diff --git a/src/adcp/protocols/mcp.py b/src/adcp/protocols/mcp.py
index fa61a20e..4fc6ce21 100644
--- a/src/adcp/protocols/mcp.py
+++ b/src/adcp/protocols/mcp.py
@@ -186,6 +186,40 @@ async def _get_session(self) -> ClientSession:
         else:
             raise ValueError(f"Unsupported transport scheme: {parsed.scheme}")
 
+    def _serialize_mcp_content(self, content: list[Any]) -> list[dict[str, Any]]:
+        """
+        Convert MCP SDK content objects to plain dicts.
+
+        The MCP SDK returns Pydantic objects (TextContent, ImageContent, etc.)
+        but the rest of the ADCP client expects protocol-agnostic dicts.
+        This method handles the translation at the protocol boundary.
+
+        Args:
+            content: List of MCP content items (may be dicts or Pydantic objects)
+
+        Returns:
+            List of plain dicts representing the content
+        """
+        result = []
+        for item in content:
+            # Already a dict, pass through
+            if isinstance(item, dict):
+                result.append(item)
+            # Pydantic v2 model with model_dump()
+            elif hasattr(item, "model_dump"):
+                result.append(item.model_dump())
+            # Pydantic v1 model with dict()
+            elif hasattr(item, "dict") and callable(item.dict):
+                result.append(item.dict())
+            # Fallback: try to access __dict__
+            elif hasattr(item, "__dict__"):
+                result.append(dict(item.__dict__))
+            # Last resort: serialize as unknown type
+            else:
+                logger.warning(f"Unknown MCP content type: {type(item)}, serializing as string")
+                result.append({"type": "unknown", "data": str(item)})
+        return result
+
     async def _call_mcp_tool(self, tool_name: str, params: dict[str, Any]) -> TaskResult[Any]:
         """Call a tool using MCP protocol."""
         start_time = time.time() if self.agent_config.debug else None
@@ -205,12 +239,15 @@ async def _call_mcp_tool(self, tool_name: str, params: dict[str, Any]) -> TaskRe
             # Call the tool using MCP client session
             result = await session.call_tool(tool_name, params)
 
+            # Serialize MCP SDK types to plain dicts at protocol boundary
+            serialized_content = self._serialize_mcp_content(result.content)
+
             if self.agent_config.debug and start_time:
                 duration_ms = (time.time() - start_time) * 1000
                 debug_info = DebugInfo(
                     request=debug_request,
                     response={
-                        "content": result.content,
+                        "content": serialized_content,
                         "is_error": result.isError if hasattr(result, "isError") else False,
                     },
                     duration_ms=duration_ms,
@@ -220,7 +257,7 @@ async def _call_mcp_tool(self, tool_name: str, params: dict[str, Any]) -> TaskRe
             # For AdCP, we expect the data in the content
             return TaskResult[Any](
                 status=TaskStatus.COMPLETED,
-                data=result.content,
+                data=serialized_content,
                 success=True,
                 debug_info=debug_info,
             )
diff --git a/src/adcp/utils/response_parser.py b/src/adcp/utils/response_parser.py
index ad869299..cd60541e 100644
--- a/src/adcp/utils/response_parser.py
+++ b/src/adcp/utils/response_parser.py
@@ -20,10 +20,13 @@ def parse_mcp_content(content: list[dict[str, Any]], response_type: type[T]) ->
     MCP tools return content as a list of content items:
     [{"type": "text", "text": "..."}, {"type": "resource", ...}]
 
+    The MCP adapter is responsible for serializing MCP SDK Pydantic objects
+    to plain dicts before calling this function.
+
     For AdCP, we expect JSON data in text content items.
 
     Args:
-        content: MCP content array
+        content: MCP content array (list of plain dicts)
         response_type: Expected Pydantic model type
 
     Returns:
diff --git a/tests/test_protocols.py b/tests/test_protocols.py
index 7ffd8c70..b10e8013 100644
--- a/tests/test_protocols.py
+++ b/tests/test_protocols.py
@@ -240,3 +240,59 @@ async def test_close_session(self, mcp_config):
         mock_exit_stack.aclose.assert_called_once()
         assert adapter._exit_stack is None
         assert adapter._session is None
+
+    def test_serialize_mcp_content_with_dicts(self, mcp_config):
+        """Test serializing MCP content that's already dicts."""
+        adapter = MCPAdapter(mcp_config)
+
+        content = [
+            {"type": "text", "text": "Hello"},
+            {"type": "resource", "uri": "file://test.txt"},
+        ]
+
+        result = adapter._serialize_mcp_content(content)
+
+        assert result == content  # Pass through unchanged
+        assert len(result) == 2
+
+    def test_serialize_mcp_content_with_pydantic_v2(self, mcp_config):
+        """Test serializing MCP content with Pydantic v2 objects."""
+        from pydantic import BaseModel
+
+        adapter = MCPAdapter(mcp_config)
+
+        class MockTextContent(BaseModel):
+            type: str
+            text: str
+
+        content = [
+            MockTextContent(type="text", text="Pydantic v2"),
+        ]
+
+        result = adapter._serialize_mcp_content(content)
+
+        assert len(result) == 1
+        assert result[0] == {"type": "text", "text": "Pydantic v2"}
+        assert isinstance(result[0], dict)
+
+    def test_serialize_mcp_content_mixed(self, mcp_config):
+        """Test serializing mixed MCP content (dicts and Pydantic objects)."""
+        from pydantic import BaseModel
+
+        adapter = MCPAdapter(mcp_config)
+
+        class MockTextContent(BaseModel):
+            type: str
+            text: str
+
+        content = [
+            {"type": "text", "text": "Plain dict"},
+            MockTextContent(type="text", text="Pydantic object"),
+        ]
+
+        result = adapter._serialize_mcp_content(content)
+
+        assert len(result) == 2
+        assert result[0] == {"type": "text", "text": "Plain dict"}
+        assert result[1] == {"type": "text", "text": "Pydantic object"}
+        assert all(isinstance(item, dict) for item in result)