fix: use image[] field for multi-reference edits

OpenAI documents the multipart array syntax as image[] for multiple
references; strict gateways reject repeated 'image' parts with
duplicate_parameter 400. Use 'image' for single, 'image[]' for 2+.

AGENTS.md

@@ -53,7 +53,10 @@ Three files do everything:
   - Upstream dispatch:
     - `referenceImages` present → `POST {baseURL}/images/edits` as
       `multipart/form-data` (blobs decoded from data URLs via
-      `decodeDataUrl` → `Uint8Array<ArrayBuffer>`).
+      `decodeDataUrl` → `Uint8Array<ArrayBuffer>`). Single reference uses
+      field name `image`; **two or more references use `image[]`** to
+      match OpenAI's documented array syntax — strict gateways reject
+      repeated `image` parts with a `duplicate_parameter` 400.
     - Otherwise → `POST {baseURL}/images/generations` as JSON.
     - Always sends `stream: true, partial_images: 2` first. On a 400 that
       mentions `stream` or `partial_images` (see

index.ts

@@ -49,6 +49,7 @@ async function callUpstream(args: {
 			form.append("stream", "true");
 			form.append("partial_images", "2");
 		}
+		const imageField = referenceImages.length > 1 ? "image[]" : "image";
 		for (let i = 0; i < referenceImages.length; i++) {
 			const dataUrl = referenceImages[i];
 			if (!dataUrl) continue;
@@ -56,7 +57,7 @@ async function callUpstream(args: {
 			if (!decoded) continue;
 			const ext = decoded.mime.split("/")[1] ?? "png";
 			form.append(
-				"image",
+				imageField,
 				new Blob([decoded.bytes], { type: decoded.mime }),
 				`ref-${i}.${ext}`,
 			);