diff --git a/go/ai/document.go b/go/ai/document.go
index 1c6407ef17..28f9fb6d19 100644
--- a/go/ai/document.go
+++ b/go/ai/document.go
@@ -298,11 +298,12 @@ func (p *Part) unmarshalPartFromSchema(s partSchema) {
 	default:
 		p.Kind = PartText
 		p.Text = s.Text
-		p.ContentType = ""
+		p.ContentType = "plain/text"
 		if s.Data != "" {
 			// Note: if part is completely empty, we use text by default.
 			p.Kind = PartData
 			p.Text = s.Data
+			p.ContentType = ""
 		}
 	}
 	p.Metadata = s.Metadata
diff --git a/go/ai/option.go b/go/ai/option.go
index d28c68e3e9..1d041e9507 100644
--- a/go/ai/option.go
+++ b/go/ai/option.go
@@ -204,7 +204,17 @@ func (o *commonGenOptions) applyGenerate(genOpts *generateOptions) error {
 func WithMessages(messages ...*Message) CommonGenOption {
 	return &commonGenOptions{
 		MessagesFn: func(ctx context.Context, _ any) ([]*Message, error) {
-			return messages, nil
+			buf, err := json.Marshal(messages)
+			if err != nil {
+				return nil, err
+			}
+
+			msgs := make([]*Message, 0, len(messages))
+			if err := json.Unmarshal(buf, &msgs); err != nil {
+				return nil, err
+			}
+
+			return msgs, nil
 		},
 	}
 }
diff --git a/go/ai/prompt_test.go b/go/ai/prompt_test.go
index cd9be3f60b..d46f16a3dd 100644
--- a/go/ai/prompt_test.go
+++ b/go/ai/prompt_test.go
@@ -3292,3 +3292,34 @@ Hello {{name}}, please help me with {{task}}.
 		}
 	})
 }
+
+func TestPromptWithDifferentInput(t *testing.T) {
+	reg := registry.New()
+	prompt, err := LoadPromptFromSource(reg, `Hello, {{name}}!`, "rawPrompt", "test-ns")
+	if err != nil {
+		t.Fatalf("LoadPromptFromRaw failed: %v", err)
+	}
+
+	if prompt == nil {
+		t.Fatal("prompt is nil")
+	}
+
+	for i := range 10 {
+		opts, err := prompt.Render(t.Context(), map[string]int{
+			"name": i,
+		})
+
+		if err != nil {
+			t.Fatalf("Render failed on iteration %d: %v", i, err)
+		}
+
+		if len(opts.Messages) != 1 {
+			t.Fatalf("Expected 1 message, got %d", len(opts.Messages))
+		}
+
+		expected := fmt.Sprintf("Hello, %d!", i)
+		if opts.Messages[0].Text() != expected {
+			t.Errorf("Iteration %d: got %q, want %q", i, opts.Messages[0].Text(), expected)
+		}
+	}
+}