docs: set default anthropic model (#21988)

`ChatAnthropic()` raises ValidationError.

docs/docs/how_to/fallbacks.ipynb

@@ -90,8 +90,8 @@
    "outputs": [],
    "source": [
     "# Note that we set max_retries = 0 to avoid retrying on RateLimits, etc\n",
-    "openai_llm = ChatOpenAI(max_retries=0)\n",
-    "anthropic_llm = ChatAnthropic()\n",
+    "openai_llm = ChatOpenAI(model=\"gpt-3.5-turbo-0125\", max_retries=0)\n",
+    "anthropic_llm = ChatAnthropic(model=\"claude-3-haiku-20240307\")\n",
     "llm = openai_llm.with_fallbacks([anthropic_llm])"
    ]
   },

libs/core/langchain_core/prompts/few_shot.py

@@ -305,7 +305,7 @@ class FewShotChatMessagePromptTemplate(
 
             # Use within an LLM
             from langchain_core.chat_models import ChatAnthropic
-            chain = final_prompt | ChatAnthropic()
+            chain = final_prompt | ChatAnthropic(model="claude-3-haiku-20240307")
             chain.invoke({"input": "What's 3+3?"})
     """
 

libs/core/langchain_core/runnables/fallbacks.py

@@ -61,7 +61,9 @@ class RunnableWithFallbacks(RunnableSerializable[Input, Output]):
             from langchain_core.chat_models.openai import ChatOpenAI
             from langchain_core.chat_models.anthropic import ChatAnthropic
 
-            model = ChatAnthropic().with_fallbacks([ChatOpenAI()])
+            model = ChatAnthropic(
+                model="claude-3-haiku-20240307"
+            ).with_fallbacks([ChatOpenAI(model="gpt-3.5-turbo-0125")])
             # Will usually use ChatAnthropic, but fallback to ChatOpenAI
             # if ChatAnthropic fails.
             model.invoke('hello')

libs/langchain/langchain/agents/self_ask_with_search/base.py

@@ -117,7 +117,7 @@ def create_self_ask_with_search_agent(
             )
 
             prompt = hub.pull("hwchase17/self-ask-with-search")
-            model = ChatAnthropic()
+            model = ChatAnthropic(model="claude-3-haiku-20240307")
             tools = [...]  # Should just be one tool with name `Intermediate Answer`
 
             agent = create_self_ask_with_search_agent(model, tools, prompt)

libs/langchain/langchain/agents/xml/base.py

@@ -147,7 +147,7 @@ def create_xml_agent(
             from langchain.agents import AgentExecutor, create_xml_agent
 
             prompt = hub.pull("hwchase17/xml-agent-convo")
-            model = ChatAnthropic()
+            model = ChatAnthropic(model="claude-3-haiku-20240307")
             tools = ...
 
             agent = create_xml_agent(model, tools, prompt)