diff --git a/sdk/translator/registry.go b/sdk/translator/registry.go
index ace97137..98909c10 100644
--- a/sdk/translator/registry.go
+++ b/sdk/translator/registry.go
@@ -3,6 +3,9 @@ package translator
 import (
 	"context"
 	"sync"
+
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
 )
 
 // Registry manages translation functions across schemas.
@@ -39,7 +42,9 @@ func (r *Registry) Register(from, to Format, request RequestTransform, response
 }
 
 // TranslateRequest converts a payload between schemas, returning the original payload
-// if no translator is registered.
+// if no translator is registered. When falling back to the original payload, the
+// "model" field is still updated to match the resolved model name so that
+// client-side prefixes (e.g. "copilot/gpt-5-mini") are not leaked upstream.
 func (r *Registry) TranslateRequest(from, to Format, model string, rawJSON []byte, stream bool) []byte {
 	r.mu.RLock()
 	defer r.mu.RUnlock()
@@ -49,6 +54,11 @@ func (r *Registry) TranslateRequest(from, to Format, model string, rawJSON []byt
 			return fn(model, rawJSON, stream)
 		}
 	}
+	if model != "" && gjson.GetBytes(rawJSON, "model").String() != model {
+		if updated, err := sjson.SetBytes(rawJSON, "model", model); err == nil {
+			return updated
+		}
+	}
 	return rawJSON
 }