Cache session file details for diagnostics to avoid repeated disk I/O

package.json

@@ -241,7 +241,7 @@
   },
   "devDependencies": {
     "@types/mocha": "^10.0.10",
-    "@types/node": "25.x",
+    "@types/node": "^25.2.0",
     "@types/vscode": "^1.108.1",
     "@typescript-eslint/eslint-plugin": "^8.54.0",
     "@typescript-eslint/parser": "^8.42.0",

src/extension.ts

@@ -74,6 +74,9 @@
 	mtime: number; // file modification time as timestamp
 	size?: number; // file size in bytes (optional for backward compatibility)
 	usageAnalysis?: SessionUsageAnalysis; // New analysis data
+	firstInteraction?: string | null; // ISO timestamp of first interaction
+	lastInteraction?: string | null; // ISO timestamp of last interaction
+	title?: string; // Session title (customTitle from session file)
 }
 
 // New interfaces for usage analysis
@@ -1986,12 +1989,126 @@
 		return analysis;
 	}
 
+	/**
+	 * Add editor root and name information to session file details.
+	 * Enriches the details object with editorRoot and editorName properties.
+	 */
+	private enrichDetailsWithEditorInfo(sessionFile: string, details: SessionFileDetails): void {
+		try {
+			const parts = sessionFile.split(/[/\\]/);
+			const userIdx = parts.findIndex(p => p.toLowerCase() === 'user');
+			if (userIdx > 0) {
+				details.editorRoot = parts.slice(0, userIdx).join(require('path').sep);
+			} else {
+				details.editorRoot = require('path').dirname(sessionFile);
+			}
+			details.editorName = this.getEditorNameFromRoot(details.editorRoot || '');
+		} catch (e) {
+			details.editorRoot = require('path').dirname(sessionFile);
+			details.editorName = this.getEditorNameFromRoot(details.editorRoot || '');
+		}
+	}
+
+	/**
+	 * Reconstruct SessionFileDetails from cached data without reading the file.
+	 * Returns undefined if cache is not valid or doesn't have all required data.
+	 */
+	private async getSessionFileDetailsFromCache(sessionFile: string, stat: fs.Stats): Promise<SessionFileDetails | undefined> {
+		const cached = this.getCachedSessionData(sessionFile);
+
+		// Validate cache against file stats
+		if (!cached || cached.mtime !== stat.mtime.getTime() || cached.size !== stat.size) {
+			return undefined;
+		}
+
+		// Check if cache has the required fields (for backward compatibility with old cache)
+		if (!cached.usageAnalysis?.contextReferences || typeof cached.interactions !== 'number' || cached.interactions < 0) {
+			return undefined;
+		}
+
+		// Reconstruct SessionFileDetails from cache
+		const details: SessionFileDetails = {
+			file: sessionFile,
+			size: cached.size || stat.size,
+			modified: stat.mtime.toISOString(),
+			interactions: cached.interactions,
+			contextReferences: cached.usageAnalysis.contextReferences,
+			firstInteraction: cached.firstInteraction || null,
+			lastInteraction: cached.lastInteraction || null,
+			editorSource: this.detectEditorSource(sessionFile),
+			title: cached.title
+		};
+
+		// Add editor root and name
+		this.enrichDetailsWithEditorInfo(sessionFile, details);
+
+		return details;
+	}
+
+	/**
+	 * Update or create cache entry with session file details.
+	 * Merges new detail fields with existing cached data if available.
+	 */
+	private async updateCacheWithSessionDetails(
+		sessionFile: string, 
+		stat: fs.Stats, 
+		details: SessionFileDetails
+	): Promise<void> {
+		// Get existing cache entry if available
+		const existingCache = this.getCachedSessionData(sessionFile);
+
+		// Create or update cache entry
+		const cacheEntry: SessionFileCache = {
+			tokens: existingCache?.tokens || 0,
+			interactions: details.interactions,
+			modelUsage: existingCache?.modelUsage || {},
+			mtime: stat.mtime.getTime(),
+			size: stat.size,
+			usageAnalysis: existingCache?.usageAnalysis || {
+				toolCalls: { total: 0, byTool: {} },
+				modeUsage: { ask: 0, edit: 0, agent: 0 },
+				contextReferences: {
+					file: 0, selection: 0, implicitSelection: 0, symbol: 0, codebase: 0,
+					workspace: 0, terminal: 0, vscode: 0
+				},
+				mcpTools: { total: 0, byServer: {}, byTool: {} },
+				modelSwitching: {
+					uniqueModels: [],
+					modelCount: 0,
+					switchCount: 0,
+					tiers: { standard: [], premium: [], unknown: [] },
+					hasMixedTiers: false
+				}
+			},
+			firstInteraction: details.firstInteraction,
+			lastInteraction: details.lastInteraction,
+			title: details.title
+		};
+
+		// Update the contextReferences in usageAnalysis with the current data
+		// usageAnalysis is guaranteed to exist here since we always initialize it above
+		cacheEntry.usageAnalysis!.contextReferences = details.contextReferences;
+
+		this.setCachedSessionData(sessionFile, cacheEntry, stat.size);
+	}
+
 	/**
 	 * Get detailed session file information for diagnostics view.
 	 * Analyzes session files to extract interactions, context references, and timestamps.
+	 * Uses cached data when available to avoid re-reading files.
 	 */
 	private async getSessionFileDetails(sessionFile: string): Promise<SessionFileDetails> {
 		const stat = await fs.promises.stat(sessionFile);
+
+		// Try to get details from cache first
+		const cachedDetails = await this.getSessionFileDetailsFromCache(sessionFile, stat);
+		if (cachedDetails) {
+			this._cacheHits++;
+			return cachedDetails;
+		}
+
+		this._cacheMisses++;
+
 		const details: SessionFileDetails = {
 			file: sessionFile,
 			size: stat.size,
@@ -2008,20 +2125,7 @@
 		};
 
 		// Determine top-level editor root path for this session file (up to the folder before 'User')
-		try {
-			const parts = sessionFile.split(/[/\\\\]/);
-			const userIdx = parts.findIndex(p => p.toLowerCase() === 'user');
-			if (userIdx > 0) {
-				details.editorRoot = parts.slice(0, userIdx).join(require('path').sep);
-			} else {
-				details.editorRoot = require('path').dirname(sessionFile);
-			}
-			// Also populate a friendly editor name for this file
-			details['editorName'] = this.getEditorNameFromRoot(details.editorRoot || '');
-		} catch (e) {
-			details.editorRoot = require('path').dirname(sessionFile);
-			details['editorName'] = this.getEditorNameFromRoot(details.editorRoot || '');
-		}
+		this.enrichDetailsWithEditorInfo(sessionFile, details);
 
 		try {
 			const fileContent = await fs.promises.readFile(sessionFile, 'utf8');
@@ -2107,6 +2211,10 @@
 					details.firstInteraction = new Date(timestamps[0]).toISOString();
 					details.lastInteraction = new Date(timestamps[timestamps.length - 1]).toISOString();
 				}
+
+				// Update cache with the details we just collected
+				await this.updateCacheWithSessionDetails(sessionFile, stat, details);
+
 				return details;
 			}
 
@@ -2174,6 +2282,9 @@
 					details.lastInteraction = stat.mtime.toISOString();
 				}
 			}
+
+			// Update cache with the details we just collected
+			await this.updateCacheWithSessionDetails(sessionFile, stat, details);
 		} catch (error) {
 			this.warn(`Error analyzing session file details for ${sessionFile}: ${error}`);
 		}
@@ -4076,6 +4187,10 @@
 		fourteenDaysAgo.setDate(fourteenDaysAgo.getDate() - 14);
 		const detailedSessionFiles: SessionFileDetails[] = [];
 
+		// Track cache performance for this load operation
+		const initialCacheHits = this._cacheHits;
+		const initialCacheMisses = this._cacheMisses;
+
 		// Sort files by modification time (most recent first) before taking first 500
 		// This ensures we prioritize recent sessions regardless of their folder location
 		const fileStats = await Promise.all(
@@ -4125,7 +4240,15 @@
 				command: 'sessionFilesLoaded',
 				detailedSessionFiles
 			});
-			this.log(`Loaded ${detailedSessionFiles.length} session files in background`);
+
+			// Calculate and log cache performance for this operation
+			const cacheHits = this._cacheHits - initialCacheHits;
+			const cacheMisses = this._cacheMisses - initialCacheMisses;
+			const totalAccesses = cacheHits + cacheMisses;
+			const hitRate = totalAccesses > 0 ? ((cacheHits / totalAccesses) * 100).toFixed(1) : '0.0';
+
+			this.log(`Loaded ${detailedSessionFiles.length} session files in background (Cache: ${cacheHits} hits, ${cacheMisses} misses, ${hitRate}% hit rate)`);
+
 			// Mark diagnostics as loaded so we don't reload unnecessarily
 			if (panel === this.diagnosticsPanel) {
 				this.diagnosticsHasLoadedFiles = true;