diff --git a/electron/agent-hooks/runtime.ts b/electron/agent-hooks/runtime.ts
index 065763dc..8239d78f 100644
--- a/electron/agent-hooks/runtime.ts
+++ b/electron/agent-hooks/runtime.ts
@@ -1,6 +1,7 @@
-import { app, type BrowserWindow } from 'electron';
+import { type BrowserWindow } from 'electron';
 import path from 'path';
 import { IPC } from '../ipc/channels.js';
+import { getUserDataDir } from '../user-data-dir.js';
 import { setAgentHookRuntime } from '../ipc/pty.js';
 import { error as logError, info as logInfo } from '../log.js';
 import { emitAgentHookEvent } from './events.js';
@@ -15,7 +16,11 @@ let server: AgentHookServer | null = null;
  * Failure is logged and otherwise ignored: the PTY heuristics keep working.
  */
 export async function startAgentHookRuntime(getWindow: () => BrowserWindow | null): Promise<void> {
-  const dir = path.join(app.getPath('userData'), 'agent-hooks');
+  // Per-instance, not raw userData: this directory holds endpoint.env (loopback
+  // port + bearer token), written only at startup, so sharing it between a dev
+  // run and an installed build hands every agent's hook events to whichever
+  // instance started last.
+  const dir = path.join(getUserDataDir(), 'agent-hooks');
   // Resolved per event: the server starts before the window exists so that no
   // Claude launch can race it, and the window may be recreated later.
   const forward = (event: AgentHookEventPayload): void => {
diff --git a/electron/ipc/persistence.ts b/electron/ipc/persistence.ts
index df1d68be..803304f4 100644
--- a/electron/ipc/persistence.ts
+++ b/electron/ipc/persistence.ts
@@ -1,19 +1,9 @@
-import { app } from 'electron';
 import fs from 'fs';
 import path from 'path';
-
-function getStateDir(): string {
-  let dir = app.getPath('userData');
-  // Use separate dir for dev mode
-  if (!app.isPackaged) {
-    const base = path.basename(dir);
-    dir = path.join(path.dirname(dir), `${base}-dev`);
-  }
-  return dir;
-}
+import { getUserDataDir } from '../user-data-dir.js';
 
 function getStatePath(): string {
-  return path.join(getStateDir(), 'state.json');
+  return path.join(getUserDataDir(), 'state.json');
 }
 
 export function saveAppState(json: string): void {
@@ -52,7 +42,7 @@ export function saveAppState(json: string): void {
 }
 
 function getThemesDir(): string {
-  return path.join(getStateDir(), 'themes');
+  return path.join(getUserDataDir(), 'themes');
 }
 
 const VALID_THEME_ID = /^[a-zA-Z0-9_-]+$/;
diff --git a/electron/ipc/register.ts b/electron/ipc/register.ts
index aadab018..3d01257e 100644
--- a/electron/ipc/register.ts
+++ b/electron/ipc/register.ts
@@ -43,6 +43,7 @@ import { buildVerifyEnv, validateVerifyCommand, verificationRunner } from './ver
 import { startRemoteServer, getMCPLogs, type RemoteProject } from '../remote/server.js';
 import type { RemoteAttentionState } from '../remote/protocol.js';
 import { atomicWriteFileSync } from '../mcp/atomic.js';
+import { getUserDataDir } from '../user-data-dir.js';
 import { buildMcpLaunchArgs } from '../mcp/agent-args.js';
 import {
   getSymlinkCandidates,
@@ -766,29 +767,20 @@ export function registerAllHandlers(win: BrowserWindow): void {
   });
 
   // --- Keybindings ---
-  function getKeybindingsDir(): string {
-    let dir = app.getPath('userData');
-    if (!app.isPackaged) {
-      const base = path.basename(dir);
-      dir = path.join(path.dirname(dir), `${base}-dev`);
-    }
-    return dir;
-  }
-
   ipcMain.handle(IPC.LoadKeybindings, () => {
-    return loadKeybindings(getKeybindingsDir());
+    return loadKeybindings(getUserDataDir());
   });
 
   ipcMain.handle(IPC.SaveKeybindings, (_e, args) => {
     assertString(args?.json, 'json');
-    saveKeybindings(getKeybindingsDir(), args.json);
+    saveKeybindings(getUserDataDir(), args.json);
   });
 
   // --- Arena persistence ---
   ipcMain.handle(IPC.SaveArenaData, (_e, args) => {
     assertString(args.filename, 'filename');
     assertString(args.json, 'json');
-    const filePath = path.join(app.getPath('userData'), args.filename);
+    const filePath = path.join(getUserDataDir(), args.filename);
     const basename = path.basename(filePath);
     if (basename !== args.filename) throw new Error('Invalid filename');
     if (!basename.startsWith('arena-') || !basename.endsWith('.json'))
@@ -800,7 +792,7 @@ export function registerAllHandlers(win: BrowserWindow): void {
 
   ipcMain.handle(IPC.LoadArenaData, (_e, args) => {
     assertString(args.filename, 'filename');
-    const filePath = path.join(app.getPath('userData'), args.filename);
+    const filePath = path.join(getUserDataDir(), args.filename);
     const basename = path.basename(filePath);
     if (basename !== args.filename) throw new Error('Invalid filename');
     if (!basename.startsWith('arena-') || !basename.endsWith('.json'))
diff --git a/electron/user-data-dir.test.ts b/electron/user-data-dir.test.ts
new file mode 100644
index 00000000..191d17d0
--- /dev/null
+++ b/electron/user-data-dir.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest';
+import path from 'path';
+import { resolveUserDataDir } from './user-data-dir.js';
+
+describe('resolveUserDataDir', () => {
+  const userData = path.join('/home', 'someone', '.config', 'parallel-code');
+
+  it('uses the userData path as-is in a packaged build', () => {
+    expect(resolveUserDataDir(userData, true)).toBe(userData);
+  });
+
+  it('appends -dev to the last segment in a dev run', () => {
+    expect(resolveUserDataDir(userData, false)).toBe(
+      path.join('/home', 'someone', '.config', 'parallel-code-dev'),
+    );
+  });
+
+  // Suffixing the whole path instead of the last segment would put dev's data
+  // in a sibling of the config root rather than beside the packaged profile.
+  it('keeps the parent directory', () => {
+    expect(path.dirname(resolveUserDataDir(userData, false))).toBe(path.dirname(userData));
+  });
+});
diff --git a/electron/user-data-dir.ts b/electron/user-data-dir.ts
new file mode 100644
index 00000000..69391ead
--- /dev/null
+++ b/electron/user-data-dir.ts
@@ -0,0 +1,27 @@
+import { app } from 'electron';
+import path from 'path';
+
+/**
+ * Per-instance data directory, given Electron's `userData` path.
+ *
+ * A dev run gets its own `<name>-dev` directory so `npm run dev` does not read
+ * or write the installed build's data. Pure and parameterised so the rule can be
+ * tested without an Electron runtime, and so there is exactly one copy of it.
+ */
+export function resolveUserDataDir(userDataPath: string, isPackaged: boolean): string {
+  if (isPackaged) return userDataPath;
+  const base = path.basename(userDataPath);
+  return path.join(path.dirname(userDataPath), `${base}-dev`);
+}
+
+/**
+ * The per-instance data directory for this process.
+ *
+ * Every file the app keeps under `userData` goes here rather than under
+ * `app.getPath('userData')` directly. The suffix is what keeps a dev run and an
+ * installed build from writing over each other, so a caller that reaches past
+ * this helper silently opts one of its files out of that separation.
+ */
+export function getUserDataDir(): string {
+  return resolveUserDataDir(app.getPath('userData'), app.isPackaged);
+}