fix(image): pass aspect ratio and size to Gemini Flash image models

The Gemini Flash image path (`generateContent`) dropped both `aspect_ratio`
and `image_size`: `generate()` never forwarded them and
`_generate_gemini_flash` did not accept them, so every request fell back to
1:1 / input-matched output. The Imagen path was unaffected.

Forward the hints and emit them under
`generationConfig.responseFormat.image` per the current Gemini API. Aspect
ratio is validated against the accepted set; `imageSize` is validated against
{512,1K,2K,4K} and only sent to Gemini 3+ image models, since
`gemini-2.5-flash-image` supports only `aspectRatio`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nanobot/providers/image_generation.py

@@ -33,6 +33,13 @@ _AIHUBMIX_ASPECT_RATIO_SIZES = {
 }
 _GEMINI_DEFAULT_TIMEOUT_S = 120.0
 _GEMINI_IMAGEN_ASPECT_RATIOS = {"1:1", "9:16", "16:9", "3:4", "4:3"}
+# Aspect ratios accepted by the Gemini Flash image (generateContent) models.
+_GEMINI_FLASH_ASPECT_RATIOS = {
+    "1:1", "2:3", "3:2", "3:4", "4:3", "4:5", "5:4",
+    "9:16", "16:9", "21:9", "1:4", "4:1", "1:8", "8:1",
+}
+# Image-size tokens accepted by Gemini 3+ image models (2.5 Flash Image ignores it).
+_GEMINI_FLASH_IMAGE_SIZES = {"512", "1K", "2K", "4K"}
 _OLLAMA_DEFAULT_SIDE = 1024
 _OLLAMA_SIZE_PRESETS = {
     "1K": 1024,
@@ -635,7 +642,11 @@ class GeminiImageGenerationClient(ImageGenerationProvider):
                 prompt=prompt, model=model, aspect_ratio=aspect_ratio
             )
         return await self._generate_gemini_flash(
-            prompt=prompt, model=model, reference_images=reference_images or []
+            prompt=prompt,
+            model=model,
+            reference_images=reference_images or [],
+            aspect_ratio=aspect_ratio,
+            image_size=image_size,
         )
 
     async def _generate_imagen(
@@ -691,15 +702,22 @@ class GeminiImageGenerationClient(ImageGenerationProvider):
         prompt: str,
         model: str,
         reference_images: list[str],
+        aspect_ratio: str | None = None,
+        image_size: str | None = None,
     ) -> GeneratedImageResponse:
         parts: list[dict[str, Any]] = [
             {"inlineData": image_path_to_inline_data(path)} for path in reference_images
         ]
         parts.append({"text": prompt})
 
+        generation_config: dict[str, Any] = {"responseModalities": ["TEXT", "IMAGE"]}
+        image_config = _gemini_flash_image_config(model, aspect_ratio, image_size)
+        if image_config:
+            generation_config["responseFormat"] = {"image": image_config}
+
         body: dict[str, Any] = {
             "contents": [{"role": "user", "parts": parts}],
-            "generationConfig": {"responseModalities": ["TEXT", "IMAGE"]},
+            "generationConfig": generation_config,
         }
         body.update(self.extra_body)
 
@@ -748,6 +766,31 @@ class GeminiImageGenerationClient(ImageGenerationProvider):
         )
 
 
+def _gemini_flash_image_config(
+    model: str,
+    aspect_ratio: str | None,
+    image_size: str | None,
+) -> dict[str, str]:
+    """Build the ``responseFormat.image`` config for Gemini Flash image models.
+
+    Aspect ratio applies to all Flash image models; image size is only honored
+    by Gemini 3+ image models (``gemini-2.5-flash-image`` ignores it).
+    """
+    config: dict[str, str] = {}
+    if aspect_ratio and aspect_ratio in _GEMINI_FLASH_ASPECT_RATIOS:
+        config["aspectRatio"] = aspect_ratio
+    if image_size and _gemini_flash_supports_image_size(model):
+        normalized = image_size.strip().upper()
+        if normalized in _GEMINI_FLASH_IMAGE_SIZES:
+            config["imageSize"] = normalized
+    return config
+
+
+def _gemini_flash_supports_image_size(model: str) -> bool:
+    """Return whether the model honors ``imageSize`` (Gemini 3+ image models)."""
+    return "2.5" not in model.lower()
+
+
 async def _aihubmix_images_from_payload(
     client: httpx.AsyncClient,
     payload: dict[str, Any],

tests/providers/test_image_generation.py

@@ -422,6 +422,63 @@ async def test_gemini_flash_reference_images(tmp_path: Path) -> None:
     assert parts[1] == {"text": "edit this"}
 
 
+def _gemini_flash_image_response() -> FakeResponse:
+    return FakeResponse(
+        {
+            "candidates": [
+                {"content": {"parts": [{"inlineData": {"mimeType": "image/png", "data": RAW_B64}}]}}
+            ]
+        }
+    )
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_forwards_aspect_ratio_and_image_size() -> None:
+    fake = FakeClient(_gemini_flash_image_response())
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    await client.generate(
+        prompt="draw a cat",
+        model="gemini-3-pro-image",
+        aspect_ratio="16:9",
+        image_size="2K",
+    )
+
+    image_config = fake.calls[0]["json"]["generationConfig"]["responseFormat"]["image"]
+    assert image_config == {"aspectRatio": "16:9", "imageSize": "2K"}
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_2_5_drops_image_size() -> None:
+    fake = FakeClient(_gemini_flash_image_response())
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    await client.generate(
+        prompt="draw a cat",
+        model="gemini-2.5-flash-image",
+        aspect_ratio="4:3",
+        image_size="1K",
+    )
+
+    image_config = fake.calls[0]["json"]["generationConfig"]["responseFormat"]["image"]
+    assert image_config == {"aspectRatio": "4:3"}
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_ignores_unsupported_hints() -> None:
+    fake = FakeClient(_gemini_flash_image_response())
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    await client.generate(
+        prompt="draw a cat",
+        model="gemini-3-pro-image",
+        aspect_ratio="7:5",
+        image_size="1024x1024",
+    )
+
+    assert "responseFormat" not in fake.calls[0]["json"]["generationConfig"]
+
+
 @pytest.mark.asyncio
 async def test_gemini_requires_api_key() -> None:
     client = GeminiImageGenerationClient(api_key=None)