Protect retained metadata from custom JSON encoders

internal/llm/openai_compatible_client.go

@@ -17,6 +17,7 @@ import (
 
 	"go.maximumdirect.net/promptkit/internal/defaults"
 	"go.maximumdirect.net/promptkit/internal/domain"
+	"go.maximumdirect.net/promptkit/internal/jsonvalue"
 )
 
 var (
@@ -125,6 +126,9 @@ func (c *OpenAICompatibleClient) PrepareGeneration(req domain.GenerateRequest) (
 		return nil, fmt.Errorf("%w: invalid endpoint path: %v", ErrInvalidRequest, err)
 	}
 
+	// Custom JSON encoders may mutate their receivers. Both payload encoding
+	// and encoding-failure diagnostics must use values detached from metadata.
+	req.Target.ExtraParams = jsonvalue.CloneTrusted(req.Target.ExtraParams).(map[string]any)
 	wirePayload, err := openAIChatRequestPayload(req)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)