diff --git a/sdk/translator/registry.go b/sdk/translator/registry.go
index 98909c10..e3d5182d 100644
--- a/sdk/translator/registry.go
+++ b/sdk/translator/registry.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"sync"
 
+	log "github.com/sirupsen/logrus"
 	"github.com/tidwall/gjson"
 	"github.com/tidwall/sjson"
 )
@@ -55,7 +56,9 @@ func (r *Registry) TranslateRequest(from, to Format, model string, rawJSON []byt
 		}
 	}
 	if model != "" && gjson.GetBytes(rawJSON, "model").String() != model {
-		if updated, err := sjson.SetBytes(rawJSON, "model", model); err == nil {
+		if updated, err := sjson.SetBytes(rawJSON, "model", model); err != nil {
+			log.Warnf("translator: failed to normalize model in request fallback: %v", err)
+		} else {
 			return updated
 		}
 	}
diff --git a/sdk/translator/registry_test.go b/sdk/translator/registry_test.go
new file mode 100644
index 00000000..1cd4fb12
--- /dev/null
+++ b/sdk/translator/registry_test.go
@@ -0,0 +1,92 @@
+package translator
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestTranslateRequest_FallbackNormalizesModel(t *testing.T) {
+	r := NewRegistry()
+
+	tests := []struct {
+		name          string
+		model         string
+		payload       string
+		wantModel     string
+		wantUnchanged bool
+	}{
+		{
+			name:      "prefixed model is rewritten",
+			model:     "gpt-5-mini",
+			payload:   `{"model":"copilot/gpt-5-mini","input":"ping"}`,
+			wantModel: "gpt-5-mini",
+		},
+		{
+			name:          "matching model is left unchanged",
+			model:         "gpt-5-mini",
+			payload:       `{"model":"gpt-5-mini","input":"ping"}`,
+			wantModel:     "gpt-5-mini",
+			wantUnchanged: true,
+		},
+		{
+			name:          "empty model leaves payload unchanged",
+			model:         "",
+			payload:       `{"model":"copilot/gpt-5-mini","input":"ping"}`,
+			wantModel:     "copilot/gpt-5-mini",
+			wantUnchanged: true,
+		},
+		{
+			name:      "deeply prefixed model is rewritten",
+			model:     "gpt-5.3-codex",
+			payload:   `{"model":"team/gpt-5.3-codex","stream":true}`,
+			wantModel: "gpt-5.3-codex",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input := []byte(tt.payload)
+			got := r.TranslateRequest(Format("a"), Format("b"), tt.model, input, false)
+
+			gotModel := gjson.GetBytes(got, "model").String()
+			if gotModel != tt.wantModel {
+				t.Errorf("model = %q, want %q", gotModel, tt.wantModel)
+			}
+
+			if tt.wantUnchanged && string(got) != tt.payload {
+				t.Errorf("payload was modified when it should not have been:\ngot:  %s\nwant: %s", got, tt.payload)
+			}
+
+			// Verify other fields are preserved.
+			for _, key := range []string{"input", "stream"} {
+				orig := gjson.Get(tt.payload, key)
+				if !orig.Exists() {
+					continue
+				}
+				after := gjson.GetBytes(got, key)
+				if orig.Raw != after.Raw {
+					t.Errorf("field %q changed: got %s, want %s", key, after.Raw, orig.Raw)
+				}
+			}
+		})
+	}
+}
+
+func TestTranslateRequest_RegisteredTransformTakesPrecedence(t *testing.T) {
+	r := NewRegistry()
+	from := Format("openai-response")
+	to := Format("openai-response")
+
+	r.Register(from, to, func(model string, rawJSON []byte, stream bool) []byte {
+		return []byte(`{"model":"from-transform"}`)
+	}, ResponseTransform{})
+
+	input := []byte(`{"model":"copilot/gpt-5-mini","input":"ping"}`)
+	got := r.TranslateRequest(from, to, "gpt-5-mini", input, false)
+
+	gotModel := gjson.GetBytes(got, "model").String()
+	if gotModel != "from-transform" {
+		t.Errorf("expected registered transform to take precedence, got model = %q", gotModel)
+	}
+}