fix(providers): support lookup-based fallback candidate resolution

pkg/providers/fallback.go

@@ -43,11 +43,26 @@ func NewFallbackChain(cooldown *CooldownTracker) *FallbackChain {
 
 // ResolveCandidates parses model config into a deduplicated candidate list.
 func ResolveCandidates(cfg ModelConfig, defaultProvider string) []FallbackCandidate {
+	return ResolveCandidatesWithLookup(cfg, defaultProvider, nil)
+}
+
+func ResolveCandidatesWithLookup(
+	cfg ModelConfig,
+	defaultProvider string,
+	lookup func(raw string) (resolved string, ok bool),
+) []FallbackCandidate {
 	seen := make(map[string]bool)
 	var candidates []FallbackCandidate
 
 	addCandidate := func(raw string) {
-		ref := ParseModelRef(raw, defaultProvider)
+		candidateRaw := strings.TrimSpace(raw)
+		if lookup != nil {
+			if resolved, ok := lookup(candidateRaw); ok {
+				candidateRaw = resolved
+			}
+		}
+
+		ref := ParseModelRef(candidateRaw, defaultProvider)
 		if ref == nil {
 			return
 		}

pkg/providers/fallback_test.go

@@ -453,6 +453,75 @@ func TestResolveCandidates_EmptyPrimary(t *testing.T) {
 	}
 }
 
+func TestResolveCandidatesWithLookup_AliasResolvesToNestedModel(t *testing.T) {
+	cfg := ModelConfig{
+		Primary:   "step-3.5-flash",
+		Fallbacks: nil,
+	}
+
+	lookup := func(raw string) (string, bool) {
+		if raw == "step-3.5-flash" {
+			return "openrouter/stepfun/step-3.5-flash:free", true
+		}
+		return "", false
+	}
+
+	candidates := ResolveCandidatesWithLookup(cfg, "", lookup)
+	if len(candidates) != 1 {
+		t.Fatalf("candidates = %d, want 1", len(candidates))
+	}
+	if candidates[0].Provider != "openrouter" {
+		t.Fatalf("provider = %q, want openrouter", candidates[0].Provider)
+	}
+	if candidates[0].Model != "stepfun/step-3.5-flash:free" {
+		t.Fatalf("model = %q, want stepfun/step-3.5-flash:free", candidates[0].Model)
+	}
+}
+
+func TestResolveCandidatesWithLookup_DeduplicateAfterLookup(t *testing.T) {
+	cfg := ModelConfig{
+		Primary:   "step-3.5-flash",
+		Fallbacks: []string{"openrouter/stepfun/step-3.5-flash:free"},
+	}
+
+	lookup := func(raw string) (string, bool) {
+		if raw == "step-3.5-flash" {
+			return "openrouter/stepfun/step-3.5-flash:free", true
+		}
+		return "", false
+	}
+
+	candidates := ResolveCandidatesWithLookup(cfg, "", lookup)
+	if len(candidates) != 1 {
+		t.Fatalf("candidates = %d, want 1", len(candidates))
+	}
+}
+
+func TestResolveCandidatesWithLookup_AliasWithoutProtocolUsesDefaultProvider(t *testing.T) {
+	cfg := ModelConfig{
+		Primary:   "glm-5",
+		Fallbacks: nil,
+	}
+
+	lookup := func(raw string) (string, bool) {
+		if raw == "glm-5" {
+			return "glm-5", true
+		}
+		return "", false
+	}
+
+	candidates := ResolveCandidatesWithLookup(cfg, "openai", lookup)
+	if len(candidates) != 1 {
+		t.Fatalf("candidates = %d, want 1", len(candidates))
+	}
+	if candidates[0].Provider != "openai" {
+		t.Fatalf("provider = %q, want openai", candidates[0].Provider)
+	}
+	if candidates[0].Model != "glm-5" {
+		t.Fatalf("model = %q, want glm-5", candidates[0].Model)
+	}
+}
+
 func TestFallbackExhaustedError_Message(t *testing.T) {
 	e := &FallbackExhaustedError{
 		Attempts: []FallbackAttempt{