diff --git a/.jules/bolt.md b/.jules/bolt.md
index 69da333..427be48 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -64,3 +64,6 @@
 ## 2024-05-15 - Electron Main Process File Operations
 **Learning:** Using synchronous `fs` methods (`readFileSync`, `existsSync`, `statSync`) in the Electron main process (e.g., `preview-server.ts`) severely blocks the Node.js event loop, preventing concurrent processing of HTTP requests and potentially causing UI thread starvation or micro-stuttering. Furthermore, buffering entire large files (e.g., `readFileSync`) consumes unnecessary heap memory.
 **Action:** Always default to async file operations (`fs.promises`) and stream piping (`createReadStream(..).pipe(res)`) when dealing with HTTP response bodies in the Electron main process to maintain responsiveness and minimize memory footprint.
+## 2025-05-17 - Replace O(N²) array search with O(N) Set lookup in session listing
+**Learning:** Using `array.some(...)` with string operations (like `replace()`) inside a loop over files creates an unnecessary O(N²) performance bottleneck, particularly when reading directories with many files (like the sessions history directory).
+**Action:** When filtering files against another list of files, pre-compute a `Set` of base names outside the loop for O(1) lookups, changing the overall time complexity from O(N²) to O(N).
diff --git a/packages/core/src/session-manager.ts b/packages/core/src/session-manager.ts
index af6412b..226eb1e 100644
--- a/packages/core/src/session-manager.ts
+++ b/packages/core/src/session-manager.ts
@@ -556,6 +556,8 @@ export class SessionManager {
 
     // Collect legacy JSON sessions
     const jsonFiles = files.filter((f) => f.endsWith('.json'));
+    // ⚡ Bolt: Pre-compute Set for O(1) lookups instead of O(N^2) array.some() string replacement in the inner loop
+    const jsonlBaseNames = new Set(jsonlFiles.map((f) => f.slice(0, -6)));
     const CONCURRENCY_LIMIT = 20;
     for (let i = 0; i < jsonFiles.length; i += CONCURRENCY_LIMIT) {
       const chunk = jsonFiles.slice(i, i + CONCURRENCY_LIMIT);
@@ -566,7 +568,7 @@ export class SessionManager {
             const raw = await fs.readFile(fullPath, 'utf-8');
             const data = JSON.parse(raw) as ChatSession;
             // Skip if already have a JSONL version
-            if (jsonlFiles.some((jf) => jf.replace('.jsonl', '') === file.replace('.json', ''))) return;
+            if (jsonlBaseNames.has(file.slice(0, -5))) return;
             const { messages: _messages, stats: _stats, ...meta } = data;
             metas.push(meta);
           } catch {