diff --git a/CHANGELOG.md b/CHANGELOG.md
index e975296d6..0452e772b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ## 2026.1.15 (unreleased)
 
-- TBD
+- Fix: guard model fallback against undefined provider/model values. (#954) — thanks @roshanasingh4.
 
 ## 2026.1.14-1
 
diff --git a/src/agents/model-fallback.test.ts b/src/agents/model-fallback.test.ts
index 160deb7e1..381da47ff 100644
--- a/src/agents/model-fallback.test.ts
+++ b/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
diff --git a/src/agents/model-fallback.ts b/src/agents/model-fallback.ts
index f3d7e7d52..469f45a2c 100644
--- a/src/agents/model-fallback.ts
+++ b/src/agents/model-fallback.ts
@@ -119,8 +119,6 @@ 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 model = params.model.trim() || DEFAULT_MODEL;
   const primary = params.cfg
     ? resolveConfiguredModelRef({
         cfg: params.cfg,
@@ -128,11 +126,15 @@ function resolveFallbackCandidates(params: {
         defaultModel: DEFAULT_MODEL,
       })
     : null;
+  const defaultProvider = primary?.provider ?? DEFAULT_PROVIDER;
+  const defaultModel = primary?.model ?? DEFAULT_MODEL;
+  const provider = String(params.provider ?? "").trim() || defaultProvider;
+  const model = String(params.model ?? "").trim() || defaultModel;
   const aliasIndex = buildModelAliasIndex({
     cfg: params.cfg ?? {},
-    defaultProvider: DEFAULT_PROVIDER,
+    defaultProvider,
   });
-  const allowlist = buildAllowedModelKeys(params.cfg, DEFAULT_PROVIDER);
+  const allowlist = buildAllowedModelKeys(params.cfg, defaultProvider);
   const seen = new Set<string>();
   const candidates: ModelCandidate[] = [];
 
@@ -160,7 +162,7 @@ function resolveFallbackCandidates(params: {
   for (const raw of modelFallbacks) {
     const resolved = resolveModelRefFromString({
       raw: String(raw ?? ""),
-      defaultProvider: DEFAULT_PROVIDER,
+      defaultProvider,
       aliasIndex,
     });
     if (!resolved) continue;