fix: default model fallback when provider/model missing (#954) (thanks @roshanasingh4)

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
 

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 = String(params.provider ?? "").trim() || DEFAULT_PROVIDER;
-  const model = String(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;