diff --git a/nanobot/providers/anthropic_oauth.py b/nanobot/providers/anthropic_oauth.py
index a840faa..c14eb03 100644
--- a/nanobot/providers/anthropic_oauth.py
+++ b/nanobot/providers/anthropic_oauth.py
@@ -148,6 +148,9 @@ class AnthropicOAuthProvider(LLMProvider):
 
             if role == "user":
                 content = msg.get("content", "")
+                # Convert OpenAI image_url blocks to Anthropic image blocks
+                if isinstance(content, list):
+                    content = self._convert_image_blocks(content)
                 # Merge text into previous user message if it has tool_result blocks
                 # (handles the "Reflect on the results" interleaved message)
                 if converted and converted[-1].get("role") == "user":
@@ -167,6 +170,33 @@ class AnthropicOAuthProvider(LLMProvider):
         system_prompt = "\n\n".join(system_parts)
         return system_prompt, converted
 
+    @staticmethod
+    def _convert_image_blocks(content: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Convert OpenAI image_url blocks to Anthropic image blocks.
+
+        OpenAI format:  {"type": "image_url", "image_url": {"url": "data:mime;base64,DATA"}}
+        Anthropic format: {"type": "image", "source": {"type": "base64", "media_type": "mime", "data": "DATA"}}
+        """
+        converted = []
+        for block in content:
+            if block.get("type") == "image_url":
+                url = block.get("image_url", {}).get("url", "")
+                if url.startswith("data:") and ";base64," in url:
+                    header, data = url.split(";base64,", 1)
+                    media_type = header.removeprefix("data:")
+                    converted.append({
+                        "type": "image",
+                        "source": {"type": "base64", "media_type": media_type, "data": data},
+                    })
+                else:
+                    converted.append({
+                        "type": "image",
+                        "source": {"type": "url", "url": url},
+                    })
+            else:
+                converted.append(block)
+        return converted
+
     def _convert_tools_to_anthropic(
         self,
         tools: list[dict[str, Any]] | None