fix: filter image_url for non-vision models at provider layer

- Add  field to ProviderSpec (default True)
- Add  and  methods in LiteLLMProvider
- Filter image_url content blocks in  before sending to non-vision models
- Reverts session-layer filtering from original PR (wrong layer)

This fixes the issue where switching from Claude (vision-capable) to
non-vision models (e.g., Baidu Qianfan) causes API errors due to
unsupported image_url content blocks.

The provider layer is the correct place for this filtering because:
1. It has access to model/provider capabilities
2. It only affects non-vision models
3. It preserves session layer purity (storage should not know about model capabilities)

nanobot/providers/litellm_provider.py

@@ -124,6 +124,32 @@ class LiteLLMProvider(LLMProvider):
         spec = find_by_model(model)
         return spec is not None and spec.supports_prompt_caching
 
+    def _supports_vision(self, model: str) -> bool:
+        """Return True when the provider supports vision/image inputs."""
+        if self._gateway is not None:
+            return self._gateway.supports_vision
+        spec = find_by_model(model)
+        return spec is None or spec.supports_vision  # default True for unknown providers
+
+    @staticmethod
+    def _filter_image_url(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Replace image_url content blocks with [image] placeholder for non-vision models."""
+        filtered = []
+        for msg in messages:
+            content = msg.get("content")
+            if isinstance(content, list):
+                new_content = []
+                for block in content:
+                    if isinstance(block, dict) and block.get("type") == "image_url":
+                        # Replace image with placeholder text
+                        new_content.append({"type": "text", "text": "[image]"})
+                    else:
+                        new_content.append(block)
+                filtered.append({**msg, "content": new_content})
+            else:
+                filtered.append(msg)
+        return filtered
+
     def _apply_cache_control(
         self,
         messages: list[dict[str, Any]],
@@ -234,6 +260,10 @@ class LiteLLMProvider(LLMProvider):
         model = self._resolve_model(original_model)
         extra_msg_keys = self._extra_msg_keys(original_model, model)
 
+        # Filter image_url for non-vision models
+        if not self._supports_vision(original_model):
+            messages = self._filter_image_url(messages)
+
         if self._supports_cache_control(original_model):
             messages, tools = self._apply_cache_control(messages, tools)
 

nanobot/providers/registry.py

@@ -61,6 +61,9 @@ class ProviderSpec:
     # Provider supports cache_control on content blocks (e.g. Anthropic prompt caching)
     supports_prompt_caching: bool = False
 
+    # Provider supports vision/image inputs (most modern models do)
+    supports_vision: bool = True
+
     @property
     def label(self) -> str:
         return self.display_name or self.name.title()