Ag2 Hitl

Human-in-the-loop

When to use

• The agent should ask for confirmation before doing something risky.
• The agent needs information from the user mid-conversation (a password, an API key, missing context).
• A specific tool call should require human approval before it runs (irreversible / expensive / sensitive).
• Quality assurance — show a draft, get human edits/approval before finalising.

Two distinct mechanisms — pick by intent:

Pattern 1 — context.input() for open questions

A tool requests input via Context.input(message, timeout=...). The agent must have a hitl_hook that knows how to collect that input.

from ag2 import Agent, Context, tool
from ag2.events import HumanInputRequest, HumanMessage

@tool
async def execute_query(context: Context) -> str:
    answer = await context.input(
        "Are you sure you want to run this query? (yes/no)",
        timeout=60.0,
    )
    if answer.strip().lower() != "yes":
        return "Query cancelled."
    return "Query executed successfully."

def hitl_hook(event: HumanInputRequest) -> HumanMessage:
    print(f"Agent asks: {event.content}")
    return HumanMessage(content=input("Your answer: "))

agent = Agent("dba", tools=[execute_query], hitl_hook=hitl_hook)

The hook receives a HumanInputRequest (the prompt is in event.content) and returns either a HumanMessage or a plain str (the framework wraps a str via HumanMessage.ensure_message). Both def and async def hooks are supported.

You can also register the hook after construction:

agent = Agent("dba", tools=[execute_query])

@agent.hitl_hook
async def async_hitl_hook(event: HumanInputRequest) -> HumanMessage:
    answer = await collect_from_ui(event.content)
    return HumanMessage(content=answer)

The decorator overrides any hook set in the constructor — but if one was already set (e.g. via the constructor), applying @agent.hitl_hook emits a RuntimeWarning ("You already set HITL hook, provided value overrides it"). Set the hook in exactly one place to avoid the warning.

The hook participates in dependency injection — Context, Inject, Variable, Depends work the same as in tools.

If context.input() is called and no hook is registered, the framework raises HumanInputNotProvidedError.

Pattern 2 — approval_required() for specific tool calls

Gate a single tool with the built-in approval middleware. The user is prompted before the tool body runs and can approve or deny.

from ag2 import Agent, tool
from ag2.config import OpenAIConfig
from ag2.middleware import approval_required

@tool(middleware=[approval_required()])
def delete_account(user_id: str) -> str:
    """Deletes a user account by ID permanently."""
    return f"Account {user_id} deleted."

agent = Agent(
    "support",
    config=OpenAIConfig(model="gpt-4o-mini"),
    tools=[delete_account],
    hitl_hook=lambda event: input(event.content),
)

When the agent calls delete_account, the user sees (with the default allow_always=True):

Agent wants to call the tool:
`delete_account`, {"user_id": "abc-123"}
Please approve or deny this request.
Y/N/Always?

The answer is lowercased before matching. y, yes, or 1 approve this one call; always approves this call and all subsequent calls of the same tool in the same context (it sets a per-context bypass flag). Anything else denies it; the agent receives denied_message (default "User denied the tool call request") and can adjust. The default timeout is 30 seconds. Set allow_always=False to drop the "Always" option (the prompt then shows just Y/N?).

approval_required() calls context.input() under the hood, so it also requires a hitl_hook — without one you'll get a runtime error.

Custom prompt

@tool(middleware=[approval_required(
    message="⚠️ Run `{tool_name}` with {tool_arguments}? (y/n)",
    denied_message="Operation blocked by user.",
)])
def send_email(to: str, subject: str, body: str) -> str:
    """Send an email to the given address."""
    ...

{tool_name} and {tool_arguments} are interpolated.

Pairing both patterns

For a tool that both gathers input mid-run and requires approval:

@tool(middleware=[approval_required()])
async def schedule_report(name: str, context: Context) -> str:
    """Schedule a report — asks the user for the cadence, then runs after approval."""
    cadence = await context.input("How often? (daily / weekly / monthly)")
    return f"Scheduled '{name}' on {cadence} cadence."

The approval middleware runs first (outermost). Once approved, the tool body executes and context.input() triggers a second human interaction.

Going deeper

• Source docs: website/docs/user-guide/context/human_in_the_loop.mdx (context.input, hitl_hook), website/docs/user-guide/tools/approval_required.mdx (approval_required middleware).
• Tool middleware in general — website/docs/user-guide/tools/tool_middleware.mdx. See also ag2-middleware for agent-wide HITL interception via BaseMiddleware.on_human_input().
• HITL hooks support dependency injection identically to tools — see ../ag2-add-custom-tool/references/dependency_injection.md.

Common pitfalls

• approval_required() without a hitl_hook — the middleware calls context.input(), so the agent needs a hook. You'll see HumanInputNotProvidedError otherwise.
• Forgetting to handle the denial path — context.input() returns whatever the hook returns. If you only branch on "yes", any other answer (including silence/default) lets the operation continue. Always validate.
• Sync input() in an async UI — input() blocks the event loop. Use an async hook (async def) and an async input collector (web socket, message queue) for any non-CLI app.
• No timeout — context.input(prompt) can wait forever. Pass timeout=60.0 (seconds) for any production path.
• Decorator hook overrides constructor hook — if you set both, the decorator wins and emits a RuntimeWarning. Pick one place.
• Expecting HumanMessage to flow into the conversation history automatically — it does for the requesting tool's return value, but mid-run inputs collected via ctx.input() are not separate user turns. They live in the tool's scope.