diff --git a/app/src/main/hl/engines/browserHarnessEnv.ts b/app/src/main/hl/engines/browserHarnessEnv.ts
index 237fe425..25c6de00 100644
--- a/app/src/main/hl/engines/browserHarnessEnv.ts
+++ b/app/src/main/hl/engines/browserHarnessEnv.ts
@@ -2,15 +2,15 @@ import { createHash } from 'node:crypto';
 import path from 'node:path';
 import type { SpawnContext } from './types';
 
-export function browserHarnessReplPort(sessionId: string): string {
-  const n = createHash('sha256').update(sessionId).digest().readUInt16BE(0);
+export function browserHarnessReplPort(sessionId: string, targetId = ''): string {
+  const n = createHash('sha256').update(`${sessionId}:${targetId}`).digest().readUInt16BE(0);
   return String(18_000 + (n % 20_000));
 }
 
 export function applyBrowserHarnessEnv(ctx: SpawnContext, env: NodeJS.ProcessEnv): NodeJS.ProcessEnv {
   const sdkDir = path.join(ctx.harnessDir, 'browser-harness-js', 'sdk');
   env.PATH = env.PATH ? `${sdkDir}${path.delimiter}${env.PATH}` : sdkDir;
-  env.CDP_REPL_PORT = env.CDP_REPL_PORT ?? browserHarnessReplPort(ctx.sessionId);
+  env.CDP_REPL_PORT = env.CDP_REPL_PORT ?? browserHarnessReplPort(ctx.sessionId, ctx.targetId);
   env.CDP_REPL_LOG = env.CDP_REPL_LOG ?? path.join(ctx.harnessDir, `browser-harness-js-${ctx.sessionId}.log`);
   env.BU_SESSION_ID = ctx.sessionId;
   return env;
diff --git a/app/src/main/hl/stock/browser-harness-js/sdk/repl.ts b/app/src/main/hl/stock/browser-harness-js/sdk/repl.ts
index 40334ca2..4504a738 100644
--- a/app/src/main/hl/stock/browser-harness-js/sdk/repl.ts
+++ b/app/src/main/hl/stock/browser-harness-js/sdk/repl.ts
@@ -34,7 +34,12 @@ async function connectToAssignedTarget(): Promise<{ targetId: string; port: numb
   if (!session.isConnected()) {
     await session.connect({ port, targetId });
   } else {
-    await session.use(targetId).catch(() => {});
+    try {
+      await session.use(targetId);
+    } catch {
+      session.close();
+      await session.connect({ port, targetId });
+    }
   }
 
   await Promise.all([
diff --git a/app/src/main/hl/stock/browser-harness-js/sdk/session.ts b/app/src/main/hl/stock/browser-harness-js/sdk/session.ts
index 4fc530ef..825bae10 100644
--- a/app/src/main/hl/stock/browser-harness-js/sdk/session.ts
+++ b/app/src/main/hl/stock/browser-harness-js/sdk/session.ts
@@ -124,10 +124,16 @@ export class Session implements Transport {
       const timer = setTimeout(() => finish(new Error(`timed out after ${timeoutMs}ms`)), timeoutMs);
       ws.addEventListener('open', () => finish());
       ws.addEventListener('error', (e) => finish(new Error(`WS error: ${(e as any)?.message ?? 'connect failed (likely 403, permission not granted, or port closed)'}`)));
-      ws.addEventListener('message', (e) => this.onMessage(String(e.data)));
+      ws.addEventListener('message', (e) => {
+        if (this.ws === ws) this.onMessage(String(e.data));
+      });
       ws.addEventListener('close', () => {
-        for (const [, p] of this.pending) p.reject(new Error('CDP socket closed'));
-        this.pending.clear();
+        if (this.ws === ws) {
+          this.ws = undefined;
+          this.activeSessionId = undefined;
+          for (const [, p] of this.pending) p.reject(new Error('CDP socket closed'));
+          this.pending.clear();
+        }
         finish(new Error('WS closed before open (likely 403 or port closed)'));
       });
       this.ws = ws;
@@ -139,7 +145,12 @@ export class Session implements Transport {
   }
 
   close(): void {
-    this.ws?.close();
+    const ws = this.ws;
+    this.ws = undefined;
+    this.activeSessionId = undefined;
+    for (const [, p] of this.pending) p.reject(new Error('CDP socket closed'));
+    this.pending.clear();
+    ws?.close();
   }
 
   /**
diff --git a/app/tests/unit/hl/browserHarnessEnv.test.ts b/app/tests/unit/hl/browserHarnessEnv.test.ts
new file mode 100644
index 00000000..cd409360
--- /dev/null
+++ b/app/tests/unit/hl/browserHarnessEnv.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+import { applyBrowserHarnessEnv, browserHarnessReplPort } from '../../../src/main/hl/engines/browserHarnessEnv';
+import type { SpawnContext } from '../../../src/main/hl/engines/types';
+
+function spawnContext(targetId: string): SpawnContext {
+  return {
+    prompt: 'Open example.com',
+    harnessDir: '/tmp/harness',
+    sessionId: 'session-123',
+    targetId,
+    cdpPort: 9222,
+    attachmentRefs: [],
+  };
+}
+
+describe('browser harness environment', () => {
+  it('scopes the REPL port to the assigned target as well as the app session', () => {
+    const firstTarget = browserHarnessReplPort('session-123', 'target-a');
+    const secondTarget = browserHarnessReplPort('session-123', 'target-b');
+
+    expect(browserHarnessReplPort('session-123', 'target-a')).toBe(firstTarget);
+    expect(secondTarget).not.toBe(firstTarget);
+  });
+
+  it('gives reruns with a replacement browser target a fresh REPL port', () => {
+    const firstEnv = applyBrowserHarnessEnv(spawnContext('old-target'), {});
+    const rerunEnv = applyBrowserHarnessEnv(spawnContext('new-target'), {});
+
+    expect(firstEnv.CDP_REPL_PORT).toBe(browserHarnessReplPort('session-123', 'old-target'));
+    expect(rerunEnv.CDP_REPL_PORT).toBe(browserHarnessReplPort('session-123', 'new-target'));
+    expect(rerunEnv.CDP_REPL_PORT).not.toBe(firstEnv.CDP_REPL_PORT);
+  });
+
+  it('preserves an explicit REPL port override', () => {
+    const env = applyBrowserHarnessEnv(spawnContext('target-a'), { CDP_REPL_PORT: '9876' });
+
+    expect(env.CDP_REPL_PORT).toBe('9876');
+  });
+});