langchain fundamentals

<create_agent>

Creating Agents with create_agent

create_agent() is the recommended way to build agents. It handles the agent loop, tool execution, and state management.

Agent Configuration Options

Parameter Purpose Example

model

LLM to use "anthropic:claude-sonnet-4-5" or model instance

tools

List of tools [search, calculator]

system_prompt / systemPrompt

Agent instructions "You are a helpful assistant"

checkpointer

State persistence MemorySaver()

middleware

Processing hooks [HumanInTheLoopMiddleware] (Python) / [humanInTheLoopMiddleware({...})] (TypeScript)

</create_agent>

@tool def get_weather(location: str) -> str: """Get current weather for a location.

Args: location: City name """ return f"Weather in {location}: Sunny, 72F"

agent = create_agent( model="anthropic:claude-sonnet-4-5", tools=[get_weather], system_prompt="You are a helpful assistant." )

result = agent.invoke({ "messages": [{"role": "user", "content": "What's the weather in Paris?"}] }) print(result["messages"][-1].content)

</python> <typescript>

import { createAgent } from "langchain";
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const getWeather = tool(
  async ({ location }) => `Weather in ${location}: Sunny, 72F`,
  {
    name: "get_weather",
    description: "Get current weather for a location.",
    schema: z.object({ location: z.string().describe("City name") }),
  }
);

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  tools: [getWeather],
  systemPrompt: "You are a helpful assistant.",
});

const result = await agent.invoke({
  messages: [{ role: "user", content: "What's the weather in Paris?" }],
});
console.log(result.messages[result.messages.length - 1].content);

checkpointer = MemorySaver()

agent = create_agent(
model="anthropic:claude-sonnet-4-5",
tools=[search],
checkpointer=checkpointer,
)

config = {"configurable": {"thread_id": "user-123"}}
agent.invoke({"messages": [{"role": "user", "content": "My name is Alice"}]}, config=config)
result = agent.invoke({"messages": [{"role": "user", "content": "What's my name?"}]}, config=config)

Agent remembers: "Your name is Alice"

&#x3C;/python>
&#x3C;typescript>
Add MemorySaver checkpointer to maintain conversation state across invocations.
```typescript
import { createAgent } from "langchain";
import { MemorySaver } from "@langchain/langgraph";

const checkpointer = new MemorySaver();

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  tools: [search],
  checkpointer,
});

const config = { configurable: { thread_id: "user-123" } };
await agent.invoke({ messages: [{ role: "user", content: "My name is Alice" }] }, config);
const result = await agent.invoke({ messages: [{ role: "user", content: "What's my name?" }] }, config);
// Agent remembers: "Your name is Alice"

Tools are functions that agents can call. Use the @tool
 decorator (Python) or tool()
 function (TypeScript).

@tool
def add(a: float, b: float) -> float:
"""Add two numbers.

Args:
    a: First number
    b: Second number
"""
return a + b

&#x3C;/python>
&#x3C;typescript>
```typescript
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const add = tool(
  async ({ a, b }) => a + b,
  {
    name: "add",
    description: "Add two numbers.",
    schema: z.object({
      a: z.number().describe("First number"),
      b: z.number().describe("Second number"),
    }),
  }
);

Middleware intercepts the agent loop to add human approval, error handling, logging, and more. A deep understanding of middleware is essential for production agents — use HumanInTheLoopMiddleware
 (Python) / humanInTheLoopMiddleware
 (TypeScript) for approval workflows, and @wrap_tool_call
 (Python) / createMiddleware
 (TypeScript) for custom hooks.

Key imports:

from langchain.agents.middleware import HumanInTheLoopMiddleware, wrap_tool_call

import { humanInTheLoopMiddleware, createMiddleware } from "langchain";

Key patterns:

- HITL: middleware=[HumanInTheLoopMiddleware(interrupt_on={"dangerous_tool": True})]
 — requires checkpointer
 + thread_id

- Resume after interrupt: agent.invoke(Command(resume={"decisions": [{"type": "approve"}]}), config=config)

- Custom middleware: @wrap_tool_call
 decorator (Python) or createMiddleware({ wrapToolCall: ... })
 (TypeScript)

&#x3C;structured_output>

Structured Output

Get typed, validated responses from agents using response_format
 or with_structured_output()
.

class ContactInfo(BaseModel):
name: str
email: str
phone: str = Field(description="Phone number with area code")

Option 1: Agent with structured output

agent = create_agent(model="gpt-4.1", tools=[search], response_format=ContactInfo)
result = agent.invoke({"messages": [{"role": "user", "content": "Find contact for John"}]})
print(result["structured_response"])  # ContactInfo(name='John', ...)

Option 2: Model-level structured output (no agent needed)

from langchain_openai import ChatOpenAI
model = ChatOpenAI(model="gpt-4.1")
structured_model = model.with_structured_output(ContactInfo)
response = structured_model.invoke("Extract: John, john@example.com, 555-1234")

ContactInfo(name='John', email='john@example.com', phone='555-1234')

&#x3C;/python>
&#x3C;typescript>
```typescript
import { ChatOpenAI } from "@langchain/openai";
import { z } from "zod";

const ContactInfo = z.object({
  name: z.string(),
  email: z.string().email(),
  phone: z.string().describe("Phone number with area code"),
});

// Model-level structured output
const model = new ChatOpenAI({ model: "gpt-4.1" });
const structuredModel = model.withStructuredOutput(ContactInfo);
const response = await structuredModel.invoke("Extract: John, john@example.com, 555-1234");
// { name: 'John', email: 'john@example.com', phone: '555-1234' }

&#x3C;model_config>

Model Configuration

create_agent
 accepts model strings ("anthropic:claude-sonnet-4-5"
, "openai:gpt-4.1"
) or model instances for custom settings:

from langchain_anthropic import ChatAnthropic
agent = create_agent(model=ChatAnthropic(model="claude-sonnet-4-5", temperature=0), tools=[...])

&#x3C;/model_config>

CORRECT: Clear, specific description with Args

@tool
def search(query: str) -> str:
"""Search the web for current information about a topic.

Use this when you need recent data or facts.

Args:
    query: The search query (2-10 words recommended)
"""
return web_search(query)

&#x3C;/python>
&#x3C;typescript>
Clear descriptions help the agent know when to use each tool.
```typescript
// WRONG: Vague description
const badTool = tool(async ({ input }) => "result", {
  name: "bad_tool",
  description: "Does stuff.", // Too vague!
  schema: z.object({ input: z.string() }),
});

// CORRECT: Clear, specific description
const search = tool(async ({ query }) => webSearch(query), {
  name: "search",
  description: "Search the web for current information about a topic. Use this when you need recent data or facts.",
  schema: z.object({
    query: z.string().describe("The search query (2-10 words recommended)"),
  }),
});

CORRECT: Add checkpointer and thread_id

from langgraph.checkpoint.memory import MemorySaver

agent = create_agent(
model="anthropic:claude-sonnet-4-5",
tools=[search],
checkpointer=MemorySaver(),
)
config = {"configurable": {"thread_id": "session-1"}}
agent.invoke({"messages": [{"role": "user", "content": "I'm Bob"}]}, config=config)
agent.invoke({"messages": [{"role": "user", "content": "What's my name?"}]}, config=config)

Agent remembers: "Your name is Bob"

&#x3C;/python>
&#x3C;typescript>
Add checkpointer and thread_id for conversation memory across invocations.
```typescript
// WRONG: No persistence
const agent = createAgent({ model: "anthropic:claude-sonnet-4-5", tools: [search] });
await agent.invoke({ messages: [{ role: "user", content: "I'm Bob" }] });
await agent.invoke({ messages: [{ role: "user", content: "What's my name?" }] });
// Agent doesn't remember!

// CORRECT: Add checkpointer and thread_id
import { MemorySaver } from "@langchain/langgraph";

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  tools: [search],
  checkpointer: new MemorySaver(),
});
const config = { configurable: { thread_id: "session-1" } };
await agent.invoke({ messages: [{ role: "user", content: "I'm Bob" }] }, config);
await agent.invoke({ messages: [{ role: "user", content: "What's my name?" }] }, config);
// Agent remembers: "Your name is Bob"

CORRECT: Set recursion_limit in config

result = agent.invoke(
{"messages": [("user", "Do research")]},
config={"recursion_limit": 10},  # Stop after 10 steps
)

&#x3C;/python>
&#x3C;typescript>
Set recursionLimit in the invoke config to prevent runaway agent loops.
```typescript
// WRONG: No iteration limit
const result = await agent.invoke({ messages: [["user", "Do research"]] });

// CORRECT: Set recursionLimit in config
const result = await agent.invoke(
  { messages: [["user", "Do research"]] },
  { recursionLimit: 10 }, // Stop after 10 steps
);

CORRECT: Access messages from result dict

result = agent.invoke({"messages": [{"role": "user", "content": "Hello"}]})
print(result["messages"][-1].content)  # Last message content

&#x3C;/python>
&#x3C;typescript>
Access the messages array from the result, not result.content directly.
```typescript
// WRONG: Trying to access result.content directly
const result = await agent.invoke({ messages: [{ role: "user", content: "Hello" }] });
console.log(result.content); // undefined!

// CORRECT: Access messages from result object
const result = await agent.invoke({ messages: [{ role: "user", content: "Hello" }] });
console.log(result.messages[result.messages.length - 1].content); // Last message content

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