Langchain Middleware

<overview>
Middleware patterns for production LangChain agents:

- **HumanInTheLoopMiddleware** / **humanInTheLoopMiddleware**: Pause before dangerous tool calls for human approval
- **Custom middleware**: Intercept tool calls for error handling, logging, retry logic
- **Command resume**: Continue execution after human decisions (approve, edit, reject)

**Requirements:** Checkpointer + thread_id config for all HITL workflows.
</overview>

---

## Human-in-the-Loop

<ex-basic-hitl-setup>
<python>
Set up an agent with HITL middleware that pauses before sending emails for approval.

```python
from langchain.agents import create_agent
from langchain.agents.middleware import HumanInTheLoopMiddleware
from langgraph.checkpoint.memory import MemorySaver
from langchain.tools import tool

@tool
def send_email(to: str, subject: str, body: str) -> str:
    """Send an email."""
    return f"Email sent to {to}"

agent = create_agent(
    model="gpt-4.1",
    tools=[send_email],
    checkpointer=MemorySaver(),  # Required for HITL
    middleware=[
        HumanInTheLoopMiddleware(
            interrupt_on={
                "send_email": {"allowed_decisions": ["approve", "edit", "reject"]},
            }
        )
    ],
)
```
</python>
<typescript>
Set up an agent with HITL that pauses before sending emails for human approval.

```typescript
import { createAgent, humanInTheLoopMiddleware } from "langchain";
import { MemorySaver } from "@langchain/langgraph";
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const sendEmail = tool(
  async ({ to, subject, body }) => `Email sent to ${to}`,
  {
    name: "send_email",
    description: "Send an email",
    schema: z.object({ to: z.string(), subject: z.string(), body: z.string() }),
  }
);

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  tools: [sendEmail],
  checkpointer: new MemorySaver(),
  middleware: [
    humanInTheLoopMiddleware({
      interruptOn: { send_email: { allowedDecisions: ["approve", "edit", "reject"] } },
    }),
  ],
});
```
</typescript>
</ex-basic-hitl-setup>

<ex-running-with-interrupts>
<python>
Run the agent, detect an interrupt, then resume execution after human approval.

```python
from langgraph.types import Command

config = {"configurable": {"thread_id": "session-1"}}

# Step 1: Agent runs until it needs to call tool
result1 = agent.invoke({
    "messages": [{"role": "user", "content": "Send email to john@example.com"}]
}, config=config)

# Check for interrupt
if "__interrupt__" in result1:
    print(f"Waiting for approval: {result1['__interrupt__']}")

# Step 2: Human approves
result2 = agent.invoke(
    Command(resume={"decisions": [{"type": "approve"}]}),
    config=config
)
```
</python>
<typescript>
Run the agent, detect an interrupt, then resume execution after human approval.

```typescript
import { Command } from "@langchain/langgraph";

const config = { configurable: { thread_id: "session-1" } };

// Step 1: Agent runs until it needs to call tool
const result1 = await agent.invoke({
  messages: [{ role: "user", content: "Send email to john@example.com" }]
}, config);

// Check for interrupt
if (result1.__interrupt__) {
  console.log(`Waiting for approval: ${result1.__interrupt__}`);
}

// Step 2: Human approves
const result2 = await agent.invoke(
  new Command({ resume: { decisions: [{ type: "approve" }] } }),
  config
);
```
</typescript>
</ex-running-with-interrupts>

<ex-editing-tool-arguments>
<python>
Edit the tool arguments before approving when the original values need correction.

```python
# Human edits the arguments — edited_action must include name + args
result2 = agent.invoke