Graph Implementation Milestones

G1: Current Sequential Runtime

• Node
• NodeOutput
• StateGraph
• direct edges
• conditional edges
• recursion limit

G2: Builder And Compile Step

• introduce GraphBuilder
• introduce CompiledGraph
• move validation to compile
• add START and END
• add input/output schema concepts
• preserve current closure-node ergonomics

G3: Commands, Sends, And Typed Events

• add Command
• add Send
• add GraphEvent
• add event recorder
• add destination hints for command-rendered edges

G4: Channels And Partial Updates

• add Channel trait
• add state update type
• add last-value, append, aggregate, topic, and message reducers
• add invalid concurrent update errors
• add partial update examples

G5: Supersteps And Parallel Execution

• add multi-active-task executor
• apply writes at step boundaries
• add waiting/barrier edges
• add concurrency limits
• add task stream events

G6: Checkpointing, Interrupts, And Time Travel

• add Checkpointer
• add in-memory backend
• add checkpoint tuples and pending writes
• add interrupt/resume API
• add get_state, get_state_history, and update_state
• add durability modes

G7: Policies, Cache, And Error Handlers

• add graph and node defaults
• add retry, timeout, cache, and error-handler policies
• replay cached writes
• add cooperative drain

G8: Subgraphs And Namespaces

• add subgraph node
• add checkpoint namespaces
• add parent command routing
• stream nested subgraph events
• expose child state in task metadata

G9: Sub-Agents And Recursion

• add SubAgentNode
• add child run hierarchy
• add context forking for parallel sub-agents
• add shared-cache fork policy
• add recursion stack
• add depth events
• add max-depth policy
• roll up child usage and cost events

G10: Full Graph Streaming And Introspection

• expose async graph event streams
• add stream projections for values, updates, messages, custom data, checkpoints, tasks, and debug
• forward harness streams with node context
• add stream transformers
• add JSON and Mermaid graph exports