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.

reimburse = Mistri::Tool.define(
  "reimburse", "Issues an expense reimbursement.",
  needs_approval: ->(args) { args["amount_usd"].to_i > 1000 },
) do |args|
  Expense.reimburse!(args)
end

result = agent.run("Reimburse Dana's $1,200 conference travel")
result.awaiting_approval?  # => true; nothing executed
result.pending             # the calls waiting on a decision

Small reimbursements go straight through; only the ones over the threshold park for a human.

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: "receipts don't match the report")

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.