Backend: Agent-scoped checkpoints via checkpoint_ns (agentId) + agentId-aware filters and socket init

[noa-lucent]

Context
Multiple agent instances write to the same collection and are only filtered by thread_id, causing collisions when agents share a thread.

Interface clarification

• Socket (and CheckpointerService filters) use { agentId?, threadId? }.
• Internally, agentId maps 1:1 to checkpoint_ns and should equal the runtime node id.

Correct approach (no saver namespace)

• @langchain/langgraph-checkpoint-mongodb MongoDBSaver does not take a namespace option. The correct way to scope writes is to set configurable.checkpoint_ns in the RunnableConfig passed to LangGraph.

Tasks

1. Pass checkpoint_ns (agentId) in RunnableConfig

• apps/server/src/agents/simple.agent.ts
  • Update SimpleAgent to accept agentId in constructor.
  • In init(), set default _config to include configurable: { checkpoint_ns: agentId } while preserving recursionLimit.
  • BaseAgent.invoke already merges thread_id each call, so checkpoint_ns remains set.

2. agentId-aware filtering in DB layer

• apps/server/src/services/checkpointer.service.ts
  • Add checkpointNs?: string to CheckpointWriteNormalized; map from raw.checkpoint_ns.
  • fetchLatestWrites({ threadId?, agentId? }, limit): apply both fields (agentId -> checkpoint_ns) to Mongo filter.
  • watchInserts({ threadId?, agentId? }): add to $match as fullDocument.checkpoint_ns.

3. Wire agentId from template registry

• apps/server/src/templates.ts: simpleAgent factory uses ctx.nodeId as agentId and passes it to constructor.

4. Socket init supports agentId

• apps/server/src/services/socket.service.ts
  • Extend InitPayload with agentId?: string and threadId?: string; pass through to checkpointer with mapping agentId -> checkpoint_ns.

Acceptance Criteria

• Two agents produce checkpoint_writes with distinct checkpoint_ns (matching agentId/nodeId) without modifying MongoDBSaver constructor.
• fetch/watch with agentId filter returns only matching writes.
• Socket init with agentId streams only that agent’s writes; no regression when agentId omitted.

Relationships

• Blocks: UI: Auto-filter checkpoint stream by selected agent (agentId; no extra inputs) #10
• Part of Epic: Epic: Isolate checkpoint writes per agent namespace and auto-filter UI by selected agent #8

[noa-lucent]

Blocking/Blocked-By:

• Blocks: UI: Auto-filter checkpoint stream by selected agent (agentId; no extra inputs) #10 (UI will rely on namespace support in the backend)
• Blocks: Ops: Ensure Mongo indexes via idempotent startup and migration script #11 (indexes are useful after the backend adds namespace filtering)