Fix model fallback crash on undefined provider/model (#946)

src/agents/model-fallback.test.ts

@@ -213,6 +213,34 @@ describe("runWithModelFallback", () => {
     expect(calls).toEqual([{ provider: "anthropic", model: "claude-opus-4-5" }]);
   });
 
+  it("defaults provider/model when missing (regression #946)", async () => {
+    const cfg = makeCfg({
+      agents: {
+        defaults: {
+          model: {
+            primary: "openai/gpt-4.1-mini",
+            fallbacks: [],
+          },
+        },
+      },
+    });
+
+    const calls: Array<{ provider: string; model: string }> = [];
+
+    const result = await runWithModelFallback({
+      cfg,
+      provider: undefined as unknown as string,
+      model: undefined as unknown as string,
+      run: async (provider, model) => {
+        calls.push({ provider, model });
+        return "ok";
+      },
+    });
+
+    expect(result.result).toBe("ok");
+    expect(calls).toEqual([{ provider: "openai", model: "gpt-4.1-mini" }]);
+  });
+
   it("falls back on missing API key errors", async () => {
     const cfg = makeCfg();
     const run = vi

src/agents/model-fallback.ts

@@ -119,8 +119,8 @@ function resolveFallbackCandidates(params: {
   /** Optional explicit fallbacks list; when provided (even empty), replaces agents.defaults.model.fallbacks. */
   fallbacksOverride?: string[];
 }): ModelCandidate[] {
-  const provider = params.provider.trim() || DEFAULT_PROVIDER;
+  const provider = String(params.provider ?? "").trim() || DEFAULT_PROVIDER;
-  const model = params.model.trim() || DEFAULT_MODEL;
+  const model = String(params.model ?? "").trim() || DEFAULT_MODEL;
   const primary = params.cfg
     ? resolveConfiguredModelRef({
         cfg: params.cfg,