diff --git a/nanobot/providers/image_generation.py b/nanobot/providers/image_generation.py
index 6cab279b1..1316f1d43 100644
--- a/nanobot/providers/image_generation.py
+++ b/nanobot/providers/image_generation.py
@@ -139,6 +139,11 @@ _IMAGE_GEN_PROVIDERS: dict[str, type[ImageGenerationProvider]] = {}
 
 
 def register_image_gen_provider(cls: type[ImageGenerationProvider]) -> None:
+    """Register an image provider at import time only.
+
+    The registry is populated by module side effects so provider discovery
+    stays lazy and consistent across the process.
+    """
     name = cls.provider_name
     if not name:
         raise ValueError(f"{cls.__name__} must set provider_name")
@@ -229,7 +234,10 @@ class ImageGenerationProvider(ABC):
         *,
         headers: dict[str, str],
         body: dict[str, Any],
+        client: httpx.AsyncClient | None = None,
     ) -> httpx.Response:
+        if client is not None:
+            return await client.post(url, headers=headers, json=body)
         if self._client is not None:
             return await self._client.post(url, headers=headers, json=body)
         async with httpx.AsyncClient(timeout=self.timeout) as c:
@@ -400,10 +408,11 @@ class AIHubMixImageGenerationClient(ImageGenerationProvider):
         model_path = _aihubmix_model_path(model)
         url = f"{self.api_base}/models/{model_path}/predictions"
         try:
-            response = await client.post(
+            response = await self._http_post(
                 url,
                 headers={**headers, "Content-Type": "application/json"},
-                json=body,
+                body=body,
+                client=client,
             )
         except httpx.TimeoutException as exc:
             raise ImageGenerationError("AIHubMix image generation timed out") from exc
@@ -585,9 +594,9 @@ class GeminiImageGenerationClient(ImageGenerationProvider):
         return "https://generativelanguage.googleapis.com/v1beta"
 
     def _resolve_base_url(self, api_base: str | None) -> str:
-        # The Gemini provider's registry default_api_base is the OpenAI-compat
-        # shim (.../v1beta/openai/), which has no image endpoints.
-        # Skip the registry lookup and use the native API base directly.
+        # Gemini chat completions use the registry's OpenAI-compatible shim.
+        # Image generation must hit the native Generative Language API, so we
+        # intentionally bypass the shared registry lookup here.
         if api_base:
             return api_base.rstrip("/")
         return self._default_base_url()
@@ -849,22 +858,16 @@ class MiniMaxImageGenerationClient(ImageGenerationProvider):
 
         body.update(self.extra_body)
 
-        client = self._client or httpx.AsyncClient(timeout=self.timeout)
-        try:
-            return await self._generate_with_client(client, body, headers)
-        finally:
-            if self._client is None:
-                await client.aclose()
+        return await self._generate_with_client(body, headers)
 
     async def _generate_with_client(
         self,
-        client: httpx.AsyncClient,
         body: dict[str, Any],
         headers: dict[str, str],
     ) -> GeneratedImageResponse:
         url = f"{self.api_base}/image_generation"
         try:
-            response = await client.post(url, headers=headers, json=body)
+            response = await self._http_post(url, headers=headers, body=body)
         except httpx.TimeoutException as exc:
             raise ImageGenerationError("MiniMax image generation timed out") from exc
         except httpx.RequestError as exc:
diff --git a/tests/providers/test_image_generation.py b/tests/providers/test_image_generation.py
index f3ca1459c..77025895c 100644
--- a/tests/providers/test_image_generation.py
+++ b/tests/providers/test_image_generation.py
@@ -410,6 +410,11 @@ async def test_gemini_requires_api_key() -> None:
         await client.generate(prompt="draw", model="imagen-4.0-generate-001")
 
 
+def test_gemini_image_client_uses_native_api_base_by_default() -> None:
+    client = GeminiImageGenerationClient(api_key="AIza-test")
+    assert client.api_base == "https://generativelanguage.googleapis.com/v1beta"
+
+
 @pytest.mark.asyncio
 async def test_gemini_no_images_raises() -> None:
     fake = FakeClient(FakeResponse({"candidates": [{"content": {"parts": [{"text": "sorry"}]}}]}))
@@ -452,6 +457,17 @@ async def test_minimax_payload_and_response_with_reference_image(tmp_path: Path)
     assert body["subject_reference"][0]["image_file"].startswith("data:image/png;base64,")
 
 
+@pytest.mark.asyncio
+async def test_minimax_base64_response_uses_detected_mime() -> None:
+    raw_b64 = base64.b64encode(JPEG_BYTES).decode("ascii")
+    fake = FakeClient(FakeResponse({"data": {"image_base64": [raw_b64]}}))
+    client = MiniMaxImageGenerationClient(api_key="sk-mm-test", client=fake)  # type: ignore[arg-type]
+
+    response = await client.generate(prompt="draw", model="image-01")
+
+    assert response.images == [f"data:image/jpeg;base64,{raw_b64}"]
+
+
 # ---------------------------------------------------------------------------
 # StepFun (阶跃星辰)
 # ---------------------------------------------------------------------------
diff --git a/tests/providers/test_litellm_kwargs.py b/tests/providers/test_litellm_kwargs.py
index 924ee0060..6e00cba19 100644
--- a/tests/providers/test_litellm_kwargs.py
+++ b/tests/providers/test_litellm_kwargs.py
@@ -458,6 +458,12 @@ def test_gemma_routes_to_gemini_provider() -> None:
     assert "gemma" in spec.keywords
 
 
+def test_gemini_spec_keeps_openai_compat_base() -> None:
+    spec = find_by_name("gemini")
+    assert spec is not None
+    assert spec.default_api_base == "https://generativelanguage.googleapis.com/v1beta/openai/"
+
+
 async def test_openrouter_sets_default_attribution_headers() -> None:
     spec = find_by_name("openrouter")
     with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI") as mock_client_cls: