Declare the tier.
The runtime does the rest.

1# agent.py — Powered by UndoLog
2 
3from undolog_sdk import undolog_tool, ToolTier
4from undolog_sdk.tier import CompensationDescriptor
5 
6# SAFE — read-only · no log entry · replayed on retry
7@undolog_tool(tier=ToolTier.SAFE)
8async def lookup_customer(customer_id: str) → dict:
9return {name: ..., plan: ...}
10 
11# COMPENSABLE — writes · logs effect · registers undo
12@undolog_tool(
13tier=ToolTier.COMPENSABLE,
14compensation=CompensationDescriptor.new(
15"undo_create_ticket",
16args={{"ticket_id": "{ticket_id}"}},
17),
18)
19async def create_ticket(customer_id: str, amount: float) → dict:
20await tools.create_ticket(customer_id, amount)
21 
22# IRREVERSIBLE — approval gate · session suspended
23@undolog_tool(tier=ToolTier.IRREVERSIBLE)
24async def send_email(to: str, subject: str, body: str) → dict:
25return await mailer.send(to, subject, body)
26