fix(agents): harden model fallback failover paths

src/agents/model-fallback.test.ts

@@ -8,7 +8,7 @@ import type { AuthProfileStore } from "./auth-profiles.js";
 import { saveAuthProfileStore } from "./auth-profiles.js";
 import { AUTH_STORE_VERSION } from "./auth-profiles/constants.js";
 import { isAnthropicBillingError } from "./live-auth-keys.js";
-import { runWithModelFallback } from "./model-fallback.js";
+import { runWithImageModelFallback, runWithModelFallback } from "./model-fallback.js";
 import { makeModelFallbackCfg } from "./test-helpers/model-fallback-config-fixture.js";
 
 const makeCfg = makeModelFallbackCfg;
@@ -581,6 +581,39 @@ describe("runWithModelFallback", () => {
     expect(calls).toEqual([{ provider: "anthropic", model: "claude-opus-4-5" }]);
   });
 
+  it("keeps explicit fallbacks reachable when models allowlist is present", async () => {
+    const cfg = makeCfg({
+      agents: {
+        defaults: {
+          model: {
+            primary: "anthropic/claude-sonnet-4",
+            fallbacks: ["openai/gpt-4o", "ollama/llama-3"],
+          },
+          models: {
+            "anthropic/claude-sonnet-4": {},
+          },
+        },
+      },
+    });
+    const run = vi
+      .fn()
+      .mockRejectedValueOnce(Object.assign(new Error("rate limited"), { status: 429 }))
+      .mockResolvedValueOnce("ok");
+
+    const result = await runWithModelFallback({
+      cfg,
+      provider: "anthropic",
+      model: "claude-sonnet-4",
+      run,
+    });
+
+    expect(result.result).toBe("ok");
+    expect(run.mock.calls).toEqual([
+      ["anthropic", "claude-sonnet-4"],
+      ["openai", "gpt-4o"],
+    ]);
+  });
+
   it("defaults provider/model when missing (regression #946)", async () => {
     const cfg = makeCfg({
       agents: {
@@ -721,6 +754,39 @@ describe("runWithModelFallback", () => {
   });
 });
 
+describe("runWithImageModelFallback", () => {
+  it("keeps explicit image fallbacks reachable when models allowlist is present", async () => {
+    const cfg = makeCfg({
+      agents: {
+        defaults: {
+          imageModel: {
+            primary: "openai/gpt-image-1",
+            fallbacks: ["google/gemini-2.5-flash-image-preview"],
+          },
+          models: {
+            "openai/gpt-image-1": {},
+          },
+        },
+      },
+    });
+    const run = vi
+      .fn()
+      .mockRejectedValueOnce(new Error("rate limited"))
+      .mockResolvedValueOnce("ok");
+
+    const result = await runWithImageModelFallback({
+      cfg,
+      run,
+    });
+
+    expect(result.result).toBe("ok");
+    expect(run.mock.calls).toEqual([
+      ["openai", "gpt-image-1"],
+      ["google", "gemini-2.5-flash-image-preview"],
+    ]);
+  });
+});
+
 describe("isAnthropicBillingError", () => {
   it("does not false-positive on plain 'a 402' prose", () => {
     const samples = [