Why opaque checkpoints on the plane?

Framework checkpointers that require direct Postgres make the control plane’s backend choice a lie.

Decision

The plane stores opaque bytes keyed by tenant/thread. Runners serialize; the plane never parses framework internals. An HTTP proxy with CAS/ETag keeps multi-replica writers honest.

Consequences

• Durable multi-turn can sit on whatever state backend the plane already uses (Postgres / MySQL / SQLite / Mongo) — not “Postgres-only because LangGraph said so.”
• Runners need not hold DB credentials in proxy mode.
• CrewAI / LlamaIndex / AutoGen / LangChain adapters use the same /internal/checkpoints HTTP contract for message-transcript continuity (set RUNKITE_HTTP_URL). LangGraph keeps its richer saver (direct Postgres or proxy).

Rejected alternatives

• Reuse Agent Protocol thread state as a LangGraph saver — different shape; wrong abstraction.
• Force Postgres everywhere — contradicts multi-backend positioning.

See architecture.md and runner-protocol.