fix: handle BinaryImage and UserContent types as attachments in cast()

src/marvin/fns/cast.py

@@ -5,13 +5,24 @@
 
 from typing import Any, TypeVar
 
+from pydantic_ai.messages import (
+    AudioUrl,
+    BinaryContent,
+    DocumentUrl,
+    ImageUrl,
+    VideoUrl,
+)
+
 import marvin
 from marvin.agents.agent import Agent
 from marvin.handlers.handlers import AsyncHandler, Handler
 from marvin.thread import Thread
 from marvin.utilities.asyncio import run_sync
 from marvin.utilities.types import TargetType
 
+# Non-string UserContent types that should be passed as attachments
+_ATTACHMENT_TYPES = (ImageUrl, AudioUrl, DocumentUrl, VideoUrl, BinaryContent)
+
 T = TypeVar("T")
 
 DEFAULT_PROMPT = """
@@ -83,14 +94,24 @@ async def cast_async(
         raise ValueError("Instructions are required when casting to string values.")
 
     task_context = context or {}
-    task_context["Data to transform"] = data
+    attachments = []
+
+    # Handle non-string UserContent types (images, audio, etc.) as attachments
+    # to avoid serializing binary data into the prompt text
+    if isinstance(data, _ATTACHMENT_TYPES):
+        attachments.append(data)
+        task_context["Data to transform"] = "(provided as attachment)"
+    else:
+        task_context["Data to transform"] = data
+
     prompt = prompt or DEFAULT_PROMPT
     if instructions:
         prompt += f"\n\nYou must follow these instructions for your transformation:\n{instructions}"
 
     task = marvin.Task[target](
         name="Cast Task",
         instructions=prompt,
+        attachments=attachments,
         context=task_context,
         result_type=target,
         agents=[agent] if agent else None,

tests/basic/fns/test_cast.py

@@ -0,0 +1,110 @@
+"""Tests for cast function - basic unit tests."""
+
+from pydantic_ai.messages import BinaryImage, ImageUrl
+
+import marvin
+from marvin.tasks.task import Task
+
+
+class TestCastWithAttachments:
+    """Test that cast properly handles attachment types like images."""
+
+    def test_binary_image_passed_as_attachment(self, test_model):
+        """Test that BinaryImage is passed as attachment, not in context."""
+        binary_image = BinaryImage(data=b"fake image data", media_type="image/png")
+
+        # We need to inspect the task that gets created
+        original_task_init = Task.__init__
+        captured_task = None
+
+        def capture_task(self, *args, **kwargs):
+            nonlocal captured_task
+            original_task_init(self, *args, **kwargs)
+            captured_task = self
+
+        Task.__init__ = capture_task
+
+        try:
+            marvin.cast(
+                binary_image,
+                target=str,
+                instructions="Describe the image",
+            )
+        finally:
+            Task.__init__ = original_task_init
+
+        # Verify the task was constructed correctly
+        assert captured_task is not None
+        assert len(captured_task.attachments) == 1
+        assert captured_task.attachments[0] is binary_image
+        assert captured_task.context["Data to transform"] == "(provided as attachment)"
+
+    def test_image_url_passed_as_attachment(self, test_model):
+        """Test that ImageUrl is passed as attachment, not in context."""
+        image_url = ImageUrl(url="https://example.com/image.png")
+
+        original_task_init = Task.__init__
+        captured_task = None
+
+        def capture_task(self, *args, **kwargs):
+            nonlocal captured_task
+            original_task_init(self, *args, **kwargs)
+            captured_task = self
+
+        Task.__init__ = capture_task
+
+        try:
+            marvin.cast(
+                image_url,
+                target=str,
+                instructions="Describe the image",
+            )
+        finally:
+            Task.__init__ = original_task_init
+
+        assert captured_task is not None
+        assert len(captured_task.attachments) == 1
+        assert captured_task.attachments[0] is image_url
+        assert captured_task.context["Data to transform"] == "(provided as attachment)"
+
+    def test_string_data_not_treated_as_attachment(self, test_model):
+        """Test that string data is still passed in context, not as attachment."""
+        original_task_init = Task.__init__
+        captured_task = None
+
+        def capture_task(self, *args, **kwargs):
+            nonlocal captured_task
+            original_task_init(self, *args, **kwargs)
+            captured_task = self
+
+        Task.__init__ = capture_task
+
+        try:
+            marvin.cast("hello world", target=int)
+        finally:
+            Task.__init__ = original_task_init
+
+        assert captured_task is not None
+        assert len(captured_task.attachments) == 0
+        assert captured_task.context["Data to transform"] == "hello world"
+
+    def test_dict_data_not_treated_as_attachment(self, test_model):
+        """Test that dict data is still passed in context, not as attachment."""
+        original_task_init = Task.__init__
+        captured_task = None
+
+        def capture_task(self, *args, **kwargs):
+            nonlocal captured_task
+            original_task_init(self, *args, **kwargs)
+            captured_task = self
+
+        Task.__init__ = capture_task
+
+        try:
+            marvin.cast({"key": "value"}, target=str, instructions="Convert to JSON")
+        finally:
+            Task.__init__ = original_task_init
+
+        assert captured_task is not None
+        assert len(captured_task.attachments) == 0
+        assert captured_task.context["Data to transform"] == {"key": "value"}