diff --git a/nanobot/providers/anthropic_provider.py b/nanobot/providers/anthropic_provider.py
index 01c5c30c..c52600f6 100644
--- a/nanobot/providers/anthropic_provider.py
+++ b/nanobot/providers/anthropic_provider.py
@@ -275,7 +275,16 @@ class AnthropicProvider(LLMProvider):
             blocks.append({"type": "text", "text": content})
         elif isinstance(content, list):
             for item in content:
-                blocks.append(item if isinstance(item, dict) else {"type": "text", "text": str(item)})
+                if isinstance(item, dict):
+                    if not item.get("type"):
+                        # Anthropic requires every content block to declare a "type".
+                        # A tool that returned a bare dict lands here; coerce it to
+                        # a text block instead of emitting one that the API rejects.
+                        blocks.append({"type": "text", "text": str(item)})
+                    else:
+                        blocks.append(item)
+                else:
+                    blocks.append({"type": "text", "text": str(item)})
 
         for tc in msg.get("tool_calls") or []:
             if not isinstance(tc, dict):