test: add tests for multi-part AIMessage content extraction

Verifies langchain_messages_to_copilotkit correctly concatenates all text
parts from list-style content blocks (Anthropic models). Includes
regression tests for the exact scenario from CopilotKit#1748 (text + image blocks)
and tests for string lists, mixed content, empty lists, and edge cases.

Author: jpr5

sdk-python/tests/test_langchain_message_content.py

@@ -0,0 +1,191 @@
+"""Tests for multi-part content handling in langchain_messages_to_copilotkit.
+
+Covers the fix in PR #3844 / issue #1748: when AIMessage.content is a list
+of content blocks (e.g. Anthropic models), all text parts must be extracted
+and concatenated — not just the first element.
+"""
+
+import pytest
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
+
+from copilotkit.langgraph import langchain_messages_to_copilotkit
+
+
+class TestMultiPartContentList:
+    """AIMessage.content as a list should concatenate all text parts."""
+
+    def test_list_of_text_dicts(self):
+        """Multiple {"type": "text", "text": "..."} dicts are all concatenated."""
+        msg = AIMessage(
+            id="ai-1",
+            content=[
+                {"type": "text", "text": "Hello "},
+                {"type": "text", "text": "world"},
+            ],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Hello world"
+        assert result[0]["role"] == "assistant"
+
+    def test_list_of_strings(self):
+        """Content list of plain strings should be concatenated."""
+        msg = AIMessage(
+            id="ai-2",
+            content=["Part A", " Part B"],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Part A Part B"
+
+    def test_mixed_strings_and_text_dicts(self):
+        """Mix of plain strings and text dicts should all be concatenated."""
+        msg = AIMessage(
+            id="ai-3",
+            content=[
+                "Start ",
+                {"type": "text", "text": "middle "},
+                {"text": "end"},
+            ],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Start middle end"
+
+    def test_non_text_parts_are_skipped(self):
+        """Non-text content blocks (e.g. images) should be ignored."""
+        msg = AIMessage(
+            id="ai-4",
+            content=[
+                {"type": "text", "text": "Sample png file"},
+                {
+                    "type": "image",
+                    "image_data": {"data": "base64data", "format": "image/png"},
+                },
+            ],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Sample png file"
+
+    def test_empty_list_returns_empty_content(self):
+        """Empty content list should produce empty string, not crash."""
+        msg = AIMessage(
+            id="ai-5",
+            content=[],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        # Empty content means message is not included (content is falsy)
+        assert len(result) == 0
+
+    def test_single_text_dict_in_list(self):
+        """Single text dict in a list should still be extracted."""
+        msg = AIMessage(
+            id="ai-6",
+            content=[{"type": "text", "text": "Only one part"}],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Only one part"
+
+    def test_dict_without_type_but_with_text_key(self):
+        """A dict with "text" key but no "type" should still have text extracted."""
+        msg = AIMessage(
+            id="ai-7",
+            content=[{"text": "no type field"}],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "no type field"
+
+
+class TestSingleDictContent:
+    """AIMessage.content as a single dict (Anthropic style) should extract text.
+
+    Note: langchain_core.messages.AIMessage validates content as str | list,
+    so a raw dict cannot be passed directly. We use a mock to exercise the
+    dict-handling code path in langchain_messages_to_copilotkit, which exists
+    to handle edge cases from deserialized or non-standard message objects.
+    """
+
+    def test_dict_with_text_key(self):
+        """A content dict with "text" key should have its text extracted."""
+        from unittest.mock import MagicMock
+
+        msg = MagicMock(spec=AIMessage)
+        msg.content = {"text": "dict content"}
+        msg.id = "ai-8"
+        msg.tool_calls = []
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "dict content"
+
+
+class TestPlainStringContent:
+    """Standard string content should still work as before."""
+
+    def test_plain_string_content(self):
+        """Normal string content passes through unchanged."""
+        msg = AIMessage(
+            id="ai-9",
+            content="Just a string",
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Just a string"
+
+    def test_human_message_string(self):
+        """HumanMessage with string content still works."""
+        msg = HumanMessage(id="human-1", content="Hello")
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+        assert result[0]["content"] == "Hello"
+
+    def test_system_message_string(self):
+        """SystemMessage with string content still works."""
+        msg = SystemMessage(id="sys-1", content="System prompt")
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["role"] == "system"
+        assert result[0]["content"] == "System prompt"
+
+
+class TestIssue1748Reproduction:
+    """Directly reproduces the scenario from issue #1748.
+
+    The original bug: when content is a list of dicts including an image block,
+    only the first element was taken via `content[0]`, which was the dict itself,
+    not a string. This caused the message to be silently dropped or mangled.
+    """
+
+    def test_text_and_image_content_preserves_text(self):
+        """The exact scenario from issue #1748: text + image content blocks."""
+        msg = AIMessage(
+            id="ai-repro",
+            content=[
+                {"type": "text", "text": "Sample png file"},
+                {
+                    "type": "image",
+                    "image_data": {"data": "aW1hZ2VfZGF0YQ==", "format": "image/png"},
+                },
+            ],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "Sample png file"
+        assert result[0]["role"] == "assistant"
+
+    def test_multiple_text_parts_are_not_truncated(self):
+        """The core bug: only the first element was kept. All text must survive."""
+        msg = AIMessage(
+            id="ai-trunc",
+            content=[
+                {"type": "text", "text": "First part. "},
+                {"type": "text", "text": "Second part. "},
+                {"type": "text", "text": "Third part."},
+            ],
+        )
+        result = langchain_messages_to_copilotkit([msg])
+        assert len(result) == 1
+        assert result[0]["content"] == "First part. Second part. Third part."