diff --git a/AGENTS.md b/AGENTS.md
index 9634f29..8bfdcc9 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -91,6 +91,14 @@ Three files do everything:
   - Targets the **gpt-image series only** (gpt-image-2 default). Do not
     reintroduce DALL·E-only fields like `response_format` — gpt-image
     always returns `b64_json`.
+  - `gpt-image-2` size constraints (per the OpenAI cookbook): both edges
+    multiple of 16, max edge **< 3840**, long/short ≤ 3:1, total pixels
+    in 655,360–8,294,400, `auto` not supported. Exact 16:9 requires
+    `16k × 9k` with `k` a multiple of 16 (so `1280×720`, `1536×864`,
+    `2048×1152`, `2560×1440`, …). Sizes above 2560×1440 are
+    experimental — the popular 4K target `3840×2160` violates the
+    `< 3840` rule, round down to `3824×2144` if you need it. Common
+    misses: `1920×1080` is **not valid** (1080 % 16 ≠ 0).
 - `client.ts` — browser entry, loaded via `<script type="module"
   src="./client.ts">` in `index.html`. Bun's bundler resolves the import,
   inlines `@microsoft/fetch-event-source`, and serves the bundle from
diff --git a/index.html b/index.html
index fd19eda..5b75a28 100644
--- a/index.html
+++ b/index.html
@@ -199,10 +199,19 @@
 				<div class="row">
 					<label for="size">Size</label>
 					<select id="size">
-						<option value="1024x1024">1024x1024 (square)</option>
-						<option value="1536x1024">1536x1024 (landscape)</option>
-						<option value="1024x1536">1024x1536 (portrait)</option>
+						<option value="1024x1024">1024x1024 (1:1 square)</option>
+						<option value="1536x1024">1536x1024 (3:2 landscape)</option>
+						<option value="1024x1536">1024x1536 (2:3 portrait)</option>
+						<option value="1536x864">1536x864 (16:9 landscape)</option>
+						<option value="864x1536">864x1536 (9:16 portrait)</option>
+						<option value="2048x1152">2048x1152 (16:9 2K landscape)</option>
+						<option value="1152x2048">1152x2048 (9:16 2K portrait)</option>
+						<option value="2560x1440">2560x1440 (16:9 QHD — experimental boundary)</option>
 					</select>
+					<small class="hint">
+						Sizes above 2560x1440 are experimental for <code>gpt-image-2</code>.
+						Both edges must be a multiple of 16; max edge &lt; 3840.
+					</small>
 				</div>
 				<div class="row">
 					<label for="prompt">Prompt</label>