diff --git a/src/state-store.ts b/src/state-store.ts
index faad0710..ffab99f4 100644
--- a/src/state-store.ts
+++ b/src/state-store.ts
@@ -39,7 +39,7 @@ import {
   TokenUsageEntry,
 } from './types.js';
 import { Debouncer, MAX_SESSION_TOKENS } from './utils/index.js';
-import { dataPath } from './config/instance.js';
+import { dataPath, CODEMAN_INSTANCE } from './config/instance.js';
 
 /** Debounce delay for batching state writes (ms) */
 const SAVE_DEBOUNCE_MS = 500;
@@ -90,8 +90,10 @@ export class StateStore {
   private _saveInFlight: Promise<void> | null = null;
 
   constructor(filePath?: string) {
-    // Migrate legacy data directory (~/.claudeman → ~/.codeman)
-    if (!filePath) {
+    // Migrate legacy data directory (~/.claudeman → ~/.codeman). Default (prod)
+    // instance only — a named instance (e.g. beta) must never touch the shared
+    // ~/.codeman / ~/codeman-cases layout, preserving instance isolation.
+    if (!filePath && !CODEMAN_INSTANCE) {
       const legacyDir = join(homedir(), '.claudeman');
       const newDir = join(homedir(), '.codeman');
       if (existsSync(legacyDir) && !existsSync(newDir)) {