resume_state

Middleware that persists per-checkpoint state needed to resume a thread.

Registers two checkpointed, schema-private channels and writes them from after_model:

• _context_tokens — total context tokens from the latest AIMessage.usage_metadata. Powers /tokens and the status bar.
• _model_spec — the provider:model spec that was effectively in use for the turn, read from runtime.context["effective_model"]. Lets dcode -r restore the model the resumed thread was actually using instead of falling back to the user's global default.

Both are facts the CLI reads back from state_values on thread resume so it can rehydrate the session without replaying or re-tokenizing history.

Persisting from inside the graph (rather than via a separate client-side aupdate_state call) keeps the write on the same checkpoint as the model response and avoids creating a standalone UpdateState run in LangSmith. Because both values are versioned channel state, resuming a specific checkpoint yields the values as of that checkpoint — not a thread-level aggregate. It also works identically against local and remote (HTTP) graphs.