    return 'text-green-400';
+  };
 
   // Format cost
   const formatCost = (cost) => {
@@ -135,19 +154,33 @@ export function StatusBar({ sessionStats, isProcessing, connected, permissionMod
           )}
         </div>
 
-        {/* Right side: Token usage */}
-        <div className="flex items-center gap-4">
-          {/* Context status - only shown when Claude reports it (context getting low) */}
+        {/* Right side: Context usage with progress bar */}
+        <div className="flex items-center gap-3">
+          {/* Context progress bar - always visible when we have data */}
           {contextRemaining !== null && (
             <div className="flex items-center gap-2">
-              <span className="text-dark-500">Context:</span>
-              <span className={`${
-                contextRemaining <= 5 ? 'text-red-400 font-medium animate-pulse' :
-                contextRemaining <= 15 ? 'text-red-400' :
-                contextRemaining <= 30 ? 'text-yellow-400' : 'text-green-400'
-              }`}>
-                {contextRemaining}% left
-              </span>
+              <Database className="w-3 h-3 text-dark-500" />
+              <div className="flex items-center gap-2">
+                {/* Progress bar container - fills up as context is used */}
+                <div className="w-24 h-2 bg-dark-700 rounded-full overflow-hidden" title={`${tokensUsed.toLocaleString()} / ${contextWindow.toLocaleString()} tokens used`}>
+                  <div
+                    className={`h-full transition-all duration-300 ${getContextColor(contextUsedPercent)}`}
+                    style={{ width: `${contextUsedPercent}%` }}
+                  />
+                </div>
+                {/* Percentage text - shows how full the context is */}
+                <span className={`min-w-[45px] text-right ${getContextTextColor(contextUsedPercent)}`}>
+                  {contextUsedPercent}%
+                </span>
+              </div>
+            </div>
+          )}
+
+          {/* Show "waiting" state if no context data yet */}
+          {contextRemaining === null && connected && (
+            <div className="flex items-center gap-2 text-dark-500">
+              <Database className="w-3 h-3" />
+              <span>Context: --</span>
             </div>
           )}
         </div>

frontend/src/contexts/SessionContext.jsx

@@ -590,6 +590,61 @@ export function SessionProvider({ children }) {
         }
         break;
 
+      case 'context_update':
+        // Live context window metrics from message_delta events
+        updateSession(sessionId, (session) => ({
+          stats: {
+            ...(session.stats || {}),
+            tokensUsed: data.tokensUsed,
+            contextWindow: data.contextWindow,
+            contextLeftPercent: data.percentRemaining,
+          },
+        }));
+        break;
+
+      case 'compacting_started':
+        console.log(`[${sessionId}] Compacting started`);
+        updateSession(sessionId, (session) => ({
+          stats: {
+            ...(session.stats || {}),
+            isCompacting: true,
+          },
+        }));
+        addMessage(sessionId, {
+          type: 'system',
+          content: '⏳ Context compacting in progress...',
+        });
+        break;
+
+      case 'compacting_finished':
+        console.log(`[${sessionId}] Compacting finished`);
+        updateSession(sessionId, (session) => ({
+          stats: {
+            ...(session.stats || {}),
+            isCompacting: false,
+          },
+        }));
+        break;
+
+      case 'compact_boundary':
+        console.log(`[${sessionId}] Compact boundary:`, data);
+        // Context was compacted - reset the metrics
+        updateSession(sessionId, (session) => ({
+          stats: {
+            ...(session.stats || {}),
+            isCompacting: false,
+            tokensUsed: 0,
+            contextLeftPercent: 100,
+            lastCompactTrigger: data.trigger,
+            lastCompactPreTokens: data.preTokens,
+          },
+        }));
+        addMessage(sessionId, {
+          type: 'system',
+          content: `✅ Context compacted (${data.trigger}). Was at ${data.percentUsedBeforeCompact}% usage.`,
+        });
+        break;
+
       default:
         console.log(`[${sessionId}] Unhandled message type:`, type, data);
     }

frontend/src/hooks/useClaudeSession.js

@@ -229,6 +229,49 @@ export function useClaudeSession() {
           blockedPath: data.blockedPath
         });
         break;
+
+      case 'context_update':
+        // Live context window metrics from message_delta events
+        setSessionStats(prev => ({
+          ...prev,
+          tokensUsed: data.tokensUsed,
+          contextWindow: data.contextWindow,
+          contextLeftPercent: data.percentRemaining
+        }));
+        break;
+
+      case 'compacting_started':
+        console.log('Compacting started');
+        setSessionStats(prev => ({ ...prev, isCompacting: true }));
+        setMessages(prev => [...prev, {
+          type: 'system',
+          content: '⏳ Context compacting in progress...',
+          timestamp: Date.now()
+        }]);
+        break;
+
+      case 'compacting_finished':
+        console.log('Compacting finished');
+        setSessionStats(prev => ({ ...prev, isCompacting: false }));
+        break;
+
+      case 'compact_boundary':
+        console.log('Compact boundary:', data);
+        // Context was compacted - reset the metrics
+        setSessionStats(prev => ({
+          ...prev,
+          isCompacting: false,
+          tokensUsed: 0,
+          contextLeftPercent: 100,
+          lastCompactTrigger: data.trigger,
+          lastCompactPreTokens: data.preTokens
+        }));
+        setMessages(prev => [...prev, {
+          type: 'system',
+          content: `✅ Context compacted (${data.trigger}). Was at ${data.percentUsedBeforeCompact}% usage.`,
+          timestamp: Date.now()
+        }]);
+        break;
     }
   }, []);