match codex image request payload

pkg/providers/oauth/codex_image_generation.go

@@ -14,6 +14,7 @@ import (
 
 const (
 	codexDefaultImageGenerationModel = "gpt-image-2"
+	codexDefaultImageGenerationSize  = "1024x1024"
 	maxImageGenerationResults        = 4
 	maxImageGenerationSSEBytes       = 64 * 1024 * 1024
 	maxImageGenerationEvents         = 512
@@ -93,9 +94,14 @@ func (p *CodexProvider) requestOptions() ([]option.RequestOption, string, error)
 }
 
 func buildCodexImageParams(req ImageGenerationRequest) responses.ResponseNewParams {
+	size := strings.TrimSpace(req.Size)
+	if size == "" {
+		size = codexDefaultImageGenerationSize
+	}
+
 	tool := responses.ToolUnionParam{OfImageGeneration: &responses.ToolImageGenerationParam{
 		Model: req.Model,
-		Size:  req.Size,
+		Size:  size,
 	}}
 	if req.Quality != "" {
 		tool.OfImageGeneration.Quality = req.Quality
@@ -104,10 +110,17 @@ func buildCodexImageParams(req ImageGenerationRequest) responses.ResponseNewPara
 		tool.OfImageGeneration.OutputFormat = req.OutputFormat
 	}
 
+	content := responses.ResponseInputMessageContentListParam{
+		responses.ResponseInputContentParamOfInputText(req.Prompt),
+	}
+	input := responses.ResponseInputParam{
+		responses.ResponseInputItemParamOfMessage(content, responses.EasyInputMessageRoleUser),
+	}
+
 	return responses.ResponseNewParams{
 		Model: "gpt-5.4",
 		Input: responses.ResponseNewParamsInputUnion{
-			OfString: openai.Opt(req.Prompt),
+			OfInputItemList: input,
 		},
 		Instructions: openai.Opt("You are an image generation assistant."),
 		Tools:        []responses.ToolUnionParam{tool},

pkg/providers/oauth/codex_image_generation_test.go

@@ -2,6 +2,8 @@ package oauthprovider
 
 import (
 	"encoding/base64"
+	"encoding/json"
+	"strings"
 	"testing"
 
 	"github.com/openai/openai-go/v3/responses"
@@ -51,6 +53,22 @@ func TestBuildCodexImageParams(t *testing.T) {
 	if params.Model != "gpt-5.4" {
 		t.Fatalf("request model = %q, want gpt-5.4", params.Model)
 	}
+	if params.Input.OfString.Valid() {
+		t.Fatalf("input uses string form, want structured message input")
+	}
+	if len(params.Input.OfInputItemList) != 1 {
+		t.Fatalf("input item count = %d, want 1", len(params.Input.OfInputItemList))
+	}
+	data, err := json.Marshal(params)
+	if err != nil {
+		t.Fatalf("marshal params: %v", err)
+	}
+	payload := string(data)
+	for _, want := range []string{`"input":[`, `"role":"user"`, `"type":"input_text"`, `"text":"make a tiny icon"`} {
+		if !strings.Contains(payload, want) {
+			t.Fatalf("payload missing %s: %s", want, payload)
+		}
+	}
 	if len(params.Tools) != 1 || params.Tools[0].OfImageGeneration == nil {
 		t.Fatalf("expected one image_generation tool, got %#v", params.Tools)
 	}