fix(models): make tier-change listener registration non-destructive

docs/plans/analysis/2026-02-16-codebase-audit-report.md

@@ -14,6 +14,7 @@ Scope: Production-risk-first audit of bugs, code improvements, and feature oppor
 - ✅ F-012 addressed: synthetic repeated-tool nudge no longer emits invalid `tool_result.tool_use_id`; nudge is injected as plain user text guidance.
 - ✅ F-009 addressed: gateway now enforces per-connection WebSocket ingress rate limits with deterministic throttle errors and close-on-repeated-violation behavior.
 - ✅ F-008 addressed: WhatsApp Chromium launch is now sandboxed by default; no-sandbox mode is behind explicit `whatsapp.no_sandbox: true` opt-in.
+- ✅ F-014 addressed: `ModelRouter.setOnTierChange` now preserves existing listeners instead of replacing them, removing destructive listener-setter behavior.
 
 ## Executive Summary
 

docs/plans/state.json

@@ -2512,6 +2512,18 @@
         "docs/plans/analysis/2026-02-16-codebase-audit-report.md"
       ],
       "test_status": "pnpm test:run src/channels/whatsapp/adapter.test.ts src/config/schema.test.ts + pnpm typecheck passing"
+    },
+    "audit-followup-modelrouter-listeners": {
+      "status": "completed",
+      "date": "2026-02-16",
+      "updated": "2026-02-16",
+      "summary": "Removed ModelRouter tier-change listener setter footgun: setOnTierChange now appends instead of replacing existing listeners, with regression test coverage.",
+      "files_modified": [
+        "src/models/router.ts",
+        "src/models/router.test.ts",
+        "docs/plans/analysis/2026-02-16-codebase-audit-report.md"
+      ],
+      "test_status": "pnpm test:run src/models/router.test.ts + pnpm typecheck passing"
     }
   },
   "overall_progress": {

src/models/router.test.ts

@@ -463,4 +463,22 @@ describe('setClient and labels', () => {
     expect(fallback.chat).not.toHaveBeenCalled();
     expect(router.isTierStrict('default')).toBe(true);
   });
+
+  it('setOnTierChange does not replace existing listeners', () => {
+    const router = new ModelRouter({
+      default: { chat: vi.fn() } as unknown as ModelClient,
+      fast: { chat: vi.fn() } as unknown as ModelClient,
+      fallbackChain: [],
+    });
+
+    const first = vi.fn();
+    const second = vi.fn();
+    router.addOnTierChange(first);
+    router.setOnTierChange(second);
+
+    router.setTier('fast');
+
+    expect(first).toHaveBeenCalledWith('fast');
+    expect(second).toHaveBeenCalledWith('fast');
+  });
 });

src/models/router.ts

@@ -67,7 +67,7 @@ export class ModelRouter implements ModelClient {
   }
 
   setOnTierChange(callback: (tier: ModelTier) => void): void {
-    this.tierChangeListeners = [callback];
+    this.addOnTierChange(callback);
   }
 
   addOnTierChange(callback: (tier: ModelTier) => void): void {