diff --git a/ui-electrobun/src/mainview/health-store.svelte.ts b/ui-electrobun/src/mainview/health-store.svelte.ts
index 9de7687..f7edf96 100644
--- a/ui-electrobun/src/mainview/health-store.svelte.ts
+++ b/ui-electrobun/src/mainview/health-store.svelte.ts
@@ -145,6 +145,7 @@ export function trackProject(projectId: string): void {
     lastActivityTs: Date.now(),
     lastToolName: null,
     toolsInFlight: 0,
+    activeToolMap: new Map(),
     costSnapshots: [],
     totalTokens: 0,
     totalCost: 0,
@@ -163,6 +164,13 @@ export function recordActivity(projectId: string, toolName?: string): void {
     t.lastToolName = toolName;
     // Fix #8 (Codex audit): Increment counter for concurrent tool tracking
     t.toolsInFlight++;
+    // Feature 10: Track per-tool starts
+    const existing = t.activeToolMap.get(toolName);
+    if (existing) {
+      existing.count++;
+    } else {
+      t.activeToolMap.set(toolName, { startTime: Date.now(), count: 1 });
+    }
   }
   if (!tickInterval) startHealthTick();
 }
@@ -171,9 +179,32 @@ export function recordActivity(projectId: string, toolName?: string): void {
 export function recordToolDone(projectId: string): void {
   const t = trackers.get(projectId);
   if (!t) return;
-  t.lastActivityTs = Date.now();
+  const now = Date.now();
+  t.lastActivityTs = now;
   // Fix #8 (Codex audit): Decrement counter, floor at 0
   t.toolsInFlight = Math.max(0, t.toolsInFlight - 1);
+
+  // Feature 10: Record duration in histogram, remove from activeToolMap
+  if (t.lastToolName) {
+    const entry = t.activeToolMap.get(t.lastToolName);
+    if (entry) {
+      const durationMs = now - entry.startTime;
+      // Update global histogram
+      const hist = toolDurationHistogram.get(t.lastToolName);
+      if (hist) {
+        hist.totalMs += durationMs;
+        hist.count++;
+      } else {
+        toolDurationHistogram.set(t.lastToolName, {
+          toolName: t.lastToolName,
+          totalMs: durationMs,
+          count: 1,
+        });
+      }
+      entry.count--;
+      if (entry.count <= 0) t.activeToolMap.delete(t.lastToolName);
+    }
+  }
 }
 
 /** Record a token/cost snapshot for burn rate calculation. */
@@ -233,6 +264,30 @@ export function getHealthAggregates(): {
   return { running, idle, stalled, totalBurnRatePerHour };
 }
 
+/** Feature 10: Get all currently active tools across all projects. */
+export function getActiveTools(): Array<{ projectId: string; toolName: string; startTime: number; count: number }> {
+  const results: Array<{ projectId: string; toolName: string; startTime: number; count: number }> = [];
+  for (const t of trackers.values()) {
+    for (const [name, entry] of t.activeToolMap) {
+      results.push({ projectId: t.projectId, toolName: name, startTime: entry.startTime, count: entry.count });
+    }
+  }
+  return results;
+}
+
+/** Feature 10: Get tool duration histogram for diagnostics. */
+export function getToolHistogram(): Array<{ toolName: string; avgMs: number; count: number }> {
+  const results: Array<{ toolName: string; avgMs: number; count: number }> = [];
+  for (const entry of toolDurationHistogram.values()) {
+    results.push({
+      toolName: entry.toolName,
+      avgMs: entry.count > 0 ? entry.totalMs / entry.count : 0,
+      count: entry.count,
+    });
+  }
+  return results.sort((a, b) => b.count - a.count).slice(0, 15);
+}
+
 /** Start the health tick timer. */
 function startHealthTick(): void {
   if (tickInterval) return;