Human approval

Mark a tool needs_approval: true, or give it a predicate on the arguments, and the run suspends instead of executing it. Nothing sleeps and no thread waits: the run returns immediately with the call parked.

send_gift = Mistri::Tool.define(
  "send_gift", "Sends a real gift.",
  needs_approval: ->(args) { args["amount"].to_i > 100 },
) do |args|
  Gifts.send!(args)
end

result = agent.run("Send Ana a $200 gift")
result.awaiting_approval?  # => true; nothing executed
result.pending             # the calls waiting on a decision

Deciding

The decision is a one-line session write from any process, any time later. A controller action is enough:

session = Mistri::Session.new(store:, id: session_id)
session.approve(call_id)
session.deny(call_id, note: "over budget this quarter")

Resuming

resume settles the parked calls and carries on. Approved calls execute; denied calls answer the model with your note, in band, so it can adjust.

Mistri.agent("claude-opus-4-8", tools: tools, session: session)
      .resume

The harness renders nothing. It emits an :approval_needed event and your app draws the buttons, which is exactly the approval demo on the front page.