akashi

Multi-agent + durable-execution milestone (additive; one noted sealed-union extension).

• Subagent-as-tool — Agent.asTool(...) (extension) turns any agent into a Tool a parent can call, running the child fresh with isolated history and tools.
• Handoffs — Handoff/handoff(...) expose transfer_to_<name> tools; the loop switches the active agent's model/instructions/tools while preserving history, emitting a new HandoffEvent. API note: HandoffEvent is a new subtype of the sealed AgentEvent — exhaustive switches without a default must add a case.
• First-class escalation — EscalationPolicy objects (escalateOnToolErrors, escalateAfterSteps, escalateOnLowConfidence, escalateWhen) folded into a prepareStep hook via escalate([...]).
• Message/Part serialization — partToJson/partFromJson, messageToJson/messageFromJson, version-tagged messagesToJson, and checkpointToJson/checkpointFromJson. Total encoding (non-JSON tool output degrades to a flagged string).
• Durable human-in-the-loop — ToolLoopAgent(durableApproval: true) with a CheckpointStore persists a suspended checkpoint and throws Suspended instead of blocking; resume(checkpointId, decision: ...) re-enters the suspended step. AgentCheckpoint gains additive pendingApproval, resolvedResults, and status (CheckpointStatus) fields.
• Adapter helpers — partsToText (flatten a message's parts to text) and encodeToolOutput (string-or-JSON-encode a tool result), shared by the first-party provider adapters and available to anyone writing their own.