fix(go/plugins/compat_oai): prevent message duplication when using media parts (#3773)

Author: hugoaguirre

go/plugins/compat_oai/generate.go

@@ -79,7 +79,6 @@ func (g *ModelGenerator) WithMessages(messages []*ai.Message) *ModelGenerator {
 		case ai.RoleSystem:
 			oaiMessages = append(oaiMessages, openai.SystemMessage(content))
 		case ai.RoleModel:
-
 			am := openai.ChatCompletionAssistantMessageParam{}
 			am.Content.OfString = param.NewOpt(content)
 			toolCalls, err := convertToolCalls(msg.Content)
@@ -113,10 +112,11 @@ func (g *ModelGenerator) WithMessages(messages []*ai.Message) *ModelGenerator {
 				oaiMessages = append(oaiMessages, tm)
 			}
 		case ai.RoleUser:
-			oaiMessages = append(oaiMessages, openai.UserMessage(content))
-
 			parts := []openai.ChatCompletionContentPartUnionParam{}
 			for _, p := range msg.Content {
+				if p.IsText() {
+					parts = append(parts, openai.TextContentPart(p.Text))
+				}
 				if p.IsMedia() {
 					part := openai.ImageContentPart(
 						openai.ChatCompletionContentPartImageImageURLParam{

go/plugins/compat_oai/openai/openai_live_test.go

@@ -16,7 +16,10 @@ package openai_test
 
 import (
 	"context"
+	"encoding/base64"
+	"io"
 	"math"
+	"net/http"
 	"os"
 	"strings"
 	"testing"
@@ -279,4 +282,64 @@ func TestPlugin(t *testing.T) {
 			t.Fatalf("expecting 2 user messages, got: %d", userMsgCount)
 		}
 	})
+	t.Run("image", func(t *testing.T) {
+		image, err := fetchImgAsBase64()
+		if err != nil {
+			t.Fatalf("failed to fetch image: %v", err)
+		}
+		resp, err := genkit.Generate(ctx, g,
+			ai.WithModelName("openai/gpt-4.1-nano"),
+			ai.WithMessages(
+				ai.NewUserMessage(
+					ai.NewMediaPart("image/jpeg", "data:image/jpeg;base64,"+image),
+					ai.NewTextPart("What's in the image?."),
+				),
+			),
+		)
+		if err != nil {
+			t.Fatalf("failed to generate: %v", err)
+		}
+		if !strings.Contains(resp.Text(), "cat") {
+			t.Fatalf("image detection failed, want: cat, got: %s", resp.Text())
+		}
+		mediaMessages := 0
+		textMessages := 0
+		for _, m := range resp.Request.Messages {
+			for _, p := range m.Content {
+				if p.IsText() {
+					textMessages += 1
+				}
+				if p.IsMedia() {
+					mediaMessages += 1
+				}
+			}
+		}
+		if mediaMessages > 1 {
+			t.Fatalf("unwanted media message, want: 1, got: %d", mediaMessages)
+		}
+		if textMessages > 1 {
+			t.Fatalf("unwanted text message, want: 1, got %d", textMessages)
+		}
+	})
+}
+
+func fetchImgAsBase64() (string, error) {
+	// CC0 license image
+	imgURL := "https://pd.w.org/2025/07/896686fbbcd9990c9.84605288-2048x1365.jpg"
+	resp, err := http.Get(imgURL)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return "", err
+	}
+
+	imageBytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", err
+	}
+
+	base64string := base64.StdEncoding.EncodeToString(imageBytes)
+	return base64string, nil
 }