anthropic[patch]: always add tool_result type to ToolMessage content (#22721)

Anthropic tool results can contain image data, which are typically
represented with content blocks having `"type": "image"`. Currently,
these content blocks are passed as-is as human/user messages to
Anthropic, which raises BadRequestError as it expects a tool_result
block to follow a tool_use.

Here we update ChatAnthropic to nest the content blocks inside a
tool_result content block.

Example:
```python
import base64

import httpx
from langchain_anthropic import ChatAnthropic
from langchain_core.messages import AIMessage, HumanMessage, ToolMessage
from langchain_core.pydantic_v1 import BaseModel, Field


# Fetch image
image_url = "https://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Gfp-wisconsin-madison-the-nature-boardwalk.jpg/2560px-Gfp-wisconsin-madison-the-nature-boardwalk.jpg"
image_data = base64.b64encode(httpx.get(image_url).content).decode("utf-8")


class FetchImage(BaseModel):
    should_fetch: bool = Field(..., description="Whether an image is requested.")


llm = ChatAnthropic(model="claude-3-sonnet-20240229").bind_tools([FetchImage])

messages = [
    HumanMessage(content="Could you summon a beautiful image please?"),
    AIMessage(
        content=[
            {
                "type": "tool_use",
                "id": "toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
                "name": "FetchImage",
                "input": {"should_fetch": True},
            },
        ],
        tool_calls=[
            {
                "name": "FetchImage",
                "args": {"should_fetch": True},
                "id": "toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
            },
        ],
    ),
    ToolMessage(
        name="FetchImage",
        content=[
            {
                "type": "image",
                "source": {
                    "type": "base64",
                    "media_type": "image/jpeg",
                    "data": image_data,
                },
            },
        ],
        tool_call_id="toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
    ),
]

llm.invoke(messages)
```

Trace:
https://smith.langchain.com/public/d27e4fc1-a96d-41e1-9f52-54f5004122db/r

libs/partners/anthropic/langchain_anthropic/chat_models.py

@@ -104,7 +104,12 @@ def _merge_messages(
     for curr in messages:
         curr = curr.copy(deep=True)
         if isinstance(curr, ToolMessage):
-            if isinstance(curr.content, str):
+            if isinstance(curr.content, list) and all(
+                isinstance(block, dict) and block.get("type") == "tool_result"
+                for block in curr.content
+            ):
+                curr = HumanMessage(curr.content)  # type: ignore[misc]
+            else:
                 curr = HumanMessage(  # type: ignore[misc]
                     [
                         {
@@ -114,8 +119,6 @@ def _merge_messages(
                         }
                     ]
                 )
-            else:
-                curr = HumanMessage(curr.content)  # type: ignore[misc]
         last = merged[-1] if merged else None
         if isinstance(last, HumanMessage) and isinstance(curr, HumanMessage):
             if isinstance(last.content, str):

libs/partners/anthropic/tests/unit_tests/test_chat_models.py

@@ -140,7 +140,19 @@ def test__merge_messages() -> None:
             ]
         ),
         ToolMessage("buz output", tool_call_id="1"),  # type: ignore[misc]
-        ToolMessage("blah output", tool_call_id="2"),  # type: ignore[misc]
+        ToolMessage(
+            content=[
+                {
+                    "type": "image",
+                    "source": {
+                        "type": "base64",
+                        "media_type": "image/jpeg",
+                        "data": "fake_image_data",
+                    },
+                },
+            ],
+            tool_call_id="2",
+        ),  # type: ignore[misc]
         HumanMessage("next thing"),  # type: ignore[misc]
     ]
     expected = [
@@ -169,7 +181,20 @@ def test__merge_messages() -> None:
         HumanMessage(  # type: ignore[misc]
             [
                 {"type": "tool_result", "content": "buz output", "tool_use_id": "1"},
-                {"type": "tool_result", "content": "blah output", "tool_use_id": "2"},
+                {
+                    "type": "tool_result",
+                    "content": [
+                        {
+                            "type": "image",
+                            "source": {
+                                "type": "base64",
+                                "media_type": "image/jpeg",
+                                "data": "fake_image_data",
+                            },
+                        },
+                    ],
+                    "tool_use_id": "2",
+                },
                 {"type": "text", "text": "next thing"},
             ]
         ),
@@ -177,6 +202,27 @@ def test__merge_messages() -> None:
     actual = _merge_messages(messages)
     assert expected == actual
 
+    # Test tool message case
+    messages = [
+        ToolMessage("buz output", tool_call_id="1"),  # type: ignore[misc]
+        ToolMessage(  # type: ignore[misc]
+            content=[
+                {"type": "tool_result", "content": "blah output", "tool_use_id": "2"}
+            ],
+            tool_call_id="2",
+        ),
+    ]
+    expected = [
+        HumanMessage(  # type: ignore[misc]
+            [
+                {"type": "tool_result", "content": "buz output", "tool_use_id": "1"},
+                {"type": "tool_result", "content": "blah output", "tool_use_id": "2"},
+            ]
+        )
+    ]
+    actual = _merge_messages(messages)
+    assert expected == actual
+
 
 def test__merge_messages_mutation() -> None:
     original_messages = [