fix(ui): inherit default model fallbacks in agents overview (#25729)

Land PR #25729 from @Suko.
Use shared fallback-resolution helper and add regression coverage for default, override, and explicit-empty cases.

Co-authored-by: suko <miha.sukic@gmail.com>

ui/src/ui/views/agents-utils.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from "vitest";
+import { resolveEffectiveModelFallbacks } from "./agents-utils.ts";
+
+describe("resolveEffectiveModelFallbacks", () => {
+  it("inherits defaults when no entry fallbacks are configured", () => {
+    const entryModel = undefined;
+    const defaultModel = {
+      primary: "openai/gpt-5-nano",
+      fallbacks: ["google/gemini-2.0-flash"],
+    };
+
+    expect(resolveEffectiveModelFallbacks(entryModel, defaultModel)).toEqual([
+      "google/gemini-2.0-flash",
+    ]);
+  });
+
+  it("prefers entry fallbacks over defaults", () => {
+    const entryModel = {
+      primary: "openai/gpt-5-mini",
+      fallbacks: ["openai/gpt-5-nano"],
+    };
+    const defaultModel = {
+      primary: "openai/gpt-5",
+      fallbacks: ["google/gemini-2.0-flash"],
+    };
+
+    expect(resolveEffectiveModelFallbacks(entryModel, defaultModel)).toEqual(["openai/gpt-5-nano"]);
+  });
+
+  it("keeps explicit empty entry fallback lists", () => {
+    const entryModel = {
+      primary: "openai/gpt-5-mini",
+      fallbacks: [],
+    };
+    const defaultModel = {
+      primary: "openai/gpt-5",
+      fallbacks: ["google/gemini-2.0-flash"],
+    };
+
+    expect(resolveEffectiveModelFallbacks(entryModel, defaultModel)).toEqual([]);
+  });
+});

ui/src/ui/views/agents-utils.ts

@@ -244,6 +244,13 @@ export function resolveModelFallbacks(model?: unknown): string[] | null {
   return null;
 }
 
+export function resolveEffectiveModelFallbacks(
+  entryModel?: unknown,
+  defaultModel?: unknown,
+): string[] | null {
+  return resolveModelFallbacks(entryModel) ?? resolveModelFallbacks(defaultModel);
+}
+
 export function parseFallbackList(value: string): string[] {
   return value
     .split(",")

ui/src/ui/views/agents.ts

@@ -24,7 +24,7 @@ import {
   parseFallbackList,
   resolveAgentConfig,
   resolveAgentEmoji,
-  resolveModelFallbacks,
+  resolveEffectiveModelFallbacks,
   resolveModelLabel,
   resolveModelPrimary,
 } from "./agents-utils.ts";
@@ -390,7 +390,10 @@ function renderAgentOverview(params: {
     resolveModelPrimary(config.defaults?.model) ||
     (defaultModel !== "-" ? normalizeModelValue(defaultModel) : null);
   const effectivePrimary = modelPrimary ?? defaultPrimary ?? null;
-  const modelFallbacks = resolveModelFallbacks(config.entry?.model);
+  const modelFallbacks = resolveEffectiveModelFallbacks(
+    config.entry?.model,
+    config.defaults?.model,
+  );
   const fallbackText = modelFallbacks ? modelFallbacks.join(", ") : "";
   const identityName =
     agentIdentity?.name?.trim() ||